Volume 7, Issue 2

C 2012 Vol.7 Iss.2

2012 ARCARC-CSI DATA DVD INCLUDED

ollision C The International Compendium for Crash Research

Volume 7 Issue 2

FALL 2012

Collision: The International Compendium for Crash Research

An Analysis of

RollOver Data dynamics of vehicle to vehicle side impact crash tests

Volume 7, Issue 2 - Fall 2012

Reconstruction Essentials for Tread Separation Accidents Involving Axle Tramp

A Comparison between a Real World Crash Test, HVE Simulation and 3D Scanning collisionmagazine.com

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Contents inside

Fall 2012

Volume 7 Issue 2 View the complete article index off all published issues of Collision Magazine at : collisionmagazine.com/articleindex.html

04

Letter From the Editor

06

NAPARS: Letter From the President

07

Organization Partners

05

Advertiser Index

10

features 10

Dynamics of Vehicle-to-Vehicle Side Impact Crash Tests

24

Pilot To Determine If Using Laser Scanning Saves Time & Improves Collision Investigations

by Lawrence A. Wilson and Sean H. Haight

by Richard Auty 32

36

My Turn At The Wheel: How To Use The Combined Speed Formula,Or Not!

by Erik Carlsson 36

A Comparison Between A Real World Crash Test, HVE Simulation and 3D Scanning

by Jeffrey Suway, Anthony Cornetto, John Swanson, Fawzi Bayan, Ronny Wahba and Alfred Cipriani 46

Using & Preserving HVEDR Diagnostic Event Data

54

Determination of Impact Force and Crush Energy Using Abductive Networks

by Timothy Austin, William Messerschmidt, And Michael Farrell

by Oren Masory and Nicolas Putod 68

68

Analysis And Application Of Rollover Data From Testing

by W. R. Rusty Haight and Sean H. Haight 88

Determination of Vehicle Orientation at Ground Contact for Rollover Accidents

by Juan M. Herrera and Anselmo Najera G. 94

School Bus Submergence Collision Investigation and Passenger Survivability

by Ray Turner 100

Reconstruction Essentials for Tread Separation Accidents Involving Axle Tramp

122

by Paul T. Semones 110

Crash Team Boot Camp - Crash Test Data Review

122

FORCE-BALANCE: A Non-Standard Application of the Tool

by James D. English, John Howell, C. Bruce Gambardella, Bob Lynn and Jeff Bangle

by Daniel W. Vomhof III 132

Yaw Marks: Past And Present

by John Howell and Mario Alfonsi

www.collisionpublishing.com

Collision Magazine 3

Letter from the editor As we close out Volume 7 of Collision magazine, we might take a moment to see what’s changed this past year. Of course, we’ve gone through another Presidential election cycle and endured a massive hurricane; however, related to our industry, CFR Part 563 is now in effect along with Bosch adding four new auto manufactures to the CDR supported list (Honda, Mazda, Suzuki and Nissan). We also hosted a newly revamped 11th annual ARC-CSI Crash Conference and added a very successful mock admissibilty hearing at the 7th annual CDR User's Summit.

But what hasn't changed? For one thing, Collision magazine. As always, this issue is largely a product of information from the 2012 ARC-CSI Crash Conference (Las Vegas, June 2012) but also reflects extensive research conducted by others in the field. That sort of thing has been a central element of Collision magazine since our first issue; the notion that we merge the proceedings of either the CDR User’s Summit or the ARC-CSI Crash Conference with fresh, new, and interesting research done by those in the field. This issue is no different featuring more than 10 articles written by a truly international group of authors and leaders in their fields.

And what’s on the horizon? If the Crash Data Retrieval technology is any indication, there would be lots to look forward to. Immediately prior to the 2012 ARC-CSI Crash Conference, the current version of Bosch CDR Tool was version 5.0. As we put this issue of Collision to press, version 8.x has been released and the coverage expanded to eight auto manufacturers. The trend (if not persistent rumor) is that, in time for the CDR User’s Summit in Houston (January 2013) is that support for additional manufacturers would be announced.

This is an ever-expanding, ever-changing, ever-growing field and as 49CFR563 took effect the first part of September of this year, it laid the groundwork for CDR technology to continue to expand.

But CDR is not the only “game in town.” What else is on the horizon? For one thing, www.CrashForum.info has taken off and is poised to be a central invaluable resource for online information exchange and technology transfer. But perhaps one of the most exciting projects on the horizon will be sponsored by Collision maga-

4

Collision Magazine

zine as what we anticipate will become a landmark research project in crash analysis.

In 1992-1993, a group of SAE members underwrote a research project which became a 1993 SAE paper on barrier equivalent velocity, delta-V and repeated testing for crash reconstruction. At the time, the subject vehicles in the test series were 10-year-old Ford Escorts - cars from a time which bears almost no resemblance to today in many ways. What is near-term referred to as the “Yaris project” is an effort supported by Collision magazine to update and expand on the solid work found in the soon-to-be 20-year-old SAE paper. Using cars only five years old, the “Yaris project” will be conducted similarly to that seen in the 1993 paper but will include capabilities not available on the mid-1980s Ford Escorts.

Finite element modeling, CDR data, addressing “air gap” and tapered vehicle ends while measuring vehicle crush are just part of the goals and anticipated product of this research project. So while things change in related industries, even if our government doesn’t, one thing remains the same at Collision: we’ll be working to bring our readers and the community the latest in technology and information as it relates to crash analysis.

If nothing else sets the “Yaris project” apart, it will be that, with Collision, we will be sure to include and make available for wide distribution the videos, photos, measurements and data of the crashes conducted as part of the project. Where other forms of publishing don’t normally foster that kind of information transfer, it is become the central focus of Collision magazine and will be so with the “Yaris project.” You won’t want to miss the 2013 ARC-CSI Crash Conference where the last crashes in this series of tests that will make up the “Yaris” project will be conducted. Mark your calendar for the week of May 13!

www.collisionpublishing.com

Managing Editor

BACK ISSUES

Collision Publishing LLC PO Box 3383 Kirkland, WA 98083-3383 Toll Free: 800-280-7940 Email: editor@collisionpublishing.com Web: www.collisionpublishing.com ISSN: 1934-8681

COLLISION STAFF Scott Baker Sean Haight Tonya Baker Carla McLeod

Owner, Managing Editor Senior Editor Advertising Manager Subscription Services

All rights reserved, © 2006-12 Collision Publishing LLC. The opinions and conclusions expressed in this publication and attached data disk and throughout the articles attributed to specific authors are the opinions and conclusions of the authors noted and not necessarily the editorial staff or anyone else for that matter. While some articles have been reviewed for content, the accuracy of reprinted models or equations cannot be fully guaranteed. It is the responsibility of the reader to apply critical thinking to an individual review of the content and make their own personal judgements as to its value to them, individually.

At the end of the day, facts belong to everybody, any other opinions to us. The distinction is yours to draw...otherwise, the opinions expressed herein are not necessarily those of any employer, not necessarily ours and probably not necessary. Dissenting opinions, discussion or conclusions which may

express an adverse position to those expressed herein by specifically cited authors can be addressed in writing by sending an email to the editor at editor@collisionpublishing.com or sending a “regular mail” letter to us at PO Box 3383, Kirkland, WA 98083-3383. By sending correspondence to either our email or snail mail

addresses listed in this publication you are agreeing that: (1) we are by definition, “the intended recipient” (2) all information in the email is ours to do with as we see fit and make such financial profit, political mileage, or good joke as it lends itself to and, (3) this overrides any disclaimer or statement of confidentiality that may be included on your original message. The entire physical universe, including this publication and attached data disk, may one day collapse back into an infinitesimally small space. Should another universe subsequently reemerge, the existence of this publication and data disk in that universe cannot be guaranteed.

SUBMIT AN ARTICLE If you would like to submit an article to be published in Collision please email us your article for review or call us toll free 800-280-7940. Email: editor@collisionpublishing.com

ADVERTISING RATES & INFO If you would like to advertise your products or services in Collision Magazine, rates, availability and other information are available online at: www.collisionpublishing.com www.collisionpublishing.com

If you would like to purchase back issues of Collision Magazine, please visit www.collisionpublishing.com or call us toll free 800-280-7940.

See all back issues on page 139

ADVERTISER INDEX

PAGE

4N6XPRT Systems Inside Back, 140 ARC-CSI Crash Conference 8 ARC Network 67 CDR Training 9 Collision Publishing 139 Collision Safety Institute 9 Crash Data Group (CDR) 120, 121 CrashTest-Service.com 138 Engineering Dynamics Inside Front Eos Systems: Photomodeler 1 Laser Technology, Inc. 87 MapScenes 2, 63-66 NAPARS 6 Northwestern University 45 PC Brake Seminars 31 SAE 35, 93 Vericom Computers 109 Visual Statement 22-23, Back

Included with this issue of Collision

is the 2012 ARC-CSI Crash Conference data DVD. The ARC-CSI DVD includes all the photos, video, instrumented crash testing data, scaled diagrams, speaker PowerPoints, and any other supporting data compiled in a easy to review, menu-driven data DVD.

Next ARC-CSI Crash Conference May 13-16, 2013 www.arccsi.com Collision Magazine 5

Premier Industry Partner Letter from the President

an ongoing desire to grow and learn. But how do you become involved?

A good start is to attend training events and network with others. Take advantage of the chance to ask questions, look at issues from a different prospective, and meet your peers. People often tend to look for a familiar face and avoid the work it takes to meet new people. Networking effectively can seem difficult for those unfamiliar with it, but with some practice it is really as easy as just talking to people. Look for new faces and join in conversations.

Introduce yourself to conference presenters and offer a business card. You never know what may come from that chance conversation. You may find a mentor that will help you expand your interests and world view. Many experienced members contribute by acting as a mentor to others. It is an opportunity for them to learn as well as teach. Mentors gain satisfaction from passing along their skills and expertise to others, while re-energizing their own professional outlook.

The dictionary defines the word association as a group of people joined together for a purpose. Membership in a professional association is an opportunity to join forces for the greater good. But how do you fit in? Are you simply a “member” of NAPARS? Now don’t get me wrong – I am very thankful for each member of our association. The collective support of our members allows us to provide this fine publication, host and sponsor conferences and training events, and generally aid in the professionalization of the crash reconstruction field. But read a few Internet postings on how to write a resume and you will find that some people see membership in a national association as just a nice entry for their resume. And while it does have some value in that regard they are missing out on the greater benefits, like the opportunity to stay up to date in our profession, to develop useful contacts, to interact, and to gain help, or to help others. Involvement in an association indicates

NAPARS also offers opportunities to become directly involved by serving on the Board of Directors or on a special committee. We have a major conference to plan for in 2014 and will be looking for presenters, planners, workers, and ideas. You could help us determine the location and topic for the conference. And we are always looking for suggestions or new content for the NAPARS web site.

Information about how you can run for a NAPARS office, assist in the conference planning, or otherwise make a contribution to the growth of NAPARS is as easy as sending us an email or stopping by the NAPARS vendors table at a conference. Contact information for the Administrator Wade Bartlett and the rest of the Board members is on the web site at www.NAPARS.org. We hope to hear from you.

Respectfully, Chuck Veppert President, NAPARS

The National Association of Professional Accident Reconstruction Specialists is a non-profit organization whose members have joined together to share the challenge of dealing with the complex problems of accident reconstruction and to upgrade and ultimately professionalize the accident reconstruction field.

6

Collision Magazine

www.collisionpublishing.com

www.napars.org

Industry Partners IAARS is one of the oldest associations dedicated to

expanding the knowledge and training of accident reconstructionists. Formed in 1980, we have members from several different countries as well as many different states across the U.S. We have three levels of membership. Our highest level, Fellow, includes a peer review of a reconstruction project. This peer review feature can be very beneficial to a member seeking expert witness qualification in court. One of the criteria a judge can consider when deciding if a person qualifies as an expert is a peer review of his or her work. We offer opportunities for a member to publish work or research in the field of accident reconstruction. This, also, can be very important when qualifying as an expert witness in court proceedings.

Membership in IAARS has other advantages, including a dis-

ASPACI is a non-profit society whose members have

joined to share in the challenge of analysing the complicated issues of road collision investigation.

ASPACI was formed in 1991 by a group of police and collision reconstructionists who felt there was a need for information sharing between police and associated institutions in the Australia, South East Asia, New Zealand and South Pacific area.

Its primary function is to assist in the spreading of knowledge and enhancing expertise in the science. The Association seeks to promote a professional approach to collision investigation and to encourage the highest standard of ethics, integrity and honesty among its members.

The ARC Network

was founded in 1998 and since has grown to be the largest and most respected organization for Accident Reconstruction and Traffic Investigation.

The ARC Network is more than just an organization it is a research and communications Internet portal for everything accident reconstruction. The ARC Network provides research tools, Expert directory, event calendar, industry news feeds, education directlory and a Member's Only area that gives access to various online databases and information.

count (from IAARS) on a subscription to Collision Magazine, member discounts at our training seminars and others listed on our web page, www.iaars.org.

We have annual training opportunities at different locations across the country. We try to have some sort of actual vehicle testing at each seminar, including full scale vehicle crashes. As a member, you would have access to a vast archive of results from many different and sometimes quite unique tests.

Sincerely, Dan Lofgren President, IAARS www.iaars.org The early days saw the number of members vary with the base of the association in Victoria, Australia. ASPACI became ASPACI Incorporated in 1993 in Victoria. Now we boast members from all over mainland Australia, New Zealand., United Kingdom, and the United States, as well as parts of Asia.

Membership to ASPACI is open to all persons engaged in collision investigation and its allied fields. The association members include Engineers, Police Officers, Independent Consultants, Scientists, Insurance Corporations, Manufacturers and Safety personnel. ASPACI is not a Trade Association or Political Lobby group, nor is it a Police organisation.

User's Summit and a free copy of Greg Russell's Excel Crush Stiffness spreadsheet program. Extend your savings and sign up as a Police Agency member or Platinum member and receive a complimentary subscription to Collision Magazine!

In addition, as a member you have the choice of being listed in the most-visited Expert Directory on the Internet. Our Expert directory boasts a #1 ranking in Google searches (a feat worth much more than the cost of membership)!

The ARC Network offers various membership levels to fit your needs and budget. Membership starts at just $79.00 per year.

However, that is where the value of the ARC Network starts.

With your membership you not only get access to everything in the ARC Network, you also receive a, $50 discount to the annual ARC-CSI Crash Conference, a $50 discount to the CDR

Scott Baker President The ARC Network (800) 280-7940 www.accidentreconstruction.com arc@accidentreconstruction.com

For information on becoming a Collision Industry Partner send an email - editor@collisionpublishing.com www.collisionpublishing.com

Collision Magazine 7

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Don’t Miss the Crash Community Event of the Year!

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For the past 11 years, the annual ARC-CSI Crash Conference has been the gold standard for excellence in live, fully instrumented practical crash testing. ARC-CSI continually delivers the widest variety of exceptional speakers and unique presentation topics designed to expand your knowledge of collision reconstruction, investigation and traffic safety.

The ARC-CSI Crash Conference sets the bar for valuable insight into how various crash scenarios occur and has become the basis for a wealth of information on vehicle systems and operations.

This is a unique opportunity to experience, hands-on, what goes into setting up and conducting crash tests. Boot Camp is limited to about 8-10 people and is offered as a first-come, first-served basis.

Nothing compares to an opportunity to see live, full scale crash tests and then, hours later, get your hands on actual data, video, photos and other documentation to take back and improve your crash investigation/reconstruction job performance.

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Lawrence A. Wilson, P.E.

Sean H. Haight

Wilson Consulting, LLC

Collision Safety Institute

A

bstract Nine vehicle-to-vehicle side impact crash tests involving high closing speeds were analyzed. In each of these tests, the front of the bullet vehicle struck the side of the target vehicle. This paper analyzes the vehicle impact dynamics associated with these crash tests with an emphasis on the following vehicle impact parameters: • • • •

10

Maximum vehicle acceleration Crash pulse duration Residual crush Coefficient of restitution

Collision Magazine

These parameters were compared with moveable deformable barrier (MDB) tests conducted by NHTSA. Of the nine target vehicles involved in these crash tests, five were tested under the NHTSA Side NCAP series of tests and two were tested under the Federal Motor Vehicle Safety Standard (FMVSS) 214 test protocol. The findings from this analysis are summarized below: • The vehicle-to-vehicle crash pulses, on average, were more than twice as long as those for the MDB-to-vehicle crash tests.

www.collisionpublishing.com

Dynamics of Vehicle-to-Vehicle Side Impact Crash Tests

• The maximum lateral acceleration for the target vehicles in the MDB-to-vehicle crash tests were more than 50% greater than those for the target vehicles in the vehicle-to-vehicle crash tests.

• Even though the maximum lateral accelerations for the target vehicles in the MDB-to-vehicle crash tests were significantly greater than those for the vehicle-to-vehicle crash tests, the change in velocities (delta-v’s) associated with the target vehicles for the MDB-to-vehicles tests were significantly less than those for the target vehicles in the vehicle-to-vehicle tests.

For the vehicle-to-vehicle crash tests, the relationship between maximum residual crush of the target vehicle and crash pulse was examined. Eight of the nine crash tests involved passenger cars as the target vehicle. The remaining target vehicle was a minivan. It was found that the crash pulse increases as the maximum residual crush of the target vehicle increases. When the eight passenger cars were considered, excluding the minivan, there was a strong correlation between maximum residual crush of the target vehicle and crash pulse duration with a small variance. If the minivan was included, the lower region of the crush-crash pulse spectrum still exhibited a small variance.

www.collisionpublishing.com

Collision Magazine 11

However, at the upper end of the crush-crash pulse spectrum, the variance increases significantly, but still maintains a direct correlation between crush and crash pulse.

The coefficients of restitution for the vehicle-to-vehicle crash tests were calculated and analyzed. Negative coefficients of restitution resulted when the bullet vehicle effectively pushed the target vehicle aside and the vehicles never attained a common velocity. The vehicle-to-vehicle tests in which the bullet vehicles struck the front axle and firewall of the target vehicle exhibited the highest coefficients of restitution. Analysis of coefficient of restitution was expanded where the inter-vehicular rebound of the vehicles involved in the vehicle-to-vehicle crash tests was analyzed.

A companion paper emphasizing vehicle-to-vehicle rear impacts with high closing speed will be presented in a subsequent publication.

I

ntroduction

Damaged-bases analysis is a fundamental technique used by accident reconstructionists to determine impact severity. Impact severity of motor vehicle crashes are typically used to determine the speeds of the vehicles involved in the collision. Often times, the impact severity is used by biomechanists as a basis to determine the occupant kinematics and the injuries associated with the occupants in the subject vehicles. Damage-based analyses are heavily dependent on crash tests. The majority of crash tests used by accident reconstructionists in the U.S. are conducted as part of Federal Motor Vehicle Safety Standards (FMVSS), the New Car Assessment Program (NCAP) conducted by the National Highway Traffic Safety Administration (NHTSA) and the Insurance Institute for Highway Safety (IIHS). These tests are summarized below: Frontal Impacts: •

FMVSS 208 – Frontal 30 mph Fixed Rigid Barrier [1]

•

NHTSA NCAP – Frontal 35 mph Fixed Rigid Barrier [2]

•

IIHS – Frontal 40 mph 40% Offset Fixed Deformable Barrier [3]

Side Impacts:

12

•

FMVSS 214 – Side 30 mph Moveable Deformable Barrier [4]

•

NHTSA NCAP – Side 38 mph Moveable Deformable Barrier [3]

Collision Magazine

•

IIHS – Side 31 mph (50 kph) Moveable Deformable Barrier [5]

Rear Impacts: •

FMVSS 301 – Rear 30 mph Moveable Deformable Barrier [6]

All of these tests involve a collision between a barrier and the test vehicle. The principal advantage of barrier-tovehicle and vehicle-to-barrier crash tests is their ability to achieve consistent test results under identical conditions.

However, the principal disadvantage is the ability of a crash test involving a barrier to replicate the impact dynamics of vehicle-to-vehicle collisions in the real world. In the NHTSA Side NCAP, the IIHS Frontal and the IIHS Side Impact tests, these differences are reduced by having the barriers moveable and deformable. However, substantial differences in moments of inertia, weight, stiffness and geometry still exist between the moveable deformable barriers (MDBs) and production vehicles driven on U.S. roadways. The purpose of this paper is to analyze the impact dynamics associated with vehicle-to-vehicle crash tests in which the front of the bullet vehicle strikes the side of the target vehicle. The emphasis of this analysis will focus on the following vehicle impact parameters: •

Maximum vehicle acceleration

•

Crash pulse duration

•

Residual crush

•

Coefficient of restitution

These parameters from the vehicle-to-vehicle crash tests will be compared to those from the NHTSA Side NCAP tests and the IIHS Side Impact tests in order to identify differences in the vehicle impact dynamics between vehicle-to-vehicle crashes compared to vehicle crash tests involving barriers.

V

ehicle-to-Vehicle Side Impact Crash Tests – Test Matrix and Set Up

Nine crash tests were analyzed that involved the front of the bullet vehicle striking the side of the target vehicle. These tests are described in Table 1. In all of the tests except Test 4, the bullet vehicle was striking a stationary target vehicle. In Test 4, the target vehicle was moving at 8 mph. The minimum closing speed for these tests was 38 mph. The maximum closing speed was 54 mph.

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Test 1

Source ARC-CSI 2011 Test 4

Bullet Vehicle 2009 La Crosse

2

ARC-CSI 2011 Test 7

2006 Crown Victoria

3

ARC-CSI 2011 Test 3

2006 Crown Victoria

4

SCARS 2007 Test 1

1999 Crown Victoria

5

SCARS 2007 Test 4

1997 Suburban

6

NAPARS 2005 Test 2

2002 Crown Victoria

7

IATAI 2005 Test 1

1994 Ford E150 Van

8

NCAC 2010 Test 10010

1996 Explorer

9

NCAC 2010 Test 10016

1996 Explorer

Target Vehicle 1997 Ford Aspire

Impact Scenario Side impact. 90 degree angle at B pillar.

2006 Toyota Sienna Side impact. 90 degree angle at B pillar.

1997 Saturn SL2 Side impact. 45 degree angle at right front door.

1991 Mazda 626 Side impact. 60 degree angle at A pillar. Both vehicles moving.

2003 Dodge Intrepid Side impact. 60 degree angle at right front fender.

1994 Ford Escort Side impact. 90 degree angle at B pillar.

2002 Oldsmobile Alero Side impact. 45 degree angle at right front door.

1997 Ford Taurus Side impact. 60 degree angle at right front door.

2002 Ford Taurus Side impact. 60 degree angle at A pillar.

Table 1: Summary of Vehicle-to-Vehicle Side Impact Crash Tests All tests were captured on standard video and high-speed video. For Test 1 through Test 7, the impact speed was measured using a VBOX GPS data logger which captured over-the-ground speed at 10 Hz. Additionally, impact speeds for these tests were captured using infrared speed trap sensors spaced 3 feet apart near the point of impact.

Acceleration was measured using an IST triaxial accelerometer placed near the center of gravity of the vehicle. The acceleration data was captured at 10kHz. The acceleration data was filtered using a SAE J211 Class 60 filter. Postimpact velocities were determined by applying a SAE J211 Class 180 filter and integrating the acceleration data. The post-impact crush was measured using a laser transit total station.

For Test 8 through Test 9, the impact speed was measured using with an optical gate placed near the point of impact that is triggered by a device attached to the vehicle. Acceleration was measured using a DTS TDAS data logger and DTS 6DX 6-axis accelerometer/gyroscope which was placed near the center of gravity of the vehicle. An identical DTS 6DX 6-axis accelerometer/gyroscope was placed next to the primary instrument as a back-up. The data logger and accelerometer set-up for these tests are shown in Photograph 1a and 1b.

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Photograph 1a: Data Logger Equipment for Test 8 and Test 9

Photograph 1b: Accelerometers for Test 8 and Test 9

Collision Magazine 13

The acceleration data was captured at 10kHz. The acceleration data was filtered using a SAE J211 Class 60 filter.

Post-impact velocities were determined by applying a SAE J211 Class 180 filter and integrating the acceleration data.

The post-impact crush was photographed and measured using the protocol dictated by FMVSS 214. [4]

V

ehicle-to-Vehicle Side Impact Crash Tests – Results and Analysis

Nine vehicle-to-vehicle side impact crash tests involving high closing speeds were analyzed. The impact acceleration and velocity traces for each test are provided in Appendix A (on attached data disk). It should be noted that the velocity plots are presented in “Delta-V” format similar to those presented in Bosch CDR reports.

The actual impact velocities and post-impact velocities from the vehicle-to-vehicle side impact tests are shown in Appendix B (on attached data disk) along with a summary of other relevant impact parameters. In each of these tests, the front of the bullet vehicle struck the side of the target vehicle. The following vehicle impact parameters were analyzed: •

Maximum vehicle acceleration

•

Crash pulse duration

•

Residual crush

•

Coefficient of restitution

"

These parameters were compared with moveable deformable barrier (MDB) tests conducted by NHTSA.

Figure 1: Impact Configuration for NHTSA MDBto-Vehicle Crash Tests (courtesy of National Highway Traffic Safety Administration Laboratory Test Procedure For FMVSS No. 214 “Dynamic” Side Impact Protection, http://www.nhtsa.gov)

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These parameters were compared with moveable deformable barrier (MDB) tests conducted by NHTSA. Of the nine target vehicles involved in these crash tests, five were tested under the NHTSA Side NCAP series of tests [7, 8, 9, 10, 11] and two were tested under the Federal Motor

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Collision Magazine

Vehicle Safety Standard (FMVSS) 214 test protocol [12, 13]. A summary of these NHTSA Side NCAP and FMVSS 214 side impact crash tests are provided in Appendix C (on attached data disk). The impact configuration for the NHTSA Side NCAP test and the FMVSS 214 test, shown in Figure 1, are identical [2]. However, the FMVSS 214 impact speed for the MDB is 33 mph; whereas, the NHTSA Side NCAP impact speed for the MDB is 38 mph. The weight of the MDB is 3,000 lb. The MDB is “crabbed” so that its heading is 27 degrees counter-clockwise relative to its forward motion. The bullet vehicle is oriented so that the face of the barrier is flat against the driver’s side of the target vehicle at impact.

rash Pulses

Figure 2 shows the crash pulses in the MDB-tovehicle crash tests conducted by NHTSA. The 1997 Ford Aspire and 1994 Ford Escort crash pulses correspond to 33 mph FMVSS 214 tests. The crash pulses for the remaining vehicles correspond to 38 mph NHTSA Side NCAP tests. The crash pulses in the Side NCAP and FMVSS 214 tests exhibited a range of 60 msec to 90 msec with a mean of 69 msec. Figure 3 shows the crash pulses from the vehicle-to-vehicle crash tests. The crash pulses in the vehicle-to-vehicle crash tests exhibited a range of 125 msec to 186 msec with a mean of 153 msec.

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Comparing Figure 2 to Figure 3, the crash pulses were, on average, 122% longer for the vehicle-to-vehicle side impacts when compared to the NHTSA Side NCAP tests.

to 33.2 g with a mean of 22.6 g. The maximum lateral accelerations for the target vehicles in the MDB-to-vehicle crash test were, on average, 52% greater than those for the target vehicles in the vehicle-to-vehicle crash tests.

Figure 2: Crash Pulses for NHTSA MDB-to-Vehicle Crash Tests

Figure 4: Maximum Lateral Accelerations for NHTSA MDB-to-Vehicle Crash Tests

Figure 3: Crash Pulses for Vehicle-to-Vehicle Crash Tests

Figure 5: Maximum Lateral Accelerations for Vehicle-to-Vehicle Crash Tests

M

aximum Lateral Accelerations

Figure 4 shows the maximum lateral accelerations for the target vehicles in the MDBto-vehicle crash tests conducted by NHTSA. The 1997 Ford Aspire and 1994 Ford Escort crash pulses correspond to 33 mph FMVSS 214 tests. The crash pulses for the remaining vehicles correspond to 38 mph NHTSA Side NCAP tests. The maximum lateral accelerations for the target vehicle in the Side NCAP and FMVSS 214 tests exhibited a range of 24.7 g to 57.3 g with a mean of 34.4 g. Figure 5 shows the maximum lateral accelerations for the target vehicles in the vehicle-to-vehicle crash tests. The maximum lateral accelerations for the target vehicles in the vehicle-to-vehicle crash tests exhibited a range of 11.1 g

D

elta-Vs

Even though the NHTSA Side NCAP and FMVSS 214 tests exhibited significantly higher maximum lateral accelerations than those in the vehicle-to-vehicle tests, the delta-Vs for the target vehicles in the Side NCAP and FMVSS 214 tests were significantly less than those for the vehicle-to-vehicle tests. The delta-Vs in the Side NCAP and FMVSS 214 tests, shown in Figure 6, exhibited a range of 13.8 mph to 18.1 mph with a mean of 15.8 mph. The delta-Vs in the vehicle-to-vehicle crash tests, shown in Figure 7, exhibited a range of 14.4 mph to 29.3 mph with a mean of 24.8 mph. The delta-Vs were, on average, 59% greater for the vehicle-to-vehicle side impacts when compared to the NHTSA Side NCAP tests.

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Collision Magazine 15

In terms of maximum lateral acceleration, the vehicle-tovehicle collisions were less severe than the MDB-to-vehicle crash tests even though their delta-Vs were significantly higher. This seemingly paradoxical result is caused by the longer crash pulses associated with the vehicle-to-vehicle crash tests.

Note: The post-impact speeds for the 2002 Oldsmobile Alero-Pontiac Grand Am crash tests were not provided in the NHTSA Side NCAP report. Therefore, the data from the 2002 Oldsmobile Alero test was not included in Figures 6 and 7.

sidual crush increases. At the lower region of the crushcrash pulse spectrum, there is small variance. However, at the upper end of the crush-crash pulse spectrum, the variance increases significantly, but still maintains a direct correlation between crush and crash pulse. Eight of the nine target vehicles were passenger cars. The remaining target vehicle was a 2006 Toyota Sienna minivan. Inspection of Figure 8 reveals that Sienna minivan, when struck by a 2006 Ford Crown Victoria, sustained a relatively short crash pulse (140 msec) given its large amount of residual crush. If the Sienna is excluded so the data set includes only passengers as target vehicles, there is a strong linear relationship, given by Equation 1, with low variance between target vehicle crush and crash pulse. (See Figure 9).

Equation 1:

Cp = 3(Cr) + 90

Where: Cp = crash pulse in milliseconds Cr = maximum residual crush of target vehicle in inches R-squared, a statistical measure of data variance or scatter, is an “average” measure of how much each data point varies from the expected value as defined by the least squares fit. Comparing Figure 8 to Figure 9, the exclusion of the Sienna from the data set improves the R-squared variance from 0.48 to 0.91. An R-squared value of 1 means that there is no variance or scatter and that each data point lies on the line defined by the least squares fit. The authors would like to emphasize that this relationship is limited to side impacts. While a direct relationship between residual crush and crash pulse duration is still maintained whether the Sienna is included or not, this data suggests that that minivans may exhibit more variance and/or a shorter crash pulses.

Figure 6: Delta-Vs for NHTSA MDB-to-Vehicle Crash Tests

Figure 7: Delta-Vs for Vehicle-to-Vehicle Crash Tests

E

ffect of Crush on Crash Pulse

The effect of target vehicle crush on crash pulse was examined. Figure 8 shows crash pulse duration plotted as a function of maximum residual crush of the target vehicle. This graph shows that the duration of the crash pulse increases as the maximum re-

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Collision Magazine

Figure 8: Crash Pulse as a Function of TargetVehicle Crush

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cle-to-vehicle crash tests. Test 7 is an example in which the bullet vehicle and target vehicle have the same post-impact velocity and therefore, a coefficient of restitution of zero.

Referring to Figure 10, Test 1 and Test 8 have negative values for coefficient of restitution and have been color-coded red as a warning that the colliding vehicles never achieved a common velocity, Newton’s Third Law of Motion has not been satisfied and therefore, COLM will not accurately predict the post-impact velocities of the vehicles.

