Volume 10, Issue 1

2015 CDR SUMMIT DVD

C

2015 Vol.10 Iss.1

ollision C The International Compendium for Crash Research

Volume 10 Issue 1

Collision: The International Compendium for Crash Research Volume 10, Issue 1 - SPRING 2015

collisionmagazine.com

Spring 2015

Crash site evidenCe

A Peterbilt 2-axle tractor pulling double belly-dump trailers uphill over a narrow bridge crossed the centerline in front of an approaching Ford sedan. Both drivers reacted, but impact occurred between the left, rear tractor drive wheels and the left, rear corner of the Ford sedan.

Expert AutoStats®

4N6XPRT BioMeknx™

Expert AutoStats® is a computer program containing over 42,000 vehicles cars, vans, utility vehicles, and light pickups. 1945-2012 model years are represented.

This program can be used alone or with our Expert Qwic Calcs® and Expert VIN DeCoder® programs. Also accessible from within REC-TEC.

(Lap top & D esktop W indow s 3 2 & 64 -bit O S C om p atible)

Enter:

M anufacturer and Year

Select:

M odel from pop-up window

Output:

Wheelbase Overall Length Height Wt. Distrib

Curb Weight Width Drive Wheels Front/Rear Track

M id-60's to present output also includes (when available): Front Overhang Rear Overhang Bumper Heights Hood Height Turning circle Wheel radius Bumper-to-Hood Bumper-to-Window

For a full reconstruction of this 8 second crash sequence, visit www.edccorp.com/wheelimpact

HVE directly answers the following questions using Newton’s laws of motion:

Questions

Answers

Impact Speed

Peterbilt: 54 – 59 mph Ford: 50 – 57 mph

Available Reaction Time Before Impact

Peterbilt: > 4 sec Ford: 2.3 sec

Peak Impact Force

Total Fx = -36,100 lb Fy = 44,000 lb Fz = 12,700 lb At Wheel Fx = -13,400 lb Fy = 7,300 lb Fz = 6,900 lb

Peak Impact Acceleration

Peterbilt: Acc Total = 3 – 4 g Ford: Acc Total = 13–18 g

5. Information was obtained by review of over 65 scientific texts, several of which are now "out of print"

www.4n6xprt.com/4n6bm.htm

PROGRAM ORDER FORM (Please Print)

Required Technologies

SIMON 3-D Vehicle Dynamics Simulation Model DyMESH 3-D Collision Model, Version 3 HVE 3-D Terrain Model

Expert AutoStats® 4N6XPRT BioM eknx™ Expert Qwic Calcs® 4N6XPRT Ped & Bike Calcs® 4N6XPRT StifCalcs® Expert VIN DeCoder® (pkg) Expert TireStuf® Order Processing

- $ 625.00 ea.

- $ 495.00 ea.

- $ 275.00 ea.

- $ 375.00 ea.

- $ 650.00 ea.

- $ 550.00 ea.

- $ 85.00 ea.

= ________ = ________ = ________ = ________ = ________ = ________ = ________

- Check = $5.00 / Credit Card = $10.00 / G ovt. P.O . = $15.00 = _______ Notarized Affidavit Requirement $25.00 per required Notarized Signature = _______

Q Deliver via electronic download (Email address Required) Q Ship on USB - $35.00 per disk per program

Other software may claim to answer these questions. Have you closely evaluated their claims and the basis for their results?

California Sales Tax Enclosed is:

- 8.50%

TOTAL (U.S. Dollars)

= 0.00 = ______

= ________

= ________

Visa ___ MasterCard ___ Am. Ex. ___ Exp ____/____ - Check ___ - Gvt PO ___

Card Number: Name on Card: Signature:

_______________________________________________ _______________________________________________ ________________________________________________

Please M ake Checks*/Purchase Orders Payable to:

4N6XPRT Systems®

To learn more about HVE and all of the available tools for crash investigators, call 888-768-6216 or visit www.edccorp.com. Our professional sales staff will help you get started using HVE today!

8387 University Ave - La M esa, CA 91942-9342 - U.S.A. FAX: (619) 464-2206 Phone: (619) 464-3478 Web: http://www.4n6xprt.com

Knowledge = Power COL-1112

www.4n6xprt.com/4n6as.htm

The program includes: 1. Over 65 printable illustrations and charts 2. 10 Separate categories or sections of information related to forensic biomechanics 3. Information on walking speeds of people from 17 months to > 65 years 4. Calculation modules for: - segment length from total body height, - segment weights and Centers of M ass (CoM ) based on total body weight, - Predicted M aximum Walking Speed based on total height

Contact Name: ________________________________________________ Company: ____________________________________________________ Street: ______________________________________________________ City:____________________ State:____ Zip:__ __ Country: __________ Phone: (_____) _______________ Fax: (_____) ___________________ E-M ail: ______________________________________________________

Peterbilt: Total Fx = - 43,000 lb Fy = 39,400 lb Fz = - 400 lb At Wheel Fx = -23,000 lb Fy = 20,900 lb Fz = -6,200 lb Ford:

Expert AutoStats® is currently owned by over 700 private individuals and 300 Law Enforcement agencies in the U.S.A., Canada, and South America. Additional information, brochure, and order from can be downloaded from:

4N6XPRT BioMeknx is designed as a collection of the Biomechanical data of importance to the Accident Investigator into one easily accessible reference location. It is designed as a program to be useful to ALL accident investigators, not just the Biomechanical expert.

Engineering Dynamics Corporation • 8625 SW Cascade Blvd., Suite 200 Beaverton, OR 97008 USA • Phone 888.768.6216 • www.edccorp.com

C hecks * M U ST * be draw n on a U.S. bank. O rders are shipp ed within 10 working days of receipt.

Prices subject to cha ng e without notice.

M ultip le orders and Package discounts availab le. Please call for details.

Contents

Spring 2015

Volume 10 Issue 1

inside 4

Letter From the Editor

6

Letter From NAPARS & Industry Partners

8

features 8

Data Mining the NHTSA NASS CDR Database by: Tobias Achstetter, Fabian Kübler, Michael Wolf and Sean Haight

30

The implications of the CRASH3 uniaxial structural response model and the nature of available collision test data in regards to work-energy relationships for oblique impacts by: Jai Singh

44

Errors and Uncertainty in Deriving Speed Estimates from Skid Tests Taken at Accident Scenes by: William C. Fischer

64

My Turn At the Wheel: A DIRTY TRICK

BY A LAW OF PHYSICS, OR WHAT? by: Erik Carlsson

74

Analysis and Performance of a Booster Seated 6 Year Old HIII Anthropomorphic Test Device When Utilizing Seat Belt Cinching For Lap Slack Control by: Michael T. Vecchio

82

A Method to Establish Delta-V and Collision Force in a Two-Vehicle Collinear Collision where Stiffness Data for One of the Vehicles is not Available by: Jai Singh

92

30

64

74

Video-Based Accident Reconstruction of TransAsia Flight by: Adam Cybanski

case study 20 94

Case Study Case Study Solution www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 3

Letter from the editor This marks the start of the 10th year of Collision Magazine. I mentioned it before in these pages but Collision wasn't actually planned but was the fortuitous product of an apparent breakdown in communication and cooperation and is now thriving beyond initial expectations. While it contractually became the official publication for NAPARS it has also become the leading publication resource of its kind for original, new and now even peer reviewed articles and papers in the industry. Putting together a publication of this kind relies on the cooperation of the authors and editing staff as well as the reviewers to make sure things come together in a timely fashion; it's a lot more than simply grabbing news from the web. But there are other benefits associated with Collision. It's a venue for new and creative ideas, cutting edge research and an opportunity for discussion.

example, copied by others, of having a tent for track side presentations and networking available, this year the media center building will be adjacent to our track area and with free WiFi, air conditioning and a huge indoor networking area will serve as the between test area for attendees. But there are other activities from demonstrations and presentations planned for the time between tests as well. To add to the options available, we also arranged for attendees to be able to take advantage of reduced price rides at the Richard Petty Driving Experience adjacent to where we'll be running the crash tests and the Media Center. This isn't some vague "also in the general geographical area" type of suggestion, this is something availSince its inception, Collision has been a signifi- able right there, adjacent to the track; something cant membership benefit for NAPARS members; to do on test day between crashes. a relationship we value and support. In addition to a subscription to Collision, NAPARS members Watch for reports in the next issue of Collision receive discounts to both the ARC-CSI Crash and elsewhere about this year's ARC-CSI Crash Conference and the CDR User's Summit. While Conference - THE conference that sets the stanthe membership numbers of associations vary dard in the crash reconstruction community for from year-to-year, Collision has continued to content, testing, experiences, and value...and the support NAPARS membership growth at the two 2016 ARC-CSI Conference should be even bigconferences and wherever possible. ger!

Speaking of conferences, by the time this reaches you we should have completed the 2015 ARCCSI Crash Conference. 2015 will be the 14th year of the ARC-CSI conference and ushers in a new phase in the growth and success of what has become the go-to Crash Conference in the field.

Over time, the conference evolved from being held at an airfield (in Bremerton, Washington) to, this year, being held on the infield of the Las Vegas Motor Speedway. While others may have tried to copy our effort to put on the leading Conference of its kind anywhere, this one's going to be hard to come close to.

The 2015 ARC-CSI Crash Conference will be held in the infield of the Motor Speedway right behind NASCAR Pit Row and adjacent to the Media Center. Where in years past we set the

4 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

Managing Editor

BACK ISSUES

Collision Publishing Inc PO Box 892885 Temecula, CA 92589 Toll Free: 800-280-7940 Email: editor@collisionpublishing.com Web: www.collisionmagazine.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, © 2015 Collision Publishing Inc. 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. Dis-

senting 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 892885, Temecula, CA 92589. 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 re-emerge, 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: sbaker@collisionpublishing.com

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

ADVERTISER INDEX 4N6XPRT Systems AR Search Engine ARC-CSI Crash Conference ARC Network CDR User's Summit Collision Magazine Collision Safety Institute Crash Data Group EDR Experts Engineering Dynamics FARO FARO MapScenes MapScenes Vehicle Scans Vericom Computers Visual Statement Visual Statement

PAGE Inside Back 19 104 63 29 68 28 81 45 Inside Front 1 42 2 69-72 29 43 102-103 Back Cover

Included with this issue of Collision

is the 2015 CDR User's Summit data DVD. The 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.

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.collisionmagazine.com www.collisionpublishing.com

Next CDR User's Summit January 25-27, 2016 www.cdrsummit.com Collision Magazine - Volume 10 Issue 1 5

Premier Industry Partner Letter from NAPARS

years ago the Internet finally made owning a physical library of crash reference books seem almost quaint and changed the way we do research. Software and cables for accessing crash data from heavy trucks became widely available for a price. Then 5 years ago we began using affordable high speed cameras, allowing almost anyone to see things at 1/10th speed in good resolution.

The times, they are a’changing. New technology for the crash investigator 25 years ago meant going out and buying an affordable accelerometer in the form of a G-Analyst, just as the advent of ABS brakes was threatening to put the crash reconstructionist out of business. NO MORE SKIDMARKS! Then about 20 years ago total stations and CAD programs were at the top of our Christmas wish lists. PLEASE DON’T MAKE ME HAND DRAW MY SCALE DIAGRAMS! 15 years ago came the shiny new Vetronix CDR kit with over 5 cables and single manufacturer coverage, once again threatening to put the average recon out of business. WHO NEEDS A RECON WHEN I ALREADY KNOW HOW FAST HE WAS GOING? 10

Today we are dealing with cars equipped with advanced stability controls that “think” where the driver wants to go and will apply a single brake to keep the car pointed in the right direction. They can parallel park themselves. Selfadaptive cruise control can maintain a safe following distance no matter how the lead vehicle changes speed. Cars can perform an emergency stop in traffic if the driver doesn’t react quickly enough. And a self-driving car just completed a 9 day, 3,400 mile cross country trip without driver input. Well, without too much help from the team of engineers surrounding it. Perhaps soon there will be no drivers to make mistakes.

Will all traffic crashes be product liability cases?

The evolution of our industry has always been about adapting and changing. More science, more technology, more advanced training….. when will it end? Probably never. I don’t think our jobs are going away anytime soon. But the times, they are a’changing…

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 - Volume 10 Issue 1

www.collisionpublishing.com

www.napars.org

Industry Partners IAARS

is one of the oldest associations dedicated to expanding the knowledge and training of automobile crash Reconstructionists. IAARS was started in 1980 as a resource for members to use when the inevitable questions arose while trying to reconstruct a crash. We have members from several different countries as well as many different states across the U.S. The annual seminars not only provide instruction on specific topics by experts in the field, they provide face-to-face discussions on how other people approach reconstructing a crash. The old saying "two heads are better than one" is definitely true in accident reconstruction.

Membership in IAARS has other advantages, including a discount (from IAARS) on a subscription to Collision Magazine; access to electronic delivery of the quarterly publication of the Society of Accident Reconstructionists, and electronic delivery of bi-monthly editions of Accident Reconstruction Journal. Members also receive discounts at our training seminars.

We have annual conferences with training opportunities across the country. IAARS was the first association to do actual crash testing in 1982 with T-Bone and Head-on collisions and we continue to try and have a dynamic full scale vehicle crash or testing at each of our seminars. As a member you have full access to the setup and test with all of its data. As a member you have We have three levels of membership. Our highest level, Felaccess to a vast archive of videos and data from many different low, includes a peer review of a reconstruction project. This and sometimes unique tests. Some of these tests include vehicle differs from many other associations in that with this process for membership it can help the member as peer review is a spe- to vehicle, truck to vehicle, motorcycle to vehicle and vehicle to pedestrian crashes, semi rollovers, tip-over testing and brake cific area of judicial consideration when admitting an expert opinion in court. This review, along with training and experi- efficiency testing. IAARS sometimes partners with other associaence, go a long way if you happen to come across a stickler for tions when providing seminars or conferences. We partnered a judge. We offer opportunities for a member to publish work with WREX2000 at Texas A&M in the year 2000 and we are working with other associations for a large seminar in Florida or research in the reconstruction field and an opportunity to in 2016. IAARS will be partnering with SCARS for this years present materials at our annual conferences. conference in Charleston South Carolina in July of 2015.

Our association has "old-timers" that have worked thousands IAARS enjoys working with all of the associations and organizaof accidents. If you have a question, chances are they have tions that are involved in helping get answers to how and why encountered a similar situation in the past and can give some insight on how they worked it out. We also have members very collisions occur and how to make our roads and highways safer. familiar with the ins and outs of new technology (Crash Data Hope to see you at one of the conferences.

Retrieval, Photogrammetry) and are willing to help with any Sincerely, Fred Rice, President, IAARS www.iaars.com questions or concerns you may have.

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.

The ARC Network offers various membership levels to fit your needs and budget. Membership starts at just $89.00 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

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)!

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 - Volume 10 Issue 1 7

E

R EER

P

D

WE E I V

>> Data Mining

The NHTSA NASS CDR Database Tobias Achstetter, Fabian Kübler, Michael Wolf and Sean Haight Center For Collision Safety and Analysis George Mason University

I

ntroduction Since 1979, the National Highway Traffic Safety Administration (NHTSA) has been implementing a nationwide crash data collection program called the National Automotive Sampling System (NASS). It is sponsored by the U.S. Department of Transportation and is operated by the National Center for Statistics and Analysis (NCSA). <<ref 1>> Since the mid 1980’s, NASS has been split into two distinct reporting systems: the General Estimates System (GES), and the Crashworthiness Data System (CDS). The GES collects data on a sample of all police reported accident reports, while the CDS collects additional detailed data on a smaller sample of accidents. For this research, the focus will be on the NASS CDS variant.