Figure 9: Crash Pulse as a Function of Target Vehicle Crush with Sienna Not Included

C

oefficient of Restitution and Inter-Vehicular Rebound

Newton’s Laws of Motion are the bases from which the principle of Conservation of Linear Momentum (COLM) is derived. Specifically, Newton’s Third Law of Motion states that when two bodies (vehicles) interact by exerting force on each other, these forces are equal in magnitude and opposite in direction. When applied specifically to vehicle collisions and the application of COLM, Newton’s Third Law of Motion stipulates the colliding vehicles must achieve a common velocity for COLM to be valid. If the vehicles do not achieve common velocity, the Newton’s Third Law of Motion has not been satisfied and the application of COLM will not provide accurate results. Also, if the colliding vehicles do not achieve a common velocity, the coefficient of restitution will be negative. Coefficient of restitution is the difference in postimpact velocities divided by the closing speed as defined by Equation 2 [14]: Equation 2:

CR = (V2 – V1)/(U1-U2)

Where: CR = Coefficient of restitution U1 = Initial velocity of Vehicle 1 (Bullet Vehicle) U2 = Initial velocity of Vehicle 2 (Target Vehicle) V1 = Post-impact velocity of Vehicle 1 (Bullet Vehicle) V2 = Post-impact velocity of Vehicle 2 Target Vehicle Equation 2 reveals the coefficient of restitution is zero if both vehicles have the same post-impact velocity. Figure 10 provides the coefficients of restitution for the nine vehi-

This scenario occurred in two instances. In both of these tests, Test 1 and Test 8, the bullet vehicle struck behind the target vehicle’s center of gravity causing significant rotation of the target vehicle. The bullet vehicle effectively pushed the target vehicle aside and never achieved a common velocity with the target vehicle. These impact dynamics are graphically illustrated in Figures 11a through 11d. In Test 8, the post-impact velocity of the bullet vehicle (blue Explorer) was 24 mph; whereas, the post-impact velocity of the target vehicle (red Taurus) was only 14.4 mph.

Figure 10: Coefficient of Restitution for Vehicle-toVehicle Crash Tests Further examination of the coefficients of restitution for the vehicle-to-vehicle crash tests shows Test 4 and Test 9 exhibited comparatively high values. In both of these tests, the bullet vehicles sustained significant engagement with the firewall and front axle of the target vehicle. Significant engagement of the target vehicle’s firewall is unique to these two tests. This data suggests that engagement of stiff structural components such as the firewall will result in higher coefficients of restitution. As an illustrative example, Figures 12a and 12b show the damage to the target vehicle for Test 9. Figure 12c shows the damage to the target vehicle for Test 4. The five remaining cases, in which no structurally significant components on the target vehicle were engaged, exhibited coefficients of restitution of no higher than 0.8.

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Collision Magazine 17

Figure 11a: Initial Contact - Test 8 (top left) Figure 11b: Maximum Engagement – Test 8 (top right) Figure 11c (bottom left) and 11d (bottom right): Blue Explorer Traveling Faster Than Red Taurus – Test 8

Figure 12a: Test 9 Target Vehicle Damage - Overview

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Collision Magazine

Figure 12b: Test 9 Target Vehicle Damage – Close Up

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Equation 4- Rebound Factor for Target Vehicle: RF2 = (V2 – VCoM)/(U1-U2) Where: RF2 = Rebound Factor for Vehicle 2 (Target Vehicle) U1 = Initial velocity of Vehicle 1 (Bullet Vehicle) U2 = Initial velocity of Vehicle 2 (Target Vehicle) VCoM = Calculated Post-Impact Velocity of Vehicle 1 and Vehicle 2 using COLM and assuming no restitution V2 = Post-impact velocity of Vehicle 2 (Target Vehicle)

Figure 12c: Test 4 Target Vehicle Damage The inter-vehicular rebound of the vehicles involved in the vehicle-to-vehicle crash tests was analyzed. By applying conservation of linear momentum (COLM) and assuming no restitution between the bullet and target vehicles, the resultant common post-impact velocity for each test was calculated. The actual measured post-impact velocities of the vehicles were compared to these calculated common velocities. The rebound factor was defined by Equations 3 and 4 as the difference between the vehicles’ measured post-impact velocity and its calculated post-impact velocity (assuming no restitution) divided by the closing speed.

Rebound factor is analogous to dividing the coefficient of restitution into a bullet vehicle component and a target vehicle component. The coefficient of restitution is the sum of the target vehicle rebound factor and the bullet vehicle rebound factor. Rebound factor for both, the bullet vehicle and target vehicle, for each of the vehicle-to-vehicle crash tests are shown in Figure 13.

Equation 3 - Rebound Factor for Bullet Vehicle: RF1 = (VCoM – V1)/(U1-U2) Where: RF1 = Rebound Factor for Vehicle 1 (Bullet Vehicle) U1 = Initial velocity of Vehicle 1 (Bullet Vehicle) U2 = Initial velocity of Vehicle 2 (Target Vehicle) VCoM = Calculated Post-Impact Velocity of Vehicle 1 and Vehicle 2 using COLM and assuming no restitution V1 = Post-impact velocity of Vehicle 1 (Bullet Vehicle)

Figure 13: Rebound Factor for Bullet Vehicle and Target Vehicles

N

egative Rebound Factors – Test 1 and Test 8

Because coefficient of restitution is the sum of the target vehicle rebound factor and the bullet vehicle rebound factor, negative rebound factors correspond to a negative coefficient of restitution. As discussed in the previous section, this indicates that the colliding vehicles did not achieve a common velocity. Test 1 and Test 8 involved impacts in which the target vehicle underwent significant rotation and effectively were pushed aside by the bullet vehicle. In Test 8, both vehicles exhibited negative rebound factors. This means that the bullet vehicle did not decelerate as much as predicted by COLM and the target vehicle did not accelerate as much as predicted by COLM. The target vehicle for Test 1 exhibited a relatively large negative rebound factor whereas the bullet vehicle for Test 1 exhibited a rebound factor close to zero. This means that the bullet vehicle achieved a deceleration close to the value predicted by COLM, but the target vehicle, like the target vehicle in Test 8, was not accelerated up to the post-impact

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Collision Magazine 19

speed predicted by COLM. An interesting similarity between Test 1 and Test 8 is that the difference in rebound factor between the target vehicle and the bullet vehicle for both tests is the very similar, approximately 0.125.

H

igh Rebound Factors – Test 4 and Test 9

Referring to Figure 10, Test 4 and Test 9 exhibited the highest values for coefficient of restitution. Considering the coefficients of restitution in Figure 10, it is expected that Test 4 and Test 9 would exhibit the highest values for rebound factor in Figure 13 because coefficient of restitution is the sum of the target vehicle rebound factor and the bullet vehicle rebound factor. In both of these tests, the bullet vehicles exhibited rebound factors that were significantly greater than the rebound factors for the target vehicles. The rebound factor for the Test 4 bullet vehicle was approximately three times greater than the Test 4 target vehicle. The rebound factor for the Test 9 bullet vehicle was approximately four times greater than the Test 9 target vehicle.

The rebound factors between the bullet vehicle and the target vehicle were approximately equal in Test 2. In the remaining tests, the rebound factor for the target vehicle was greater than the rebound factor for the bullet vehicle.

Based on the data in Figure 13, it should not be assumed that the target vehicle and the bullet vehicle have equivalent influences on the coefficient of restitution.

"

The vehicle-tovehicle crash pulses, on average, were more than twice as long as those for the MDBto-vehicle crash tests.

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C

onclusions

Nine vehicle-to-vehicle side impact crash tests involving high closing speeds were analyzed. In each of these tests, the front of the bullet vehicle struck the side of the target vehicle. The following vehicle impact parameters were analyzed: •

Maximum vehicle acceleration

•

Crash pulse duration

•

Residual crush

•

Coefficient of restitution

These parameters were compared with moveable deformable barrier (MDB) tests conducted by NHTSA. The findings from this analysis are summarized below: •

The vehicle-to-vehicle crash pulses, on average, were more than twice as long as those for the MDB-to-vehicle crash tests.

•

The maximum lateral acceleration for the target vehicles in the MDB-to-vehicle crash tests were more than 50% greater than those for the target vehicles in the vehicle-to-vehicle crash tests.

•

Even though the maximum lateral accelerations for the target vehicles in the MDB-to-vehicle crash tests were significantly greater than those for the vehicle-to-vehicle crash tests, the delta-V’s associated with the target vehicles for the MDB-to-vehicles tests were significantly less than those for the target vehicles in the vehicle-to-vehicle tests.

•

In the vehicle-to-vehicle tests, there is a direct relationship between the maximum residual crush on the target vehicle and the length of the crash pulse, the crash pulse increases as the amount of crush increases.

•

In the vehicle-to-vehicle tests, bullet vehicles that struck behind the target vehicle’s center of gravity causing significant rotation of the target vehicle.

The bullet vehicle effectively pushed the target vehicle aside as the vehicles never attained a common velocity, resulting in a negative coefficient of restitution.

•

In the vehicle-to-vehicle tests, the bullet vehicles that struck the front axle and firewall of the target vehicle exhibited the highest coefficients of restitution.

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A companion paper emphasizing vehicle-to-vehicle rear impacts with high closing speed will be available in a subsequent publication.

A

cknowledgements

The authors would like to thank the ARC-CSI Crash Conference and Rusty Haight for providing the crash test data for most of the vehicle-tovehicle crash tests analyzed in this paper. The remaining tests were conducted through a collaborative effort between the Federal Highway Administration (FHWA) and the National Crash Analysis Center (NCAC). We would like to thank Dr. Shaun Kildare, Dr. Kenneth Opiela, Dr. Kennerly Digges, Dr. Dhafer Marzougui and Mr. Christopher Story for their patience, effort and support. Also, we would like to thank Mr. Wade Bartlett for suggesting that we investigate the concept of rebound factor and Mr. Daniel Godrick for his thoughtful insight and comments.

R

eferences

1.

Department of Transportation Office of Safety Compliance “U.S. Department of Transportation National Highway Traffic Safety Administration Laboratory Test Procedure for FMVSS 208-Occupant Crash Protection, FMVSS 212-Windshield Mounting, FMVSS 219-Windshield Zone Intrusion, FMVSS 301F-Fuel System Integrity – Frontal”, DOT Report TP208-14, April 16, 2008.

2. Hershman, L. L. “The U.S. New Car Assessment Program (NCAP): Past Present and Future.” Proceedings from the 17th Enhanced Safety Vehicle Conference, Paper # 390. June 4-7, 2001.

3. Insurance Institute for Highway Safety. “Frontal Offset Crashworthiness Evaluation - Offset Barrier Crash Test Protocol (Version XIII).”

May, 2008.

4. Department of Transportation Office of Safety Compliance “U.S. Department of Transportation National Highway Traffic Safety Administration Laboratory Test Procedure for FMVSS 214-Dynamic Side Impact Protection.” DOT Report TP214D-08, December 15, 2006.

6. Department of Transportation Office of Safety Compliance “U.S. Department of Transportation National Highway Traffic Safety Administration Laboratory Test Procedure for FMVSS 212 - Windshield Mounting, FMVSS 219 - Windshield Zone Intrusion, FMVSS 301 Fuel System Integrity (Passenger Cars, MPV’s and Light Trucks with GVWR’s under 4,536 kg).” DOT Report TP301-04, January 17, 2007.

7. Mc Diarmid, P. J. et al. “Final Report of FMVSS 214 Compliance Side impact Testing of a 2004 Toyota Sienna MPV.” Report 214-CAL04-02, September 30, 2003.

8. Winkelbauer, D. et al. “New Car Assessment Program Side Impact Test of a 2000 Saturn SL2 4-Door.”, Report 214D-MGA-2000-06, July 25, 2000.

9. Winkelbauer, D. et al. “New Car Assessment Program Side Impact Test of a 2000 Pontiac Grand Am 2-Door.”, ES-2 Full Scale Vehicle Tests Report No. 6, November 16, 2000.

10. Fleck, J. “New Car Assessment Program Side Impact Test of a 1997 Ford Taurus 4-Door Sedan.” Report 214D-MGA-97-04, December 3, 1996.

11. Kosloske, D. ““New Car Assessment Program Side Impact Test of a 2000 Ford Taurus 4-Door Sedan.” Report 214D-MGA-2000-04, January 13, 2000.

12. Hauschild, H. “Final Report of FMVSS 214 Compliance Side Impact Protection of a 1997 Ford Aspire 3-Door.” Report 214DMGA-97-010, December 6, 1996.

13. Winkelbauer, D. “Final Report of FMVSS 214 Compliance Side Impact Protection Testing of a 1995 Ford Escort LX.” Report 214DMGA-2001-013, May 3, 2001.

14. Fricke, L. Traffic Accident Reconstruction – Volume 2 of the Traffic Accident Investigation Manual. Northwestern University Traffic Institute.

5. Insurance Institute for Highway Safety. “Side Impact Crashworthiness Evaluation - Crash Test Protocol (Version V).” May, 2008. www.collisionpublishing.com

Collision Magazine 21

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Pilot to determine if using laser scanning saves time & improves Collision Investigations

Richard Auty Metropolitan Police London 24

Collision Magazine

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I

ntroduction

In 2009, with the ongoing development of laser scanning technology, the Metropolitan Police Road Death Investigation Unit (RDIU) took the decision to review the potential of laser scanning for capturing data at collision scenes and to compare it against the use of Total Station Robotic Theodolites.

In particular, the question was to determine if the use of laser scanners at collision scenes could speed up survey times and if it did, which type of laser scanner would be most suitable for gathering collision scene data. I was asked to manage this project on behalf of the MPS Traffic Command Road Death Investigation Unit.

P

ulse or Phase

The first stage of the pilot study was aimed at establishing the differences between Phase and Pulse scanners and which, if either, was more suitable for Collision Investigation. Phased based scanners compare the phase of each frequency of the received signal against the transmitted signal. The scanner software then calculates from this phase difference, for all frequencies, the distance to each point. A pulse based laser scanner uses the time taken for each individual pulse to be returned and the known value of the speed of light to calculate the distance of the feature from the unit.

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Collision Magazine 25

An open invitation was made to all the major suppliers of such equipment to attend Hendon, where three collision scenes were staged on the training roads. The only requirements were that the unit had to be self-contained.

No assembly should be necessary, other than putting the unit onto a tripod. There should be no external batteries or GPS receiver; it should not require a laptop computer to operate the equipment and it must have an integral camera.

The pulse scanner had a longer effective range of over 500 metres but to attain a reasonable density of points for the settings used, 80 metres was found to be a good distance between scanning positions, therefore less scans were required. It provided good quality images with no interference.

To test the equipment three scenes were set up and the scanning time was recorded, the time starting when the box was opened and stopped when the equipment was put away.

From this initial comparison, we determined a pulse scanner was the preferred system for Collision Investigation.

•

Scene 1 - A complicated scene with multiple vehicles in close proximity

•

Scene 2 - A bend and junction with parked cars and a tunnel.

•

Scene 3 - An open section of road with two cars and transient data.

The analysis of the data provided some interesting differences between the equipment. The phase scanners were reasonably fast but with a limited effective range and examination of the scans afterwards showed that where individual scans were more than 20 metres apart, it provided a diminished end product. Thus the phase scanners were quicker but required more scans from more locations to capture a complete scene.

All the transient marks that the phase scanner recorded, were also recorded by the pulse scanner.

P

ilot

The 2010 pilot was set to run for 6 months and was designed to identify if there was any potential for significant time savings at scenes through the use of laser scanning technology and if there were, what was the extent of these gains. The pilot compared workflows and scene methodology in order to determine the best and most effective means of scene data capture.

O

bjectives

The primary objective of the trial was to quantify the time savings achieved, if any, through the use of laser scanning equipment and to identify any problems capturing the scene data whilst using the equipment. In addition a number of secondary questions were posed at the start of the trial to be answered, over the full course of the project, in order to answer questions placed into three categories; Scene, Processing and Miscellaneous.

The phase scan had ‘interference’ in the form of a wave pattern, caused by the phased nature of the scan and this required remedial work to resolve it during processing.

Ariel view of road showing Phase Scanner Interference which can be seen as a ‘ripple’ effect 26

Collision Magazine

Ariel view of road showing Pulse Scanner with no Interference and better data collection at greater distance

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S

use the Leica Robotic Theodolite Total Station at any scene they attended.

3. An effective marking system to assist in scan only scene capture

L

cene

1. An effective scene examination and processing methodology using laser scanning

2. The extent of a ‘scanning only’ option for scene data capture

4. The occasions when Total Station and laser scanning should / could be combined 5. A workflow that maximises accurate data capture in the shortest time 6. The limitations, if any, of scanning systems in Road Death Investigation

P

rocessing

1. The best workflow process to ensure scene data continuity 2. The best software packages for each process

3. What final products are attainable within the abilities of most Collision Investigators (CI’s) 4. Which products would be acceptable for criminal courts, coroners & outside agencies 5. Which methods would deliver a high quality finished product for court 6. How to present and use the products in court to best effect

M

iscellaneous

1. A suitable training package and methodology for future CI’s

2. The means to maximise the equipment’s use during investigations The equipment test had identified the two most suitable pulse scanners for Collision Investigation. The Highways Agency (HA) agreed to provide the Metropolitan Police Service (MPS) both pieces of equipment; the Leica C10 was used by the Road Death Investigation Unit (RDIU) Northwest at Alperton (TDQ) and the Riegl VZ400 was used by the RDIU Southwest at Hampton (TDT).

CI’s from the two units were trained to use the respective equipment and associated software.

The two remaining RDIU’s, Northeast at Chadwell Heath (TDJ) and Southeast at Catford (TDP) would continue to

Every callout from the four units was recorded. In total the laser scanners were deployed to 145 scenes and the Total Stations to 128 scenes. aser Scanning: Operational Issues

Some limitations were identified during the pilot.

Scan data requires photographs to colour the data points and in low light, the Leica C10 could not take photographs using its internal camera at night. This caused all captures in low light to be in greyscale only. The photographic capture on the Leica C10 took an array of high quality photographs but this in turn slowed the scene capture down. To speed the process up, an external camera solution using a ‘Zero Point Mount’ was trialled.

Once the scan was completed, the scanner was taken off the tripod and the mount put on. The Collision Investigator (CI) could then take a number of photographs, as the camera was moved through 360 degrees. However, the time taken to turn these photographs into a format the software could use to colour the scan was protracted.

It was envisaged that the longer scan time of the Leica C10 might not be an issue if the scans were collected whilst the CI was dealing with other aspects of the collision.

The reality was that officers tended to deal with the scene in a methodical manner, did not tend to scan and simultaneously complete other aspects of the investigation and when they did, often caught themselves in the scans, requiring these images to be removed in processing; which caused the processing time to be extended. By excluding the ‘scanning whilst completing other work’ workflow, the scan time became significant for this unit.

Captured data is normally stored on an internal hard drive or on a connected laptop. The MPS chose on the Riegl VZ400 to store data direct onto a 128Gb USB memory stick, which presented a compatibility issue causing a ‘Buffer Overrun’. This issue was resolved by storing direct onto the internal hard drive and transferring it to the USB memory stick once the scans were completed.

L

aser Scan Quality

The quality of captured scans is determined by the spacing between individual points, both in the vertical and horizontal axis, at a set distance. The standard setting of the two units differed at a set distance of 10 metres. The Leica C10 provided a spacing of 10mm horizontal and 10mm vertical whilst the Riegl VZ400 was set at 7mm horizontal and 7mm vertical. These settings can be changed but the increase in point density does in-

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Collision Magazine 27

crease the capture time. An alternative is to have a lower density scan for most of the scene and a higher density on important areas. The result of this method is that the final product is not pleasing to the eye as the density of the scan changes as you move through it. The standard settings are a compromise between speed of capture and a scan with sufficient detail to be of use.

M

ark Capture

The examination of the scene by the CI and the highlighting of marks did not differ greatly from the use of Total Station. Marks that are visible to the human eye and have contrast from the surface are captured by the scanner. The lighter marks, such as ABS tyre marks and those that require a more oblique observation remain elusive to the scanner.

In testing, both the Phase and Pulse Scanners identified the same clear marks but failed to collect ‘hard to see marks’.

Numbered cones, cosmic crayon and liquid chalk were used to mark the scene and any ephemeral data, to ensure capture. A uniform method was developed to ensure these could be accurately highlighted later in ‘fly through’ presentations or marked out on scale plans.

S

can Speed

For the MPS, the speed of the scanning was paramount whilst ensuring that all the data was captured to allow the road to be re-opened in the shortest time possible. The Riegl VZ400 provided in full daylight a colour scan in approximately 4½ minutes, with night scans taking about a minute longer at its standard 7x7x10 setting. The Leica C10 could provide, in full daylight, a colour scan in 15 minutes, with greyscale only at night or during low light levels, at its standard 10x10x10 setting.

S

can Angle

In a standard configuration, the Leica C10 could complete a full dome scan, useful if there was an overhead structure at the scene or if the underside of a vehicle needed to be scanned, while the Riegl VZ400 required an optional attachment to do this. This optional extra has never been required at a scene and has been used once during a vehicle examination.

P

ilot Results

During the pilot, every attendance at a collision scene within the MPS was recorded in a set format by the attending CI. Any calls resulting in no survey being completed were removed and where both scanners and Total Stations were used at the scene, only the scanner survey time was used.

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No two scenes are the same and road closure times are affected by a number of factors such as the size and complexity of the incident and investigation. The results include delays incurred as a result of actions by third party organizations including body and vehicle recovery, repair and cleaning operations. The data cannot allow for these factors to be isolated within the total road closure time but as they occur at most if not all scenes, their inclusion is unlikely to be significant, particularly as the dataset was large.

The road closure time was from the moment the first road closure was put in by Police, to when the final road was opened. The survey time was noted by the CI’s.

Data from a total of 273 valid scene examinations has been considered, with laser scanning used at 145 scenes and Total Stations used at 128 scenes.

Total Station Surveying TDP TDJ Number of call outs 68 60 Average survey time 70 60 (minutes) Laser Scanning TDQ Leica C10 Number of call outs 65 Average survey time 59 (minutes)

TDT Riegl VZ400 80 31

Total 128 65

Total 145 45

The two units using Total Station equipment showed similar survey times, which considering the number of variables discussed would tend to support a belief within units that surveying with a Total Station takes about an hour to complete at most scenes.

The two laser scanners however provided significantly disparate results.

The survey time for the Leica C10 (TDQ) was within the time range of the units using the Total Stations, whereas the Riegl VZ400 (TDT), had a survey time about half that of either the Total Stations or the Leica C10.

The above time savings would also appear to have a direct impact on road closure times: Average road closure time (Hrs:Min) Total Station 4:25 Riegl VZ Scanner (TDT) 3:36

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Once a scene has been cleared and the road reopened for normal use, there is no means of data verification or additional capture for traditional surveying methods.

Approximate scan time 360 (Minutes) Leica C10 (TDQ) 15 Riegl VZ (TDT) 4½ The first comparison of the above data should identify the disparity between the two laser scanners. The speed of the Riegl VZ400 to complete a full scan in colour is roughly twice as fast as the Leica C10, if the internal camera is not used, as and more than three times faster if the internal camera is used to capture data to colour the scans.

In addition the Riegl VZ400 provided a greater level of point density and provided coloured scans both during the day and in low light conditions at night. There is no tangible difference between the processing times of data captured by the two devices.

When comparing the data capture times of the best performing laser scanner against the Total Station, an ‘on scene’ time saving in excess of fifty percent is evident. It can be assumed that this in turn, translates into a saving of over an hour of road closure time, per incident.

The total station can provide scaled line plan of the scene with minimal back office time, i.e. the time taken to process the raw data collected at the scene into meaningful information for presentation in court.

The back office time is currently estimated at one hour to produce a basic 2D plan. It is currently taking CI’s in the region of 1½ hours, including registration, to produce the same level of information from the laser scanned data, although it is hoped, with experience, this figure may reduce.

The speed of a laser scanner at the scene needs to be considered in conjunction with the ‘back office’ time, the time required to process the raw data. Back office time is about 1¼ hours for each scene that requires processing, although only an estimated 1 in 4 scenes actually require processing for a full investigation. The time saved at the scene, which is on average 31 minutes, equates favourably to the additional back office time required to process data.

The magnitude of data collected by laser scanners in comparison with total station surveying has a number of additional benefits. The integrity of the data is indisputable.

A total station survey relies on operator skill and an individual’s interpretation of the scene including start and end of ‘marks’ and identification and positioning of evidence.

This is also true when considering the use of scanners but unlike total station, the scan provides verification by capturing the whole scene with greater detail.

As an investigation develops it may be necessary to revisit a scene to provide clarification. The ‘virtual’ scenes, as collected by the laser scanners, have removed the need for these repeat site visits.

The quality of the laser scanned data is also much greater than required for most cases. It is possible to recreate the scene in high definition 3D with quality visuals for court use, simulations, animations and a fly through.

A

dditional Considerations

The initial training and ongoing support for CI’s is essential when considering adoption of new technology. The best system can and will save time gathering data at the scene yet if this does not contribute to the creation of high quality information, suitable for use in court, any savings in field data capture time are irrelevant.

The style of training received was an initial three days of ‘hands-on’ training using the software and the scanner, followed by the use of comprehensive guides to create a support product specifically tailored to the group’s ability and requirements. This process of amending the basic guides and manuals during the basic training helped CI’s develop a working knowledge of the software while additional training days with one to one coaching were invaluable.

M

ultiple Returns

The way in which the data is recorded was of note. Multiple returns, where the laser struck two objects and both beams returned, were dealt differently by the two scanners trialled. The Leica C10 took both returns, created a point but used the average of the two distances taken to calculate the location for that point. This provided some concern as a distance from an averaged point cannot be relied upon. The Riegl VZ400 accepted both points and located them in their respective positions. This second method is called ‘Full Wave Analysis’.

The laser of the two scanners also varied in class. The Leica C10 was a visible light; whilst the Riegl VZ400 was invisible to the human eye. The visible laser was tested at night to ensure it had no effect on our Air Support Unit. They hovered about 100 feet above the unit and reported no difficulty or distraction to them.

Some uses for the scanner, such as creating products for pre-planned operations, require the scanning of locations with minimal fuss and attention. The use of a scanner with

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Collision Magazine 29

Pulse Scanner without ‘Full Waveform Analysis’ can result in the plotting of features in mid air no visible light helped prevent attention being drawn to this work.

C

ross Contamination

The cleaning of the scanner between crime scenes to prevent cross contamination requires all crevices to be accessible. The differing ‘Ingress Protection’ or IP rating of the two scanners highlighted this issue. The Riegl VZ400 was easily cleaned with all buttons being flush and no inaccessible areas. The Leica C10 had vents on the underside of the unit with fans mounted behind them for cooling. This allowed the potential for particles to be lodged within this housing area, which could not be cleaned and could allow the particles to be transported to subsequent scenes.

M

aintenance

Good maintenance support is also essential with the need for a replacement scanner, in case of breakdown at short notice. Both of these companies provided exemplary support for the scanners.

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C

ost

The cost of the scanning equipment is a significant issue. There appears at first review, great disparity in the cost of scanners but if the running costs are considered over 5 years, this apparent disparity is less obvious. The Leica C10 required annual servicing whilst the Riegl VZ400 had a much longer service interval.

The cost of the software is an additional concern. Some scanners are sold with software; other products are sold with a basic program that requires further modules to be purchased to complete other required processes. It is important to determine which modules are required for your workflow as this can have a profound effect on the overall cost.

The assessment of the software products is integral to part 2 of the pilot and is being undertaken at this time with a number of products being reviewed. This includes Pointools which has proved a very powerful, intuitive and cost effective product, compared with some of the software

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aligned to particular scanners and Edge FX, which allows the whole point cloud to be imported. A further report covering this area for the Metropolitan Police and Highways Agency is expected in the autumn.

Setting aside the 5-year + purchase and service cost analysis, the difference in the two scanner speeds is reflected in the purchase price difference of about £13,000 (In 2010).

The Leica C10, the less expensive and slower unit did not provide any discernable scene survey time savings but provided a higher level of data captured when compared to the total station.

C

onclusions

The pilot project has clearly shown that laser scanning technology can provide measurable and repeatable time savings when capturing raw data at a scene, with an average time saving of 50% when comparing the faster scanner to the total station.

It also demonstrated that the ‘survey time’ saving can be directly translated into reduced road closure times.

If these time savings of the Riegl VZ400 were to be extrapolated out across the remaining scenes attended by the other units MPS RDIU’s involved in this pilot, 199 in total, this would equate to a saving in survey time of about 100 hours. The MPS uses a rate for the economic cost of road closures of £100,000 per hour. Assuming the saving in survey times is directly transferable to a reduction in road closure, the use of laser scanning systems on these incidents alone would save an estimated £20,000,000. If this figure were to be extrapolated across all 1,200 incidents attended by the MPS CI’s it would be in the region of £60,000,000 per annum.

The Riegl VZ400 was a higher speed scanner, provided full colour scans in all light conditions at a higher intensity, is self levelling and has an in-built compass and GPS, but has a higher initial purchase price. If time was a factor, this scanner would provide a high level of data capture at a higher speed.

This article has been written as an overview of the potential time savings the use of laser scanning may provide.

Prospective users should make their assessment of the available equipment in relation to their own individual requirements and budget. The Metropolitan Police does not recommend or endorse any particular system over the other.

Further information can be sought on this project from the author at richard.auty@met.police.uk

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he T t

l e e Wh

HOW TO USE THE COMBINED SPEED FORMULA, Erik Carlsson OR NOT!

A n ur

T y M

Following are two very different examples where traffic accident analysts have used the combined speed formula to determine the speed of vehicles involved in collisions. The first case went to trial in Cook County, Illinois, while the second case, involving the U.S. Justice Department in New Jersey, settled. The readers are invited to determine when reading the two cases whether they would have done anything different. If so, they are encouraged to send their comments to the author at erikcarlsson@live.com. To make the calculations easy to follow, I have rounded the numbers.

of the skidmarks, the point of impact, and the point where the truck came to rest. The two intermediate differences in road level could serve as distances in the combined speed formula, with f= minus 1.0.] The firm’s subsequent computation of the truck’s post-impact speed, based on the length of the skidmarks from the point of impact to the point of rest, and the calculated rate of deceleration, showed a speed of 50 mph.

Based on the truck’s post-impact speed, the car, stuck in front of the truck, reached a lateral speed of 50 mph during the impact. Hence the car’s delta-V in the direction the truck was moving was CASE I. THE CHICAGO CASE. 50 mph. (Naturally, the car’s lateral speed prior A car and an empty dump truck were involved in to the impact was zero.) The weight of the truck a two-fatality traffic accident at a 4-way intersec- was 5 times that of the car and the truck’s delta-V tion close to the crest of a hill on a major highway therefore 1/5th of that of the car, or 10 mph. outside Chicago. The accident happened in daylight on a dry road. The car was crossing the high- The calculation of the truck’s pre-impact speed, way when it was broadsided by the truck coming based on the length of the skidmarks from incepover the hill. The collision was very severe, and tion to point of impact, showed a speed of 40 the car became wrapped around the truck’s front mph. (This speed is of course neither the actual end and was pushed, as if mounted sideways to speed, nor the speed reduction during the brakthe truck’s front end, a substantial distance down ing, but the speed at which the truck would have the road. The truck, which pre-dated the use of stopped at the observed distance and the calcuABS, left heavy continuous skidmarks from a lated rate of deceleration. This is the speed to be point before the collision to the truck’s point of used in the Combined Speed Formula.) rest. The point of impact could be established through debris on the road, though in particular The engineering firm decided to apply the Comthrough a small but distinct change of direction bined Speed Formula to the case. The square of of the otherwise perfectly straight skidmarks from the three calculated “speeds” (50, 10, 40) are 2500, 100 and 1600 respectively, for a total of the truck’s front wheels.

4200. The square root of 4200 is 64.8. Hence, as An engineering firm in Chicago performed a very the engineering firm stated in its report, the speed detailed analysis of the accident, which included of the truck was 64.8 mph at the point where the determination of the length of the skidmarks skidmarks began. prior to and following the point of impact, the coefficient of friction between the truck’s tires and While the speed 64.8 mph was over the limit of the road surface, and with a land surveyor mea- 55 mph, it was within the excess speed many drivsuring the slope of the road along the full length ers no doubt consider “safe.” The speed compuof the skidmarks. [It had in fact been sufficient to tations thus showed a speed that was just about measure the road level at there points – the start what probably most people involved in the inves32

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tigation expected, considering that the paved road was dry and the accident happened in daylight.

There is no doubt that the two gentlemen at the engineering firm who signed the detailed and wellwritten accident report recalculated the numbers several times before finalizing the report. After all, the report was going to be a crucial document at the impending trial in Cook County, Illinois.

Well, was there anything wrong with the speed computations? Yes, indeed! There was a fundamental error. The error probably would have been caught, had the analysts simply asked themselves what if?

What if the effect of the collision is disregarded, and the truck’s calculated pre-accident speed were based entirely on the length of the skidmarks?

The coefficient of friction between the tires and the road surface would of course be the same, as would the effect of the slope of the road. The only difference would be the omission of the squared delta-V, i.e., 100, in the combined speed formula.