The data for the NASS CDS database is collected via 24 geographical locations which are called Primary Sampling Units (PSU). The collected data is then processed and sanitized to ensure privacy, then it is stored on the NHTSA website for analysis. The NHTSA website, which provides the ability to search the database, can be found at http://www.nhtsa.gov/NASS. The portal allows the user to search for specific cases, by vehicle, by accident type, damage, delta-v, etc. Once a case is selected, the user can view information about the accident such as: photographs, vehicle information, occupant information, injuries, scene diagrams, crush measurements, and more. In addition to the web portal, database software suites, like SAS, can also be used to interface with the NASS database.

8 Collision Magazine - Volume 10 Issue 1

While most of this is known to the average safety researcher or crash reconstructionist, what is not typically known is that many of these cases also include Bosch Crash Data Retrieval (CDR) reports. Starting in sampling year 2000, NHTSA began providing PDF copies of the CDR reports from supported vehicles involved in NASS CDS accident cases. These files are located on the NHTSA website via file transfer protocol (FTP) at ftp://ftp.nhtsa.dot.gov/NASS/EDR_Reports/. Anyone with internet access can download packaged collections of CDR reports for a given sampling year. Each package contains all of the CDR generated reports for the specified year.

Each file is organized by a reference number, which is also the filename. This filename includes the sampling year, the PSU, the case number, and the vehicle number.

For example, on CDR report is named: 2000-12-193V1.PDF. Using the file name, the specific case contents, like photographs, can be accessed using the NHTSA NASS CDS webpage. From sampling year 2000-2012, there are approximately 7,450 CDR reports.

M

otivation Currently, there is little to no research dedicated to the analysis of this large and valuable data set. The primary motivation of this research is to provide a roadmap for future research using the CDR data from the NASS CDS dataset. This research will outline one possible method for data extraction and organization, as well as some sample data mining techniques that can be used to extract meaningful data.

www.collisionpublishing.com

Most of the analysis and research currently being done involving the NASS CDS is based on the data that is hardcoded by the NASS investigator. Due to incorrect/ incomplete information given to the investigator or human error, this objective method of analysis can be another piece of the puzzle for analysts. For example, there can sometimes be contradictory information between the NASS CDS data and the CDR report. For example, in case number 2001-45-064-V2, the police report indicates that the “belted driver and three nonbelted passengers of V2 were transported to the hospital …” However, the Bosch CDR file clearly states that the driver’s belt switch circuit status is listed as unbuckled.

This information, which was passed to the NASS investigator via the police report does not match the data provided by the Event Data Recorder (EDR). While it is technically not conclusive either way (perhaps the driver was sitting on the buckled seatbelt in this case), using the CDR reports can shed some additional light on the NASS CDS case information.

M

ethod for extracting data from CDR PDFs In order to make the data of the CDR PDF reports more accessible and to statistically examine the information that are provided in the CDR files, one goal of this research was to generate and populate a SQL database. The methodology that was used to achieve this goal is explained in some detail in the following paragraphs.

C

DR Data Elements The first step was to select the data which should be extracted from the CDR PDFs. Figure 1 shows an example of an early 2000's General Motors CDR report file. On the left side, important parts of the PDF file are shown which holds the CDR file information such as Vehicle Identification Number (VIN), NASS Case Number (file name), the CDR software version number and the type of events recorded.

For the system status at event, the authors extracted the seat belt status and the ignition cycle count. Similarly, crash parameters like vehicle speed, percent throttle and brake switch circuit status were extracted from the CDR reports.

D

ata Extraction Schemes The database extraction scheme in Figure 2 was used to extract the data, shown in Figure 1, and to generate the database that can be used to propose scientific queries. The first step, was to convert the PDF to text files using a free Python module PDFminer <<ref 2>>, which is available for Python 2.7. Python 2.7 was the primary programming language used in this project. The most laborious part of the project was the several hundred lines of Python-code that were written to extract the data from the text files. To deal with the different types of CDR report formats, many

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 9

CDR File Information: • VIN Number • NASS Case No. (filename) • CDR Version • Events Recorded System Status at Event: • Seat belt status • Ignition cycle count Pre-Crash Data: • Vehicle speed • Percent throttle • Brake switch circuit status

Figure 1: Early 2000’s GM SDM data extraction example overview subroutines were needed to extract the right data from the right place. In the last step, a database was created and populated using sqlite3 <<ref 3>>. In the end, the authors were able to extract data from 5,519 files and add this extracted data to the generated database.

Convert PDF to Text First, the method for converting the PDF to text and to extract the data from the text is explained. Using the module PDFminer, the whole PDF is first converted to one long text string, which can be subdivided with the Pythoncommand “readlines()”. The readlines() command is able to read in an entire file at once. It automatically parses the contents of a file into a list of lines. This results in a list, which later is appended to a global list that holds the data of all the text files in one directory. One of the main problems that occurred during the conversion is that the content of data structures, like tables in the PDF, are getting moved around despite similar PDF formats. For example, sometimes the content is read-out column wise and in some other formats line wise. Figure 3 shows a section of the resulting text file that could be converted from the PDF file of an early 2000's General Motors CDR report file that was shown earlier as an example in Figure 1.

Extract Data from Text The second step is to extract the data from the text files to populate the database tables. The recognition of the data parameters was based on line numbers and the digit num-

10 Collision Magazine - Volume 10 Issue 1

ber within the line. Oftentimes keywords were used to find the specific line number that belonged to the specific information. This was primarily accomplished by using the index('search_string') method for the list class in Python.

It returns an integer, which represents the location of the defined keyword. Some information of interest could be found by following a format specific numeric pattern after finding the position of the keyword. An example of the used methods is shown in Figure 4, where the location of the VIN, the information about the year from the NASS Case number, and the information on the type of events are highlighted in the text file.

D

atabase Design The creation of the CDR database requires three major steps. First is finding the relevant parameters to extract and store. Second, create the Entity-Relationship-Diagram and Relation-Schema to get the logical structure of the database. Third, is to combine step one and two to create the final CDR database.

Selected data parameters The design of the database depends heavily on the available crash data parameters. The authors attempted to extract and store as many parameters as possible. This would ensure that interesting queries can be constructed to generate valuable statistical results. In turn, these parameters should be completely searchable in the final database design.

www.collisionpublishing.com

Figure 2: Data extraction scheme CDR File Information Vehicle Identification Number Investigator Case Number Investigation Date Crash Date Filename Saved on Collected with CDR version Reported with CDR version Event(s) recovered 1G8ZK5275XZ******

Figure 3: Example text file after PDF extraction

2000-02-130-V1.CDR xxxxx Crash Data Retrieval Tool 1.00 Crash Data Retrieval Tool 2.900 Non-Deployment SDM Data Limitations SDM Recorded Crash Events: There are two types of SDM recorded crash events. The first is the Non-Deployment Event. A Non-Deployment Event is an event severe enough to ?wake up? the sensing algorithm but not severe enough to deploy the air bag(s). It contains Pre-Crash and Crash data. The SDM can store up to one Non-Deployment Event. This event may be overwritten by another NonDeployment Event. This event will be cleared by the SDM after the ignition has been cycled 250 times.

The second type of SDM recorded crash event is the Deployment Event. It also contains Pre-Crash and Crash data. The SDM can store up to two different Deployment Events, if they occur within five seconds of one another. Deployment Events cannot be overwritten or cleared from the SDM. Once the SDM has deployed the air bag, the SDM must be replaced.

The data in the Non-Deployment Event file will be locked after a Deployment Event, if the Non-Deployment Event occurred within 5 seconds before the Deployment Event unless a Deployment Level Event occurs within 5 seconds after the Deployment www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 11

CDR File Information Vehicle Identification Number Investigator Case Number Investigation Date Crash Date Filename Saved on Collected with CDR version Reported with CDR version Event(s) recovered

Information from VIN in line 11 is saved in database in the vehicle table

1G8ZK5275XZ****** 2000-02-130-V1.CDR xxxxx Crash Data Retrieval Tool 1.00 Crash Data Retrieval Tool 2.900 Non-Deployment SDM Data Limitations SDM Recorded Crash Events:

Sampling year information from NASS Case No. is stored in the vehicle table Information on the type of event is stored in the first_event or second_event table

Figure 4: Example of text recognition from extracted document The crash data parameters that were extracted for this database are: •

Pre-impact vehicle speed

•

Pre-impact brake status

•

Delta-V vs. time for frontal airbag deployment event

•

Engine speed

•

Ignition cycle count at event

•

Ignition cycle count at investigation

•

Maximum Delta-V

•

Passenger's airbag status (enabled/disabled)

•

SIR Warning Lamp status (on/off)

•

Status of driver's seat belt switch (buckled/unbuckled)

•

Throttle percent

•

Time between non-deployment and deployment event

•

Time from Algorithm Enable (AE) to airbag deployment

•

Time from AE to maximum Delta-V

Entity-Relation Diagram and Database Schema The basic structure of the database design is reflected in the Entity-Relationship-Diagram (E-R Diagram) shown in Figure 5. This diagram provides entities: objects which are distinguishable from other objects. <<ref 4>> Each en12

Collision Magazine - Volume 10 Issue 1

tity is classified by attributes and one identifying attribute.

Within the diagram the relationship imparts the interconnection between each entity. For the CDR Entity‑Relationship‑Diagram, six entities are created which can be seen in Figure 5. Each of these entities are assigned with attributes extracted from the CDR files.

In detail, the entity “vehicle” with the identifier “filename” are described by the attributes “manufacturer” and “year_ of_data_extraction”. The “vehicle” entity can have a “first_ event” and/or a “second_event”. Thus, “vehicle” is associated with the entities “first_event” and “second_event” by the relationship “can have”.

Using the E-R diagram as a starting points, the database Relationship Schema is developed. The Relationship Schema (Figure 6) provides more detail on the logical design of the database as well as the content being stored for each entity. The entity tables are populated with a primary key and foreign keys. The primary key is classified as the unique identifier for the entity table. Whereas, the foreign keys build the connection with primary keys from other tables. <<ref 4>> For example the connection between the entities “vehicle” and “events”, first or second, is based on the key “event_ ID”. Each report has an event ID, labeled as foreign key, which relates to the primary key of the entity first or second event.

The use of normalization, creating IDs to connect tables, makes the structure of the database more efficient. Because

www.collisionpublishing.com

Figure 5: Entity Relationship Diagram

Figure 6: Database Schema www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 13

if all event data would have been stored in one table a file with only one recorded event would use more space than required. Another example of normalization is the record of pre-crash event data. For each attribute, five values are recorded (one for each second for example). By assigning an identifying key to the attribute the values can be stored separately in an additional table.

Final Database design In the final database design, the structure from the E‑R Diagram and Relationship Schema are implemented with SQlite. The entities of the database design previously described are used as names for the tables. For the attributes, the specific value type is added. If possible, the unique CDR filenames are used for the entity IDs. That makes the connection between the tables easier and reduces the amount of total IDs.

R

esults After designing the database, extracting the data, and populating the database tables, research can then be conducted using the database. The primary purpose of this research was to develop a proof of concept for data mining this large dataset. In addition, some “research questions” were proposed to show how SQL data mining can produce interesting statistics. In this case, 10 research questions were answered using the created database.

Since the database was created in the SQL format, SQL queries can be written to retrieve information from the database. Typical SQL queries used, contained mainly these SQL operators: “Select” retrieves data from one or more lines in a table.

“From” indicates from which table to extract data from.

“Where” is a clause that can restrict the extraction and “Join” is used to combine two tables into one to combine the data.

The following section of this paper includes the ten proposed research questions, the SQL query and the result.

Q1: What is the average speed before airbag deployment occurs (at -1 s time value)?

In this query, the average of the “one-second before deployment” column of the table “vehicle_speed” of all the “Deployment” events was selected.

Query:

Result: 14

SELECT AVG(one) FROM vehicle_speed WHERE type = 'Deployment'; 25.65

Collision Magazine - Volume 10 Issue 1

The result, directly taken from the query, was an average velocity of 25.7 Mph (41.3 km/h) out of the 3,416 datasets used in this query. This result seems to be reasonable since it is a common average speed for vehicles (for city and highway driving).

Q2: What is the percentage of events report seatbelt status as “Buckled”?

The query written joined the tables “vehicle”, containing the basic information, with the table “vehicle_status” which provided the information about the seat belt status. To answer the actual question, this join would not be necessary. However, to analyze the seat belt status more precisely for each year separately, the inner join was necessary.

Query:

Result:

SELECT COUNT(vehicle.file name) FROM vehicle INNER JOIN vehicle_status ON vehicle.filename = vehicle_status.filename WHERE vehicle_status.seatbelt_status = 'BUCKLED’ 'Buckled': 4022 'Unbuck led': 2315

The results showed, that of all events recorded (whether deployment or non-deployment) 63% of all drivers were reported as buckled. This percentage seems to be very low compared to survey data published by the NHTSA in May 2014 with 87% nationwide seat belt use for the year 2013 <<ref 5>> This large discrepancy might be explained by the fact that the police and subsequently the NASS investigators are only alerted in more serious (tow-away) crashes that tend to have severe injuries, which are more likely to happen to unbuckled occupants.

One reason that can create incorrect values for the reported seat belt use is power interruption during an event. Therefore these cases were removed by checking the ignition cycles count. An ignition cycle count of 0 is an indicator of a power interruption. The term “ignition_cycles != 0” was added to remove roughly 200 cases. The new result was, that 65% of drivers was reported as buckled.

In addition, the percentage of seat belt use for each year in the examined time frame was calculated to see how the percentage of buckled drivers has developed over the years. Figure 7 shows the resulting total num-

www.collisionpublishing.com

Year 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

Buckled Unbuckled 13 15 134 129 277 208 372 224 538 300 441 258 436 208 327 168 311 139 301 139 249 134 400 259 214 131

Total

%

28 263 485 596 838 699 644 495 450 440 383 659 345

46 51 57 62 64 63 68 66 69 68 65 61 62

Figure 7: Reported buckled and un-buckled results by year (left) Figure 8: Reported buckled and un-buckled results by year (above)

bers and for better visualization the percentage of each year was plotted in Figure 8.

Q4: What is the average (maximum) Delta-V for all deployment events recorded?

To get the results, the previous query had to be changed and the line 'AND vehicle.year = 2000;' added for each year. The black line in Figure 8 shows the results before removing the cases in which power interruptions occurred, whereas the gray line shows the results after removal.

This question was answered by selecting the largest magnitude (because delta-v is typically negative) of each delta-v point in the delta-v table and then averaging all the resulting values from each case. In other words, the peak delta-v value was selected for each case and then all of those values were averaged across all the deployment events. For this query, only older GM SDM (with 150 ms of total delta-v data) reports were used because they were very common in the database, and were most comparable. In total, there were about 1,200 reports in this format. Additionally, some cases with zero delta-v were reviewed and excluded. The result was that the average deployment delta-v for a deployment event is about -16.5 Mph.

An almost continuous increase from the years 2000 to 2009 can be observed, which is not surprising.

However, the decline from 2009 to 2011 is difficult to explain. One reason might be due to the reduction in the total number of CDR report samples for years after 2004.

Q3: What is the percentage of deployment events for all reported events?

All events were separated into first and second event recorded and considered separately.

Query:

SELECT COUNT(filename) FROM first_event WHERE type = 'Non-Deployment'; Result: 2839 For the first event the percentage of non-deployment events was 41.8% and for the second event 91.9%.

Overall, about one third of the events had airbag deployments.

Query:

SELECT AVG(MIN(ten,twenty, thirty,forty,fifty,sixty, seventy,eighty,ninety,hun dred,hundredten,hundredtwe nty,hundredthirty,hundredf orty,hundredfifty)) FROM velocity_change WHERE type= 'Deploy ment'; Result: -16.54 Mph Q5: What is the average (maximum) Delta-V for all nondeployment events recorded?