Thus the calculated speed of the truck would in such a case have been the square root of 4100, not 4200.

2500w, where w represents the car’s weight. The square of the truck’s speed, with and without including the effect of the collision is 4200 or 4100 respectively, for a difference of 100, which is simply the square of the delta-V used in the combined speed formula. The truck’s weight was five times that of the car, i.e., 5w, so the difference in the truck’s kinetic energy between including and not including the effect of the collision is 500w.

The computations thus show that in order to account for the 2500w energy transferred to the car during the impact, the truck’s preaccident kinetic energy only had to be 500w higher. This is of course impossible since no energy was created at the impact! In fact, in addition to the energy transferred from the truck to the car during the impact there was also a substantial dissipation of energy due to the extensive deformation of the car.

This energy all had to come from the truck’s own kinetic energy during the impact. Thus the additional kinetic energy of the truck (required to account for the 2500w transferred to the car during the collision) had to itself be much higher than 2500w.

The square root of 4100 is 64.0. The computations thus would have shown that if the effect of the severe collision were omitted the computed pre-accident speed would be 64.0 mph, compared to 64.8 mph when the effect of the collision is included. This small difference, 0.8 mph, certainly would have raised a yellow flag. Is it possible that the severe collision that caused a virtually instantaneous reduction of the truck’s speed by 10 mph, an equally fast lateral speed increase of 50 mph for the car, and the total destruction of the car, corresponds to a pre-accident speed difference of the truck of less than 1 mph?

It may be argued that the car was moving at the time of the impact, as there otherwise would not have been a small change in the direction of the tire marks from the truck’s front wheels. Thus the car had a certain kinetic energy prior to the impact.

While this is true, none of that energy could have contributed to the car’s lateral acceleration when the car was broadsided by the truck. The car’s forward movement ceased when it became wrapped around the truck’s front end. The car’s pre-impact kinetic energy at that point was dissipated primarily through deformation of the car, with a lesser amount causing the short lateral displacement of the truck’s front wheels against the road friction.

That there is a fundamental error in the computations can be shown through very simple energy calculations, so simple that we probably make the calculations faster in our heads than on a calculator. We know that the kinetic energy of a moving object is jointly proportional to the square of the speed and to the object’s weight, irrespective of which units we use. Therefore, we can use the shown speed in mph for our energy calculations.

The car reached a lateral speed of 50 mph during the impact. The square of 50 is 2500, so the car’s kinetic energy at that point can be written as

The error in the described speed computation is that the delta-V did not represent a specific energy unrelated to the other two “speeds,” which were based on braking distance and braking force (the latter being proportional to the truck’s weight and rate of deceleration.) The combined speed formula is an energy formula, and the numbers used in the formula have to correspond to dissipated energy. The delta-V used by the engineering firm was based on conservation of momentum, not energy, and should therefore not have been used in the

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Collision Magazine 33

combined speed formula as a speed based on dissipated energy.

What the delta-V meant in the described case is that the difference between the truck’s speed at the point of its initial contact with the side of the car and at the end of the impact was 10 mph. Hence the truck’s speed when it contacted the car had to be 60 mph since it was moving at a speed of 50 mph at the end of the impact. Thus, the numbers that should have been used in the combined speed formula are the squares of 40 and (10+50), i.e., 1600 and 3600, for a total of 5200.

The square root of 5200 is 72.1. Thus the computation would have shown a speed of the truck of 72.1 mph where the skidmarks began, again had the combined speed formula been used correctly.

It may be of interest to repeat the simple energy calculations used above, using the corrected speed of the truck.

The difference between the squares of the truck’s speed when including and not including delta-V in the computation is 5200-4100, or 1100. Multiplying that by the truck’s weight of 5w shows a difference of kinetic energy of 5500w. The energy transferred to the car during the impact was 2500w, which means that the damage energy amounted to 3000w, not including the car’s own pre-impact energy.

If anyone reading this article questions whether it is acceptable to simplify energy computations to the extent I have done in this article (instead of using established units, such as the old British foot-pound), I would suggest that he or she takes the time, when his workload permits, and do the required conversions and computations. It is an interesting exercise! The weight of the car and the truck must be included in the computations, and I would suggest any reasonable weight for the car, and five times that for the truck. Anyone going through the trouble to establish actual numbers in foot-pounds in the computations would find that the car gains a kinetic energy of exactly five times the difference shown for the truck when the delta-v is included versus not included in the incorrectly used combined speed formula, as shown above. The reason for this even number of five is that the delta-V for the car was five times that of the truck. Five squared is 25, but the weight of the car was 1/5th of that of the truck, so the result was 25/5, which we know is exactly five. This also means that we can indeed simplify the energy computations, as done above, since all we need to know is whether the computed numbers are compatible.

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CASE II. THE NEW JERSEY CASE. An SUV collided with a minivan on a Sunday morning in a residential area in a small New Jersey town with narrow streets and limited sight distance. The driver of the Government-owned SUV had started to pull out from a side street when the minivan approached from the SUV driver’s left. The minivan driver braked with screeching tires, but could not slow down fast enough to enable the slow-moving left-turning SUV to clear the intersection.

There was an eyewitness to the accident, who called the local police station immediately. He stated during the subsequent investigation that he was on the second floor of his residence, situated some distance from the intersection, when he heard screeching tires. He looked out through a window and saw the vehicles before they collided. He realized that a collision was imminent because the minivan was approaching the slow-moving SUV too fast to enable the SUV to get out of the way.

Based on the damage to the vehicles, the speed of the minivan was calculated to be 30 mph at the time of the collision. While this was 5 mph over the limit, it was not considered excessive on the dry road and in the clear weather, and with little traffic on a Sunday morning. The investigating police officer did therefore not considered the cause of the accident a big deal, and saw no reason to spend time establishing presence and length of tire marks and measuring sight distance.

When the case later reached legal considerations, the speed of the minivan at the time the driver applied the brakes became the issue, not the speed when the vehicles collided.

As in many traffic accident investigations, reaction time, or perception/response time, plays an important part of the analysis. (This also explains the large number of publications dealing with drivers’ reactions in unanticipated situations.) From my experience, most accident reconstructionists tend to use a perception/response time of 1.5 seconds, possibly based on a SAE-paper sometimes referred to as the Olson paper. Some analysts apparently think 1.5 seconds is on the high side, and use 1.0 second instead, while yet some others use a longer time than 1.5 seconds.

In this case, 1.0 second was used since it was determined that the lowest probable speed, rather than the actual speed of the minivan, should be considered due to the obvious uncertainty in such factors as perception/response time and coefficient of friction between tires and road pavement. However, it was not the two drivers’s reaction times that were of interest in this case, but that of the eyewitness. He reacted to the sound of screeching tires, looked out through a window at the second floor of his residence,

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and saw the vehicles approaching each other all before the collision occurred. It is therefore clear that the duration of the minivan’s braking was longer than the witness’s own perception/response time. How much longer is of course impossible to determine. The additional time (plus the small time lag before the sound of the tires reached the witness’ residence) was therefore disregarded. Hence the collision was considered to have happened 1.0 seconds after the minivan’s brakes were applied and the tires started to slide and make the screeching sound.

The coefficient of friction between car tires and dry pavement is usually considered to be in the range 0.7 to 0.8, and most accident reconstructionists probably use the mean value 0.75, unless friction tests show a different number.

Since it was the lowest speed of the minivan that the computations would yield that was of interest, the coefficient of friction 0.7 was used in the speed computations, which is equivalent to a deceleration rate of 0.7 g. This corresponds to a speed reduction of 15.4 mph per second, which was rounded down to an even 15 mph per second. Thus, given that the speed of the minivan was 30 mph after the driver had braked for one second at a deceleration rate of 15 mph per second, the van’s speed one second before impact was determined to be 45 mph.

An investigator trained in accident reconstruction at a specific school in Florida subsequently presented a detailed report, in which he stated that the combined speed formula applied, and that the speed of the minivan was not 45 mph, but 33.54 mph, when it approached the intersection. He further stated that the minivan traveled a distance of 98.41 feet during the last two seconds before the driver applied the brakes (making the point that the minivan was then within the view of the SUV driver, who should therefore not have entered the intersection in front of the approaching minivan.)

Apart from the question whether the speed of the minivan, based on the given numbers, was 33.54 or 45 mph one second before impact, the investigator’s report was unusual in one particular respect – it is very rare to find a report in which a distance has been determined to be, or implied to be, accurate to the nearest 1/100th of a foot. This implies an accuracy of slightly better than 1/8th of an inch, or about 3 mm, which is pretty good, especially when the computations include rounded numbers!

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Collision Magazine 35 4/10/12 3:21 PM

A Comparison Between A Real World Crash Test, HVE Simulation and 3D Scanning Jeffrey Suway, Anthony Cornetto, John Swanson, Fawzi Bayan, Ronny Wahba, Alfred Cipriani S-E-A Ltd. 36

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A

bstract: A method for matching surveyed scene evidence, 3D scanning of post-impact vehicles and HVE simulation is presented. This method is useful for creating a full reconstruction of automotive accidents. This method utilizes 3D scans of the vehicles involved in the accident as well as exemplar vehicles, and a survey of the accident scene. The accident scene survey is used to create a Computer Aided Design (CAD) drawing of the physical evidence. This CAD drawing is imported into

HVE to match the impact location, scene evidence, and final vehicle positions. The 3D scans of the exemplar vehicles and the crushed vehicles are compared to determine the vehicles' crush, which is compared to the predicted crush from the HVE simulation and used in traditional calculations to calculate change in velocity (DV) and pre-impact speed. As an example, the above method was used to reconstruct a crash test from the 2012 ARC-CSI Crash Conference, with a comparison of results from the reconstruction with the crash test data.

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Collision Magazine 37

I

ntroduction:

In accident reconstruction, specific cases warrant a different approach and a different level of reconstruction.

This paper presents a complete method for vehicle accident reconstruction, including simulation and crush analysis, and compares the reconstruction to the real world crash test, upon which the reconstruction was based. To fully validate this method, the initial reconstruction was completed prior to analyzing the crash test data. During the actual crash test, the authors did not have much data about the crash, such as pre-impact speeds. The subject crash test involved a 2003-2010 Ford Crown Victoria that was driven into the passenger's side of a 1996-2002 Saturn SL2. The Saturn was angled at impact.

The crash test was reconstructed with both EDSMAC4 and SIMON simulations within Engineering Dynamics Corporation's HVE simulation software. The reconstructed crash pulse, DV, and vehicle crush were then compared to the real world test data. EDSMAC4 does not output 3D crush and SIMON does not output the simulated crush in x,y,z coordinates; therefore the simulated crush cannot be directly compared to the scan data. Future work will be performed to transform the SIMON outputted

crush into global x,y,z coordinates for direct comparison to the scan data points. Therefore, this paper only presents a comparison between planar scan data points and the simulated EDSMAC4 crush profile. While, traditional calculations of DV and pre-impact speed are not discussed in this paper, the full three-dimensional crush volume can make these calculations more accurate by incorporating every level and every point of crush into the average crush calculation.

S

cene Evidence

For this example, the scene evidence was provided

by Kent Boots as part of the 2012 ARC-CSI Crash Conference. The scene evidence collected included the post-impact tire marks and final rest positions of both vehicles. Prior to the crash test, the location of the Saturn's wheels were marked on the pavement with paint. This allowed for the pre-impact position of the Saturn to be included in the scene evidence. The authors took this collected scene evidence and placed the evidence onto a scaled Google aerial image of the impact location. Figure 1 shows the scene evidence collected at the accident site location.

Figure 1: Scene Evidence from the Subject Crash 38

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V

ehicle Crush

The authors used a 3D laser scanner to scan the subject vehicles prior to the crash and after the crash test. Scanning the vehicles prior to the crash is similar to scanning an exemplar vehicle. Using 3D scan data allows the user to measure and analyze the entire volume of crush. The traditional method for reconstruction is to use six evenly spaced measurements at the vehicle bumper level. This method assumes that the same amount of crush occurs at every elevation and that the crush between each measurement is linear. While this provides reliable and accurate reconstructions, the measurement method can be improved upon. 3D scan data allows the user to measure the entire crush profile very quickly and very accurately. A more accurate vehicle crush profile can allow the reconstructionist to learn more about the subject crash.

Figure 2 shows the scan data from the uncrushed Saturn.

Figure 3 shows the scan data from the crushed Saturn.

This scan data can be assessed in many different ways for different analyses. These analyses range from measuring specific points in 3D space to planar analyses, to full volumetric analyses of the crush.

The most basic crush analysis used with the scan data only looks at the elevation of maximum crush. Figure 4 shows a horizontal slice through the scan data points at the point

Figure 2: Scan of the Uncrushed Saturn with a Mesh of the Side of the Vehicle

of maximum crush of the Saturn. This slice allows the user to see the crush profile and extent of the deformation; however, this slice does not give the reconstructionist complete data about the crush profile. In order to get the complete crush profile, the scan data must be analyzed in 3D. Figures 5, 6, and 7 show the entire crush volume.

This volume is created by meshing the uncrushed vehicle, as seen in Figure 2, and meshing the crushed vehicle. The blue mesh lines represent the mesh from the uncrushed vehicle, and the red mesh lines represent the mesh from the crushed vehicle. To further illustrate the volume of crush measured, Figure 8 shows different slices of the crush volume. The slices are viewed from the front of the vehicle, and the slices are along a plane perpendicular to the longitudinal axis of the vehicle.

S

imulation

The scene evidence was imported into HVE's environment as a terrain file. This allows the user to match the pre-impact conditions, vehicle motion that is consistent with scene evidence, and final rest positions. For this case, HVE has both of the subject vehicles in its existing database, allowing for the standard HVE vehicles to be used for this simulation with minor changes.

The Crown Victoria had a human driver (Rusty Haight) and a 5th percentile female Anthropomorphic Test Dummy (ATD), as well as the data acquisition systems. Therefore, the Crown Victoria's weight was modified to reflect this difference.

Except for the weight of the vehicles, the HVE default vehicles were unmodified, including the default stiffnesses.

Initially, SIMON was used to recreate the subject accident. The initial position of the Saturn was known from the collected scene evidence. The Crown Victoria's speed, steering and braking inputs, and impact location and angle were iterated to match the scene evidence. In order to get the Saturn's and Crown Victoria's motion to match the scene evidence, braking and steering inputs were required for the Crown Victoria. After the simulation was completed, the authors reviewed the test video and found that

Figure 3: Scan Data from the Crushed Saturn

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Figure 4: Planar Slice of Scan Data from the PostCrash Saturn at the Level of Maximum Crush Collision Magazine 39

Figure 6: Top-view of the Crush Volume of the Saturn

Figure 5: Front-view of the Crush Volume from the Saturn

Figure 7: Crush Volume of the Saturn

Figure 8: Cross sections of the Volume of Crush 40

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there was, in fact, similar braking and steering applied to the Crown Victoria.

A reasonable match for the physical evidence was found with a speed of 44 mph for the Crown Victoria. Figure 9 shows the vehicle's at three different time steps: pre-impact, 1 second after impact and final rest. The red vehicle is the Crown Victoria and the blue vehicle is the Saturn. It can be seen that the pre-impact, post-impact and vehicle motion matches the scene collected evidence.

After a pre-impact speed was determined from SIMON, the same pre-impact conditions were used in EDSMAC4.

While there were slight changes in steering and braking timing, a pre-impact speed of 44 mph was still found to produce the best match to the scene evidence.

At this stage, the real world crash test data was analyzed and compared to the simulations. It was determined that the EDSMAC4-calculated DV and crash pulse (acceleration versus time) did not match the real world crash test data as closely as expected, however, the pre-impact speed matched very closely.

As noted earlier, the default HVE stiffness coefficients were used in the initial simulations. However, the analysis can also be performed by modifying the default HVE stiffness coefficients (A and B), prior to running any HVE simulation. The authors calculated stiffness coefficients based on

the Campbell [1] method and crash test data compiled by Neptune Engineering. It was assumed that the vehicles would experience almost zero static crush during a 5 mph and 2 mph barrier crash test for the front and side of the vehicles, respectively. Based on this work, and the known locations of vehicle damage, the stiffness coefficients for the front and side of the Crown Victoria and the stiffness coefficients for the side of the Saturn were recalculated.

Using the updated stiffness coefficients, the EDSMAC4calculated DV, crash pulse, and vehicle speed matched more closely to the measured crash test data.

R

esults

As previously mentioned, the HVE simulation was initially created when the only information available to the authors were the scene evidence and vehicle crush. This initial work yielded a pre-impact speed of approximately 44 mph, while the real world impact had a pre-impact speed, measured from a speed trap, of approximately 44.5 mph. The HVE-simulated DV, crash pulse and vehicle speed versus time did not match the real world test data as closely as expected. The authors then calculated the stiffness coefficients using the Campbell method with Neptune Engineering-published data from NHTSA crash tests of the subject vehicles.

At the ARC-CSI 2012 Crash Conference, the data presented for the subject crash shows that the Crown Victoria

Figure 9: HVE Simulation prior to impact, 1 second after impact and final rest www.collisionpublishing.com

Collision Magazine 41

experienced a 16.3 mph DV longitudinally over 140 ms as measured by the installed control instrumentation. The same type of control instrumentation was installed in the subject Saturn. This yielded an 18 mph DV in the longitudinal direction and a 23 mph DV in the lateral direction over 140ms for the Saturn.

Prior to updating the stiffness coefficients using the Campbell method, the simulated longitudinal DV for the Crown Victoria was 18.8 mph over 100 ms., the simulated longitudinal DV for the Saturn was 15.8 mph and the lateral DV was 23.7 mph, over 100 ms. Figure 10 shows the comparison between the calculated DV from EDSMAC4 prior to updating the stiffness coefficients, and the measured DV from the real world test, over time.

After the stiffness coefficients were updated, the EDSMAC4 simulation for this impact produced a longitudinal DV of 17.0 mph over approximately 100 ms. Figure 11 shows a plot of the DV from the test versus the DV from EDSMAC4 over time for the Crown Victoria. For the Saturn, HVE produced a longitudinal DV of 17.8 mph and a lateral DV of 23.4 mph over 100 ms.

Another metric of interest is the longitudinal acceleration. Based on the control instrumentation mounted in the Crown Victoria, the peak longitudinal acceleration was 12.98 g's at 70 ms. Prior to updating the stiffness coefficients, and at 70 ms, the EDSMAC4 simulated longitudinal acceleration is 15.55 g's. However, the acceleration continues to increase, and peaks at 17.68 g's at 75.5 ms.

Figure 10: DV versus Time for the real world crash test and the HVE simulation prior to updating stiffness coefficients

Figure 11: DV versus Time for the Real World Crash Test and the HVE simulation after updating the stiffness coefficients 42

Collision Magazine

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Figure 12 shows the comparison between the crash pulse for the real world test and the EDSMAC4 HVE simulation prior to updating the stiffness coefficients.

After updating the stiffness coefficient, the EDSMAC4 simulated longitudinal acceleration at 70 ms is 12.41 g's.

Again, the acceleration continues to increase, and peaks at 17.01 g's at 91 ms. Both the peak acceleration and the shape of the crash pulse improves with the updated stiffness coefficients. It should also be noted that the duration of the crash pulse is only around 100 ms in HVE, while it is 140 ms in the real test. Figure 13 shows the longitudinal crash pulse for the Crown Victoria from the EDSMAC4 simulation and the real world test.

As previously mentioned, HVE does not export 3D crush measurements in a easily, comparable, form from SIMON, and EDSMAC4 does not calculate crush in 3D. Therefore, for the time being, it is necessary to compare crush in 2D. For this, HVE's traditional vehicle damage output was used and then plotted in CAD software. All HVE crush measurements used for this comparison were used from simulation after the stiffness coefficients were modified. The previously shown planar slice from the Saturn's crush as well as a similar slice from the Crown Victoria were used to compare with the HVE crush. Figure 14 shows the planar slice from the Saturn scan data points in red, the plotted HVE crush with default stiffness coefficients in green, and the plotted HVE crush with updated stiffness coefficients in blue. Figure 15 shows the planar

Figure 12: Longitudinal Crash Pulse for the Crown Victoria prior to updating the stiffness coefficients

Figure 13: Longitudinal Crash Pulse for the Crown Victoria after updating the stiffness coefficients

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Collision Magazine 43

slice from the Crown Victoria scan data points in red, the plotted HVE crush with default stiffness coefficients in green, and the plotted HVE crush with updated stiffness coefficients in blue

C

onclusions

In many reconstructions, one of the most important questions is, how fast were the vehicles traveling prior to impact? HVE accurately matched the pre-impact speed of the Crown Victoria, knowing only the scene evidence and the vehicle type and weight. The supplied HVE-specific vehicles were used, with updating only the vehicle weight to match the as-tested vehicle weights.

This confirms that HVE is modeling the vehicle dynamics correctly and that EDSMAC4 and SIMON can successfully be used to calculate pre-impact speeds by matching the scene evidence using known vehicle parameters.

Using the supplied HVE specific vehicles, the crash pulse duration was shorter and had a higher peak acceleration when compared to the real world test. The peak of the acceleration also had a longer duration, more similar to a square wave. Therefore, the DV was higher than the real

Figure 14: Saturn Scan Data Planar Slice Compared to HVE Simulated Crush

world test. After the stiffness coefficients were updated, the crash pulse matched the real world test more closely; however, the peak is still greater and the duration of the entire crash pulse is shorter. This leads to a slightly higher simulated DV than in the real world. HVE uses the stiffness coefficients to create equal stiffness values across the entire front of the vehicle. Accordingly, this is not true for real world vehicles. Vehicles in the real world have stiff frame rails, weaker sheet metal, and air space between objects in the engine compartment, this resulting in a more sinusoidal crash pulse. However, this sinusoidal crash pulse can be modeled by a square or triangular wave. While the crash pulse and DV were not a perfect match, they were sufficiently close for the authors to consider them to be very good matches for reconstruction purposes. The simulated longitudinal DV for the Crown Victoria was 4.3% over predicted, while the simulated longitudinal and lateral DV for the Saturn were 1.1% and 1.9% over predicted, respectively. The initial speed for the Crown Victoria matched to within 1.1%.

When using the HVE default stiffness coefficients, the EDSMAC4 simulated crush of the Saturn does not exactly match the area of the most crush from the scan data. After the stiffness coefficients were updated, the EDSMAC4 simulated crush matches the entire crush profile more closely. Both EDSMAC4 simulated crush profiles for the Crown Victoria are very similar. This is a result of several factors, including the fact that the Crown Victoria's frontal stiffness is significantly higher than the Saturn's side stiffness coefficients as well as the fact that the updated frontal stiffness coefficients for the Crown Victoria were not substantially different from the default HVE stiffness coefficients. When the stiffness coefficients are calculated for the specific vehicle, and the impact conditions are modeled correctly, the simulated crush matches the real world crush closely. Furthermore, matching the simulated crush to the real world crush is a good way to further validate a reconstruction.

A

cknowledgments

The authors would like to extend their gratitude to the entire ARC-CSI group. Specifically, Rusty and Sean Haight, who allowed the authors time with each vehicle, before and after the crashes and helped interpret the ARC-CSI test data. The authors would also like to thank Kent Boots for providing the collected scene data.

R

eferences:

Figure 15: Ford Scan Data Planar Slice Compared to HVE Simulated Crush 44

Collision Magazine

Campbell, K., "Energy Basis for Collision Severity," SAE Paper Number 740565, 1974.

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Collision Magazine 45

Using & Preserving

HVEDR Diagnostic Event Data Timothy Austin, Wisconsin State Patrol William Messerschmidt, Messerschmidt Safety Consultants Michael Farrell, Fox Valley Technical College

I

ntroduction On an early fall morning in 1996, a rural Wisconsin school bus stopped in front of a farm house to pick up several students. Its flashing red lights were activated, and a station wagon stopped behind it to wait for the kids to board.

It was foggy morning, and difficult to see any great distance. Because of the reduced visibility, an approaching tractor-semitrailer driver did not

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recognize the stopped bus or its lights. Instead, it continued ahead at speeds of just over 45 miles per hour. The tractor, a conventional Peterbilt, struck the station wagon, killing its occupants.

It then drove over the car, struck the bus, and took the life of a teenager seated in the back.

This tragic incident was played out in the local

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press and in the court system, with the key being electronic data preserved in the truck engine’s Electronic Control Module (ECM). The court accepted the use of Heavy Vehicle Event Data Recorder (HVEDR) technology, and both sides of the courtroom aisle used the information to present their respective cases.

Since then, there have been numerous published studies illustrating the accuracy and use of Cummins “Sudden Deceleration” files, Detroit Diesel “Hard Brake” incidents, and Caterpillar “Quick Stop” data. More recently, articles and research have looked at “Incident Reports” from Mack and Volvo engines, as well as “Accel/Decel” re-

ports from International Maxxforce engines. In most cases, data are written when the wheel speed changes by a set amount, such as the deceleration associated with pre-collision braking. Moreover, some engine ECMs store data surrounding the last time the vehicle came to stop (“Last Stop”).

In some cases, however, these expected files are not available, even in situations where battery power is maintained throughout the collision sequence. Or, with respect to Caterpillar engines, it is possible that the “Quick Stop” feature is not activated at all. Because of this, it is important that investigators consider other HVEDR data types, such as Diagnostic Records, for use in collision analysis.

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Collision Magazine 47

C

oncern with “Hard Brake” and “Last Stop” Files

It is well known that Detroit Diesel ECMs store both Hard Brake and Last Stop incidents. This, of course, assumes the ideal conditions required for data recording. With respect to Hard Brake events, not only does the wheel speed need to change by the programmed parameter within one second of time, but it cannot be preceded or followed by an acceleration of more than four miles per hour per second [1]. In other words, drive wheels that may be only temporarily locked or momentarily obstructed could prevent a Hard Brake file from being written.

In another rural Wisconsin crash, a Freightliner driver “stabbed” and released his brakes prior to impact with a small passenger car. The involved truck is shown in Figure

1. This hard application caused the drive wheels to slip at a rate consistent with the hard brake threshold. However, when the driver released the brake, the wheel speed was observed to suddenly increase as it returned to its actual rolling state. Because of this, a Hard Brake file was not written by the ECM. Fortunately, however, a “Last Stop” record did show the actual crash event.

Last Stop files generally document the last time the wheel speed returned to 0.0 mph. The Last Stop data from the Freightliner’s Detroit Diesel engine has been partially reproduced below in Table 1. About 19 to 20 seconds before the truck comes to a stop, the driver “stabs” the brakes, causing the drive wheels to temporarily slip. The wheel speed is shown to fall from 62.5 to 30.5 mph. When the driver releases the brake, the wheels are free to roll, and the observed wheel speed increases to 50.5 mph. While the 32 mph change should have been cause for a Hard Brake file to be written, the immediate upward change created the condition wherein the ECM did not log the file.

Figure 1: This Freightliner driver "stabbed" the brakes, causing the wheel speed to rapidly decrease then increase from wheel slip. A Hard Brake file was not recorded.

While the Last Stop file in the previous example helped investigators to analyze the crash, these event records are not guaranteed, either. As Smith, et al, explained in a past issue of Collision, some Mercedes Benz engines consider the Last Stop to be the last time the engine speed achieved zero, as opposed to wheel speed [2]. If the example Freightliner had employed a Mercedes Benz power plant as opposed to a Detroit Diesel, there would have been no HVEDR crash record at all. Other situations can also be cause for the “Last Stop” to not log the actual crash event.

Police investigators in Appleton, Wisconsin found this to be true in a recent fatal incident that involved a rollover.

When the truck tipped onto its side (Figure 2), the engine

Time -0:24 -0:23 -0:22 -0:21 -0:20 -0:19 -0:18 -0:17 -0:16 -0:15

Speed (MPH) 66.5 66.5 67.0 67.0 62.5 30.5 50.5 49.5 49.0 48.0

RPMs

Brake

Clutch

Eng. Load

Throttle

Cruise

1674 1681 1688 1679 1575 763 1296 1244 1242 1215

No No No No Yes Yes Yes No No Yes

No No No No No No No No No No

59.50 % 53.00 % 35.5 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 %

66.00 % 61.60 % 50.80 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 % 0.00 %

No No No No No No No No No No

Table 1: The Detroit Diesel Last Shop File showing a hard brake condition being met. However, because it was followed by a sudden increase in wheel speed, a Hard Brake File was not written.

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Figure 2: In this rollover crash, the wheels continued to rotate, thereby preventing a Last Stop file from being written when the crash occurred. stayed running and the drive wheels continued rotate. So, a last stop was not “observed” until sometime after the crash even though the truck had in fact come to a stop.

Here, a “Hard Brake” was available to reconstructionists, thereby helping to analyze the crash. But, had the conditions have been slightly different, the possibility exists that no file may have been preserved. Similarly, after a crash in Tuscaloosa County, Alabama, the driver of a Mercedesequipped Freightliner braked to a stop and allowed the truck to idle for several minutes at final rest. In this case the “Last Stop” showed only the 1:44 seconds of idling before the police arrived and the engine was turned off.

D

iagnostic Records

Beyond the Hard Brake and Last Stop file types typically associated with HVEDRs, there is also the “Diagnostic Record.” While most engines will log these occurrences, Detroit Diesel, Mercedes Benz, and Caterpillar engines provide a significant amount of information associated with such events. Although these data include wheel speed and driver inputs, they are oftentimes overlooked by investigators. Worse yet, current methodologies for data extraction can actually overwrite the crash-related files, thereby permanently losing valuable information. In continuing to use Detroit Diesel as an example, a typical Diagnostic Record documents about 55 seconds of data prior to an engine or monitored system fault. If the ECM detects a condition such as low oil pressure, low coolant level, high coolant temperature, or other problematic situation, a record is written to the HVEDR. Figure 3, Figure 4 and Table 2 show case samples involving recorded fault conditions. Likewise, if a sensor or its wiring is damaged, the occurrence may be documented.

In many collisions, damage to the engine or other components does indeed trigger a Diagnostic Record containing valuable crash data.

Figure 3: Loss of engine coolant is evident in a visible fluid trail. This fault is typical of many crash situations.

Most ECMs have two plugs on their side or face, which are commonly referred to as the “vehicle side” and “engine side.” The “vehicle side” has connections for Original Equipment Manufacturer (OEM) sensors and options such as the wheel speed sensor, throttle position sensor, cruise control, and others that are part of the truck itself.

The “engine side” connects to the various sensors on the engine, such as those for fuel temperature, oil temperature, engine speed, fuel pressure, and similarly monitored parameters. A typical “engine side” harness is shown in Figure 5. Depending on the manufacturer and specific engine, most of these sensors will trigger a Diagnostic Record should they be damaged in a crash.

In some cases, there is a time period wherein the ECM must observe a fault condition for a certain duration before the fault is logged and data are written. The purpose of this waiting period is to avoid the writing of data for intermittent conditions. With Caterpillar, for example,

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Collision Magazine 49

Figure 4: In this case example, a Freightliner's oil pan was damaged during a collision sequence. A Diagnostic Record resulted, thereby detailing the event.

Time 11:12:22 11:12:17 11:12:12 11:12:07 11:12:02 11:11:57 11:11:52 11:11:47 11:11:42 11:11:37 11:11:32 11:11:27

Speed (MPH) 40.0 40.0 69.0 71.0 72.0 72.0 72.0 72.0 72.0 72.0 72.0 72.0

RPMs

Brake

Clutch

Eng. Load

Throttle

Oil Pressure

1386 1508 768 1579 1624 1622 1623 1624 1620 1619 1624 1619

No No Yes No No No No No No No No No

No No No No No No No No No No No No

16.0 % 10.0 % 0.0 % 21.5 % 58.5 % 69.0 % 77.5 % 79.5 % 70.0 % 66.0 % 65.0 % 62.0 %

62.8 % 100.0 % 0.0 % 42.8 % 100.0 % 100.0 % 100.0 % 100.0 % 100.0 % 100.0 % 100.0 % 70.8 %

2.9 psi 17.0 psi 34.3 psi 47.4 psi 45.9 psi 45.8 psi 45.8 psi 45.6 psi 45.8 psi 45.9 psi 46.0 psi 46.1 psi

Table 2: This table shows a portion of the information captured in the Detroit Diesel's Diagnostic Record. After the oil pan was damaged, the subsequent loss of pressure caused the fault data to be written and saved by the ECM. a Diagnostic Record typically consists of data logged for about nine seconds before the noted time, and about three seconds after. If a fault occurs with respect to a loss of power at the Boost Pressure Sensor, most engine model ECMs will document data after the fault has been active for two seconds. Here, assuming that the condition occurs during the crash sequence, it is likely that approximately seven seconds of pre-crash data would be captured. However, if the wiring is damaged to the Barometric Pressure Sensor, that waiting period may be extended to approximately 30 seconds. In this case, the data would likely be from well after the actual collision event. Exact wait-time durations can be found in engine service manuals, and, in 50

Collision Magazine

most situations, they are short enough for pre-crash data to be observed.