Similarly, for the non-deployment events, the average (maximum) delta-v was queried. Only the operator

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 15

“WHERE type='Deployment'” had to be changed to “Non-Deployment”. Like query 4, the cases with zero delta-v were excluded from the analysis. The total sample size for this query was about 800 reports. The average delta-v for non-deployment events was about -4.7 Mph.

that doesn't trigger airbag deployment (like a postbox) before a serious crash occurs. Being able to recognize such events can improve the preparation of the car for the crash (for example faster airbag deployment and additional braking assistance).

Q6: How many crashes occurred with the airbag warning lamp on?

Q8: How many reports have a deployment event and a non-deployment event?

As expected, the number is very low with 4.59% (282 out of 6138) of the events. The low number is not surprising, because it is an indicator of a damaged or incorrectly replaced SRS system.

Simlar to query number 7, the tables are again joined on the filename and additionally only the first_events that are non-deployment and only the second events that are deployment events are picked. The constraint involving the time between the events is removed. The result was that 25.5% (1410 out of 5519) of reports have a non-deployment and a deployment event.

Query:

Result:

SELECT COUNT(filename) FROM vehicle_status WHERE SIR='ON'; 282

Q7: How many reports have two events within one second?

Queries 7 is about consecutive events that were recorded. In query 7, the number of times in which an event followed a previous one within 1 second, as well as the total number of multiple events recorded was calculated. Of all data-sets provided, in 7.3% (404 out of 5519) of events a second event follows a first one within 1 second, and in 27.8 % (1536 out of 5519) of the files multiple events were recorded.

To answer this question, an inner join of the tables first_event and second_event had to be used to get information about both events in the same query. In an SQL join, the tables have to be joined on one of the tables value. As previously explained, we used the file name as keys and therefore joined the tables on the file name.

Query:

Result:

SELECT COUNT(first_event. filename) FROM first_event INNER JOIN second_event ON first_event.filename = second_event.filename WHERE second_event.time_ between_events < 1.0; 404

The pie chart in Figure 9 shows the results more clearly. 72.2% of reports had only one event recorded. Of the remaining 27.8%, 26.3% (404 out of 1536) followed within 1 second after the first event. This can be interesting to know for scientists that work on improving the algorithms which trigger airbag deployment. It might occur often that a car hits something 16

Collision Magazine - Volume 10 Issue 1

Q9: What percentage of reports show brake use prior to impact?

For this query all five seconds for each case are checked if they contain an ON value and then all files are counted that fulfill this constraint.

Query:

Result:

SELECT COUNT(filename) FROM brake_switch_circuit_ state WHERE five = ‘ON’ OR four=‘ON’ OR ...

3269

In 72.8% of all events (3269 out of 4491), the brake switch was ON at some point prior to the impact.

Additionally, the percentages of events with brake use for each second prior to the crash was analyzed. For better visualization, the results of this query and question number 10 are shown in Figure 10.

From the figure, it can be seen that the percentage of drivers that brake (shown in black) increases between the 3rd and the 2nd second before impact and is increasing even more dramatically between the second and first second before impact.

Q10: What is the average percentage of throttle observed for each time sample for the five seconds before the crash?

The answer to the last query shows similar results. The red curve in Figure 10, shows the average percent of throttle used in the last five seconds of a crash. Here again the usage drops within the last three seconds before the crash occurs. Both curves show a significant change in slope from second three to one. It is important to mention that these results don't allow to draw any conclusions about the reac-

www.collisionpublishing.com

Figure 9: Pie chart showing distribution of number of events

Figure 10: Plot of average brake and percent throttle for each time increment tion time of the drivers. The results just represent the times (relative to some time zero) when the driver takes action and releases the throttle (query 10) or brake (query 9).

D

iscussion The largest achievement of this study is that it is able to provide large amounts of naturalistic observations of crashes at a low cost to the analyst. This naturalistic study allows the researcher to analyze data from

an actual crash and compare it to the data collected by a seasoned crash investigator. More importantly, however, this method provides thousands of CDR reports, from real world collisions, that can be compared to each other. If nothing else, this research provides an explicit method for other analysts to provide non-judicial research in the area of Event Data Recorder technology. Since E.I. du Pont de Nemours & Co. v. Robinson (923 S.W.2d 549, 557 (Tex. 1995)), one of the factors considered in Daubert hearings

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 17

is if there has been any “non-judicial uses which have been made of the theory or technique”. This paper not only produces non-judicial application of EDR data, but it lays out the framework for future work.

Since EDRs are typically thought of as objective data, this method can provide a comparison to some of the subjective data frequently found in the NASS CDS. For example, the data found in the CDR report can often be compared to reliable eye witness data with accurate measurements that is commonly found in a police report.

Naturalistic studies can often be very expensive and time consuming. For example, a researcher might spend thousands of dollars designing, manufacturing and installing crash recorders in a hundred vehicles. It might take years to collect enough information on those vehicles to come to any viable scientific conclusions. However, utilizing the CDR tool as a method for data collection, the analyst can collect crash parameters from thousands of crashes in a short amount of time.

To accomplish this, a data extraction and organization scheme was designed by the authors. Using a PDF to text converted in conjunction with a data mining script, the authors were able to create a large database of valuable crash data. Based on the information provided in this paper, other researchers can do their own investigations into this large data set.

There are a few limitations on the data and analysis provided in this paper. First, the authors did not fully investigate special cases, or outliers, in the data set. For the purpose of this paper, the data provided in the CDR report was directly extracted into the database pool. For example, the authors did notice some cases with odd reported values (ie. 56 MPH non-deployment delta-v). These anomalies should be dealt with in order to achieve a more scientific understanding of specific statistics. The authors did provide a method for removing some of these anomalies, as in

“”

The largest achievement of this study is that it is able to provide large amounts of naturalistic observations of crashes at a low cost to the analyst.

18 Collision Magazine - Volume 10 Issue 1

the case of the power interruption. It should be noted that the statistics shown are solely based on the reported values, not necessarily what happened in the actual crash.

Another limitation in this study is based on the fact that there are many types of report formats used in the CDR Tool. The Bosch CDR program takes the raw data collected from the vehicle and formats it in a way that the user can easily read it. However, since many vehicles provide different amounts/types of data parameters, the formats have to be customized for some specific vehicles. For this research, each report template that was used was hard-coded into the data extraction script. Therefore, the more format types available meant more programming time. In turn, some of the CDR report formats were excluded because they were a lower percentage of the overall data set. In fact, about 94% of the total CDR reports were of similar report formats from General Motors vehicles. The other 6% were from Chrysler, Ford and other manufacturers.

An additional limitation of this study is based on the fact that the reports provided by NHTSA were in the PDF format generated at the time of the investigation. As any certified CDR Analyst would know, some translations/reports can change over time. There can be corrections or additional data provided in one report that was not available in a previous release of the Bosch CDR tool software.

Since the reports were only provided in the PDF format, the data cannot be re-translated using the newest version of the Bosch CDR software.

In retrospect, this data would have been more valuable if NHTSA had been archiving the original *.CDR or *.CDRx file from the accident investigation. However, future improvements to the NASS CDS organization are coming. Based on presentation provided by NHTSA at the 2015 Government Industry meeting <<ref 6>>, the NASS CDS (which will be renamed to Crash Investigation Sampling System) will include a “usable, sanitized EDR file”.

The authors assume that this means that these accident investigations will include the raw *.CDRx files. In addition, it would be very valuable if there was a XML export function, with XML tags, in the CDR software. If there was a way to export this crash data, in an organized format, it would allow researchers to extract data into a database more efficiently.

There can also be some improvements to the next iteration of this CDR database structure. For example, more CDR report formats can be hardcoded into the extraction script. This can be easily implemented with some additional programming time. An alternative method could be developed where a machine learning approach is used to automatically populate new formats in the data-

www.collisionpublishing.com

base; therefore reducing programming resources. A similar algorithm can be developed to search and flag possible outliers or anomalies in the data set. This would be useful to analysts and would provide more accurate results. Lastly, a more complex schema could be designed to account for more types of data, and in turn, more complicated analyses.

Further improvements to the data mining methods can also be implemented. For example, the program can be redeveloped so that it interfaces directly with the NASSCDS online database. Therefore the analyst can extract investigation data, like crush, along with CDR data like delta-v. Additionally, more complex data mining queries can be developed so that more accurate and complete statistics can be derived. Finally, the program can be redeveloped so that the translations are re-formatted in the most recent versions of the CDR tool.

C

onclusion In conclusion, this study attempted to extract crash data parameters from thousands of CDR files located in the NHTSA NASS-CDS database. In turn, a database was created to store many of the data parameters found in the CDR reports. This database was then queried to show how crash statistics can be determined from the large dataset.

In total, 10 proposed research questions were answered using the SQL computing language. While these results provide interesting insights into driver behavior and crash severity statistics, there are many limitations to consider.

As stated, the statistics generated in this paper was simply extracted from the raw data in the CDR reports. Anomalies and technical issues with the reported data were not considered in this research (unless otherwise specified).

However, the authors feel that this research might spark a new interest in vehicle safety research. A more detailed study, with more advanced data mining and machine learning algorithms can provide even more accurate accident statistics. As Event Data Recorders become a more common part of every NASS-CDS investigation, the larger this data set becomes. In the future, this kind of crash data could be be a part of the decision making policy in vehicle and road design.

R

eferences 1. National Highway Traffic Safety Administration. National Automotive Sampling System — Crashworthiness Data System 2013 Analytical User’s Manual. Rep. no. DOT HS 812 066. Washington, DC: U.S. Department of Transportation, 2013. Print.

2. Shinyama, Yusuke. "PDFMiner." PDF Parser and Analyzer. Python.org, 28 Mar. 2014. Web. 27 Mar. 2015. <https://pypi.python.org/pypi/pdfminer/>.

3. SQLite Home Page." SQLite Home Page. N.p., n.d.

Web. 27 Mar. 2015. https://www.sqlite.org/ 4. Silberschatz, Abraham, Henry F. Korth, and S. Sudarshan. Database System Concepts. New York: McGraw-Hill, 2011. Print.

5. National Highway Traffic Safety Administration.

Crash Stats: Seat Belt Use in 2013 - Use Rates in the States and Territories (n.d.): n. pag. U.S. Department Of Transportation, May 2014. Web. <http://wwwnrd.nhtsa.dot.gov/Pubs/812030.pdf>.

6. Morgan, Tina. Update on NHTSA's Data Modernization Project. Proc. of Society of Automotive Engineers Government Industry Meeting, Washington, DC. NHTSA, Jan. 2015. Web.

Accident Reconstruction

Search

Engine

The one click resource for all Accident Reconstruction industry information.

www.ARSearchEngine.com Powered by Google and Collision Publishing.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 19

dy

tu S e

Cas

Three car, in-line crash analysis with CDR Data Rusty Haight Collision Safety Institute

F

or this case study, the Crash Analyst is presented with a case involving three cars; an otherwise “typical” in-line crash involving three vehicles. The involved vehicles are: a 2005 Ford 4wd F-150 SuperCrew (assumed weight is 5800lbs/2630kg), a 2010 Toyota Corolla (assumed weight 3000lbs/1361kg), and a 2007 Toyota RAV4 (assumed weight 3700lbs/1678kg).

The Human Component The driver of the RAV4 said they were stopped in a line of traffic and heard tires skidding then felt an impact.

The driver of the Corolla said they were in stop-and-go traffic behind the RAV4, at or near a stop when they heard skidding behind them then they were rearended and pushed into the RAV4. The driver of the F-150 said they thought the driver of the cars ahead braked unnecessarily.

The Vehicle Component You may adopt that the RAV4 is in the “lead” position; numbered from rear-to-front in the group, the #3 vehicle. The Corolla is in the middle or #2 position and the F-150 is in the rearmost position in the line or the #1 vehicle. The damage to the involved vehicles can be seen in the photos accompanying this case problem. None of the vehicles had pre-existing damage in the areas involved in this event. All are seen in the accompanying photos at their controlled points of rest on the shoulder of the road post-impact having been moved there by the respective drivers.

In terms of potential CDR Tool retrievable data, the truck is supported for PCM data but there was no deployment of airbags and one may adopt that, after the crash, the driver left the truck running so there would be no data remaining associated with this event. The RAV4 is supported for EDR/CDR data but is, for whatever reason, unavailable to the analyst for imaging. The remaining vehicle, the one in the middle of the three, is imaged and excerpts of the CDR report are provided in these pages of Collision for your review in this analysis.

20 Collision Magazine - Volume 10 Issue 1

The Environment Component The roadway in the area of this collision is a straight, relatively level 3 lane divided highway which was clear and dry at the time of the collision. The collision took place at approximately 9:00AM in the right-most travel lane for the direction all three had been moving and there was some stop-and-go traffic approaching an offramp about a quarter mile head of the area of impact(s).

The Challenge: Given the available information, the CDR Data Analyst should be able to answer the following questions about this collision. No inference should be made from the order of the questions and, where appropriate, more than one value might be offered for a given question if the Analyst interprets that the question applies to, for example, multiple impacts.

1. What is the order of the impact(s)? Or, which vehicle hit which vehicle first? What information supports your conclusion(s)?

2. When the F-150 hits the Corolla, what is the closing speed of the F-150?

3. When the F-150 hits the Corolla, what is the speed of the Corolla?

4. When the F-150 hits the Corolla, what is the impact speed of the F-150?

5. What is the delta-V for the Corolla when it is hit by the F-150?

6. When the F-150 hits the Corolla, approximately how far is the Corolla away from (behind) the RAV4?

7. What is closing speed of the Corolla relative to the RAV4?

Answers and a discussion are found later in this issue of Collision.

www.collisionpublishing.com

Editor’s note: This case problem is drawn from an actual crash but where the vehicle history for the Toyota Corolla from which the CDR Data is drawn is known and documented. This is not drawn from a litigated case so this case problem is not a justification of an analysis, as is sometimes seen in published papers elsewhere.

The CDR report offered for this case problem is displayed on these pages in the latest version of the CDR Tool software (version 16.0.1).

Normally, of course, one would work from the CDR file but for this case problem so everyone reviewing it is on the same page, so to speak, the relevant pages of the report has been reproduced here and edited to efficiently fit the pages of Collision. For example, the data found on the page associated with “TRG 3” would normally span three pages in a CDR report but here have been condensed to one page making the delta-V graph smaller but allowing for normal print size on the remaining data elements. The hexadecimal data pages have been eliminated solely in favor of space considerations for this example.

Since there is the possibility that a data element may be reported differently in a later version of the CDR Tool software, it is possible that there may be variations of this data, as represented, if translated with a future version of the CDR software, but, for this example, we will adopt it is the most current, accurate translation and reliable representation of the Data Limitations text we might work from.

This case problem is offered as an example of how CDR data might be used in a crash where, for whatever reason, there’s just not a lot of other information to work with other than a few at-scene photos and the CDR report for one of the involved cars. Whether as a function of a limited amount of at-scene documentation or a designed case problem, this analysis relies on the information available and nothing more. Ideally, there might be more documentation such as crush measurements and perhaps even CDR data from the other two involved vehicles but then that wouldn’t be in keeping with the focus of this case problem’s design. www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 21

22 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 23

24 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 25

26 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 27

3D Vehicle Point Clouds Vehicle Scans is a 3D library of exemplary vehicles, offering 3D point clouds at multiple resolutions, vehicle photo documentation and 2D orthographic, scale renderings. Vehicle Scans utilizes the latest in 3D LiDAR technology resulting in highly accurate data sets.

www.vehiclescans.com Use this data to: ·Take specific measurements ·Create 3D models ·Verify the accuracy of an existing 3D model ·Determine damage to an accident vehicle ·Create technical graphics and animations Download a Free sample today

E

P

R EER

D

WE E I V

The implications of the CRASH3 uniaxial structural response model and the nature of available collision test data in regards to work-energy relationships for Jai Singh oblique impacts Biomechanical Engineering Analysis & Research, Inc.