T

he Challenge of Preservation

Although the Hard Brake and Last Stop data file types are generally not volatile with certain precautions, Diagnostic Records can easily be overwritten.

Caterpillar engines typically store two records while Detroit Diesel and Mercedes Benz will save three. When a new fault condition is observed, it will generally overwrite the oldest in the ECM’s memory. Therefore, it is important for investigators to ensure that no additional faults are created during the imaging or download process. Generally, accessing the ECM via the truck’s in-cab Diagnostic

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Figure 5: The "Engine Side" wiring harness and sensors are assembled on a Caterpillar C15 at Fox Valley Technical College.

Figure 6: Using the truck's Diagnostic Connector will generally prevent new faults from being recorded.

(Deutsch) Connector will prevent current records from being overwritten (Figure 6). Unfortunately, in many postcrash cases, the electrical system has been compromised or collision damage prevents this technique from being employed. In these situations, the ECM is removed from the engine for a “bench” extraction. However, as research has shown, this practice will most likely overwrite any Diagnostic Records that are currently stored in the module.

equipment and options (Figure 9). Given the wide range of configurations for some engines, it is important to ensure that the vehicles and engines are indeed exemplar. Options such as Optimized Idle (Detroit Diesel) and coolant level sensor type (Caterpillar) should be carefully examined through personal inspection of the crash vehicle and its potential surrogate. In these cases, the ECM is removed from the exemplar truck’s engine, and replaced with the subject module. Imaging is then completed through the Diagnostic Connector.

In a 2011 study of extraction methodologies, David Plant of D.P. Plant and Associates in Washington, D.C. found that using any method other than establishing communication with the ECM through the Diagnostic Connector will overwrite existing data [3]. This includes the use of cables, programming harnesses, and adapters available from engine manufacturers (Figure 7). These systems typically connect only to the “vehicle side” of the ECM, leaving the “engine side” empty (Figure 8). Furthermore, sensors typically connected at the “vehicle side” are not in place. When the ECM is powered and data are not returned from the missing “engine” and “vehicle” sensors, faults are logged.

In the Plant study using Detroit Diesel and Mercedes Benz engines, Diagnostic Records were created to be typical of a crash scenario. When the modules were imaged using standard cables, all original files were overwritten with new faults. Given issues of evidence spoliation, the use of these cables is not recommended for Caterpillar, Detroit Diesel, and Mercedes Benz engine ECMs that contain EDR data associated with diagnostic faults.

U

sing Sensor Simulators (“Truck in a Box”)

A second way of imaging ECMs and recovering data when communication through the vehicle’s Diagnostic Connector is not possible is through the use of a Sensor Simulator. Commonly referred to as a “Truck in a Box,” these devices simulate the sensors connected to both the “engine side” and “vehicle side” of

U

sing Exemplar Vehicles

To avoid the potential loss of evidence associated with removing the ECM from an engine, it is possible to use an exemplar, or surrogate, vehicle. In situations where the involved unit is part of a large fleet, there may be additional trucks available that have the same www.collisionpublishing.com

Figure 7: A "bench" download using manufacturer cables may cause data to be overwritten.

Collision Magazine 51

Sample Engine and Vehicle Harness Sensors

Fuel Temperature Boost Pressure

Throttle Position

Oil Temperature

Coolant Level

Oil Pressure

Demonstrative Only

Figure 8: The standard “bench” cable connected to the “vehicle side” does not include the sensors typically found “engine side.”

Figure 9: A large fleet with many similar trucks can facilitate location of a surrogate vehicle.

Figure 10: Sensor values are read on a Mercedes Benz MBE4000 at Fox Valley Technical College

the ECM. This can be accomplished through the use of actual wiring harnesses and sensors, or by returning the appropriate sensor values to the ECM. A typical sensor receives and returns a range of voltage values to the ECM, depending on the pressure, temperature, or position of the parameter being measured. By supplying the ECM with a value within that expected range, no conditions are created wherein diagnostic records are overwritten and lost.

While this requires thorough research and testing of individual sensors (Figure 10), once completed, ECMs from individual engine families can be safely imaged off-truck using the “Truck in a Box” device.

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Sensor Simulator applications create a controlled environment for imaging where main power, ignition power, and all associated sensors are set and maintained at known val-

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Figure 11: A sample "Truck in a Box" system designed by the authors is shown connected to a Caterpillar ECM. ues. As shown in Figure 11, a “Truck in a Box” designed by the authors is shown connected to a Caterpillar ADEM 2000 ECM. By simulating all sensors associated with the truck and engine configuration, existing Diagnostic Records can be safely viewed and preserved. Without this system, it would be necessary to locate an exemplar surrogate vehicle to extract any information contained in the ECM. The Sensor Simulator, however, saves time while ensuring that all sensors and options are accounted for.

C

onclusion

Although the reconstruction community has become familiar with using HVEDR information in the form of “Hard Brake” and “Last Stop” files, there are conditions where these may not be written. In such situations, the use of Diagnostic Records may provide insight into the crash. These types of files are written when an engine or system fault occurs, and, with respect to Detroit Diesel, Mercedes Benz, and Caterpillar engines, generally contain information surrounding the particular event. In certain cases, a loss of oil pressure, a loss of coolant, or the severing of a sensor wire occurring during the collision sequence can be cause for a Diagnostic Record to be written.

While valuable, however, these file types are potentially volatile, and can be overwritten simply by attempting to image data.

To preserve Diagnostic Records, it is recommended that communication with the ECM be established through the vehicle’s Diagnostic Connector. This six or nine-pin port is usually located in the cab near the driver, and is the preferred method of data extraction. If this is not possible, investigators should seek an exemplar vehicle or use a “Truck in a Box” system. At the present time, these are the only “safe” ways of imaging HVEDR data, and such reasonable precautions should be taken whenever possible. Diagnostic Records have proved to be valuable records of a crash event, and their presence should always be considered when using HVEDR data for collision reconstruction.

R

eferences

1. Detroit Diesel Corporation, “Extracting and Analyzing Data from Electronic Control Modules and ProDrivers,” Document Number 7SE397, 2002.

2. Smith, B., et a.l, “Testing the Last Stop Record in the Mercedes MBE 4000,” Collision. 4(1), 2009.

3. Plant, D., et al., “Data Extraction Methods and their Effects on the Retention of Event Data Contained in the Electronic Control Modules of Detroit Diesel and Mercedes Benz Engines,” Society of Automotive Engineers Technical Paper 2011-01-0808, 2011.

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Collision Magazine 53

Determination of Impact Force and Crush Energy Using Abductive Networks

Oren Masory

Nicolas Putod

Mechanical Engineering Department, Florida Atlantic University

IFMA Institut Francais De Mechanique Advancee, Les Cezeaux

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A

bstract Sensory based methods as well as approximation methods are currently being used to determine the impact force and the crush energy pertained to vehicles’ collision. This paper describes a method, based on Abductive Networks, that can be used to develop explicit models by which the above quantities can be estimated. Similar to Neural Networks Abductive Networks are “trained”, using experimental data and upon convergence, a model is established. A comparison between the results, obtained by the different methods, indicate that the models obtained with this method provide more accurate results. www.collisionpublishing.com

Collision Magazine 55

I

ntroduction

The values of the impact force and the crush energy, generated in cars’ collisions, are used to determine the vehicles’ pre and post crash speeds as well as the severity of the accident from the passengers’ injuries point of view. Both quantities can be calculated using a set of coefficients, which characterize the stiffness of the vehicle’s structure and were predetermined experimentally, and the measurements of the crush in both vehicles.2,4 If sensory data are provided, in particular accelerometers’ readings from the airbag module, the value of the impact force is readily available and the crush energy can be calculated.5 The difficulties with the first method include inaccurate stiffness coefficients 4 and simple models for the impact force and the crush energy. As a result, large errors in the estimation of the vehicles’ speeds are expected. To improve accuracy, stiffness coefficients are provided for different categories of vehicles, such as passenger car or van, and within each category for different range of wheel bases dimension.

"

This paper proposes a different approach by which models for the impact force and the crush energy will be obtained automatically by training abductive networks using a set of experimental data.

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This paper proposes a different approach by which models for the impact force and the crush energy will be obtained automatically by training abductive networks using a set of experimental data. Once the model is obtained it can be used to determine these quantities for cases which were not included in the training set.

In this study one set of crush and sensory data from crash tests, performed by NHTSA, were used for training and another set for verification of the model. The results obtained by this model were compared with the results obtained by CRASH3 both relative to the actual sensory data. To demonstrate this method this study deals only with front collision with a fixed rigid barrier covering the whole width of the vehicle.

The results indicate that the estimation for the impact force and the crush energy obtained by the proposed method is more accurate than the ones obtained by CRASH3 method.

A

IM Overview

AIM is a powerful supervised inductive learning tool for automatically synthesizing network models from a database of input and output values. The model emerging from the AIM synthesis process is a robust and compact transformation implemented as a layered abductive network of feed-forward functional elements as shown in Figure 1.

Figure 1: Example Abductive Network All the functional and connection elements are learned from the input data. Currently AIM has seven types of elements. The algebraic form of each element is a polynomial where Wn are the coefficients determined by AIM and Xn are the input variables (Table 1 shows sample elements).

All terms in an element’s equation may not appear in a node since AIM will throw out or carve terms which do not contribute significantly to the solution.

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C

urrent Estimation Methods

Method based on crush measurements CRUSH3 algorithm 2 is a simple, most commonly used, model for the estimation of the impact force and the crush energy during collision.

Table 1: Example of elements definition.

The eligible inputs for each layer and the network synthesis strategy are defined in a set of rules and heuristics which are an inherent part of the synthesis algorithm.

AIM automatically determines the best network structure, element types, coefficients and connectivity by minimizing a modeling criterion which attempts to select as accurate a network as possible without over fitting the data.

The modeling criterion used within AIM is the Predicted Squared Error (PSE). The PSE is a heuristic measure of the expected network squared error for independent data not in the training database. The PSE is given by:

The estimation is based on measurements of the actual crush depths and a set of stiffness coefficients characterizing the particular vehicle. It is assumed that these coefficients are roughly the same for each class of cars, characterized by wheelbase dimensions.

Different stiffness coefficients, A and B that were found experimentally 3, are used to accommodate frontal, side or rear impacts.

The impact force, F, is determined by:

where L is the crush indentation width and is the average depth of crush, measured according to a protocol detailed in.1 The crush energy, E, is given by:

where FSE is the fitting squared error of the model on the training data and KP is a complexity penalty term determined in AIM by the equation:

where K, N and sp2 are determined by the database of examples used to synthesize the network and CPM, the Complexity Penalty Multiplier, is a variable the user can select.

The default value of CPM is 1; a lower value decreases the complexity penalty impact and results in a more complex network and inversely for a higher value.

To create a model using AIM one has to follow these steps: 1 Decide what are the inputs and the output of the model.

2. Create a database which includes sets of inputs and the corresponding outputs from the process being modeled.

3. Train the abductive network using the above database.

4. Evaluate model performance of the model using sets of inputs/outputs which were not used to train the network.

5. Once the network (model) performs to satisfaction an explicit model can be derived and implemented.

where Ci are equally spaced crush indentation values along the indentation length.

The accuracy of this algorithm depends on the accuracy of the stiffness coefficients as well as the accuracy of the crush indentations. The stiffness coefficients as well as the vehicles classification are being updated from time to time to accommodate for major changes in vehicle’s structure designs and the introduction of new designs [4].

Sensory Based Method This method is based on the crash test experiments realized by the NHTSA or acceleration data available from the airbag module. It is assumed that the acceleration signal is sampled with sufficient rate to capture the actual acceleration the vehicle has experienced at impact. The maximum value of impact force can be obtained directly from this signal:

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Collision Magazine 57

T

where m is the vehicle’s mass.

Similarly, the average value of the impact force can be obtained by:

where a is the acceleration, N is the number of samples and T is the sampling period and ΔT is the impact duration time.

The acceleration signal can be integrated with respect to time in order to determine the vehicle velocity, v. The sampling rate is usually very high, so any numerical integration method can be used. Using trapezoidal integration, the velocity at instant k, is given by:

he Modeling Process

Create a database A data base, that contains the information items shown in Table 2, for 50 different passenger vehicles was constructed. Vehicle information and the crush indentation were extracted directly from NHTSA data base and the impact force and crush energy was calculated using acceleration data given at the same data base. The assignment of these items as input to or output from the model is also indicated in table 2.

Parameters Wheelbase Weight Indentation Length Crush Depth 1 Crush Depth 2 Crush Depth 3 Crush Depth 4 Crush Depth 5 Crush Depth 6 Max Force Crush Energy

where v0 is the approach velocity.

Symbol Wheelbase Weight Indentation

Unit mm kg mm

State Input Input Input

C1 C2 C3 C4 C5 C6 Force Energy

mm mm mm mm mm mm N Joules

Input Input Input Input Input Input Output Output

Table 2: Data base information

A second integration will yield the vehicle displacement s(kT):

Using the above results, the crush energy, E, can be determined by:

where kT is the time when the vehicle’s velocity is zero.

This method has been demonstrated in Masory/Favrot 5 using sensory data provided by NHTSA. In this particular reference, it has shown that most of the signal energy is contained in very low frequencies (below 50 Hz) and therefore in this study a Low Pass Filter with a cutoff frequency of 100 Hz was used to filter the raw acceleration signal.

Obtaining models through Training The network was trained using 50 sets of data from the data base. The best models have been obtained by adjusting the CPM value. The effect of the CPM values on the models’ performance are shown in Tables 3 and 4. The errors indicate the model capability to predict the output given data which was used for training, CPM Average Error (%) Max Error (%) Min Error (%)

1 11.99 31.7 0

0.5 11.14 29.27 0.08

0.25 9.68 31.71 0

Table 3: Impact Force CPM Average Error (%) Max Error (%) Min Error (%)

1 10.53 37.58 0.00

0.5 8.45 28.02 0.00

0.25 7.86 27.57 0.00

Table 4: Crush Energy 58

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0.1 9.65 32.49 0

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0.1 7.46 24.44 0.00

Based on the results in Tables 3 and 4, for impact force model the CPM value was set to 0.25 and for crush energy to 0.1.

C

omparison With Other Methods

To compare the performance of the Abductive network model to other methods the following steps were taken:

1. A new set of 50 passenger vehicles was selected and the relevant data was extracted from the NHTSA data base.

Figure 2 illustrates the absolute error in percentage of the impact force between the results obtained by CRUSH# and the results obtained by Abductive model with respect to the results obtained by the sensory method. As shown for most cases the Abductive model provides slightly better results and its average error is about 3% lower than the average error produced by the CRUSH3 method.

Table 5 provides the statistics related to these results.

2. The impact force and the crush energy were determined using the three methods mentioned above.

Average Maximum Minimum

3. The results from by the CRUSH3 method and the model obtained from the trained Abductive network were compared to each other using the result obtained by the sensory method as a reference.

The reason for selecting the results obtained by sensory method as a reference is to due the fact that the values were obtained directly from sensors that were mounted on the vehicle during the collision. These sensors were carefully calibrated and were mounted on the vehicles’ chassis and as result the acceleration readings are very accurate.

Force Crush3 AIM 21.34 18.15 60.28 86.00 0.06 0.20

Crush Energy Crush3 AIM 22.87 16.74 61.71 47.77 1.86 0.83

Table 5: Comparison results Figure 3 demonstrates the direct relationship between the results obtained by the sensory and the Abductive methods.

Similarly, Figure 4 shows the absolute errors obtained by both methods in the estimation of the crush energy. In this

Figure 2: Impact Force Absolute Error To Experiment

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Figure 3: Impact force results – Abductive method versus Sensory method.

Figure 4: Crush Energy Absolute Error To Experiment

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Figure 5: AIM/Sensor Comparison

case the Abductive method provides better result than the CRUSH3 method. On the average, the Abductive method error are 6% lower than the ones produced by Crush3 one.

where Vinitial and Vfinal are the pre and post collision speeds of the vehicle. Arranging Eq. 11 yields the required speed:

Similar to Figure 3, Figure 5 shows the relationship between the results obtained by the sensory and the Abductive methods.

I

mpact Speed Calculations

In most accident reconstruction cases the pre-collision speed is sought. The Principle of Energy Conservation is used for this purpose and it is formulated for this case as follows:

where Ek is the kinetic energy and ECrush is the crush energy.

Explicitly it is given by:

In most cases, the kinetic energy that corresponds to the post impact speed is very small (few percents) and can be neglected. Thus, the initial speed value is determined by:

The crush energy results obtained by the Abductive and the CRUSH3 methods the corresponding pre-speeds were calculated by Eq. 13. These results were compared with the pre-speed information that is provided in the NHTSA database. The errors in the pre-speed estimation are shown in Figure 6. As indicated the average error produced by

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Collision Magazine 61

Figure 6: Velocity Absolute error.

the Abductive model is below 5% while the average error produced by the CRUSH3 model is above 14%.

C

onclusions

The results obtained in this paper indicate:

1. The estimated values for the impact force and the crush energy obtained by the Abductive model are more accurate than the ones obtained by the CRUSH3 model.

2. The development process of the Abductive model is straight forward and does not require any assumptions.

3. The results obtained by the Abductive model can be improved by including additional inputs, e.g. vehicle classification by wheel base.

R

eferences

1. AIM User’s Manual, AbTech Corporation, 1990

2. CRASH3 User’s Guide and Technical Manual, NHTSA, 1981 3. NHTSA Web site: www.nhtsa.com 4. Donald E. Siddall, Terry D. Day, “Updating the Vehicle Class Categories”, SAE Paper #960897, 1996 5. O. Masory, C. Favrot, “Determination of Impact Forces and Crush Energy Using Sensory Data and Comparison with Other Algorithms”, Proc. 15th Florida Conference on Recent Advances in Robotics, May 23-24, 2002.

4. Since the model development process is so easy it will be possible to update the model as more experimental data become available

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ANALYSIS AND APPLICATION OF

ROLLOVER DATA FROM TESTING W. R. RUSTY HAIGHT SEAN H. HAIGHT COLLISION SAFETY INSTITUTE

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R

ollovers represent a small fraction (approximately 3%) of all motor vehicle crashes; however, they account for roughly 22% of crash fatalities for occupants of passenger cars, light trucks, and SUVs. 1 As much as they are disproportionately represented in the overall body of fatal crashes, rollovers remain one of the more difficult events to analyze or reconstruct. With the advent of “event data recorder” (EDR) technology, and the Bosch Crash Data Retrieval (CDR) Tool as one of the major emerging weapons in the crash reconstructionist’s arsenal, the investigation and analysis of rollovers has the potential of becoming somewhat less daunting and results of the analysis more meaningful. This narrative will examine the background for, the conduct of and the resulting data from a full scale

rollover crash test involving a 2011 Jeep Grand Cherokee conducted at the 2012 ARC-CSI Crash Conference.

B

ackground For those vehicle lines accessible using the Bosch CDR Tool, General Motors (GM) was the first to make rollover event data part of the data set available to the reconstructionist.2 Starting with model year 2007, many GM light trucks and SUVs were equipped with a RollOver Sensor (ROS) which was installed in the vehicle and closely associated with its Sensing and Diagnostic Module (SDM) or airbag control module (ACM). The ROS (together to one degree or another with the ACM/SDM) controlled the deployment of occupant containment (“roof rail side”) curtains and then, where conditions permitted, together they may record several data elements associated with either a “Rollover” or “Non-rollover” event. 3

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Collision Magazine 69

Other CDR Tool supported manufacturer’s (OEs) lines also include rollover event related data in their potential data sets for recent vintage vehicles. For example, while introduced to the CDR Tool set of accessible OEs/vehicles later than GM, Toyota line light trucks and SUVs have the potential for recording rollover related data back to calendar year 2003.2 Where available for certain vehicles, rollover related data is found more frequently in relatively "563" Data parameter 1 Speed, vehicle indicated Steering input Stability control (on, off, engaged) Ignition cycle, crash Lateral acceleration Normal acceleration Vehicle roll angle Pretensioner deployment, time to fire, driver Pretensioner deployment, time to fire, right front passenger Side curtain/tube air bag deployment, time to deploy, driver side Side curtain/tube air bag deployment, time to deploy, right side Complete file recorded (yes, no)

Table

newer model year vehicles across the many lines supported by the CDR Tool (see Table 1).

About model year 2010, GM line vehicles which offered occupant containment protection for rollover events controlled those components using an ACM which incorporated rollover sensing and logic in the ACM rather than as a function of a separate ROS as had been the previously

GM ROS 2 Table 1 Yes Table 2 No Table 2 No

GM ACM 5 Yes Yes Yes

Ford Chrysler Toyota Honda 5

5

5

5

Nissan Mazda Suzuki 5

5

5

Yes Yes Yes

Yes Yes Yes

Yes 6 Yes 6 Yes

Yes Yes Yes

Yes Yes Yes

Yes Yes Yes

Yes Yes Yes

Table 1 Table 2 Table 2 Table 2 Table 2

Yes 3 Yes Yes Yes 4 No

Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes

Table 2 No

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Table 2 Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Table 2 Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Table 1 Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Table 1 1 - Select data parameters from 49CFR563 representative of those which might otherwise be commonly sought after in the analysis of a roll over event. While other parameters (i.e.: RF occupant classification) may be available in certain circumstances, this list is more narrow for the purposes of this analysis.

2 - ROS for 07-09 GM vehicles would "share" some data elements with those which might be found in SDM data 3 - "Ignition cycles" would be a value found in the SDM, a "synchronization counter" (comparable to an ignition cycle counter) would be common to the SDM and ROS 4 - Records "roll rate" data rather than "roll angle" but angle could, of course, be calculated from the roll rate 5 - Starting with modules/vehicles which would have to be "563" compliant (built for sale after Sept 2012) and which may have "occupant rollover protection" of some sort. Some may be subject to the "if equipped" limitation.

6 - Toyota "Gen 3" modules which record pre-crash data have a different resolution than other OE modules/systems

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used process. This approach is in line with other CDR Tool supported OEs including Toyota, Honda and Chrysler - including, for this analysis, Jeep.

In September 2012, the long anticipated 49CFR563 (“563") went into effect.4 In that regulation, there is no requirement under “Table 1" (563.7 Data elements. (a) Data elements required for all vehicles) that an OE record data which might be considered directly descriptive of a rollover event; however, in “Table 2" (563.7(b) Data elements required for vehicles under specified conditions) there are several elements which would clearly be associated with rollover events and which would, “if recorded (by the OE’s system),” be part of the data set available to the CDR Tool user. Ideally, this additional data would help the crash analyst better understand the event(s) under investigation, in this context, specifically the rollover event.

That is not to say that some of the elements found in “Table 1” aren’t useful and wouldn’t be at all related to a rollover event. For example, “Speed, vehicle indicated” might often be useful in the overall analysis of a rollover event but data elements such as “lateral acceleration” 5 or “normal acceleration” 6 are directly associated with the dynamics of a rollover rather than with a frontal (negative “X” axis) event. Table 1 is a simplified overview of some of the data which might be available for select vehicles supported by the current version of the CDR Tool 2 . Notably, most OE systems for model years 2012 and newer have, at least, the potential for this data (and more) in part as a function of “563.”

Within “563” we also find section 563.8 (a); “Data format” which, as a function of “Table 3 - Reported Data Element Format” lays out for the OEs how the data wil be recorded/reported with respect to range, accuracy, and resolution. “Table 3" is applicable to those elements found listed in Tables 1 and 2 in “563.”

In “563,” at “Table 3,” we find that the reporting range, accuracy and resolution for normal acceleration and lateral acceleration are left to “(the) option of (the) manufacturer.” “Vehicle roll angle” 7; however, has specified range, accuracy and resolution requirements and, notably, the accuracy requirement applies “within the range of the physical sensor.” From the regulation: “If measurements captured by a sensor exceed the design range of the sensor, the reported element must indicate when the measurement first exceeded the design range of the sensor.8 ” Ultimately, data from the dynamic test which is the basis for this narrative illustrates why an understanding of what that is and is not and where or how that indication may be significant in a given analysis.

In application, this means that (after September 2014) the “563 compliant” “EDR” report will have to indicate where data was/is “clipped” (where a sensor is “saturated,” when that occurs) but, in the meantime, it appears to be left to the OE or the end user analysts to evaluate whether or not that has occurred. Historically, “clipping” has been observed in CDR data since the earliest GM accessible ACMs and is demonstrably an issue where it relates to liner acceleration and/or delta-V whether on the longitudinal or lateral axis (although observed in the earliest cases along the X axis in frontal delta-V data from GM vehicles).

Where “clipping” has been observed as it relates to either linear acceleration or delta-V, a natural extension of that phenomenon has been that the cumulative quantity(s) tends to be otherwise generally under reported. A careful reading of “563” suggests that NHTSA anticipated that may be the case and they have called for an indication of the first point where “clipping” occurs as part of the “563” data set. As will be seen later in this narrative, “clipping” may be observed with angular data sets and may impact cumulative integrated quantities but would not necessarily render the data after the first observed “clipping” grossly inaccurate. There are other considerations, particularly as it relates to a rollover, which would come into play first, potentially making rollover related data unreliable - in either application or appearance.

Not the least of these might be the ACM’s recognition of the onset of the event. In “563,” there is a definition for “time zero” 9 which includes a reference to (1) control module “wake up” or “algorithm enable,” (2) a linear delta-V (on either the “X” or “Y” axes during a specified period or (3) the deployment of a “non-reversible deployable restraint.”

While not listed as a definition in “563,” the meaning and intent of the Agency with respect to “non-reversible deployable restraint(s)” was addressed as recently as August 9, 2012 in the Federal Register where NHTSA replied to various petitions for reconsideration (of the rule). NHTSA writes:

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“...We are denying the Alliance and ASC petitions to clarify how the deployment of non-reversible restraints other than air bags are recorded. We believe that section 563.9(b) is clear that when a memory buffer is available, EDRs must capture and record current event data that does not involve deployment of an air bag. ...manufacturers may prioritize the capture of events that do not involve deployments of air bags, but do deploy other non-reversible restraints regardless of type.” 10

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It appears from the context and petitions for reconsideration that the generally accepted use of the phrase “nonreversible deployable restraint” may - depending on the entity or individual using the phrase - describe any “one time use” restraint related device controlled by the ACM except frontal or side/curtain airbags. Specifically, some would categorize safety belt pretensioners as a “non-reversible deployable restraint” which is not an “airbag.” Others consider the deployment of any pyrotechnic device a “deployment.” In addition to safety belt pretensioners, other devices which may now, or at some time in the future, be found on vehicles and may be controlled by the ACM include active head restraints and “pedestrian protection devices.” At present, the deployment of the active head restraints in the Jeep Grand Cherokee used in the underlying testing resulted in “non-deployment” events (although Chrysler line vehicles don’t apply that naming convention). These were not “locked” and could be overwritten; a condition which factors into later analysis in this narrative.

In “563” at “Table III” we find a reference to reporting accuracy where the tolerance is given as a percentage of an unspecified number but which would/should reasonably be based on a “band” along the curve of the data set rather than a value based on a single easily identified number. For example, we might consider the data parameter “vehicle roll angle” 7 where the minimum reporting range called for is -1080deg to +1080deg. The “accuracy” requirement is specified as +/-10% and the reporting resolution is 10deg.

On first blush, one might ask if is the “accuracy” requirements based on the full value of +/-1080deg or based on 10deg? Of it is based on the full value (1080deg) then the band would be +/- 108deg. Of course, if it were based on the 10% resolution, then the band would be +/-1deg.

What about basing it on a midrange quantity (i.e.: 540deg, which would result in a +/-54deg band)?

Without other specificity in “563,” it would seem that this data element’s accuracy is open to individual interpretation. While an SAE committee comprised primarily of representatives from the various OEs and interested parties has been working on (a) standards and definitions to address these sorts of apparent inconsistencies or deficiencies within “563,” still in all, we are left with large range values and a requirement for accuracy to a certain percentage without further discussion or clarification on how that percentage may relate to relatively larger or smaller absolute values within the range.

This authors’ understanding of the application, as of this writing, would be that the OEs and data end users are, at least for now, left to apply the “reasonable man test” to some of the data parameters. For example, as it relates to

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“speed, vehicle indicated” the range is 0 to 200km/h with an accuracy of +/-1km/h. Obviously then, where there are like units, quantities or descriptors, the understanding or perhaps the intent becomes more clear (it’s not a percentage of “some speed”). Where we have an angle which might be, for example, 50deg or it might be 500deg, a simple 10% of each of those numbers gives us a much wider range if we apply the percentage to those quantities. Depending on interpretation, at smaller values, the accuracy requirement becomes ever more difficult to meet whereas applied to the larger numbers, it seems almost an irrelevant value and brings to mind the notion that “even a blind squirrel finds a nut sometimes.”

Within this narrative, the assumption of the authors will be that the accuracy provision will be based on the quantity at each individually captured point where those points are normally associated with time. For example, in evaluating roll rate later in this narrative, the quantity observed using the researcher’s control instruments would be compared at the same time interval with the data reported by the airbag control module. The percentage would be taken relative to the control device’s reported quantity at an individual point.

Assuming the test instrumentation provides an accurate portrayal of the vehicle's roll rate and integrated roll angle, the percent error could be defined as the calculated roll angle from the CDR report data less the roll angle given by the test instrumentation. That number is then normalized by (divided by) the test instrumentation roll angle. The resulting decimal would be adjusted to represent a percentage.

Ultimately, the application of some of the requirements found in “563,” in some instances, may serve to make an understanding of or presentation of recovered data difficult to evaluate if not generally inconsistent in its application from one end user to the next. The dynamic test which is the basis for this narrative may be useful in addressing that notion. It is important to point out that the vehicle involved in the underlying dynamic test was not, by definition, subject to the requirements of “563” when the test was conducted but that, for the instant evaluation, a comparison of the recovered data to control data may be useful later.

T

esting overview

References to rollover related testing has conducted by the OEs and, to one degree or another as anticipated by the government, are varied and found widely in the literature. Static stability tests, half corkscrew ramp tests (see sidebar) and tests involving vehicles propelled toward a point of trip on a dolly are all found

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within the larger body of the literature describing this type of test protocol.

This researcher has been involved in various types of rollover testing over the last 14 years. Such testing usually involves a vehicle under its own power or being pushed and where the sudden application of steering to either cause a “slalom” type movement or where the vehicle is intentionally driven off the roadway and a rapid attempt to recover resulting in a rollover or near- rollover occurred. With a wide variety vehicles and under a broad group of scenarios, the results varied.

It was not until the 2012 ARC-CSI Crash Conference that such testing by this researcher and his able team of colleagues making up the ARC-CSI Conference Crash Test Team was undertaken successfully with a vehicle which not only rolled but was equipped with an airbag control module which recorded roll related data which could be compared to the control instruments installed in the vehicle for the testing by the Team.

At the 2012 ARC-CSI Crash Conference, the test markedly similar to a “corkscrew ramp” as found in the sidebar to this article as conducted by General Motors at their facility in Milford, Michigan was undertaken using a 2011 Jeep Grand Cherokee. While not subject to the requirements of “563,” the subject Jeep nonetheless was capable of storing what can only be described as voluminous and significant data associated with the planned (anticipated...) rollover event.

The Jeep was a subject in several different tests during the conference. First, it was subjected to a series of “low deltaV” tests to examine the recording characteristics and reliability of the Jeep data under those conditions and to examine performance of the tire pressure monitoring system (TPMS) as well as active head restraints. 11

an “everyday” occurrence, this type of collision is not entirely unheard of in the “real world” (not to mention in the movies and on television...). The design; however, was not to simulate an “every day” occurrence. Rather, following the GM corkscrew ramp example (see the sidebar), the intent was to configure the test such that the left side wheels on the Jeep would be lifted and the Jeep would begin a rotation around the longitudinal axis resulting in a rollover.

Given that there would be control instrumentation inside the Jeep during the test including accelerometers and roll rate sensors, the research team believed that this configuration would result in a rollover event and yield meaningful comparative data as seen in the referenced GM testing.

The Jeep was driven toward the X19 using the “Rosenfield PhantomDriver” system. This radio controlled system is the most sophisticated and reliable system of its kind available for this type of testing allowing for a accurate realtime corrections in steering and accelerator application as well as, when necessary, braking. Using redundant remote controls to minimize the potential loss of control of the vehicle in this scenario, the Rosenfield system performed virtually flawlessly in this and other tests conducted during the 2012 ARC-CSI Crash Conference.