30 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

A

bstract Presented in the subject work is an investigation into the theoretical basis for the tangential correction factor utilized in the internal work absorbed formulation of the CRASH3 model, for oblique collisions, in light of the implications of the uniaxial nature of the structural response model and the nature of available controlled collision test data. Implicit in the original CRASH3 derivation is the treatment of the resultant collision force vector, for an oblique impact, based upon the component of the collision force in the normal direction to the deformed vehicle surface, as being along the relevant undeformed vehicle principle axis. The correction of this limiting assumption produces a collision force decomposition that mirrors the collision impulse decomposition found in planar collision mechanics formulations. The uniaxial nature of the empirical

structural response model, relating the peak collision force to the residual damage depth, while precluding the ability to explicitly model force and deflection in the induced damage region, presents a more pressing problem in regards to both the original CRASH3 formulation and the nature of available controlled collision test. The latter is predicated on the provision of residual damage depths along the undeformed vehicle relevant principle axis. As a result, for a uniaxial load path, the stiffnesses are quantifiable in axial compression only and with the transverse displacement of points on the geometry boundary treated as being constrained to zero. Not only does this preclude the quantification of the component of collision force vector in a normal direction, when the normal is not axial, for an oblique impact, but reduces the workenergy relationships for the oblique case to those of the non-oblique case.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 31

I

ntroduction The damage analysis algorithm of the Calspan Reconstruction of Accident Speeds on the Highway (CRASH3) model is a generally accepted and widely utilized approach for quantifying the severity of a collision based upon the residual damage present to the collision partners. The reconstruction of motor vehicle collisions, within the field study context, for inclusion in the National Accident Sampling System (NASS) Crashworthiness Data System (CDS) involves either the use of a damage analysis algorithm or the use of a trajectory algorithm. The damage analysis algorithm used therein, unchanged due to the recasting of the model coefficients (McHenry and McHenry, 1998) as per the NHTSA’s reformulation (Prasad, 1990, 1991abc), is the CRASH3 damage analysis algorithm and represents the basis for the majority of collisions that are reconstructed and from which the estimates of collision severity are coded (Sharma et al., 2007).

Collision configurations falling within the purview of the CRASH3 damage analysis algorithm can generally be classified as planar impacts, inclusive of collinear collisions.

The vehicle geometry models utilized are based upon the plan (i.e. top down) projection of the actual three dimensional vehicle geometry. In the plan projection, the projected geometry can readily be bounded by a rectangle and it is the bounding rectangle that served as the implemented vehicle exterior surface boundary model in the development of the damage analysis algorithm. While the plan projection allows for the consideration of collisions in two dimensions, the structural response model is both uniaxial and empirically based. The empirical relationship employed in the CRASH3 damage analysis algorithm is that of equating the peak collision force, normalized per unit length of direct damage as per the undeformed vehicle geometry, to a linear function of the residual damage depth.

(1) In equation (1), F is the peak collision force, L is the length of direct damage as per the undeformed vehicle geometry, A is the crush resistance (units of force per length), B is the slope of the linear relationship (units of force per unit of residual damage depth per unit of direct damage length as per the undeformed vehicle geometry) and c is the residual damage depth. The collision force and residual damage depth have direction and thus are vector quantities. This relationship between peak collision force and residual damage depth is not a time-parametric relationship and is not the equivalent of a time-parametric forcedeflection relationship. If one considers the relationship between collision force and deflection, both generated in a time-parametric manner, from a New Car Assessment

32 Collision Magazine - Volume 10 Issue 1

Program (NCAP) collision test, the dynamic data for the entire test reduces to a single point when one compares the peak collision force and the residual damage depth present at the terminus of the separation phase. By design, the formulation of the CRASH3 damage analysis algorithm is predicated upon the achievement of a common velocity between the collision partners at the terminus of the closure phase of a collision and does not account for the separation phase response. From the strict Newtonian mechanics work-energy perspectives, the empirical CRASH3 relationship, in regards to its linearity, for collisions that produce non-zero magnitude residual damage depths, can be reproduced, only, under the following conditions (Singh, 2014): 1. The closure phase response is uniformly and ubiquitously linear.

2. The separation phase response, for collisions exceeding an elastic limit (i.e. that result in non-zero valued residual damage depth magnitudes), must also be uniformly and ubiquitously linear.

3. The separation phase response cannot be, in part or in whole, linear with zero slope or infinite slope.

For the former, such would suggest an increasing deflection while the collision force remained constant and for the latter, such would suggest that the deflection remained constant while the collision force changed magnitude. There exists no physical or explicit modeling based mechanism to support either extreme.

Clearly, in opposition to suggestions made in the literature, the empirical CRASH3 linear relationship is not a force-deflection response that has simply been shifted over on the abscissa. While the focus of previous work in regards to the CRASH3 damage analysis algorithm has been on the structural response model itself, the focus of the subject work is on the manner in which oblique impacts are considered. Figure 1, with additional annotations, is an accurate representation of Figure 9-6 of Noga and Oppenheim (1981). Because of the rectangular bounding box treatment of the undamaged vehicle geometry, the orthogonally arrayed solid lines, herein shown in the blue-colored style and labeled, represent the undamaged vehicle geometry. Directly following from this is the fact that the vehicle-fixed axes, in the plan projection, for the undamaged vehicle geometry, when represented as being rectangular Cartesian, are coincident in orientation with the undamaged vehicle geometry. The axes labeled as one and two in Figure 1 represent the longitudinal axis and lateral axis, respectively, for an end impact and the lateral axis and longitudinal axes, respectively, for a side impact.

www.collisionpublishing.com

Figure 1: Oblique impact model for the CRASH3 damage analysis algorithm.

The modeling of oblique impacts in the CRASH3 damage analysis algorithm is posited upon the development of deformed vehicle surface boundary, at maximum engagement and over the direct damage region, which is congruent and common with the maximum engagement direct damage region of the opposing collision partner. In the plan view, this common surface is reduced to a line that can be represented in the rectangular Cartesian sense by an orthogonally arrayed coordinate system comprised of a normal axis and tangent axis. The CRASH3 damage analysis algorithm initiates the analysis for an oblique impact by resolving the collision force into its normal and tangent components. Implicit in the CRASH3 approach, as shown in Figure 1, is that the resultant collision force (i.e. the force vector with a zero-valued component in the plan projection), lies along the relevant vehicle principle axis. This resultant collision force is determined from the normal component of the collision force using basic trigonometry.

a force-deflection response, the internal work absorbed (IWA) during closure by the collision partner due to the collision force, is determined by integrating the force-residual deformation relationship.

(2)

(5)

Another incumbent step in the CRASH3 damage analysis algorithm for oblique impacts is that the starting residual damage depth is in the normal direction. The resultant residual damage depth is then determined from basic trigonometry.

In the CRASH3 approach, the Eabs, on a resultant basis, is:

(3)

(4)

In equation (4), δ is the time-parametric deflection δ(t) that is work-conjugate with the collision force, the force for the IWA computation is the time-parametric collision force F(t), δc is the deflection at the terminus of closure, Eabs is the CRASH3 equivalent, energy absorbed, to the IWA and the terms F and c in the operand under the integral operator to the right of the last equality are not timeparametric. Because the angle between the normal force and the resultant is constant, the cosine of the same is also constant. Applying the differential operator to equation (3), therefore, yields the following:

(6) Substitution of equations (2) and (5) into equation (6) coupled with a change of the limits of integration yields the following relationship:

Because of the CRASH3 treatment of the empirical forceresidual deformation relationship as being equivalent to www.collisionpublishing.com

(7)

Collision Magazine - Volume 10 Issue 1 33

Making note of the following:

(8) Substitution of this result into equation (7) reproduces equation (3) of Section 9.1.f of Noga and Oppenheim (1981).