The Jeep was driven remotely toward the X19, climbed the front of the smaller car with the left side wheels, began a clockwise rotation around the longitudinal axis rolling over onto its roof and then ultimately sliding to rest.

In the test, the initial contact between the Jeep and the X19 resulted first in a longitudinal event for which the Jeep’s airbag control module deployed the frontal airbags.

That event developed quickly into a rollover and the airbag

For the test which is the subject of this narrative, the research team selected a vintage Fiat X19 as a “target” vehicle and “natural ramp.” Interchangeably described as “a doorstop” or “a wedge” on wheels, the X19 was remarkably similar to the General Motors “half corkscrew” ramp although shorter and, of course, somewhat more mobile: where the corkscrew ramp was fixed to the ground, the X19 was relatively mobile. The angle of the front half of the X19 and height near the X19 roof was; however, similar to the GM ramp (see the sidebar on the following page).

The design of the tests was, following the example of the GM corkscrew ramp tests and others, to drive the Jeep up and over the X19 where only two wheels on the Jeep would pass over the top of the smaller car. While admittedly not www.collisionpublishing.com

Figure 1: Taken of the interior of the Jeep after “Test 6.” This photo shows the “deployed” passenger AHR which was not reset after “low speed/ low delta-V” Test E and the deployed passenger frontal, side and curtain airbags as deployed.

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control module commanded the occupant containment curtains long before the Jeep made a full quarter turn or came in contact with the roadway surface.

The Jeep airbag control module data was then imaged using the Bosch CDR Tool. Additionally, roll rate sensors and accelerometers installed in the Jeep were downloaded and the comparison made of the data from the control instruments to that found in the airbag control module. It is that data and that comparison that constitutes the balance of this narrative.

T

est Dynamics Overview

In this test, the Jeep was driven into the stationary X19 at 40-42mph (66-68km/h). At that point, the left front of the Jeep at the left side tires began climbing the hood of the X19. There was sufficient longitudinal contact between the Jeep and the X19 such that, in about the first 65 to 70 ms, the Jeep’s airbag control module commanded the deployment of the frontal airbags. A review of the Jeep ACM’s recorded data suggests that the frontal event “ended” at approximately 160ms.

Following that, the Jeep continued up and over the X19 by and by approximately 770 ms into the overall event

(roughly 600ms after the frontal event ended), the airbag control module commanded deployment of the occupant containment curtains (referred to in the Jeep’s “system configuration at retrieval” data table as the “left curtain airbag” and “right curtain airbag(s)”).

By about 2400 ms into the event, the Jeep was fully upside down on its roof sliding toward rest. That the Jeep went from the normal upright position (wheels on the ground) to a fully upside down position (wheels in the air and roof on the ground), suggests it experienced at least two “quarter turns” where a 360deg rotation would have four quarter components or quarter “turns.” In this regard, and by way of comparison to the GM “half corkscrew ramp” testing O’Brien-Mitchell 12 wrote: “The larger proportion of laboratory tests that rolled only one quarter turn is indicative of the objective of the rollover testing itself - to develop a rollover sensing calibration.”

To that end then, this test more than sufficiently met the objective of evaluating the ACM-based data inasmuch as that data relative to the rollover portion of the test could be examined against the Crash Test Team’s instrumentation.

Figure 2: Photos from Test 6 showing the Jeep at different positions relative to the CDR report data.

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A FIAT X19 AS A “RAMP” SURROGATE? The first thought that comes to mind when someone looks at the photos of this crash test is often along the lines of “sure, the X19 is a ‘door stop on wheels’ why wouldn’t the Jeep roll?” or “the X19 as a ramp? How realistic is that?”

To the casual observer, the Grand Cherokee mounting the Fiat X19 in this crash test seems to create an unrealistic scenario; one that you might not find often in the “real world.” (Not to mention how infrequently one might find a 1980's vintage Fiat X19 these days!) But the underlying question is, was this test designed to create a “real world rollover scenario” or examine rollover associated data that may be found in the Grand Cherokee airbag control module?

The answer to this question lies in understanding what the test was conducted for, of course. This crash test was not conducted to examine “role propensity” or create a “normally” encountered rollover scenario, rather, it was designed to more narrowly examine the data that might be found in an airbag control module after a rollover and consider its reliability and application. Nonetheless, the question remains, is this type of testing “normal” in any way or might it lead to otherwise “inapplicable” data?

One answer to this question might be found in examination of the way an auto manufacturer may conduct similar testing: rollover testing for the examination of data which might be recorded. By way of one example, we may look to the widely publicized General Motors indoor rollover crash test facility at the Milford Proving Ground (MPG) in Michigan.

General Motors conducted the industry’s first rollover tests by running a 1934 General Motors car up and angled ramp and, while there was of course no airbag control module data, this first-of-its-kind testing illustrates that, even early on, testing design didn’t necessarily mimic a “normally encountered” on-highway roadway

scenario. More recently, General Motors rollover testing at MPG includes the use of a “half corkscrew ramp” to induce a rollover in a focused test scenario to examine, among other things, Roll Over Sensor data, ACM data and sensor calibration. The “corkscrew ramp,” while longer than the Fiat X19 used in this test, is designed with an initial ramp section at approximately a 10deg angle followed by a “kick up” section at approximately 15deg.

By comparison, the “X19 ramp” hood section is at about 15deg and the windshield acts as sort of a “kick up” although the X19 is shorter than the ramp overall. Undeniably, the X19 moved as a function of this test and the GM corkscrew ramp does not, but what remains is that the end result - as it relates to the vehicle to be tested - is effectively the same: there was a rollover in a documented crash test, the airbag control module recorded data, and we are able to evaluate it post-test.

In the GM “corkscrew ramp” test, the tires on one side of the subject vehicle ride up the ramp inducing an initial role of the vehicle around its long axis; the elevation of the ramp toward its end and the “kick up” section ensure that the tested vehicle will roll. When vehicles were selected for the testing conducted at the 2012 ARC-CSI Crash Conference, the X19 was seen as an obvious surrogate for the GM “corkscrew ramp.” During the test, the left side wheels of the Grand Cherokee Road up the X19 elevating that side of the vehicle in a manner similar to that seen in the GM “corkscrew ramp” testing and the end result was the same.

The underlying concept is that this wasn’t designed as a test of a vehicle systems ability to avoid a rollover (for example the performance of an “ESP” type system) or to simulate a “real-world” scenario one might readily encounter; rather, this test was designed to examine data that would be recorded in the Grand Cherokee’s airbag control module once a rollover had actually occurred.

To that end, this fully documented test with a well instrumented 2011 Jeep Grand Cherokee easily satisfied the testing goals.

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However, within the literature, the description or definition of what constitutes a “quarter turn” may be confusing in the context of other analysis particularly where it might be applied by an agency further studying airbag control module based data. For example, we see this in recently published NHTSA literature:

as roof contacts. Roof contacts are the number of times a vehicle roof (is) subjected to rollover touches, brushes, or merely faces the ground. ... (sic)” 13

“...The goal is to amass all available information and either to populate the parameters required to accurately run a multiple-parameter rollover crash model or to identify missing attributes deemed useful.

Some missing attributes, constrained by recording time and storage size, may be technologically impossible to obtain owing to the state-of-the-art present in the vehicle fleet. Other information, however, is currently available or could be made available, in the absence of privacy concerns, from the current fleet of electronic data recorders (EDRs).

The last part of that passage describes “quarter turns” associated with “roof contacts” such that if a vehicle were, for example, to move from a normal, “upright wheels on the ground position” and roll directly to an upside down orientation to where it’s roof is in contact the ground, that would (under this description) apparently be seen as a single quarter turn. Presumably then, if it subsequently rolled back onto its wheels from that inverted position, since there would be no further roof contact, there would be no more quarter turns. In that regard then, conceivably and again applying this description, a 360 deg rotation around the longitudinal axis of the vehicle could be described as a single quarter turn.

The rollover crashes are further resolved by an aggregation of vehicle quarter turns, referenced

Editorially, one would hope that the author(s) of the passage above recognize that roof contact with the roadway

Figure 3: Roll rate and roll angle from test instrumentation and CDR report. Also shown are the approximate vehicle positions at given time points.

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or ground need not even happen in a rollover event where a vehicle rolls onto its side without making any contact between the roof and the roadway. A vehicle changing its orientation relative to the ground such that, for example, the driver’s side might come in contact with the ground/ roadway would, assuming a frame of reference around the longitudinal axis of the vehicle in a 360 deg circle, have rolled one quarter turn however under the definition above, the vehicle would not have even rolled one quarter turn inasmuch as the roof had not made contact with or “faced” the ground.

If, as suggested by Eigen 13 , such collected (EDR) data might be useful “to accurately run a multiple-parameter rollover crash model or to identify missing attributes deemed useful,” then one would hope that the conventions used to define data parameters that are collected for that analysis or simulation model would be consistent with similar data parameters found elsewhere within the community, the literature, and as a function of the underlying technology itself. Where Eigen might want to describe quarter turns and associate data from, for example an airbag control module, definitions consistent with the remaining body of literature by way of description and the application of specific terms is crucial to effective communication. (Nonetheless, one anticipates that the discussion of the description of the “quarter turn” will turn out to be an example of “Muphry's law” 14 .)

Even within “563” after its implementation in September 2012, there remain contradictory and/or poorly defined terms or concepts. As previously mentioned, to simply say that the data parameter has to be “accurate” to +/-10% is insufficient. Clarification of what quantity that percentage is relative to, how it might be applied at both ends of a range and under what circumstances would reasonably be required to help the end user, OE, researcher or even regulator ensure that the data is “... record(ed) in a readily usable manner.” 4 As previously described, but not specified in “563,” one suggested method of quantifying a “percent error” would be to examine corresponding values for an “experimental value” versus a “theoretical value.” For a given pair or range of data points, the theoretical value (control data) subtracted from the experimental (CDR data) value divided by the theoretical value would yield a decimal fraction of 100% the “error” as previously described. Other conditions such as “clipping,” power interruptions and other data anomalies should still be considered.

Where data from actual crashes in the field and from event data recorder components or capabilities might be used together in a future analysis aimed at efforts toward reducing

injury risk potential, consistency between data elements evaluated and a uniform approach to the documentation, collection, evaluation and application of event data recorder data cannot be stressed in strong enough terms.

T

he Jeep Rollover data

It may assist the reader to refer to the data DVD included with this issue of Collision and which contains the crash data collected at the 2012 ARC-CSI Crash Conference. The disk includes more than 3.5 GB of photos, video, drawings, and presentation information from the conference as well as the voluminous amount of crash test data compiled during the Conference crash tests.

In that series of 12 tests, the Jeep rollover test is identified as “test 6" and the Jeep as “vehicle number 5.” In total, there are more than 700 Mb of crash data, drawings photos and videos of the Grand Cherokee and X19 as used in “test 6” alone.

During the preparations for the rollover test as part of the crash day during the Conference, several “low speed/low delta-V” tests were conducted with the Jeep. The Jeep was the “bullet” in “low speed” tests A and B and the target in “low speed” tests C, D and E. It may be useful for the enduser to review the CDR data for the Jeep as found in the “low speed tests” folders as well as in the “test six” folder for the ensuing portion of this narrative.

Another valuable part of the data available on the disc is that from the Crash Team accelerometers, roll rate sensors and other instrumentation data which has been exported from its native format into spreadsheet-compatible documents. While the many photos and videos give the enduser a feel for what happened in the tests, it’s no substitute for having been there and watched the tests first hand. Still in all, the data found on the DVD is voluminous, valuable and useful in many ways.

The testing conducted using the Jeep will be evaluated in two parts: first, the “low speed/low delta-V” test series and then second, the rollover test. Ultimately, a complete understanding of parts of the rollover data and some of the later conclusions will be made more clear by an understanding of the “low speed” test series involving the Jeep.

The ACM installed in the 2011 Jeep Grand Cherokee used in the test series described in this narrative is considered to be “about 90% ‘563’ compliant.” The ACM may record longitudinal and lateral delta-V, data from non-deployment event(s) and data from rollover event(s). It may also record other data parameters found on “563" Tables I and II but there are restrictions. Some of these restrictions are found in the “Data Limitations” section of a CDR report generated by the Bosch Crash Data Retrieval Tool after data is recovered from the ACM.

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In the Data Limitations text generated as part of the report for this Jeep (labeled 03001_Chrysler_r 012), 2 we may find the following entries which are relevant to this analysis whether as part of the “low speed/low delta-V” tests series or the rollover tests: “T0 (where “zero” is subscript) (-0.1 sec.) is defined as either: •

The last sample point in the vehicle data buffer when the ACM commanded a deployment

•

The algorithm wake-up.

•

Please note that the algorithm wake-up may be different for front, side, and rollover events and their associated parameters.”

The entry in the Data Limitations text above labeled “T0" is largely consistent with the “563" parameter “time zero” 9 with an additional reference to “pre-crash data” where we see the mention of the “last sample point in the vehicle data buffer.”

“If power to the ACM is lost during an event, all or part of the event data record may not be recorded. ...

An event may be displayed in the “Event (s) Recovered” section of the report and “Interrupted” will be displayed for Vehicle Event Recorder Status.”

“Active Head Restraint (AHR) - This refers to the active head restraint systems that are electronically controlled by the ACM. For applicable vehicles, a “Yes” for a particular item in the Deployment Command Data section of the report indicates that the ACM commanded the deployment of the associated device.”

“For ACMs that store non-deployment events, the minimum delta-V required to store an event is a delta-V of 5 mph (8 km/h) within a 150 ms interval.”

The CDR Tool end-user will also find a table in the Data Limitations text which provides an explanation of the sign notation for data elements which may be included in that report. Specific to the test series being evaluated in this narrative, longitudinal linear acceleration and delta-V, lateral linear acceleration and delta-V, and “Vehicle Roll Angle” are all reported in accordance with the provisions of SAE J211 and SAE J 1733. Normal acceleration is reported where positive is “upward” which conforms to ISO rather than SAE convention.

Notably, the data elements found in the table in the Data Limitations text labeled “Vehicle Roll Angle” should rea78

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sonably labeled “Vehicle Roll Rate.” As may be seen in a copy of the CDR report generated by the latest version of the CDR Tool software using the data imaged from the subject Jeep, the reported data parameter is actually “Vehicle Roll Rate” not a “roll angle.” 14 The roll angle, of course, may be calculated from the reported roll rate as will be seen later in this narrative.

What is not explicitly stated in the Data Limitations amplifies information relative to the recording of a non-deployment event, and the AHR and may be found - in part - in the CDR Tool Help file. If one were to drill down through the Help file menu or directory tree under the category “Using Collected Data” then “Data from Chrysler vehicles” then “Airbag Control Module Data” then “F00K108228 (MY2011+) they would find the following entry: “These ACM’s are capable of storing event data from both deployment (rollover events - if equipped, side and frontal impacts) and non-deployment events in up to two event records. Stored deployment data cannot be overwritten and non-deployment records can.”

The reference to “F00K108228” is, of course, a reference to the direct-to-module cable required for such access to the module found in this vehicle; the Jeep, when DLC imaging is otherwise not possible. What is not indicated in either the Data Limitations text or the Help file is whether the deployment/activation of the active head restraint (AHR) constitutes a “deployment” or “non-deployment.”

14 This brings us back to the discussion of what is a “deployment of a non-reversible deployable restraint” and, since the AHR could be manually reset, is it truly a “nonreversible deployable restraint?” 10 Information the authors were given in this respect by Chrysler representatives was that the “activation” of the AHR would be recorded as a non-deployment. The extension of this information would be an understanding that and AHR “deployment” would be recorded as an unlocked, volatile event which could be overwritten by a later occurring event. Similarly, where a non-deployment is recorded we do find in the Data Limitations text is that there is a recording threshold for non-deployment events. As previously noted, for this type of ACM, a non-deployment events will only be recorded where the module “observes” a delta-V of 5 mph (8 km/h) within a 150 ms interval.” 2 Notably, there is no restriction on or requirement for the direction (polarity) of that delta-V. This information will be addressed further in the abbreviated review of the “low speed/low delta-V the tests.”

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Another table or section within the Data Limitations text that may play into the analysis of the rollover event later has to do with the indication of whether or not data is “complete(ly)” recorded or “interrupted.”

Skilled users of the CDR Tool will recognize that this section constitutes only a brief review of the larger Data Limitations text, Help file and body of knowledge that would be available as a function of thorough training. At the very least, a further review of the remaining relevant provisions of the Data Limitations text, and Help file is assumed as this narrative moves forward.

T

he “low speed/low delta-V” test series

The “low speed/low delta-V” test series was designed to address several issues. In no particular order, some of those issues included non-deployment recording thresholds and associated system requirements (i.e.: AHR activation as a non-deployment), tire pressure monitoring system data, event recording order and priority, and the accuracy of some of the reported data elements (i.e.: “speed, vehicle indicated” and other elements called out in “563” Tables I and II).

For the later analysis of the rollover data, this section will focus on recording thresholds and observations associated with the AHR activation. An understanding of the events in this “low speed” series in terms of order and event over writing potential will be particularly useful toward a more complete evaluation of the rollover data.

For the “low speed/low delta-V ” series, the subject Jeep was used in tests A and B as the “bullet vehicle” (“striking” vehicle). In tests C, D, and E the Jeep was the “target vehicle.” Throughout this test series, the opposite vehicle was a 2011 Chrysler Town and Country. Many of the same areas of interest associated with the Jeep were also tested or investigated in other ways on the Town and Country.

In “Test A,” the Jeep delta-V from the Crash Team’s instrumentation was less than 5 mph (8 km/h). As anticipated, the Jeep’s ACM did not record an event in this test. In”Test B,” where the Jeep was again a “bullet” vehicle, striking the Town and Country, the Jeep’s delta-V exposure was recorded at the ACM as -6 mph (-9.7 km/h). This event was recorded in the ACM as “event number” 1 and the “total number of events recorded” parameter also reflected 1. There were no observed airbag deployments or apparent activations of other systems (i.e.: AHR).

For the remaining tests (Tests C, D, and E), the Jeep was the target vehicle and was rear-ended by the Town and Country. In Test C, the delta-V for the Jeep was less than 5 mph (8 km/h). Imaging the ACM after Test C indicated

that no new event had been recorded and the only event present remained Test B (as the “Most Recent Event”).

In Test D, the Jeep’s delta-V exposure was +5 mph (8 km/h). This was recorded in the ACM as “event number” 2 and the “total number of events recorded” parameter reflected 2. In the report data table labeled “Deployment Command Data” the “event recorder status” showed “complete,” and the indications for all the devices labeled as “airbags” were that they were again not deployed.

The Data Limitations text and the Help file both indicate that this module is capable of storing 2 events and that deployment events are locked and cannot be overwritten.

Non-deployment events are unlocked and may be overwritten. Up to this point in the series, that information held true insofar as the module, at the conclusion of Test D, had 2 recorded events stored. If the Jeep were to be exposed to another low delta-V event, and it was considered a non-deployment by the ACM, the data from Test B should be eliminated from the ACM and the remaining 2 recorded events would reflect data from Tests D and E; assuming Test E qualified for recording as a non-deployment in terms of the observed delta-V in the specified period.

Test E was conducted and the Jeep’s delta-V exposure was +5 mph (8 km/h). The ACM recorded a new event showing a delta-V of +5 mph (8 km/h). This event was identified as “event number” 3 in the “total number of events recorded” similarly reflected 3. As is the case with Chrysler line event naming convention, the data associated with Test E was labeled “Most Recent Event.” The module also contained a “First Prior Event” which was Test D. The examination of the “System Status at Event (First Prior Event)” indicated that this was “event number” 2 and that the “total number of events recorded” was 3. The data in this “First Prior Event” was reviewed and confirmed it was that which had been recorded in Test D. At the conclusion of Test E then, the module had been exposed to three events which qualified for recording but, because it can only hold two events at a time, it only retained the last two chronologically identified as the “Most Recent Event” (data from Test E) and the “First Prior Event” (data from Test D).

In both of these events as recorded at the conclusion of Test E, the “Deployment Command Data” data table indicated that none of the airbags (frontal or side) had been commended for deployment. However, for both of these events, the parameter “active head restraint deployment, driver (if equipped)” and “active head restraint deployment, passenger (if equipped)” displayed the word “yes.”

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A physical inspection of the vehicle revealed that, in fact, the AHR had been activated/deployed after both of these tests. It was physically reset after Test D and “redeployed/ reactivated” in Test E by the ACM. A further examination of the related human subject data from these tests as well as vehicle data associated with the three “AHR events” will be presented at the CDR User’s Summit in January 2013 in Houston.13 Another significant aspect of the data from Tests D and E is that in the “system status at event” data table for both of those events the data parameter “event recorder status” reflects “complete.”

At the conclusion of the “low speed/low delta-V” series, the ACM in the Jeep had 2 completely recorded events, both of which indicated there had been an AHR “deployment” (activation”) but no airbag deployments. Another notable observation is that in order to record a more recently occurring but lesser severity event (Test E), the ACM eliminated/overwrote the event that had been recorded from Test B. This is consistent with the notion that the next occurring non-deployment event could overwrite a previously recorded non-deployment event and non-deployments are “unlocked.”

But, since the data recorded in Test B did not result in the activation of an AHR, whether or not that (activation/deployment) event is truly an unlocked non-deployment or not cannot be directly answered from the series as it stood at the conclusion of Test E. The next test, the rollover test, would be better suited to address that issue.

One other aspect of this series was the observation of which might be called “pre crash” data, particularly as it relates to “speed, vehicle indicated.” In Test B, the “closing speed” was planned for about 10-12mph (16-19 km/h).

Observed externally at 12mph-13mph (19-21 km/h), the data set for this test shows the “speed, vehicle indicated” as 12mph (19km/h) for the last 2.4seconds before the crash.

Test D and E were planned such that the Jeep would be stationary when hit from behind by the Town and Country. Pre-crash “speed, vehicle indicated” in that case was... zero. It was accurately reported as such for both tests.

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ollover test: what was recorded & what was not?

The remaining test conducted with the Jeep as part of the ARC-CSI Crash Conference was the rollover test identified on the conference DVD as Test 6. Going into this final test, the Jeep’s ACM held

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two recorded events: the chronologically second and third events it had been exposed to numerically identified as event numbers 2 and 3 from Tests D and E. For the purposes of this analysis, data parameters including “speed, vehicle indicated” were/are not the primary focus of the rollover test as designed. While those data parameters were not completely ignored, for the bulk of this portion of the analysis, the focus will be on data recording potential, linear crash pulse, and roll rate then later the integrated roll angle.

At the conclusion of the rollover event previously described in this narrative, the ACM was imaged and the indication was that there were (still) 2 “Event (s) Recovered.” These events are identified as the “Most Recent Event” and “First Prior Event” - all consistent with the typical Chrysler event naming convention.

Reviewing the “System Status at Event (Most Recent Event)”, we find that the “most recent event” is “event number” 4. The “total number of events recorded” had also incremented to 4. Interestingly enough, the “event recorder status” indicates “interrupted” (it had indicated “complete” for all the events recorded in the “low speed/ low delta-V” series).

In the “Deployment Command Data” data table, we also see indications for the majority of the pyrotechnic driven airbags show that “yes,” these devices had been deployed.

For example, the parameter “frontal airbag deployment, first stage, driver” indicated “yes.” Of course the interior of the Jeep had been examined and photographed post test and the frontal airbags and side curtains had all actually been deployed.

Notably, in the “Deployment Command Data” data table for this “most recent event” the parameters observed during the previous non-deployment series of tests which related to the AHR (“active head restraint deployment, driver (if equipped)” and that for the passenger ) were no longer present in that report table. Where a parameter is identified as not being relevant for the current data set to be translated to a “plain language report,” it is not unheard of within the context of “CDR technology” to see the data parameter simply eliminated from the report “form.” Inasmuch as this is demonstrably a deployment event, and a frontal event at that, the AHR would reasonably have no real relevance as a device or data element so eliminating that particular parameter from the report template should

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not be seen as particularly unusual. Its omission as a parameter name in the report is not a power interruption.

In that context, we might look back a the data from Test B. In that test, the Jeep was the “bullet” vehicle striking the target Town and Country from behind. The effect on the Jeep in that case acted on the Jeep’s X axis in the negative direction. The AHR - effective for positive X (rearender) events - would, again, not be terribly effective in this frontal event. If the data parameter was going to be eliminated from the data table where it was irrelevant to the rollover event dynamics, it would make sense that would be eliminated in “low speed” Test B as well as test six (the rollover event). That parameter does not appear in the CDR report for Test B. It is useful to keep in mind here that the CDR report layout is like a form or a template; that a given parameter label doesn’t appear in a given report may be an indication the data parameter is not applicable to the data set being translated, not a function of a “power interruption.” The data translation and completion of what the end user sees as the CDR report is done within the CDR software not in the ACM. However, other indications within this particular report suggest the possibility of a “power interruption” having affected the data but these particular parameters not being included in the template for the actual frontal airbag deployment and for the frontal Test B event further suggests that an AHR activation is a “non-deployment” associated with positive acceleration on the X axis - an event which may be recorded and overwritten by a subsequent event.

Before Test 6, the ACM held two recorded events: both were assumed to be “non-deployments” but AHR “activations.” Data was recorded for both events and, when the data from Test E was recorded, the ACM eliminated data from Test B so that Test E - the most recently occurring could be recorded.

After Test 6, the ACM still held two events: the rollover (event number 4) and the “first prior” event, the non-deployment which included the activation of the AHR from Test E. Both events reported the “number of events recorded” had incremented to 4.

The obvious conclusion would be, at this point, that the AHR activation/deployment is treated as non-deployment and, at least where the delta-V threshold of 5mph (8 km/h) was met and there is sufficient power available, the data

would be recorded. As a non-deployment it is unlocked, it can be overwritten by a later occurring event. What cannot be determined from the data set before us is whether or not an AHR activation with a delta-V of less than 5 mph (8km/h) would, by virtue of the “deployment” of the AHR - despite not meeting the delta-V the threshold - still be recorded or not. The question is one looking for an opportunity to do some additional testing.

Not that it should come as a surprise to those familiar with this technology, but not all OEs take this same approach (perhaps one of the reasons for the petition addressed by NHTSA on the issue of recording where there was a deployment of a “non-reversible deployable restraint 10 For example, in some of the GM systems of the same year as the Jeep which is the subject of this test series, where a beltpretensioner deployment or the GM’s equivalent of the AHR is deployed, GM designates this a “non-deployment” and applies a “conditional, temporary lock” on that data set where, among other conditions, until a certain number of ignition cycles has elapsed, that non-deployment is “soft locked.”

At the end of Test 6, what was recorded was a “Most Recent Event” (Test 6) and a “First Prior Event” (Test E). The recording logic anticipated for the types of events observed generally follows what is expected (before one looks closely at the data actually recorded as a function of Test 6). It supports the notion that non-deployments are unlocked and can be overwritten and AHR “activations” are recorded (where they “qualify”) as non-deployments.

Another area which might be considered at this stage of the analysis would be the “pre-crash data” recorded by the ACM in Test 6. As previously discussed in the lower severity test series the “speed, vehicle indicated” where the Jeep was a “bullet” vehicle, was demonstrably accurate to within a 1km/h range. In the case of Test 6, the anticipated closing/impact speed was 40 mph (64km/h). The last data point recorded by the ACM prior to impact in this test (at time -0.1s) was 41 mph (65 km/h).

While there are several other data elements in the 5 precrash data tables for each event available for this analysis, only two or three rate comment in this instance. Two of these parameters taken together which rate notice are identified in table 1 of 5 in the pre-crash data as “engine throttle, % full” and “accelerator pedal, % full.” In the data from this test, the “engine throttle” at approximately 2 seconds before the impact shows essentially repeating values

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of 76.4% uniformly for approximately the last two seconds before the event. Similarly, the “accelerator pedal” up to about two seconds out before the event also shows relatively high values reaching a maximum of 77.2%. This being a crash test, and the Jeep being driven by the “phantomdrive” we know that in the last short period of time before impact, there was 100% accelerator pedal application. In the Data Limitations text, on this point, we find this entry: “... On vehicles equipped with ETC, “accelerator pedal, percent full” and “engine throttle percent full” are relative values - relative pedal position and relative engine throttle. These parameters may record values of less than 100% when the pedal/throttle was actually at its maximum.”

Another data element which ties in to the significance of this event and the notion that the data as recorded might be accurately reported is “steering input (degrees).” As we look at that data element on table 3 of 5 for the Most Recent Event, we notice that in the last 1.5 seconds out from impact, there appears to be an application of 7 increasing to 9 increasing to 18 deg of steering. The reported polarity of the steering input is positive. The significance of the polarity as it relates to a steering input is discussed in the Data Limitations text and in a review of a small table within that section of the CDR report, we find that the subject vehicle from this crash test does not report steering wheel angle in accordance with the SAE J standards 211 and 1733. Instead that angle is reported following an ISO standard where positive steering is, from the driver perspective, counterclockwise.

Armed with this knowledge we can see that during what is approximately the last 1.5 seconds before the event, the “remote driver” applied “positive” steering. If this were reported in accordance with SAE J211 or J1733, then the steering application would be clockwise relative to the driver’s position as a positive value. In this cas, it is counterclockwise (as a positive number). One reason for an analysis of this information is as it relates to some of the rollover related data. One quantity in particular is labeled as the “Rollover Crash Pulse (Most Recent Event)” where for most of the 2500 ms prior to the module “waking up” we see effectively 0deg roll rate applied to the vehicle. Normal steering and body roll of a vehicle on its suspension might be expected with steering maneuvers and, in this case, we see a developing increase in body roll over what appears to be approximately the last 220 ms prior to the module waking up.

With counterclockwise steering, one would expect to see body roll from the vehicle measured in a positive direction (where the driver side would be effectively coming up over 82

Collision Magazine

the passenger side). Here, in the pre-crash data, we see just that. For about the same time prior to the event onset that we see relatively significant steering input (there is counterclockwise steering) we see some positive body roll reflected in the “rollover crash pulse” data.

This basic review of the data from Test 6 up to this point highlights the need for a common frame of reference, a common language in this technology. Identification of polarity and, where possible, standardization of elements is important to the analysis.

In summary, and to provide a frame of reference moving the final analysis of this data, after Test 6, the CDR there are two events recorded in the ACM: a frontal deployment (from Test 6) and a non-deployment (AHR “activation/ deployment” from Test E. To record the frontal deployment from Test 6, the AHR non-deployment which was from Test D was eliminated from the module so a AHR “deployment” (“activation”) is recorded but is an unlocked event.

Select pre-crash elements evaluated, particularly those associated with Table 1, are demonstrably reported within the accuracy tolerance shown in Table III in “563.”

There are indications in the “Most Recent Event” (the frontal deployment from Test 6) that the event data recording was impacted by a power interruption and subsequent power disruption such that the event was incompletely recorded. That knowledge plays into an analysis of the rollover data.

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ollover test: how many events were there?

On its face, Test 6 appears to reflect data from a single event as a function of the Jeep striking the X19 primarily with its left (driver’s) side, climbing the front of the Jeep with its left side wheels and then, with the driver’s side coming over the passenger’s side, rolling over onto the ground. A cursory review of the data imaged from the Jeep’s airbag control module supports that basic scenario. However...

Deployment of frontal airbags is reportedly treated as a separate event from the deployment of side airbags/curtains by the airbag control module. If we were to assume that the longitudinal, lateral and roll rate data seen in the report using data imaged after Test 6 were all from the same deployment event then we should similarly expect to see, in the Deployment Command Data data table from the “Most Recent event,” that the side curtains were commanded for deployment. We do not.

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In the Deployment Command Data data table for the Most Recent Event (the frontal deployment), an indication that the side curtains were deployed is lacking and, if the data parameters are to be believed on face value, the ACM did not command side curtains for deployment. One other aspect of the data is that this data table is marked “interrupted” suggesting the potential for a power interruption.

If we are to adopt the notion that a frontal event (particularly a deployment) is considered by the airbag control module as a distinct event, separate from a rollover, it follows that there should have been two new events recorded as a function of Test 6; one would relate to the frontal deployment and the other to the rollover. As previously stated, only one new event was recorded.

There are several indications of a power interruption in the Most Recent Event data parameters. It is possible that the reason there is no indication of a rollover event having been recorded independently of the frontal event is that the power interruption and subsequent power depletion prevented the ACM from writing any roll event specific data to a separate event area in the module.

Normally, where there is a frontal deployment, it is treated as a single, independent event. If there is a rollover, it is treated as a single, independent event. In this instance, we know there were side curtains deployed and their deployment is NOT reflected in the frontal deployment event data. It follows that, had there been sufficient power available to write data associated with the rollover, given the sequence, it should have been recorded as an independent event and would have been labeled “Most Recent Event” and the frontal deployment would have been recorded as “First Prior Event.” The data we see in the current set imaged from the ACM from Test E would have been eliminated.

That there are indications in the frontal deployment event we do have data from that there was a power interruption and that we do not see any data from the rollover, further suggests that the system was simply unable to record the rollover event.

The ACM was clearly capable of evaluating data and deploying the side curtains so it did it’s primary job as an “airbag control module” (Chrysler’s specific name: “Occupant Restraints Controller”). It simply ran out of power writing part of the frontal deployment event and was unable to “switch roles” and become an “event data recorder” (to initiate that sequence of instructions within the ACM).

The time between the frontal event and the rollover is less than 5 seconds (from a review of the data, likely closer to

between 600 and 700ms between the events) and, as such, should have been recorded (assuming available power).

An examination of the longitudinal and lateral delta-Vs suggests that event (the frontal event) begins and may even end before the module experiences a “rollover event wakeup” and begins to address the dynamics of that event (resulting in, of course, the side curtain deployments).

The longitudinal and lateral delta-V’s both appear to end at about “T-zero” plus 166ms where the delta-V data seen in the report’s longitudinal axis data table and graph “bottom out” at -8.7 mph (-14 km/h). At that point, the data table values “drop” to “zero” (both for the X and Y axes) where as the normal convention which might be expected from a Chrysler data set would be that the delta-V would “flat line” at -8.7mph. That BOTH the X and Y axes data reset to “zero” which is consistent with the observation that, in the “Most Recent Event” “System Status” data table we see that: “Event Record Status - Delta-V, Longitudinal = Inter rupted Event Record Status - Delta-V, Lateral = Interrupted Event Record Status - Angular rate = Interrupted” It is virtually impossible to escape the conclusion that the frontal event was incompletely recorded and that was a function of a power interruption early in the crash sequence followed by a power depletion during the period when the ACM was trying to write the data for the frontal event and before it began to address the potential of writing data for the rollover event.

If there was a power interruption, one might wonder why and when? The alignment of the jeep to the X19 was such that the left side of the Jeep struck the X19 and then continued “up and over” initiating the rollover. Where the Jeep and X19 came in contact was at or around the left front headlight. On the Jeep, there is a power distribution center (PDC) immediately behind the left front headlight.

Another consideration is the potential for other impacts effecting the power. In the figures highlighted in this article showing the motion of the Jeep relative to the data from the CDR Tool report, we can see that, where there are apparent “drop outs” in the ACM collected data (where the roll rate drops to “zero” in two places but the control instrumentation from the Crash Team’s roll rate sensor continued to collect data) those “drop outs” correspond to points where the right side of the Jeep strikes the ground.

The battery on the Jeep is on the right side behind the right front seat. Either or both (a strike at the PDC or the bat-

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Collision Magazine 83

tery being effected by the right side impacts) could play a role in the data for the rollover going unrecorded.

As it relates to “563,” this test still lends itself to evaluation of the accuracy of the data from roll rate sensor(s) in the ACM. Taken on its face, trying to associate the timing of the frontal event to the rollover event is somewhat difficult as a function of the concepts of “where is ‘T-zero’” and the lack of other data from the rollover itself.

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art “563" Analysis

Test 6 raises two significant areas of interest relative to “563” accuracy keeping in mind (a) this Jeep is not subject to “563” by virtue of its vintage and (b) we’ve already addressed the probability of a power interruption impacting the data and the accuracy of elements like “speed, vehicle indicated” elsewhere.

These areas are the accuracy of the reported roll rate (which might then be used to calculate a roll angle, the element called for in “563”) and what the aforementioned “dropouts” are in the roll rate data set.

Figure 2, is a comparison of the roll rate data imaged from the ACM plotted against the Conference Crash Team instrumented roll rate. Clearly, the team’s instrumentation collected data during the “drop out” periods. At first blush, we notice that the data represents relatively higher roll rates during those periods when the ACM reports “no data” (the rate is actually reported as “zero” during those periods). Since the Team’s instrumentation demonstrates a relatively higher rate, one potential explanation would be that the ACM’s sensor has “saturated” and “clipped” the data. However, on closer examination and when taken by way of comparison with data from another Jeep Grand Cherokee, (Figure 4) we see that saturation/clipping would occur at 240deg/sec whereas the Team’s data from this test only approached that level once in the +800ms range (admittedly during the first “drop out”) but it did not during the longer second “drop out” at about +1000ms. So, saturation or “clipping” can reasonably be eliminated as an explanation (in this case). The most reasonable explanation (again referring to the jeep’s dynamic motion as seen in the figures associated with this narrative) is that it was during these “drop outs” that either (a) the Jeep’s ground impacts disrupted power in the Jeep or (b) those sections (coincidentally matching those ground contacts) were simply unwritten because of the later occurring power depletion caused by the event’s power interruption(s).

What might then be addressed in the context of “563” then is the impact on the roll rate and resultant roll angle data accuracy (if we were to assume that the Jeep were subject to “563”). In Figure 5 we find the Team’s instrumenta84

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tion roll rate, ACM roll rate and resultant roll angles calculated fro both data sets. Clearly, the integrated roll angle is less than that angle calculated from the Team’s control instrument data. In Figure 6, we see the roll angles plotted in isolation against one another and a band as previously described in this narrative representing the accuracy range of +/-10%. Obviously, the ACM data falls well below that range; however, one might ask, is the comparison “fair?”

The authors would argue is it not a reasonable comparison and believe that a comparison arguably based on the provisions of “563” would be more realistic. In the regulation, we see that data should be reported from “Time-zero” 4, 9 a point (in time) left to the OE to identify. In this case, “Time-zero” is identified in the data set as relative to the onset of the frontal event (because we only have the frontal event recorded) but looking at the roll rate from that data set, either data before about +200s was that part of the data set impacted by the power depletion in the frontal event and left unwritten (notably the indication previously discussed of “Event Record Status - Angular rate = Interrupted” may explain this) or it was before the module “woke up” to the rollover event while “working on” the frontal event.

In either case, if one were to calculate the roll angle from the CDR Report data starting at the first data available indicating roll rate being collected (or reported, at +200ms) and compare that to the Team’s calculated roll angle as seen in Figure 6 it’s clear that the rollover sensing data in the ACM falls almost exactly on top of the Team’s control instrumentation data as it relates to the integrated roll angle.

But what of the “drop outs?” While it would not seem that these were a function of sensor saturation or “clipping,” once they impact the data, as can be seen in Figure 5, there is a corresponding impact on the integrated roll angle. Al-

Figure 4: Data from another Jeep Grand Cherokee,which shows saturation/clipping would occur at 240deg/sec

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though not anticipated in “563,” applying what might be called the “legislative intent” of the provision for a clipping flag as called for in the regulation, one might otherwise exclude data after that point. The authors would hope that, as the “clipping” flag becomes part of the regulation in 2014, that this occurrence will be addressed in some way in the regulation or, at least considered when making comparisons for an evaluation of accuracy after a test.

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ummary

The test series conducted using the 2011 Jeep Grand Cherokee gives the CDR Tool user insight to the data writing process, the effect of power interruptions, accuracy of data as recorded under certain conditions and more. Caparisons made of this data relative to provisions of “563” in terms of accuracy show that, when considered reasonably, the ACM recorded data in this vehicle meets the accuracy requirements of the regulation.

Figure 5: Instrumentation roll rate and calculated roll angle plotted with CDR data

Figure 6: Instrumentation roll angle and calculated CDR roll angle (with a 10% error band) www.collisionpublishing.com

Collision Magazine 85

In practical application, data in rollover event should be carefully evaluated in the context of the vehicle dynamics and a careful review and an understanding of the data as recorded (what do the parameter names actually mean or refer to?) Because, sometimes, what’s NOT in a CDR Report can be as important as what IS.

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eferences

1. NHTSA Traffic Safety Facts 2005, National Center for Statistics and Analysis, National Highway Traffic Safety Administration, DOTHS-810-631 2. Bosch Crash Data Retrieval (CDR) Tool software Help File listing of supported vehicles through CDR version 8.1, the latest version available at the time of this writing.

(1) For systems with “wake-up” air bag control systems, the time at which the occupant restraint control algorithm is activated; or (2) For continuously running algorithms, (i) The first point in the interval where a longitudinal cumulative delta-V of over 0.8 km/h (0.5 mph) is reached within a 20 ms time period; or (ii) For vehicles that record “delta-V, lateral,” the first point in the interval where a lateral cumulative delta-V of over 0.8 km/h (0.5 mph) is reached within a 5 ms time period; or (3) Deployment of a non-reversible deployable restraint.

3. As used in the context of GM RollOver Sensor (ROS) Data, a “Rollover event” may be recorded where there is the deployment of occupant containment curtains. A “NonRollover event” is where the system “recognized” there was a potential roll-type event but, given the information available, did not deploy occupant containment curtains.

Essentially a “Non-Rollover” event is the equivalent of a frontal “non-deployment” without respect to whether or not the vehicle ended up over turned in some fashion.

In this regard, see also in 49CFR563.5; Definitions: “Event means a crash or other physical occurrence that causes the trigger threshold to be met or exceeded, or any non-reversible deployable restraint to be deployed, whichever occurs first.”

4. Title 49: Transportation, Code of Federal Regulations; Part 563 “Event Data Recorders” as referenced chiefly in the Federal Register at 71 FR 51043, Aug. 28, 2006. The latest revisions to this part may be found at the U.S. Government Printing Office website at http://www.ecfr.gov

11. An analysis of the tests and the data associated with the “low delta-V” crash tests using this Jeep and the active head restraints will be the subject of later study and a presentation at the CDR User’s Summit in Houston in January 2013. The data was made available at the ARCCSI Crash Conference in June 2012.

5. From 49CFR563.5; Definitions: “Lateral acceleration means the component of the vector acceleration of a point in the vehicle in the y-direction. The lateral acceleration is positive from left to right, from the perspective of the driver when seated in the vehicle facing the direction of forward vehicle travel.”

6. From 49CFR563.5; Definitions: “Normal acceleration means the component of the vector acceleration of a point in the vehicle in the z-direction. The normal acceleration is positive in a downward direction and is zero when the accelerometer is at rest.”

7. From 49CFR563.5; Definitions: “Vehicle roll angle means the angle between the vehicle's y-axis and the ground plane.”

10. Federal Register Vol 77, No 154, August 9, 2012; 49 CFR Part 563 [Docket No. NHTSA–2012–0099] RIN 2127–AL14

12. “Data Analysis Methodology and Observations from Rollover Sensor Development Tests,” Bridget M. O’BrienMitchell et al General Motors Corporation, United States, Paper Number 07-0308 13. DOT HS 811 235 January 2010, “Review of NMVCCS Roll over Variables in Support of Roll over Reconstruction,” Ana Maria Eigen, D.Sc. 14. Muphry's law – "If you write anything criticizing editing or proofreading, there will be a fault of some kind in what you have written." - John Bangsund, the Society of Editors (Victoria, Australia). Muphry's Law also dictates that, if a mistake is “as plain as the nose on your face,” everyone can see it but you.

8. From 49CFR563.8, “Table III” as footnote 1 9. From 49CFR563.5; Definitions: “Time zero means whichever of the following occurs first:

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Determination of Vehicle Orientation at Ground Contact for Rollover Accidents Juan M. Herrera PhD, P.E Anselmo Najera G. MSME

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bstract This paper presents a graphical model and a mathematical model to analyze and define vehicle orientation at ground contact while in a rollover accident. A rollover is commonly the last part of a chain of events during a single vehicle collision. On the average, the rollover sequence occurs after the vehicle has gone through other events such as avoidance maneuvers, braking, steering, yawing, etc. Usually those pre rollover events take place at a significantly higher speed than the initial vehicle’s rollover speed. Also, during the rollover event, the vehicle suffers structural damage, including scratches. This damage provides with valuable information to analyze and reconstruct the rollover accident.

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ntroduction The accident reconstructionist bases the accident analysis on evidence found on the accident site and on the vehicle. It is well known that the scratch angles measured on the vehicle’s surface deviate from its actual relative orientation at contact with the ground as the velocity of the vehicle’s perimeter has translational components and rotational components [4]. A graphical and/or a mathematical solution can be applied after information is obtained from the vehicle inspection and speed analysis .

This information is based on the scratch orientation/angle with respect to the axis of the vehicle, the angular velocity and the linear speed

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of the vehicle's center of gravity [8]. It aids in determining the orientation of the vehicle at each ground contact while it is rolling.

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ollover Documentation A vehicle inspection is a must to identify as much as possible all families of scratches and then to relate them to a roll position. Evidence left at the accident site should also be related to the evidence found on the vehicle. Together they provide a close match to find the vehicle’s ground contact and orientation, it also may help determine distances between roll positions. It is necessary to collect as much evidence as possible from the accident site, the vehicle, scene photos, witnesses, etc.

D

iscussion The motion of a rigid body in space is the combination of two components. One component is a translational movement and the second component is a rotational movement [1]. In the case of a vehicle involved in a rolling event the translational component is given by the linear velocity of the center of gravity of the vehicle. The rotational component is due to the angular velocity of the vehicle about its axis of rotation. When a vehicle contacts the ground, the difference in velocity between the contact point on the vehicle (CP) and ground (G) results in a scratch [4]. As the vehicle continues to roll, several

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Collision Magazine 89

scratches are generated to the external surfaces of the vehicle. These scratches are part of the evidence available to the accident reconstructionist to conduct a roll accident analysis. The velocity of a point (P) on the external surfaces of a vehicle, is given by two components. One component is the linear velocity of the center of gravity of the vehicle; the second component is given by the angular velocity of the vehicle and the position vector of the point P with respect to the axis of rotation of the vehicle. In vector form the linear velocity of the vehicle is given by the magnitude of the velocity vector and by a direction. The rotational component is given by the cross product of the angular velocity vector and its position vector [7]. The two components add in vector form to give the overall velocity of point P as shown below

[1]

[2]

Where VP is the magnitude of the velocity of point P and uP is the unit vector of said velocity. Similarly, for the linear velocity of the center of gravity.

[3] Where VCG is the magnitude of the linear velocity of the center of gravity of the vehicle And uCG is the direction of the translational motion of the vehicle.

Then:

[4]

Where ω x rP is the rotational component of point P If we were to take a look at a point on a vehicle that is about to make contact with ground as seen in Fig 2.

Where: •

Vp is the velocity of point P.

•

VCG is the linear velocity of the center of gravity of the vehicle.

•

ω is the angular velocity of the vehicle in vector form.

•

rp is the relative position vector of point P with respect to the axis of rotation.

Figure 2: Scratch analysis on Vehicle.

The scratch is generated in the opposite direction of VP. Looking at the vector diagram shown on figure 3 below:

Figure 1: Velocity components The scratches on the vehicle are the result of the ground being stationary while the vehicle slides over it. The scratch is generated in the opposite direction of the velocity vector Vp. During a vehicle inspection the direction of the scratch on the vehicle can be determined.

The velocity vector of point P has two elements i.e. magnitude and direction as shown below:

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Figure 3: Vector diagram

[5]

Where VS is the relative velocity of ground with respect to the point of contact on the moving vehicle:

2. Draw a line parallel to the scratch direction at the heel of the (-ωr) vector.

[6]

Therefore

[7]

Equation (7) as observed on the surface of a vehicle is a two dimensional equation. Typically in doing accident reconstruction it is normal to calculate linear velocity [1], [2], [3] & [6] and angular velocities [5], [7] & [8], as a result of those calculations and the geometry of the vehicle; ω, rP and VCG are known variables. From visual inspection of the vehicle, the scratch direction uS is known. That leaves two unknowns in equation (7). Those unknowns are: uCG, which is the orientation of the vehicle with respect to the ground; and VP(-VS) the magnitude of the velocity of point P with respect to the ground. In this paper two methods for solving equation (7) will be presented: a graphical solution and an analytical solution.

G

Figure 6: Scratch from vehicle.

3. Draw a circle with radius equal to the magnitude of VCG. Center of circle located at the head of (– ωr) vector.

raphical Solution:

The graphical solution is based on drawing the velocity vectors to scale. If equation (7) is rewritten as:

[8]

The graphical representation is given below on figure 4:

Figure 7: magnitude of VCG. 4. There are two intersections on the diagram above. The correct solution is found from looking at the directions of the scratches as observed on the vehicle.

Figure 4: Vector diagram The steps to solve equation (8) are: 1. Start by drawing the vector (– ωr) to scale.

Figure 5: Tangential velocity (– ωr). www.collisionpublishing.com

Figure 8: Direction of Scratch.

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nalytical Solution:

The vector equation can be solved analytically as well. The known variables are the same and the two unknowns remain the direction of the velocity vector for the CG and the magnitude of the scratch velocity.

If we rewrite equation 7 as:

[9]

3. Scott Altman, Dean Santistevan, Clarence Hitchings, Jerry G. Wallingford and Bill Greenlees. “Comparison of Rollover Characteristics for Passenger Cars, Light Duty Trucks and Sport Utility Vehicles”. SAE Paper No. 2002-01-0942.

4. Jon E. Bready, Andrew A. May, Douglas “L” Allsop.

“Physical Evidence Analysis and Roll Velocity Effects in Rollover Accident Reconstruction”. SAE Paper No. 2001-01-1284.

The solution to the above equation is found in engineering textbooks such as (Shigley) [9] and is as shown below:

5. H. Fred Chen, Dennis A. Guenther. “Modeling of Rollover Sequence”. SAE Paper No. 931976.

[10]

6. Peter A. Luepke, Jarrod W. Carter, Kevin C. Henry and Geoff J. Germane. “Rollover Crash Test on Dirt: An Examination of Rollover Dynamics”. SAE Paper No. 2008-01-0156.

Where: θCG = Angle of Vehicle at contact with ground θωr = Either +90° or - 90° depending on vehicle rotation θS = Scrape Angle as observed on vehicle VCG = Linear Speed of CG obtained from speed calculations And:

Where: VS= Magnitude of sliding velocity with respect to ground. ummary

The graphical and the analytical methods provides with a tool to analyze and understand the behavior of a vehicle during a single vehicle rollover accident. These methods are intended to determine the orientation of the vehicle during the roll over sequence. The appendix at the end of this paper gives an example on the use of the model.

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eferences

1. Orlowski, K. R., Moffatt, E. A., Bundorf, R. T. & Holcomb, M. P. “Reconstruction of Rollover Collisions”. SAE Paper No. 890857.

2. Jones, Ian S., Wilson, Lawrence A. “Techniques for the Reconstruction of Rollover Accidents Involving Sport Utility Vehicles, Light Trucks and Minivans”.

SAE Paper No. 2000-01-0851.

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8. James R. Funk & Peter A. Luepke. “Trajectory Model of Occupants Ejected in Rollover Crashes”. SAE Paper No. 2007-01-0742.

9. Joseph Edward Shigley and John Joseph Uicker, Jr. “Theory of Machines and Mechanisms”. McGraw hill Book Company, 1980.

[11]

S

7. Juan M. Herrera and Anselmo Najera. “Model for Analyzing Single Vehicle Rollover Accidents”. Collision, the International Compendium for Crash research, Volume 5, Issue 2, Fall 2010.

E

xample:

In this particular example the vehicle overturned four times following a passenger leading roll. A vehicle inspection was conducted followed by an analysis for the rollover event applying known techniques solving for the linear and angular velocities. Roll position 3.75 was selected to show the application of the methods presented in this paper. Figure 9 and Figure 10 show the actual vehicle scratches and orientations left as evidence during the rollover. The application of the graphical method and the analytical formula are shown below: Known Variables: Scratch angle: θS=130.0 Deg. Passenger leading roll gives us ωr upward with respect to the vehicle. θωr= - 90° Speed @ CG: VCG = 12.8 ft/sec Roll Radius: r = 3.79 ft

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Angular Velocity: ω = 2.8 rad/sec

Solution for Analytical Method:

Peripheral Speed: ωr = 10.4 ft/sec (obtained from linear & angular velocity analysis [7].

Figure 9: Scratch Direction.

Figure 10: Graphical Method

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I

nvestigating school bus Type B (short bus), Type C (conventional front engine school bus) or Type D (transit bus with front or rear engine placement) collisions and their points of rest when submerged needs to be carefully reviewed. There are several assumptions held by the general public that the accident reconstructionist community should correct regarding school bus passenger safety once a school bus has been partially or fully submerged with collision victims onboard. This article will challenge those most commonly held assumptions based on 37 collision investigation factors identified by this author during two school bus submergence tragedies. 1, 2 Partial Submergence Bus Comes to Rest at the Upright Position Assumption: “When your bus is submerged wait until the bus comes to the point of rest, move to the air bubble trapped in the upper compartment area and then escape out the opened window above the waterline and swim to safety.”

Facts: 1. The assumption above may be valid for passenger cars and some passenger vans (Type A school buses) but does not apply to conventional school bus design and functionality (Type B, C or D school buses).

2. If a school bus is upright and is partially submerged when at the point of rest a significantly different set of rules apply to facilitate passenger safely exiting from the bus up to the waterline and then making it on to shore safely.

3. A partially submerged bus would allow passengers to open the windows above the water line. Only one passenger could escape at a time per open window. Windows below the water line would have too much weight of water on them to be easily opened. If opened the water would rush in and raise the waterline level in the bus and quickly put more passengers underwater. The average school bus is fully submerged in 18 seconds.3 www.collisionpublishing.com

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4. If there are 45 passengers on board able to move to the one or two windows above the waterline and each person takes 20 seconds after floating or swimming toward the opened windows to exit the bus then most passengers would not be able to get out in time before drowning.

Those passengers who had drowned would be a significant impediment to others seeking to exit from the same opened windows.

5. Two minutes may be a realistic time limit for passengers to hold their breath and make it out of the school bus and up to the water surface. A few passengers may be forced to hold their breath for more than 2 minutes while exerting maximum physical effort to reach the opened windows.

If it takes 1 minute to swim, crawl or step across bus seatbacks to the two open windows and 20 seconds to exit either window then only 12 passengers would be able to survive.

6. A standard school bus evacuation drill using the rear exit door takes 45 students more than 2 minutes to evacuate.

Evacuation drills for bus submergence are not practiced by school districts or transit providers. For passengers who are submerged this would be a unique and terrifying experience without any prior knowledge of how they could save themselves.

7. Every passenger may not know how to swim. Other passengers who can swim may not be able to swim underwater with low water visibility toward the opened windows for their escape. Other students attempting to escape at the same time and through the same opened windows may block or be stranded within the same exit window frame.

8. Some passengers would perhaps be able to stand on top of the seats and progress quickly to the opened windows above the waterline. The successful passengers would have to compete with other equally panicked passengers attempting the same escape at the same time. Lifeguards are trained to avoid the drowning person’s response to drag them down. There are no lifeguards on the school bus to take control of any individuals, let alone all other passengers, many of whom would be panicked while drowning.

9. Based on the seating location of each passenger at the time of the collision and their displacement within the seating compartment many passengers some or all would have been ejected out of their seats and onto the bus floor.

Some students may be pinned underneath the seat frame in front of them.

10. Any of the passengers with injuries, who were in shock or who were disoriented by suddenly being underwater and confined inside a school bus would have the high-

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est probability of drowning. Most would have the lowest probability of successfully escaping their submerged school bus.

11. It is a simple process to calculate the walking speed of various sized passengers proceeding down the narrow bus aisle (in school buses the aisles may be 12” wide or more) in single file who are evacuating bus in an orderly fashion.

The accident reconstructionist can readily determine how many seconds it would take for each of the passengers to leave the front or rear exit doors or both. There are no known calculations to determine the speed of submerged bus passengers proceeding to any point of escape within the bus. There are no known calculations that would depict the probability of their successfully escaping or of drowning in this tragic scenario.

12. If the bus submergence occurs in low light or at nighttime this scenario becomes much more dangerous to the passengers onboard. Interior overhead lighting and in the school bus aisles may not be activated by the bus driver during nighttime driving. A school bus may not have passenger compartment lighting on at night since interior lighting may significantly interfere with the bus operator’s exterior nighttime traffic view. If the interior lighting is on then passenger visibility underwater would be improved but no normal safe route of escape would be provided.4 13. Buses with interior emergency light striping along the both sides of the aisle would enable those passengers to follow that striping to the either end of the bus aisle. Some passengers who would move along quickly underwater to the front of the bus where there is maximum submergence and risk of drowning. Realizing that the front area is blocked these passengers may not make it back to the opened windows to the rear. Other passengers would move to the rear of the bus along the narrow aisle and there successfully exit the bus through the opened windows.

14. Moving 45 passengers along a 12” wide school bus aisle that is approximately 30” high (12” x 30”) would make a complete evacuation impossible in less than two minutes.

The two minute standard is the time that it would take from being able to hold their breath before drowning.

15. School buses are now often equipped with seat belts or three-point securement systems, child safety restraint systems and other specialized student securement equipment that may require an adult to release them from their seat. If there is no adult present then the secured student will drown where they are seated. If each student is able to release themselves they still face the challenge to reach any opened windows and safety by their own means.

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16. Multiple objects, passengers alive but struggling, and passengers already drowned would block the clear pathway toward the opened windows. Every seat base on a school bus may also be a flotation device. Visualize for a moment what 20 floating passenger seat bases floating at the top of the school bus ceiling. The seat bases would be displaced by the passenger and trapped air outflow and block those passengers attempting to escape.

17. School buses have two escape hatches allowing access to the school bus roof exterior. When opened these escape hatches cause immediate and substantial water intrusion.

There is an equally rapid depletion of any trapped air spaces for passengers to keep their heads above the waterline while attempting to exit. Roof hatches may are forced open by the air pressure created within the bus during the process of submergence which removes any pockets of air along the bus ceiling.5

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onclusion

Out of 45 passengers and one school bus driver at least 30 passengers would perish in a partially submerged bus with only two open (rear seat area) windows through which those survivors would escape. 30 passengers drowning are equal to or greater than the worst school bus vs. train collision of record.

Full Bus Submergence with the Point of Rest with the Bus Upside Down Assumption #2: When your bus is submerged wait until the bus comes to the point of rest, move to the air bubble trapped in the compartment area and then escape out the opened window above the waterline and swim to safety.

Facts: 1. Consider the center of gravity (CG) of a fully loaded school bus. The CG is significantly higher when the bus is fully occupied with passengers than when empty.

2. The forces involved in the collision prior to the bus entering the water may result in the bus being submerged on its side or upside down.6

3. Extensive testing of school buses submerged in Michigan indicated that each trial resulted in the school bus going “nose down” into the 40 foot deep water and coming to rest upside down.7 There is no research demonstrating what submergence process would be done with a rear-engine Type D school bus. front-engine school buses would settle to the bottom upside down.

4. Clarity of the water after the bus is submerged cannot be assumed. The opposite may be a safer assumption: The

bus when upside down is in water that has been roiled and muddy would prevent clear passenger viewing of anything inside or outside the bus.

5. Passenger windows may be submerged in the muddy bottom which would disallow any exit from school bus side windows and no exit from either of the roof vents.

6. Exit from the rear door exit (if not locked); from the driver side window or from the front windshield area would be possible. The school bus rear exit door would be able to be opened by a passenger seated to the rear of the bus since the top of the door is well above the “mud line” when the school bus is inverted and submerged. The rear exit door could not be opened until and unless there is equal amount of water inside the door as outside in order to relieve the water pressure or reduce the exit door implosion factor.

7. The school bus front exit door and stairwell would not be operable due to the door being blocked by the muddy waterway bottom.

8. Optimally the same number of passengers evacuating through the rear exit door during a daylight bus evacuation drill would be able to escape through the rear exit door when the school bus is submerged. The standard expectation is that half or 25+ students would use the rear exit door and be seated in the last five or six seat rows with six elementary students seated per row on both sides of the bus aisle.

9. Students seated in mid-bus row locations and those positioned forward up to the driver’s compartment have far less chance of evacuating out of the driver’s window and would be better able to escape through the imploded front windshield area.

10. An alarm claxon may sound for a short period before being disabled by the water since it is designed to be activated after 60 seconds after the engine stops running. The rear exit door has to be been opened and then re-secured by the school bus driver during normal shutdown procedures for the alarm not to be activated. The claxon is extremely loud, very disorienting to passengers and may not provide the source of the sound to help the passengers locate a safe bus rear door exit.

11. School bus windshields are designed to permit being kicked out by the driver or other passengers only when that windshield is not submerged. When underwater the windshield would implode into the driver’s compartment and may partially block the exit stairwell. When the windshield implodes then any trapped air pocket in the forward compartment would be immediately lost for all passengers

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Collision Magazine 97

onboard. The bus driver would likely be injured during the windshield implosion and would not be able to release her seat belt and shoulder restraint to assist others to escape through the opened windshield area. The driver would remain upside down and secured as well as injured before reorienting to save herself and assist others to escape. The driver like all of the passengers has two minutes of holding their breath before they drown. During that two minute period the driver may be able to save herself rather than to save others. 8 12. The school bus driver side window would be an accessible exit point for the driver but not for the other 45 passengers. This driver side window would be above the muddy bottom level but would likely provide marginal or no driver or passenger visibility. Passengers escaping by feel alone would make this driver side window escape route highly improbable.

"

Daytime bus full submergence would be as dangerous and tragic as nighttime submergence

Collision Magazine

14. The floating bus seat bases could indicate which way was up to the alert passenger who was able to see the seat bases. The downside to the floating seat bases would be that they would eliminate any clear view inside the bus to see other access points through windows or the rear door exit.

15. Roof hatches may blow out that is caused by the pressure of water entering the passenger compartment.10 Windshields may implode with the immense water pressure and at water depths thereby injuring the driver and reducing the life-saving abilities of the only adult on the bus. 11 16. This author’s experience indicates that the bus headlights would be on and remain on though submerged for several minutes before ceasing to operate.12 This factor would enable persons outside the submerged bus to locate it and attempt underwater rescue.

17. Adults attempting underwater rescue are likely to drown in their attempts if they enter the submerged bus interior. Rescue by outsiders must take place within two minutes of full bus submergence to prevent mass drowning. [A heroic submerged bus rescue occurred on August 7, 2004 when an off-duty deputy sheriff came upon a submerged Type B church bus, entered the bus and pulled several passengers to safety.]

18. Emergency aisle lighting would remain on if the driver had activated this lighting feature before the collision occurred. There is no emergency “glow-in-the-dark” highly reflective tape placed on the bus interior ceiling to point the way to front, rear and mid-bus exit points. No interior bus windows, rear door and front exit door frame reflective tape is provided within school buses. While the exterior of school buses have highly reflective tape there is no interior reflective tape required on school buses.

13. Daytime bus full submergence would be as dangerous and tragic as nighttime submergence.9 Daylight may not reach down into the inverted bus. Passengers would be disoriented and not know which way was up to the water line and to safety. Some might have the presence of mind to view the direction of their own air bubbles. If any air was trapped in the inverted bus it would be trapped in and around the upside down passenger seats which would

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increase the possibility of passengers being caught in the seat frame or seat legs.

19. Seat bottoms when used as flotation devices would work if they were outside the submerged bus and accessible to passengers who had already escaped. Inside the bus the seat bottoms are more of a hazard than a safety feature. A seat bottom could easily block a window exit and stop all further exiting from that previously open portal.

There may be no standard seat base size that would also fit through an opened school bus side window during the school bus submergence.

20. There must be careful consideration of the vertical distance from the passenger seated in an inverted submerged school bus and the distance and time required to escape

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through a bus opening to reach the surface. Skilled swimmers may not move upward through the water more than 3.5 feet per second which may be the standard walking speed on dry, flat ground. Consider a 45 foot swim within a bus and a 30 foot rise to the surface at 3.5 feet per second.

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onclusion

Out of 45 passengers and one school bus driver at least 25 passengers would likely perish in a fully submerged bus with only the rear exit door, the driver’s window and the windshield area accessible and through which some would escape. ummary

Thirty-seven (37) factors were provided by this author for the accident investigator or reconstructionist to consider when a school bus is partially or completely submerged with the point of rest upright or inverted. How close to the rear exit door after being submerged and opened will dictate how many students seated toward the rear of the school bus will be likely to escape to the surface and survive. Those students seated at mid-bus or in front seat rows have far less chance of survival when their bus is submerged and inverted.

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ecommendations

1. This author knows of no underwater evacuation exercises that have been conducted or even contemplated by school districts. There remains a critical need for further study and an evacuation drill procedure established and implemented to minimize passenger fatalities and maximize their ability to survive school bus submergence.

2. School bus manufacturers should review their safety standards and recommend changes to the Federal Motor Vehicle Safety Standards (FMVSS) that are currently promulgated to provide some additional significant, specific and workable school bus submergence countermeasures.

3. This author envisions a school bus flotation device to prevent a school bus from submerging lower than the opened side windows and one that prevents a school bus to become inverted or to roll over in water.

bus will not sink below the opened side window line even when full of water up to that level.

7. This would permit all side windows to be opened by the passengers and enable an escape at the waterline when they exit.

8. Some students would not know how to float, tread water or swim. Handles placed along and above the exterior school bus window line would permit students to exit via the side window, turn and grasp the handle and pull themselves up on top of the bus roof and there wait to be rescued. This would apply only to a school bus that remained upright while submerged.

9. School districts with bus routes near or adjacent to canals, over bridges, or along deep waterways should equip those school buses with bus submergence prevention technology.

10. Make all future school buses sold in the United States ones that literally will float when partially submerged.

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eferences

1. Collision between Consolidated ISD Bus and Valley Coca-Cola Bottling Co., Inc. Tractor-Semitrailer Intersection of Bryan and Texas FTM Road 676, Alton, TX September 4, 1989. http://www.ntsb.gov/ investigations/summary/HAR9002.html 2. Ft. Lauderdale Baptist Church Bus Submergence.

Deputy receives Heroism Award for Bus Rescue. Broward County Sheriff’s Office. August 7, 2004.

3. Officer Leon Pratt, Michigan State Police. Personal Communication, May 25, 2006.

4. FLBCBS. 5. FLBCBS. 6. Alton, TX. Bus was on its side.

7. Michigan State Police.

8. FLBCBS. Driver was only able to save himself.

4. Compressed air bladders under the front end engine area would prevent “nose down” school bus submergence.

9. FLBCBS. Bus submergence occurred at nighttime in zero visibility water.

5. All luggage compartments could be converted to flotation devices with additional undercarriage devices installed triggered when water flows into the bus interior.

10. Pratt.

6. Enclose all passenger under seat areas and activate an expanding air flotation cushion to assure that any school

11. Pratt.

12. FLBCBS. Bus headlights remained on during the rescue activities.

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Collision Magazine 99

Reconstruction Essentials for Tread Separation Accidents Involving Axle Tramp Paul T. Semones, M.S.M.E.

Engineering Institute, Farmington, AR

I

ntroduction Accident reconstructionists are often called upon to investigate crashes resulting from tire failures. One subset of this category of crash involves rear tire tread separations that lead to rear axle tramp, and a resulting controllability challenge for the driver in attempting to avoid an accident. Tramp is defined as an axle motion where the tires are alternately bouncing out of firm contact with the ground. The loss of rear stabilizing traction during axle tramp can present an extreme difficulty for the average driver in attempting to maintain control of the vehicle, particularly when the tire failure occurs at highway speeds. The indicators of this axle tramp phenomenon may not be widely understood within the accident reconstruction and police investigator communities, and these accidents are sometimes mischaracterized as “blowouts” or blamed on the driver for “unsafe turning movement.”

This article presents several essential issues that should be investigated in determining the causes of certain kinds of tread separation accidents. It is hoped that this article will serve as an introductory guide for determining whether or not an axle tramp event may have contributed to the ultimate crash.

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xle Tramp Background Axle tramp is a general condition under which a solid axle experiences oscillating rotation about a longitudinal axis. This takes the form of the two tires bouncing in an alternating sequence – when one tire is bouncing up in the air, the tire on the opposite side of the axle is in contact with the ground. Axle tramp may be induced by the vehicle traveling over a rough surface, such as an unpaved washboard road, or by the presence of an imbalance or bump at one tire. Rear axle tramp can result in a significant loss of rear stabilizing traction, and a resultant difficulty for the driver in maintaining directional control.

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oundaries of the Problem This article considers axle tramp under the particular circumstances of highway-speed, rear tire tread separation failures, occurring on Hotchkiss-suspension vehicles such as pickup trucks and SUVs. The limiting conditions under which such an axle tramp event may occur fall into three primary categories: the tire, the suspension, and the traveling speed.

The Tire: Axle tramp becomes possible when the failing tire stays inflated and becomes imbalanced due to tread loss.

The Suspension: Axle tramp only occurs on a solid axle, and is influenced in severity by the placement and stiffness of the shock absorbers.

The Traveling Speed: Overall axle tramp is dependent on the rotational speed of the imbalanced tire producing a cyclic centrifugal force near the tramp harmonic frequency of the rear axle. Typically this occurs at highway speeds, where the imbalanced tire is rotating at 10-15 Hz.

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ire Considerations Tread separation failures typically occur when a radial tire with two steel belts suffers a separation in the rubber between the belts. Frequently, the top steel belt and outer tread assembly begins to peel away from the carcass, leaving the bottom steel belt intact and the tire inflated throughout a substantial portion of the accident sequence. This kind of tread separation sometimes results in the tread strip peeling off the tire and becoming fragmented, ejecting from the carcass in multiple pieces. This mode of tire failure can also result in a portion of the full tread circumference remaining attached to the carcass throughout the entire accident sequence, never being ejected from the tire before the ultimate crash.

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Collision Magazine 101

From: Barak, P., "Magic Numbers in Design of Suspensions for Passenger Cars," SAE Technical Paper 911921, 1991.

Axle tramp is possible for as long as the tire remains both inflated and imbalanced, allowing a bouncing action.

Three modes of tread ejection allow for periods of imbalance, and thus axle tramp: 1. In cases where a portion of the tread remains attached to the carcass, the imbalance will persist throughout the entire accident sequence.

2. In cases where the full circumference of the tread is ejected in multiple pieces, a temporary imbalance will exist during the portion of the accident sequence where some of the tread has been ejected, but some remains attached.

3. In cases where the full circumference of the tread is ejected in one continuous piece, a temporary imbalance will exist during the time in which the tread is peeling away from the full circumference, as the lengthening strip of tread flails about.

During the unfolding accident sequence, tread-separated tires do sometimes become deflated. This may occur as an artifact of a high vehicle yaw angle causing the tire to debead. Other times, as a high yaw angle develops, road abrasion on the exposed lower steel belt may cause some of the steel cords to pull away from the relatively weak body plies, allowing a sudden rupture. In both such cases, the “blowout” of the tire is best described as being caused by the loss of control, rather than the common misdiagnosis of the loss of control being caused by the “blowout.”

When the forensic evidence shows the full circumference of tread was ejected from the tire in a relatively short period of time – a fraction of a second, perhaps two to four revolutions of the tire – a destabilizing axle tramp event is not likely. On the other hand, testing has shown that even a full-circumference tread detachment, with the tread coming off in one continuous piece, sometimes takes at 102 Collision Magazine

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least a full second – perhaps ten to twelve revolutions of the tire – and this is a sufficient duration to cause a significant yaw disturbance to the rear of the vehicle. Tread separations involving multiple pieces of tread being ejected from the tire can be expected to last significantly longer.

Even seemingly short tread separation events, on the order of two to three seconds, should be considered relatively long events, and have been shown in some testing to cause a significant deviation of the vehicle from its intended path – on the order of one or two lane widths – before the driver can effectively begin to exert control over the vehicle.

Sudden deflation of the tire would tend to eliminate all potential for further axle tramp, as a flat tire would not be expected to bounce.

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uspension Considerations

A Hotchkiss rear suspension involves a solid axle connecting the two wheels, with leaf springs connecting the axle to the vehicle body and constraining it against lateral and longitudinal movements relative to the body. The springs allow vertical translation of the axle relative to the body, with shock absorbers damping this sprung motion. Because the shock absorbers must necessarily be mounted closer together than the distance between the wheels, the shocks are less able to damp rotational tramp motions of the axle than level hop motions of the axle.

The wider apart the shock absorbers, the greater their effectiveness at controlling axle tramp. A simple measure of the shock absorbers’ effectiveness at damping axle tramp is the ratio between the shock distance versus the track width.

This has been called the Roll/Ride Ratio. A ratio of 0.5 or less has been viewed within the automotive and shock absorber industries as “untuneable” for effectively controlling axle tramp. For example, a vehicle with a 60” rear track width and shocks placed 30” apart has a Roll/Ride Ratio of 30/60 = 0.5. Such a vehicle would be highly susceptible to severe axle tramp. Due to tire width and the general size limitations of typical on-road passenger vehicles, the maximum practical Roll/Ride Ratio in axle design tends

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to be roughly 0.8. Beginning in the mid-2000’s, the design philosophy for most full-size pickup trucks sold in the U.S. began to tend toward more widely spaced shock absorbers, sometimes mounted outboard of the frame rails and clearly visible within the rear wheel wells.

A second geometric consideration for axle tramp control is the angle of the shock absorbers off the vertical. Shocks angled inwards in rear view, with the upper mounts closer together than the bottom mounts, lose a significant portion of their effectiveness in tramp motion. The inboard angle of the shock results in less total motion (and thus lower shock velocity and lower damping force) for a given tramp motion of the axle than if the shock was mounted vertically in rear view. The more vertical the shock absorbers – in both rear-view and side-view – the more effective they are at providing full damping capability during axle tramp motions.

Because shock absorbers are velocity-dependent force members of the suspension, it is important to understand the velocities experienced at the shock during highwayspeed tramp events. For an imbalanced tire producing a bouncing input once per revolution while rotating at 12 Hz – typical for common tire sizes on vehicles traveling at highway speeds – the shock absorber has been observed in typical testing to oscillate through a stroke length of an inch or less. This has been observed to produce a common shock absorber peak velocity on the order of 20 to 40 in/s.

The higher the shock absorber force characteristics, the better their ability to control axle motions. The shock absorber forces produced at roughly 30 in/s have been surveyed across dozens of typical parts installed on solid rear axle vehicles sold in the U.S., and range from about 200 lbf to 1,200 lbf. At lower cyclic velocities of the shock, lower forces are produced. An accurate dyno measurement of shock characteristics is relatively inexpensive to obtain from suspension component testers, for those situations in which a reconstructionist may wish to measure the actual performance of a particular shock.

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raveling Speed Considerations

Higher vehicle traveling speeds obviously tend to result in more severe accidents and present drivers with more difficult challenges in situations where the vehicle is tending toward loss of control. For the purposes of an axle tramp investigation, vehicle speed is directly related to the oscillation frequency of the axle in tramp.

Testing has shown tread-imbalanced tires produce essentially one vertical bouncing input for every rotation of the tire at highway speeds. From the tire size, the reconstructionist can calculate the nominal circumference of the tire and thus derive the rotational frequency of that tire for a given vehicle speed. For example, a P235/75 R15 tire has a nominal circumference of 7.6 ft and rotates at approximately 11.6 Hz for a vehicle speed of 60 mph. Thus, such a tire would be expected to produce a bouncing input at one side of the axle of approximately 12 Hz when it has suffered a tread loss imbalance at 60 mph.

Typical tramp natural frequencies for Hotchkiss-suspended pickup trucks and SUVs sold in the U.S. tend to fall within a 10-15 Hz range, with several studied vehicles having axle tramp being most pronounced at 12-14 Hz.

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verall Effects

The onset of a tread separation event typically results in a sudden drag at the failing tire. Thus, a right rear tread separation will tend to begin pulling the vehicle to the right. Axle tramp, where both tires are bouncing in an alternating fashion, has been observed in testing to begin essentially instantly with the onset of tread detachment. Thus, the driver may immediately be faced with a vehicle that requires a sudden countersteer to attempt to maintain the lane of travel, all while the vehicle’s rear stabilizing traction is suffering a violent disturbance due to both rear tires bouncing out of firm contact with the ground. The driver may be faced with ineffectual countersteer results that require atypically large steering inputs.

As tread is ejected from the tire and deposited along the roadway (in the case of multiple pieces of tread), the severity of the axle tramp response may vary, and the driver may begin to “fishtail” in a series of countersteers as the vehicle over-responds to each steering input due to the sudden reduction in rear traction. The driver may also be decelerating from a relatively high traveling speed where the tramp input is above the worst-case harmonic range of the axle.

Theoretically, the driver may find the decelerating vehicle entering the range of most severe axle tramp frequency, thus causing the overall effect to become potentially worse even as the vehicle slows down.

Depending on the yaw angle developed by the swerving vehicle and the tire/pavement characteristics, the axle tramp may or may not leave forensic tire marks of intermittent “skipping” contact. Testing has shown that in a terminal yaw with the imbalanced tire at the outside (i.e. a counterclockwise yaw with a right rear tread imbalance), the partially detreaded tire may remain in contact with the pavement while still oscillating vertically, and the balanced tire on the opposite end of the axle may be bouncing completely out of contact with the pavement.

Tread imbalances occurring at higher vehicle traveling speeds, with bouncing input frequencies above 15 Hz, may still produce significant axle tramp simply due to the increased centrifugal force of the tread imbalance at higher wheel angular velocities, even though the input is occurring above the tramp natural frequency range.

When a vehicle has decelerated sufficiently to bring the tire rotational frequency below the vehicle’s natural tramp frequency, the severity of the axle’s tramping response may be expected to drop off significantly, and axle tramp will likely no longer be a concern.

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In situations where the full circumference of tread ultimately is ejected from the tire, the sudden departure of the final tread segment may cause the bouncing tires to settle back into firm contact essentially instantly. This can confront the driver with an abrupt transition in vehicle characteristics – from a condition where countersteering is ineffectual, to a condition where the vehicle is over-responsive. An uncontrolled yaw and subsequent crash may easily result.

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ategories of Evidence

The task of the reconstructionist is always to evaluate the available evidence and offer conclusions as to vehicle motions throughout the accident sequence. Some evidentiary techniques in evaluating tread separation axle tramp events may not be immediately obvious, however, and some investigations may be hampered by inadequate photographic coverage of any tire marks that may be on the roadway, or misdiagnosis of the type of tire failure by the on-scene investigators. The following discussion points will provide recommendations for evidence evaluation in determining whether an axle tramp event may have contributed to the cause of the ultimate crash.

ing tire, both from the pieces’ momentum at the time of ejection and from being struck by any passing vehicles that may roll over pieces deposited in the roadway.

Debris Indicating Beginning of Tread Separation Small tread fragments may be found as the earliest evidence of the beginning of the tread separation. These pieces often are too small to cause any sensible tire imbalance just from their ejection from the tire, but they may indicate the onset of a large length of tread peeling away from the tire and causing an imbalance. This is particularly important in cases where the entire circumference of tread is ejected in one piece later in the accident sequence.

Another kind of early evidence debris may be shattered fragments of the tail light, or portions of fender trim.

These items may be broken off the vehicle near the begin-

Tread Status An obvious indication of a potential axle tramp scenario is the presence of tread still attached to the tire. If a substantial fraction of the tread’s circumference never peeled free of the tire, then an imbalance would have persisted throughout the accident sequence.

Tramp Marks The clearest forensic evidence of an axle tramp event is the presence of skipping tire marks, or marks that show vertical oscillation of the tires by their oscillating width.

These marks are not always necessarily present, however, or they may not be captured in scene photos. Sometimes only a mark from the tire at the outboard side of the yaw is visible; when such a skipping, wavy, or oscillating mark is from the balanced tire, axle tramp during that portion of the accident sequence is highly likely.

Zone of Tread Separation An important consideration for understanding if and when an axle tramp event took place during the accident sequence is the point of rest for any identified pieces of tread. These locations will provide a general understanding of the distance, amount of time, and traveling speeds over which the axle may have been tramping. Tread pieces may be expected to travel downstream some distance from the point at which they were actually ejected from the failwww.collisionpublishing.com

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ning of the tread separation event as the developing flap of tread slaps against the vehicle, and they serve as a strong indicator of where in the accident sequence the tread began to peel away from the tire.

Tread Located in Rollover Path Since many tread separation events lead to an ultimate rollover of the vehicle, the debris field throughout the tumble path should be searched for tread pieces. In some instances the final length of tread may only pull free of the tire in the final moments of yaw prior to rollover, or due to the violence of the tumble. In these situations, the final tread piece may come to rest within the tumble path. In at least one accident investigated by the author, the tire was completely free of the entire circumference of tread at the vehicle’s point of rest, yet the final visible tire marks clearly showed skipping marks indicating an ongoing tire imbalance; these facts seemed contradictory until a sizeable strip of tread was spotted within the police photos of the rollover debris field.

Tread Piece Size For any strips of tread that may be located throughout the accident sequence, and confirmed as associated with the subject detreaded tire, it is important to document the size of each. Typically, tread strips break away from the tire with a characteristic parallelogram shape, with each end angled at approximately 20 degrees (ranging typically from 17 to 24 degrees) from the circumferential direction due to the angle of the steel cords. The length of each strip is properly measured along one sidewall edge only, so as not to overestimate the total portion of the circumference the strip represents. Length may be expressed in conventional inches/feet or centimeters/meters, but this can lead to ambiguity of real size; a better expression of tread size is in terms of degrees of a circle or percentage of total circumference, as measured along a single sidewall edge.

Traveling Speeds at Tire Marks The region of potential axle tramp is obviously bounded by the speed as determined by the reconstruction. Once the vehicle traveling speed is below the point at which the tires’ rotational frequency would be roughly 10 Hz, axle tramp due to the imbalance would not be expected. Tire marks during the terminal yaw may be essentially smooth and continuous, but this is not a counterindication of axle tramp if the traveling speed at those yaw marks is below the expected tramp speed.

Tread Contact Evidence on Vehicle The separating length of tread often flails about in and around the wheel well, breaking pieces off the vehicle such as fender trim and tail light fragments. These flaps of tread also typically leave black rubber transfer marks within the wheel well and cause various body and frame components to be deformed.

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tire mark evidence exists to indicate this motion. Such evidence may be taken into consideration with other evidences to suggest the path of the vehicle upstream of the first forensic indicators on the roadway.

Implied Evidence of Fishtailing In some cases, witnesses may testify that they recall multiple “fishtailing” motions of the vehicle prior to a terminal yaw. When no tire marks exist to support this testimony, other evidences may imply the “fishtailing” occurred.

One significant forensic indicator on the vehicle can provide important evidence of a substantial tire imbalance, even when little other evidence supports this conclusion: black rubber transfer on the exterior body panels. In some instances, sweeping arcs of black rubber transfer may be visible on the side of the vehicle, centered on the detreaded tire’s wheel hub. These marks provide an indication of the length of the tread strip that was flailing about prior to ejection from the tire, and show that it was flailing about outside the confines of the wheel well. Such evidence can provide a general sense of both the severity and duration of the imbalance being relatively large.

Implied Evidence of Tread Separation Initiation Occasionally eyewitness testimony can provide a strong indication of where the tread separation began, and thus its duration, even in the absence of any other forensic evidence. In one case investigated by the author, the only documented tire marks were from the terminal yaw; however, multiple eyewitnesses consistently described the vehicle veering in the opposite direction and crossing the fogline before reversing direction and yawing out of control. This motion toward the fogline would have necessarily occurred shortly after the onset of the tread separation. By projecting the known terminal yaw marks further upstream from their first documented point, until they intersected with the fogline, a rough estimate of the time and distance since the onset of the tread separation could be provided.

Implied Evidence of Initial Pull Typically, a vehicle experiencing a tread separation would be expected to pull initially toward the side with the failing tire. Evidence along the accident sequence, such as vehicle debris and tread fragments, may be found consistent with an early yaw motion to that side even when no other www.collisionpublishing.com

One such evidence may be a tread piece from the subject tire being found on the road shoulder on the opposite side from the failure – for example, a right rear tire failure leaving a tread piece along the left side of the road. This could indicate an initial rightward pull of the vehicle at the onset of the tread separation, followed by a countersteer that moved the vehicle significantly to the left; the tread fragment likely was ejected from the tire during this leftward motion and continued its trajectory in that direction, even as the vehicle reversed its yaw direction once again with further countersteering.

Another such evidence may be an unusually high yaw angle at the onset of terminal yaw marks. This could indicate that the vehicle entered its final yaw from beyond the lane in which the tire marks are first seen, suggesting one or more additional yaw motions further upstream.

Additionally, a front tire mark may exist in the final yaw that is clearly narrower than the rear tire mark, suggesting that the driver was countersteering without effect. This may be further indication of a multiple-steer “fishtailing” event, and is an important counterpoint to any citation by investigating officers that the driver simply made an “unsafe turning movement.”

Occurrence of Blowout Tread separation tire failures are sometimes misdiagnosed by the on-scene investigators as “blowouts”, particularly when the detreaded tire does eventually rupture or become debeaded late in the accident sequence. The location of first rim contact on the roadway should be carefully documented, and may be evidence that supports the conclusion that the tire remained inflated prior to that point. Axle tramp would be expected to cease upon loss of air from the detreading tire.

Other Apparent Tramp Marks Sometimes a series of skipping tire marks may be seen late in the accident sequence, but are not properly associated with axle tramp caused by a tire imbalance. When the Collision Magazine 107

reconstruction shows such marks to occur at a vehicle traveling speed well below the expected tramp speed, other causes must be considered, even if tread remains on the detreading tire to provide a persistent imbalance. These marks can occur due to a very high yaw angle as the tire on the leading side of the yaw scrubs and tucks under, causing a bouncing action, or due to rim contact from the leading side’s wheel after that tire has blown out or debeaded. Such marks should not be viewed as evidence of an axle tramp event further upstream, although other evidences may suggest it occurred.

Duration of Tread Separation In cases where the entire circumference of tread was ultimately ejected from the tire, the reconstructionist should attempt to estimate from the evidence the length of time over which the tread separation occurred. If all indications are that the tread separated relatively quickly – within a very few revolutions of the tire – then a destabilizing axle tramp event is not likely.

Vehicle Susceptibility One factor that a reconstructionist may wish to establish early in the investigation is whether the vehicle’s suspension architecture is potentially susceptible to axle tramp. While this judgment is typically reserved for a vehicle dynamicist, the reconstructionist can develop a preliminary assessment of the need to investigate the evidence of axle tramp further by taking some simple measurements of the rear suspension. A Roll/ Ride Ratio (as described earlier in this article) below about 0.65 would suggest a high potential for axle tramp, and greater mounting angles for the shocks off the vertical would increase this susceptibility further. Shock absorber part numbers should be identified for future analysis of the damping forces they provide. Some investigators consider leaky shocks to be worn out components that lead to exaggerated axle tramp.

The cause of any such leaks should be carefully evaluated to determine whether the leak was caused by accident damage or pre-existent, and the extent of leakage should be photographically documented.

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m a e T Crash p m a C Boot

T

he 2012 ARC-CSI conference was held in Las Vegas Nevada during the warmish month of June. As the tradition goes, there were a total of seven vehicle crash tests conducted on the first day of the conference. Among these was the crash test that was designed and performed by the group known as the “Boot Camp.” This team included experienced collision Reconstructionists that wanted to become more involved in the methodology of instrumented crash tests. The team was mentored by several veterans of instrumented crash tests. This article will provide the data and analysis of crash test #4 from the conference.

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w e i v e R a t sh Test Da onference

SI Crash C 2012 ARC-C

Cra

T

est Protocol This test was the simulation of an intersection crash where the bullet vehicle is trying to “make that yellow light,” and the target vehicle is stopped while trying to make the left turn. The angle of intercept between the two vehicles was designed to be between 30 to 35 degrees. The target was placed with a southeast heading on the test pad. See Figure 1. The left turning vehicle was stopped and the target speed of the striking vehicle was 40 MPH. A 2007 Toyota Yaris was chosen as the target vehicle and a 2003 Kia Rio was assigned to be the bullet vehicle. See Table 1 and Photos 1 through 4 www.collisionpublishing.com

Collision Magazine 111

Both vehicles were instrumented for the test, along with a fully instrumented 50th percentile male crash test dummy in the driver position of the target vehicle. The bullet vehicle was being driven via remote control. The remote system controlled the steering, braking and acceleration. The Toyota Yaris was equipped with a supplemental restraint system and data from the Airbag Control Module was expected. A “ride-along” module from the same make and model vehicle was installed as well for comparison.

Information which was attempted to be collected from the test was impact speed, Delta V for both vehicles, and for the driver, along with relative motion on the interior of the struck vehicle with respect to the driver’s knees and shoulders. There was an attempt to capture the dynamic crush restitution to the front of the vehicle along with restitution for the front of the vehicle.

V

ehicle Instrumentation

Instrumentation was installed in both vehicles for the test. The data collected would eventually be analyzed and conclusions could be made about force, acceleration, distance, time, speed and other data commonly used in collision reconstruction. The type of instrumentation used for the collision was determined by the type of collision analyzed, the measurements required to collect the data and the type of equipment available for use.

Careful considerations were also made in the location and mounting of the instrumentation so the instruments did not become damaged during the collision. Some of the instruments were expensive and can be reused if care is taken to protect them from damage. See Tables 2 and 3.

Figure 1: Draawing of pre and post crash vehicle positions

Model Make Year Model 2007 2003

Vehicle Identification Number (VIN)

Toyota JTDBT923071053555 Yaris Kia Rio KNADC125236241385

Test Vehicles Published % Weight Weight Distribution (lbs) (Front/Rear) 2309 61/39

Test Weight (lbs) 2275

Wheel Base

2403

2375

56/44

Table 1: Test Vehicles 112 Collision Magazine

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Overall Width

100.5"

Front Overhang 31"

167"

33"

66"

67"

Test Vehicles (pre and post test) The instruments chosen for use in this test were: 1. Accelerometers ( IST, OEM ACM and “ride along” module )

Instrument Tape Switch and Strobe

2. Transducers ( String Potentiometers ) 3. Tape Switches with Strobe Lights 4. Video ( Contour HD POV, VBOX GPS Video Datalogger ) 5. Load Cells (Seat belt application ) 6. Instrumented 50th percentile crash test dummy.

VBOX Cameras

Contour HD GPS Cameras

7. Data Collectors (TDAS) A speed trap was used on the test pad to establish a closing speed for the collision test.

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50th Percentile Crash Test Dummy

Location The tape switch was mounted horizontally across the front bumper and the Strobe was placed on the dashboard.

The cameras were mounted on the exterior of the vehicle. One on the roof on the front passenger door facing forward and the other on the driver door facing toward the interior.

One camera was mounted on the exterior front passenger door and was facing towards the vehicle interior.

The other camera was mounted to the exterior of the roof and was facing forward.

The Crash Test Dummy was positioned in the operator position in the vehicle and the shoulder and lap belt were buckled.

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Load Cell

Two load cells were mounted to the seat belt of the test dummy at the left hip and left shoulder area String A string pot was mounted on the Potentiometers left leg of the dummy and connected (String Pots) to the knee bolster in the passenger compartment. A second was mounted to the right shoulder of the dummy and connected to the right side “B” pillar. The third string pot was mounted in the engine bay from the firewall to the top of the radiator support.

TDAS The TDAS and power source was mounted to the floorboard to the rear of the right front seat. The dummy instrumentation was connected to the TDAS. “Ride Along” The ACM was mounted to the center ACM tunnel on the interior of the vehicle between the front seats.

Table 2: Instrumentation for the 2007 Toyota Yaris The Tape Switch was used in conjunction with the strobe light to establish time zero synchronization. The VBOX Video Cameras captured video images of the occupant (dummy) movement during the collision and the Contour HD Cameras captured an exterior and exterior video perspective. The load cells on the seat belt measured the loading on the belt during the collision. The crash test dummy was also instrumented to measure the forces experienced by an occupant during a collision.

The string potentiometer connected between the dummy legs and the knee bolster was used to measure movement of the dummies legs during the collision and the string potentiometer connected from the shoulder to the “B” pillar measured lateral movement of the upper torso. The third string potentiometer mounted in the engine bay measured dynamic crush to the front of the vehicle. The “Ride Along” ACM was used measure lateral forces on the vehicle during the collision. The TDAS allowed for connection of the various instruments to a central location for data collection and storage. The 2003 Kia Rio was used as the “Bullet” vehicle for the collision.

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Instrument Location VBOX Cameras One camera was mounted to the exterior of the roof and was facing forward. The other camera was mounted to the exterior of the driver door and was facing toward the interior.

Contour HD The Contour camera was mounted to Camera the roof and was facing forward.

IST The IST Accelerometer was mounted (Accelerometer) on the floorboard to the rear of the right front seat.

Table 3: Instrumentation for the 2003 Kia Rio The VBOX Cameras were used to capture video images during the approach to the collision and the interior of the vehicle during the collision sequence. The Contour Camera was used to capture high definition video during the collision. The IST measured acceleration experienced by the KIA during the collision. It was powered by an external battery and possessed an onboard memory.

A A

ΔV data.

cceleration Data Graphs

Figure 2 shows the Toyota Yaris Acceleration data. Figure 3 shows the Toyota Yaris angular (roll/pitch/yaw) data. Figure 4 shoes the Toyota

irbag Control Module Analysis

The Kia was equipped with an ACM, but was not supported by the Crash Data Retrieval Tool.

The ACM would have to be removed and sent to the manufacturer for imaging. At this point, without contacting Kia, there is no way of knowing what, if any, data might have been recorded in that ACM. The Toyota had been involved in a previous collision and was specifically acquired for this crash test. The module was imaged prior to the test to establish a baseline record.

It had previously recorded two Non-Deployment events, both showing a ΔV of less than 5 mph.

The “ride-along” module was also “bench” imaged on the day prior to the test to establish a baseline record. While on the bench, blows from a mallet were used to make the ACM "think" it was involved in events to the front and rear. During this testing, it only recorded impacts identified as "side impacts" and didn’t "recognize" or at least didn’t record the strikes from the front or rear. Once the EDR Recording trigger (TRG) baseline was established, the ACM was ready to mount in the Toyota. The center

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Figure 2: Toyota Yaris Acceleration Data

Figure 3: Toyota Yaris Angular (Roll/Pitch/Yaw) Data

Figure 4: Toyota Yaris Delta-V Data www.collisionpublishing.com

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console was removed and it was securely mounted to a metal bracket. Since there was some uncertainty as to the reliability of recording front and rear impact data, the decision was made to rotate it 90° clockwise to its normal position. This would record a frontal impact as a “driver’s side” impact. The ACM was “relatively level,” but was very slightly elevated toward the front of the vehicle. To validate the report of Occupancy Status Sensor of the factoryinstalled module, a 60lb weight was placed on the right front seat and secured.

Just prior to the crash test, the ignition key was turned to the “on” position, and the dash lights were reported to be on. The installed module did not send a Deployment Command and the ACM did not record data for the crash.

It appears that, in the time between the ignition being turned "on" and time of the crash, the battery ran down and the system powered down. The ACM requires something greater than about 9-volts to operate, which was apparently not available from the vehicle’s electrical system at the time of the test so nothing was recorded.

The “ride-along” did record data from both the mallet blows the night before and the test crash. As predicted, the crash data recorded a “driver’s side” impact. It reported a maximum ΔV of 13.8 mph at 71msec as recorded by the airbag Electronic Control Unit (ECU). Also as expected, there were multiple DTCs reported, along with no data recorded from the “B” and “C” pillar sensors. The “ridealong” module was powered by a small 12 volt battery that was connected only to it.

B

iomechanical Analysis

The instrumented 50th percentile male crash test dummy provided an abundance of data during the crash. The dummy was outfitted with three accelerometers, one each in the pelvis, thorax, and head. Other internal sensors included, neck force and moment sensors; though their data will not be included in this analysis. Beyond sensors housed within the dummy, two force sensors were attached to the driver’s 3-point lap/shoulder restraint.

The team also attached two string potentiometers to assess movement of the test dummy’s shoulder and knee.

Subjective comparison of accelerometer data at the pelvis, thorax, and head, provides some interesting, and perhaps intuitive, results. Our analysis assesses the magnitude and timing of peak acceleration at these locations. The pelvis and thorax reached their peak-G at nearly the same time (approximately 80 milliseconds) while peak acceleration for the head occurred some 30 milliseconds later. With respect to magnitude, peak-G for the pelvis was the greatest and the thorax was the least, though not much less than the head. Considering the test dummy was only restrained by the lap/shoulder belt (e.g. no airbag deployment), peak head acceleration would expect to be later. Reviewing the X, Y, & Z axis components of the resultant head acceleration, the downward (+Z component) was the greatest as the head ‘whipped’ down. Restrained by the lap belt, the quicker and high peak-G for the pelvis would be expected.

However, why the thorax peak-G (with the shoulder belt) was less than that of the head remains a question for study.

Further along in the crash, the head endured a brief, but substantial, spike in X & Y acceleration at approximately

Figure 5: Toyota Yaris Occupant Acceleration Data 116 Collision Magazine

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0.2 seconds into the collision. This spike was not reflected in thorax or pelvis data and was likely a head strike with some interior object. Precisely what that object was, remains to be determined. See figures 5 through 8 Two force sensors were attached to the lap/shoulder restraint. During the collision, force on these sensors was applied near the same time and at a nearly equivalent rate.

Both sensors reached their peak loads close to the same time. However, magnitude of the peak loads varied considerably. The lap restraint sensor reached a level almost three times that of the shoulder restraint sensor. Peak loads on

the lap sensor were relatively short in duration (~15 milliseconds) compared to the shoulder sensor which persisted for some 50 milliseconds. As one might expect, timing of the peak-G’s for the pelvis and thorax fell within the time frames of the peak lap and shoulder loads. Peak-G for the head occurred well after the lap restraint load began to decrease; but, was close to the end of the shoulder restraint peak load.

The crash team chose to place string potentiometers at two locations on the test dummy to assess gross movement.

Suffice it to say our analysis assumes the driver’s dash and

Figure 6: Toyota Yaris Occupant Thorax Acceleration Data

Figure 7: Toyota Yaris Occupant Neck Force Data www.collisionpublishing.com

Collision Magazine 117

Figure 8: Toyota Yaris Occupant Neck Moment Data ‘B’ post remained stable platforms and were not displaced during the collision. During the initial crash pulse and corresponding with the pelvis & thorax peak-G’s, the dummy’s left thigh moved forward, toward the dash, by over 6 inches. Thereafter, the knee moved away from the dash by as much as 16 inches from that point of closest approach (displacement includes any lateral/vertical movement). After reaching that peak distance away, the knee returned toward the dash by some 6 inches before ‘reversing’ direction and moving away again. This point of reversal may very well have been the thigh moving laterally (right to left) across the face of the potentiometer. Video recordings did not capture the test dummy’s legs to assess this point. Test video did capture the dummy’s right-shoulder movement which traveled some distance as the upper body and head collapsed forward and to the right at impact. Distance measured by the potentiometer was less dramatic however.

By the point the head obtained peak-G, there was only a net displacement of less than an inch toward the ‘B’ post.

Again, the potentiometer measures displacement of the string and the net distance is the combination of lateral, vertical, and horizontal movement relative to the working end of the instrument. After the point of the head peakG, the dummy’s head/shoulder moved toward the driver’s door and some 8 inches of displacement away from the instrument was recorded. This movement was consistent with the video.

C

ollision Reconstruction

As part crash analysis, one of the team members performed a reconstruction utilizing the pre-impact speed. The normal protocol that a

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Reconstructionist uses is to get a preliminary handle on the change in velocity based on a crash-type analysis. The Reconstructionist typically uses air gap adjusted stiffness coefficients and if possible in the speed range of interest.

There was a DOT crash test # 6069 which was run at an effective velocity of 24.7 mph resulting in a an average crush of 10.9 inches .The average crush in the subject Yaris crash was about 8.25 inches, which is reasonably close to the crush in the staged crash. Given that circumstance, the air gap adjusted stiffness coefficient from that crash test, DOT # 6069 was utilized. The A and B stiffness values for that crash were A=290, B=130. There is no stiffness data for the Kia Rio, which was the opposing vehicle in the subject staged crash. The average crush on that vehicle was fairly similar to the crush on the Yaris, therefore the stiffness for the Yaris was initially applied to the stiffness for the Rio. The EDCRASH computer program was utilized for the analysis. Appropriate dimensional data for the vehicle, including estimated weights and dimensions were utilized.

The crash profile measured on the Rio was not taken to the sheet-metal/body structure. Therefore, it is a non-air gap stiffness. Adjustments were made to the stiffness of the Rio to comply with Newton's third law of motion to make the forces in the collision equal and opposite. The collision force in the collision for the Rio was approximately 96,196 pounds, which was closely matched by the collision force in the Yaris at 96, 268 pounds. The results of this analysis indicated a change in velocity at 23.6 mph for the Rio and 22.3 mph for the Yaris. The PDOF that was utilized for

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this collision was -30 degrees for the Yaris and 0 degrees for the Rio. The above information was utilized for a starting point for an MSMAC simulation. The AKV values from the staged crash (DOT 6069) for the Yaris were utilized. An adjusted AKV value for the Kia was utilized. This adjustment was in line with the adjustment made to the A and B stiffness coefficient for the EDCRASH run, so as to match the intervehicle forces. The results of the MSMAC analysis indicate that the Yaris was stationary and that the speed of the Kia was approximately 39 mph. A reasonably good correlation was obtained to both, the travel path and the collision damage profile of the vehicles. The tire marks from the front wheels of the Kia were also closely matched as well.

It was noted that the front side tire friction for the Yaris had to be substantially reduced for a fraction of a second in order to produce the dynamics observed in the crash.

This was due to the unloading of the front tires, due to the interaction between the two vehicles. See figure 9, showing the output of the MSMAC analysis. The figure shows the underlying drawing and the damage profiles, as calculated

by MSMAC and the measured damage profiles.

It is worthwhile to note that the ΔV for the analysis between the EDCRASH analysis and the MSMAC analysis were approximately 1 mph or about 5 percent. The available information based on the instrumentation was that the Rio was traveling at 38.6 mph, just prior to the collision. It is worthwhile to note that the operator of the "phantom driver" indicated that he stayed on the throttle to and through the collision. Given the spacing between

the timing traps when the speed data was collected and the location of the impact, additional forward speed is likely on the part of the Rio. There was an auxiliary ACM mounted in the Yaris. This was mounted slightly aft of the center of gravity of the vehicle. The MSMAC run, which was done for the subject accident, was run for 52 milliseconds. At the 52millisecond point the MSMAC data correlated quite closely within substantially less than 1 mph to the ΔV, as measured by the auxiliary event data recorder. A plausible explanation for this difference would be the fact that the bumper structures of both vehicles use a bumper system consisting of bumper fascia with a foam energy absorber and a bumper bar. This compliant structure has substantially lower stiffness than the underlying structure of the vehicle. This characteristic could well explain the disparity between the point in time at which MSMAC calculates that the 14 mph change in velocity would occur, and when the event data recorder actually recorded that data. The difference in time is about 18 milliseconds.

Some of the “take-aways” from the above analysis would indicate that the combination of using EDCRASH and MSMAC work reasonably well for reconstructing two vehicle accidents, adjustments to A and B values, when they do not exist as in the subject case, seem to work reasonably well, and that air gap crush should be measured where possible. The concept of making similar adjustments to AKV values for MSMAC simulations appears to be reasonable.

Boot Camp Team Members James D. English James D. English & Associates San Diego, CA John Howell John Howell & Associates Las Vegas, NV C. Bruce Gambardella P.E. Consultant Automotive Engineer Spring Valley, NY Bob Lynn Nevada Highway Patrol Las Vegas, NV Jeff Bangle Las Vegas Metro PD Las Vegas, NV www.collisionpublishing.com

Collision Magazine 119

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FORCE-BALANCE: Application of the Tool for the Determination of Closing Speeds Daniel W. Vomhof III 4N6XPRT Systems 122 Collision Magazine

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W

ithin the realm of Vehicle Accident Investigation and Reconstruction, one method of speed determination is the application of conservation of energy and determination of energy (speed) loss due to vehicle crush.

One of the more popular tools for analyzing speed from crush is the CRASH 3 program within various wrappers - EDCRASH, WIN CRASH, M-CRASH, RecTec, etc. One requirement of the CRASH 3 program is the A-B stiffness values for the vehicles that are appropriate to the vehicle and the crush location - front, side, or rear.

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A common way of obtaining these stiffness values is from NHTSA crash tests.

However, NHTSA does not cover all of the “cases” which arise, either due to a lack of crash testing for a particular vehicle/ surface combination, or due to an impact configuration. Under/over ride, impact over a hard spot, or impact over a corner with a force vector from corner inwards, generally towards the center of mass, can all make the application of the normally applied stiffness values questionable, and therefore challengeable. When obtaining stiffness values directly from NHTSA crash tests becomes questionable for one of the vehicles, application of the ForceBalance calculation tool is a much better option than resorting to “generic” values from a table.

Collision Magazine 123

F

orce-Balance Tool Requirements

The Force-Balance tool requires the following items of information: Crush to both vehicles (objects) involved in the impact and A-B stiffness values that are “known” or judged to be valid for one of the two vehicles.

where Favg = the average force magnitude A = the A stiffness value B = the B stiffness value

Beyond that, it becomes a matter of quality = quality. If you have accurate crush measurements for the two vehicles, you will get better results. If you have correct weight information for the two objects (vehicles), you will get better results. If you have reliable “known” stiffness values, you will get better results. Lack of quality in any part of the data input does not necessarily render the calculated results invalid, it just means that greater care must be taken when utilizing them while forming your opinions to speeds within the crash.

Cavg = the trapezoidal average of the crush measurements, and

F

Knowing the force applied to the one vehicle, we apply the restated Newton’s Third Law (equal and opposite force) as the force on the second vehicle. With that force, and the Vehicle 2 known (or assumed) values of weight, crushavg depth, “No Damage” impact speed, and crush length, the A-B values for Vehicle 2 (the questionable vehicle) can be determined for that specific crash.

orce-Balance .... what is it?

It has been established that Force-Balance is a tool available to the vehicle accident investigator/reconstructionist, and the minimum data we need to use the tool, but WHAT IS IT (the Force-Balance tool, not the data)?? Force-Balance is the application of Newton's Third Law of Motion: “For every action there is an equal and opposite reaction,” 1 which has also been stated as “For every force there is an equal and opposite force,” along with Newton's Second Law of Motion: “The relationship between an object's mass (m), its acceleration (a), and the applied force (F) is F = ma. Acceleration and force are vectors; in this law the direction of the force vector is the same as the direction of the acceleration vector.” 2

Using the obtained crush measurements and the A-B stiffness values, the average force to crush one of the vehicles can be determined through the following calculation: Favg = [(A+B*Cavg) * L ] / 2

L = the Length of the crush indentation The above assumes that the application of the force is perpendicular to the undamaged collision surface. When it is not, the force calculated is then divided by cos α where α is the angle between the applied force and the vector perpendicular to the collision face.

The A-B values determined for Vehicle 2 for that specific crash can then be entered into your CRASH 3 based program to determine energy loss in crushing the vehicles, which ultimately feeds into the calculations of initial vehicle speeds just prior to impact.

The determination of appropriate A-B values for the “unknown” vehicle, as detailed above, is the typical usage of the Force-Balance tool.

Expansion of the Force-Balance Tool - With the addition/incorporation of a couple more items of data input for each vehicle from the CRASH 3 model, the usefulness

Figure 1: Force Vector and α angle

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of the basic Force-Balance tool can be greatly expanded.

With the input of a PDOF lever arm for each vehicle, and the Yaw Moment of Inertia for each vehicle, the ForceBalance tool within the 4N6XPRT StifCalcs program will calculate a linear “delta-V” speed for each vehicle in the direction of the application of the force to the collision surface, and a linear closing speed in the direction of the application of the force to the collision surface can also be calculated. This can QUICKLY tell you if your other data inputs are QUALITY inputs, or if one or more of them are GIGO (Garbage In, Garbage Out).

A quick comment on the PDOF lever arm and α values is in order here. When these values are not known, entering values of “0" insures that you are calculating the minimum speeds. These entries (one of each for each vehicle) can be refined as more data is gathered, thus refining your ForceBalance results.

E

xample Scenario

The following crash is “real” though some of the situations have been restated to take this from a test crash situation into something the reader might encounter within an actual reconstruction.

The Crash - The collision being studied was a nominal head-on collision between an Eastbound 2003 Kia Rio Sedan and a stopped 2007 Toyota Yaris Sedan. At impact, the YARIS was stopped waiting to make a left turn across eastbound traffic. The posted speed limit for the area was 35 mph . Impact occurred between the left front (driver side) of the RIO and the right front corner (passenger side) of the YARIS.

The vehicles were weighed post collision and found to have weights of: Kia Rio = 2375 pounds, Toyota Yaris = 2275 pounds.

Obtaining Stiffness Values - Using the 4N6XPRT StifCalcs computer program, the NHTSA crash test database was searched to find crash tests and stiffness values based on those tests. The first stumbling block was encountered

The crush profiles as contained within the Traffic Collision Report were as follows:

Figure 2: Kia Rio Post Impact

Figure 3: Toyota Yaris Post Impact www.collisionpublishing.com

Collision Magazine 125

at this time, in that the front bumper cover of the Toyota Yaris was essentially ripped off of the vehicle during the collision event. Since this would result in no crush measurements being taken in a NHTSA crash test, the Kia Rio was determined to be the better vehicle to use as the “known” vehicle. Immediately, the second stumbling block was encountered in that there are no frontal impact tests for the Kia Rio within the year range of 2003-2005 identified as being “the same” by the Vehicle Interchange list.

The database search was then broadened to look at frontal tests for all Kia Rio’s, four tests were found. A summary of

those test stiffness values, based on reported Average Crush and Reported Vehicle Width, is shown in Table 1A. Importing the statistical summary of these tests to the Force-Balance module contained in the 4N6XPRT StifCalcs program, and inputting the crush and weight data contained in the Traffic Collision report, allows us to calculate a range of closing speeds for the RIO which are shown in Summary Table 2A. An examination of the post crash photographs in Figure 4 clearly illustrates that this was not a collision where the

Table 1A: Available Test Results from 4N6XPRT StiffCalcs

Summary Table 2A: Force-Balance closing speed results 126 Collision Magazine

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closing speeds were in the range of 40-77 mph with a “likely” closing speed of 61-62 mph, as determined by the Kia Rio Average A-B values.

So perhaps there is a problem with the Kia Rio Stiffness values which were “assumed” to be valid. To that end, despite the problem with the Toyota Yaris bumper cover, a search of the NHTSA crash test database for Toyota Yaris crash tests and stiffness values based on those tests was completed. A summary of the resulting three tests and the

test stiffness values, based on reported Average Crush and Reported Vehicle Width, is shown in Table 1B. Importing the statistical summary of these tests to the Force-Balance module contained in the 4N6XPRT StifCalcs program, and inputting the crush and weight data contained in the Traffic Collision report, allows us to calculate a range of closing speeds for the RIO based upon the YARIS stiffness values as “known good”. These results are shown in Summary Table 2B.

Table 1B: Available Test Results from 4N6XPRT StiffCalcs

Figure 4: Kia Rio and Toyota Yaris - Post Crash www.collisionpublishing.com

Collision Magazine 127

Summary Table 2B: Force-Balance closing speed results Again, the post crash photographs of the two vehicles are inconsistent with the calculated closing speed range of 5696 mph with a “likely” closing speed of 78-79 mph, as determined by the Toyota Yaris Average A-B values.

and re-measured using a total station. An exemplar of each vehicle was also located and mapped, allowing the undamaged exemplar to be overlaid with the damaged vehicles, and crush measurements to be taken.

A question of “Could the stiffness values obtained from the NHTSA database be subject to an “air gap” issue?” was raised. (“Air Gap” is when something like a bumper cover pulls away from the bumper bar, popping out so that there is an air gap between the surface measured to and the rest of the damage.) This would result in shorter than actual crush depths, and thus overly stiff values for the “known” vehicle, and thus high speed calculations. After due consideration, this was determined to not be the case based on the following points:

Crush measurement considerations - Because the ForceBalance module contained in the StifCalcs program, and many of the Crash 3 programs, are not constrained by equally spaced crush measurements, a decision was made to take the crush measurements at the inflection points, rather than at equal spacings.

1. The A-B values for the Kia Rio tests were consistent with each other and did not have any spikes indicative of an air gap issue.

2. The A-B values for the Toyota Yaris tests were consistent with each other and did not have any spikes indicative of an air gap issue.

3. The A-B values for the Kia Rio tests and Toyota Yaris were consistent and similar with each other.

4. The likelihood that 7 tests spread across multiple years, and most probably multiple testing facilities, would ALL have air gap issues was highly unlikely.

This resulted in the next most likely “failure point” to be examined .... the crush measurements. In order to determine the validity of the crush measurements documented in the Traffic Collision report, the vehicles were located

128 Collision Magazine

Due to the bumper cover being ripped off of the Yaris during the collision event, 3 inches was subtracted from the crush depth measured between the undamaged vehicle and the damaged bumper reinforcing bar to account for the space from the outside of the bumper cover to bumper bar in an undamaged condition.

The radiator support bar was also measured for crush even though, due to its pre-impact position roughly 6-12 inches behind the front bumper, it was determined that it would give conservative crush measurements. Although the measurements would be conservative, they could also be reliably determined. Further, conservative measurements would result in conservative speed estimates, and therefore worthwhile to examine.

The resulting crush measurements are shown in Data Table 1 through Data Table 3.

Using these “mapped” crush measurements in conjunction with the previously determined A-B values from NHTSA crash tests for the RIO yields the results found in Summary Table 3.

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Data Table 1-3: Resulting crush measurements Using these “mapped” crush measurements in conjunction with the previously determined A-B values from NHTSA crash tests for the YARIS yields the results found in Summary Table 4.

In both cases we have shown two sets of results due to the two different sets of crush measurements, front bumper bar and radiator support bar, taken for the YARIS. Analysis of Results - The speeds that were calculated through the Force-Balance approach using the followup crush measurements are in much better agreement within themselves and with the expectations from viewing the post impact photographs.

Given that the stiffness values calculated from the RIO crash tests found in the NHTSA crash test database are within the expected range of frontal test stiffness values, and given that the RIO had a more “normal” crush profile, the closing speeds calculated in Summary Table 3 are the most likely to be closest to the actual closing speeds. The calculated closing speed based on the RIO Average A-B set is 35-37 mph, with a +/- 1 standard deviation A-B range of 30-42 mph.

As expected, using the YARIS as the “known” vehicle results in more conservative speed estimates due to the crush damage it sustained that ripped off the bumper cover. If the bumper cover had remained intact, it is expected that the closing speeds calculated would be in even closer agreement to each other, as well as with the closing speeds calculated using the RIO as the “known” vehicle, than is the case here.

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However, if a “minimum speed” is desired, the speeds calculated and shown in Summary Table 4 are certainly appropriate for use in the reconstruction.

The calculated closing speed based on the YARIS Average A-B set is 19-26 mph, with a +/- 1 Standard Deviation A-B range of 17-28 mph.

Re-emphasizing, the larger range in speeds when using the YARIS as the “known” vehicle is due to the more conservative crush measurements that were taken. This in turn results in lower force, which means lower energy expenditures and thus, lower calculated speeds. Given these calculated closing speeds, it is likely that the Kia Rio was traveling at or near the hypothetical speed limit of 35 mph stated at the beginning of our scenario.

So, enough “guessing” - As noted at the beginning of this scenario, this analysis is based on a test crash, and so we have the luxury of knowing (at least in theory) what the speeds are “FOR REALS”. In this collision, a speed trap captured a closing speed just before impact of 38.6 mph.

Instrumentation in the Kia Rio recorded a delta-v range of 16.1-19.1 mph. Instrumentation in the Toyota Yaris recorded a delta-v range of 18.9-21.9 mph.

This is VERY good agreement with the speed ranges calculated using the RIO as a “known” vehicle. It is noted that the Average A-B set closing speed is a little below the speed trap speed, and the calculated YARIS delta-v’s are slightly less than the values recorded by instrumentation. Both of these results are again attributed to the conservative crush measurements that were used for the YARIS.

Collision Magazine 129

Summary Table 3: Force-Balance closing speed results based on the Kia Rio Tests

S

ummary

Within this review use of the Force-Balance tool as incorporated in the 4N6XPRT StifCalcs program to determine delta-v and closing speeds outside of a full blown CRASH 3 calculation analysis has been demonstrated.

Further, it has been demonstrated that this tool allows one to quickly determine when there are gross errors within the crush data, thus requiring the investigator to review the work done up to that point in time, and possibly complete further investigation and documentation.

Finally, it has been demonstrated that when “quality” data goes in to a project, “quality” results can be obtained. The entry and analysis time to determine the first set of crush measurements had problems took no more than 5 minutes. Re-measuring the crush profiles from diagrams and the data entry into the 4N6XPRT StifCalcs module took

130 Collision Magazine

an additional 20 minutes. In comparison, data entry for a full blown CRASH 3 analysis can take 30 minutes in itself when using a computer program .... longer if done by hand.

Additional photographs, complete Force-Balance calculation sheets, and the diagrams showing the crush measurements taken from the mapped vehicles pre & post impact can be found on the web at www.4n6xprt.com/ARCCSI_2012_Test_4.htm

F

urther reading

Additional material on the Force-Balance model, both theory and practice, can be found in the following sources.

For more information on the 4N6XPRT StifCalcs program and the Force-Balance calculation module contained within the program, we refer you to: "4N6XPRT StifCalcs® Manual" by Daniel Vomhof available from the 4N6XPRT

www.collisionpublishing.com

Summary Table 4: Force-Balance closing speed results based on the Toyota Yaris Tests Systems web site - http://www.4n6xprt.com/4N6XPRT_ StifCalcs_Manual_2011.pdf .

Several SAE papers discuss the Force-Balance method, among them - 970942, 1999-01-0079, 2005-01-1188, and 2005-01-1205.

For the formulas used to calculate the Yaw Moment of Inertia, we refer you to SAE # 881767, “Vehicle Inertial Parameters - Measured Values and Approximations” by W.R. Garrott, M. W. Monk, and J. P. Chrstos. For more information on the Force-Balance process, we refer you to: "Balancing Collision Forces in Crush / Energy Analysis"

by Nathan Shigemura and Andrew Rich available from the IPTM Webstore http://www.iptm.org/webstore/ under Crash Investigation Publications.

general, we refer you to: Fundamentals of Traffic Crash Reconstruction Volume 2 of the Traffic Crash Reconstruction Series by John Daily, Nathan Shigemura, and Jeremy Daily available from the IPTM Webstore http://www.iptm.org/ webstore/ under Crash Investigation Publications.

Thanks is extended to the following people for their assistance both with the technical aspects and in making this more readable: Andrew Rich, Michelle Rich, Daniel Vomhof, Ph.D., Marion Vomhof, J.D., and Sheryl Vomhof.

R

eferences

1. http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

2. http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

For more information on crush energy calculations, as well as accident investigation and reconstruction calculations in www.collisionpublishing.com

Collision Magazine 131

Yaw Marks: Past And Present

John Howell

Mario Alfonsi

John Howell and Associates LLC

Y

aw marks, centrifugal skid marks, critical speed scuff marks or side slip marks.

Whichever name you utilize for them the basic principal for their creation is the same. Strictly defined they are scuff marks left on a surface by a rotating tire that is slipping parallel to its axel.

task and can be performed in several ways, including but not limited to a direct measurement of the entire length along with overlapping chord and middle ordinate measurement of all marks involved, use of laser measuring devices in a baseline offset type measurement, and the use of a total station to locate points along the marks.

There are distinguishing characteristics of these particular marks which make them readily recognizable when seen on a roadway, which include striations which appear at the beginning of a yaw mark and are made by sidewall ribs from the tire crosswise of the mark and the fact that the mark is curved, which indicates it is due to steering input are just two of these.

Accident reonstructionists are sometimes faced with a dilemma that those at the scene are not.

This is the fact that the marks are long since gone from the roadway in question and the only data they may have remaining are scale diagrams or raw data from a total station to re-create the mark and obtain measurements.

Questions have surfaced recently within the field Yaw marks are a reliable, accepted, and often uti- which has brought to the forefront the error rate lized method of determining speed of a vehicle that may be created in such a circumstance. pre crash. When found at the scene of a crash site In order to answer some of these questions we measurement of these marks is a relatively simple have designed a series of tests.

We have taken two vehicles and instrumented them with a V-Box Mini, which will allow us to capture speed, acceleration on both the X and Y axis of the vehicles and angles of drift. The first vehicle, a 2011 Chevrolet HHR was driven at speeds of 35 MPH, 45 MPH, and 55 MPH and steered in such a manner as to produce “Yaw Marks” on a level paved surface. The second vehicle a 2008 Ford Crown Victoria Police Interceptor was driven only at 50 MPH for the test.

132 Collision Magazine

www.collisionpublishing.com

Each of these yaw marks was measured utilizing the overlapping chord method, and the 35 MPH marks were measured with a total station and a mini prism placed directly on the mark, utilizing three points, five points, ten points, and twenty points of measurement to define the mark. This information was captured utilizing a TDS Recon with Pocketzone software as the data collector and Crashzone 9 for CAD.

ing an HP Designjet 5000 PS at a scale of 1” = 5’ and printed on a laser printer at 1” = 20’, which is recommended by J. Stannard Baker from Northwestern University.

The files had drawing exchange format (.DXF) files created and the diagram was transferred to Map Scenes software, much in the manner of another reconstructionist receiving a file for study, and again plots at 1” = 5’ and prints at 1” = 20’of After the yaw marks were created in the CAD the diagram were made. program scale plots of the marks were made us-

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Collision Magazine 133

Yaw Test 35 HHR 45 HHR 55 HHR

Length Chord 1 141.5’ 50’

Middle Radius 1 Chord Ordinate 1 2 3.75’ 85.20’ 50’

Middle Radius 2 Chord Ordinate 2 3 4’ 80.12’ 50’

Middle Radius 3 Ordinate 3 13.6’ 29.77’

164.7’

50’

2.83’

111.84’

50’

3.75’

85.20’

50’

12.83’

30.77’

258.7’

100’

6.67’

190.74’

100’

12.67’

104.99

100’

25.5’

61.77’

Table 1: Data from the tests performed by the authors Table 1 shows the individual yaw marks created, their length, chords and middle ordinates measured physically at the scene from three points on the yaw, and radius of the curve calculated from these measurements using the formula:

A scale diagram of the three point record was made and shown in Figure 1. Utilizing the built in function in Crashzone a radius of 46.521 feet was obtained. By measuring and using calculations on the scale diagram a radius of 46.72 feet was obtained.

134 Collision Magazine

A scale diagram of the yaw marks was then created from the five point method which gave a closer rendering of the continuingly decreasing curve of the yaw mark, and is shown in Figure 2.

Using the built in function in Crashzone a radius of the first 1/3 of this curve was found to be 130.176 feet. Measuring from the diagram and calculating found the radius to be 113.00 feet.

A scale diagram of a 10 point rendering of the curve further smoothed the diagram with the following results shown in Figure 3.

Built in calculations were used with a radius over the first 1/3 showing as 89.608 feet, and measuring on the scale diagram and calculating gave a radius of 85.20 feet.

www.collisionpublishing.com

Figure 1

Figure 2

Figure 3

Figure 4 www.collisionpublishing.com

Collision Magazine 135

A final scale diagram of a 20 point rendering of the curve further smoothed the diagram is shown in Figure 4.

Built in calculations were used with a radius over the first 1/3 showing as 96.597 feet, and measuring on the scale diagram and calculating gave a radius of 99.896 feet.

Graphing of speed and route taken from the V-Box Mini, seen in figures 5 and 6, show a starting speed of 34.82 MPH and the route of the vehicle to be as graphed which matches quite well with the diagrams of the yaw present.

Shooting five points with a total station and rendering the yaw marks with both automatic and manual calculations from the diagram yielded a speed range of plus 2.00 to plus 4.57 MPH (Figure 8).

Shooting ten points for the diagram and doing both automatic and manual calculations from the diagram yielded speed range of a negative 1.98 to negative 2.14 MPH (Figure 9).

Speed (mph)

Speeds were calculated from the various renderings, manual, automatic, and at scene measurements using a measured co-efficient of friction of .85 utilizing the listed formula with the graphed results:

Results for the 35 MPH yaw mark shows that measurements taken at the scene yielded a speed calculation within negative 1.98 MPH of those shown by instrumentation. A three point shot of the yaw done with a total station and calculated both automatically and by manual measurements on CAD yielded a speed calculation range of negative 10.50 to negative 11.28 MPH (Figure 7).

Distance (feet) Figure 5: Speed graph durring test Diagram

Manual

35 HHR 3 Point 35 HHR 5 Point 35 HHR 10 Point 35 HHR 20 Point 35 HHR At Scene 35 HHR Instrumented

N/A

CZ CAD On Automatic Diagram 23.54 MPH 24.32 MPH

N/A

39.39 MPH 37.82 MPH

N/A

32.68 MPH 32.84 MPH

N/A

34.97 MPH 35.56 MPH

Figure 6: Graph of location during test

136 Collision Magazine

32.84 MPH 34.82 MPH

Table 2: Calculated speeds www.collisionpublishing.com

Figure 7

Figure 8

Figure 9

Figure 10 www.collisionpublishing.com

Collision Magazine 137

Diagrams, measurements, and speed calculations for the remaining yaws at 45, 50 and 55 MPH used in this study followed the same trends as listed for the 35 MPH yaw.

Diagrams translated to .DXF files and rendered through another CAD program also showed precise and the same trends as those from the direct import into the first CAD program.

A twenty shot point yielded a speed range of plus 0.74 to plus 0.15 MPH, by far the closest and most precise to the instrumented speed (Figure 10).

All speed calculations are based on measurements taken from the first 1/3 of the yaw.

138 Collision Magazine

In conclusion we found that the utilization of three points, the minimum necessary to create an arc grossly under reported the critical speed, utilizing five shot points allows for a flattening of the arc when rendered and over reports critical speed. Utilizing a minimum of ten or more points gives the most precise rendering of the curve and with proper and careful measurement on a scale diagram precise radius determination and critical speed calculation can be effectively done, but be sure of your source.

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

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             

“ 2 in - 3 out ” Use our FX3 realmotion 3d to instantly animate your reconstruction results on your 3D scene.

  



Use our SmAc FX to simulate or predict collision results.

Present powerful time distance reports and persuasive graphical output.

Overwhelm them!

All FX3 output is detailed and court acceptable.

The International Compendium for Crash Research

Two-Dimensional Analysis of EDR Information

2007 CDR User’s Conference CD

Analysis of Collisions:

Conservation of Linear Momentum

Presentations & Data

The Accuracy of GM EDRs in NHTSA Crash Tests

Reconstruction & Analysis of Steering-induced, On-road, Untripped SUV Rollover

Use our crash FX momentum tool right on the diagram to generate impact speeds.

2006 CDR User’s Conference

www.CollisionPublishing.com

PDOF

Principal Direction of Force

1.888.828.0383

www.sokkia.com

www.arccsi.com

www.cdrsummit.com

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In this issue

“Multiple Case Studies” Plus . . .

For a full reconstruction of this 46 second chase sequence, visit www.edccorp.com/bullitt

HVE directly answers the following questions using Newton’s laws of motion:

The Accuracy of Speed Recorded by a Ford PCM

Questions

and the Effects of Brake, Yaw and Other Factors

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Plus: Cable cA�� uS For A �I�e demoOptions And We WI�� AnImATe for theyour “CDRcrASh� System �����������3�3 Handyman”

Mustang: 2-3 lb brake pedal force, 200 deg left steer onto Filbert, then 25-65 %WOT Charger: 3-5 lb brake pedal force, 175 deg left steer onto Filbert, then 70-75 %WOT

Maximum Undercarriage Impact Force

Mustang: Fx= -1730 lb, Fy= -13 lb, Fz= -2120 lb Charger: Fx= -8610 lb, Fy= -120 lb, Fz= -11100 lb

Maximum Suspension Force

Mustang: 4330 lb @ L/F; 3270 lb @ R/R Charger: 4810 lb @ R/F; 3530 lb @ L/R

Hubcaps Lost

Mustang: N/A (American Racing Mags) Charger: Five

Vehicle Speed Sensor Calibration

Basic Integral Calculus

for Crash Reconstruction

Other software may claim to answer these questions. Have you closely evaluated their claims and the basis for their results?

Bus Braking Tests:

To learn more about HVE and all of the available tools for crash

With and Without ABS

investigators, call 503-644-4500 or visit www.edccorp.com. Our also Close-Range Photogrammetry professional sales staff will help you get started using HVE today! & Forensic Photography COL-SP09

Forensic Photography

& Crash Reconstruction

MINIMIZE ROAD CLOSURES AND SAVE TIME WITH mFX 2008 Accuracy of Selected

Chrysler ACM EDR’s During Braking

30 – 37 mph (both vehicles)

Driver Inputs

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CDR Evidence: FAQs

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Crash Data Collection Guide for GM Airbag Electronic Control Units

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Filament Analysis of Light Bulbs

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Commercial Vehicle Dynamics

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Death Investigations & Their Psychological Effect

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VEHICLE & OCCUPANT MOTION in Far-Side Impacts ESTIMATING MAXIMUM MOTORCYCLE Acceleration Rates INSURANCE APPLICATIONS FOR CDR Part Two in a Series THE PERFORMANCE OF SEAT BACKS in High Speed Rear Impacts

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FALL 2014

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Using Inequality Contraints in the Probability Analysis of

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Ignition Switch Data in CDR Reports

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