(9) From a strict theoretical perspective, this approach is partially correct and partially incorrect. It is correct in that a resultant force vector can readily be determined from its component along the normal axis. It is incorrect in regards to the resultant force vector, outside of happenstance, for a general oblique impact, being aligned with a vehicle principle axis in the plan projection. A second issue is the uniaxial nature of the CRASH3 model. Because of this uniaxial nature, force due to a collision is only explicitly modeled in axial compression over the direct damage region. There is no mechanism present in the model for explicitly accounting for deformation in the induced damage region. Because the model predicts zero force in the induced damage region, it is inappropriate, from a strict work-energy perspective, to include such deformation in the calculation of Eabs. In any actual case under consideration, the physical problem, under plan considerations, reveals that a boundary point in the direct damage region may displace in a biaxial manner with respect to the undeformed vehicle local coordinate system (LCS). One can readily model this using uniaxial load paths, without transverse connections, if the load paths are arrayed in an angled manner with respect to the undeformed vehicle LCS.

This raises a difficult problem, however, when it comes to quantifying model parameters as it relates to the primary source of controlled collision test data from which such a quantification, on a make, model and year range basis, is made. This primary source of data is the Vehicle Crash Test Database (VCTB), maintained by NHTSA, which contains the test results of controlled collision testing, conducted under contract, for vehicle safety compliance, assessment and research purposes. The residual damage depths reported in these controlled collision tests follow an in-practice implementation of the Society of Automotive Engineers (SAE) J224 standard (Society of Automotive Engineers, 1980) for individual residual damage depth measurements, only report the component along the relevant undeformed vehicle principle axis (i.e. only along the longitudinal axis for end impacts and only along the lateral axis for side impacts). The resultant is that from a quantitative basis, the load paths are treated as being solely aligned along the relevant undeformed vehicle principle axis and the response is one of axial compression. The actual response, the measured response and the resultant are shown in Figure 2.

With model stiffness parameters being quantified on an axially compressive basis and the CRASH3 damage analysis algorithm, for oblique impacts, requiring a force calculation along the normal direction, this poses a rather interesting issue.

Having introduced the topic, we may now elucidate the objectives of the subject study.

1. Determine the implications of the uniaxial nature of the CRASH3 damage analysis algorithm coupled with the extant reporting of collision test

Figure 2: Shown on the left is the actual case in which a boundary geometry point can displace in both directions with respect to the undeformed vehicle coordinate axes. Shown in the center is the typical reporting case in which only the displacement along the relevant vehicle principle axis is provided.

Shown on the right is the resultant implication with respect to the load path (denoted as LP).

34 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

results from the primary data source, in regards to the work-energy relationships.

2. Derive correctly, the force transformation relationships for an obliquely directed collision force for the general case of the resultant force not being solely aligned along one of the two principle vehicle axes with respect to the undeformed geometry.

The results from the second objective are presented first as they aid in the presentation of the results of the first objective.

C

ollision force decomposition: When working from the perspective of a single collision partner, there are generally three reference frames that are of interest. The choice is made, herein, to characterize each reference frame using a rectangular Cartesian representation. The LCS, noted previously, is the vehicle-fixed frame of reference with origin located at the center of mass of the same. The undeformed vehicle longitudinal axis is denoted as the x-axis and the undeformed vehicle lateral axis is denoted as the y-axis. The sign convention used herein is a mirror of the SAE vehicle dynamics sign convention with +x being directed towards the fore and +y being directed to the left. The normal and

tangent directions, also noted previously, form the normaltangent coordinate system (NTCS). Finally, we define an inertial frame of reference as being the ground fixed reference frame with coordinate axes X and Y and referred to as the global coordinate system (GCS). The three frames of reference are shown in Figure 3.

We define the following three angles: the angle from the X-axis to the x-axis is denoted as ψ. The angle from the X-axis to the n-axis is denoted as Γ. Finally, the angle from the x-axis to the collision force vector is denoted as θ. These three angles, for the example shown in Figure 3, are shown in Figure 4.

We may readily define a transformation matrix that converts a vector quantity, given in terms of a GCS basis, into its components in the LCS basis.

(10) The inverse of this transformation matrix converts a vector quantity, given in terms of a LCS basis, into its components in the GCS basis.

(11)

Similarly, the transformation from the GCS into the NTCS and its inverse (i.e. from the NTCS to the GCS) are: (12)

(13)

Figure 3: An example oblique front impact showing the LCS, NTCS and GCS.

Figure 4: Relevant angles and relationships between the frames of reference. www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 35

The transformation matrix for a vector quantity in the LCS into the NTCS is given by:

For a side impact, substitution from equations (17) and (20) yields the following result:

(23) (14) This result may be written more simply by using the sum and difference relationships:

In a situation in which the force components are known, first, in the NTCS, the corresponding components in the LCS are:

(15) (16) As a result: (17) The inverse transformation is: (18) Stiffness coefficients derived from the above-referenced source of controlled collision test data represent axial compressive coefficients (i.e. in the x-direction for an end impact and in the y-direction for a side impact). The initial quantification generated by applying equation (1) yields the force in the LCS and not in the NTCS. From Figure 4 and basic trigonometry, the following can readily be seen for an end impact: (19) Similarly, for a side impact:

(24) In comparison to the CRASH3 formulation, for a frontal impact, the force component Fx of equation (24) equates to FR whereas for a side impact, the same force component from equation (24) equates to FR. The normal force from the CRASH3 formulation is equal to the normal force component from equation (23). The angle Γ - ψ in the subject formulation equates to the angle α in the CRASH3 formulation. This result clearly differs from the CRASH3 presumption of the collision force having only one non-zero component, for an oblique impact, in the LCS.

W

ork-energy relationships The definition of the dot product, applied to equation (4), yields the following result.

(20)

The collision force components in the NTCS can then be determined by simple vector-matrix multiplication.

(21) For an end impact, substitution from equations (17) and (19) yields the following result:

(25) In equation (25), the terms on the right of the equality are the magnitudes of the vectors and the cosine of the angle between the vectors. For each of the four collision configurations (front, rear, left side and right side) we can readily reduce equation (25). For each case, there are two options in regards to measuring the angle between the force and residual damage vectors. Both options are shown even though they reduce to the same result. The case for an oblique front impact is presented herein while the other three cases are presented in Appendix A. Oblique front impact:

(22)

36 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

The vector diagrams for an oblique frontal impact are shown in Figure 5. The direction of the residual damage vector, as well as its differential, are constrained due to the uniaxial limitations of the model and due to the fact that the quantified collision force from the subject controlled collision test data is only known in regards to axial compression along the x-axis.

The angle between the two vectors, for each case, is: (26)

(29)

For the first option, the cosine of the angle can be reduced by using the difference formula:

(27)

The Eabs on a LCS basis is: (30) If the problem is approached from the perspective of the NTCS, the residual damage vector transformed from the LCS into the NTCS is:

This result also holds for the second option.

(28) Substitution into equation (25) yields the following result:

(31)

Figure 5: Vector diagrams for the oblique front impact case.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 37

The magnitude of this vector is: (32) The magnitude of the force vector, using equation (22) is: (33) Applying the differential operator to equation (32) and substituting the resultant along with (33) into the Eabs relationship reproduces equation (30). Collectively, the derivation shown for the case of an oblique frontal impact coupled with the findings shown in Appendix A, show that for all four impact configurations, the work-energy relationships for the oblique impact case, due to the manner of stiffness coefficient quantification and the uniaxial nature of the model, reduce to the non-oblique cases.

D

iscussion and Conclusions Of the issues presented herein in regards to the manner in which oblique collisions are handled by the CRASH3 damage analysis algorithm, the simplest issue to mitigate is that of the presumption that the resultant force vector lies solely along one of the principle axes of the undeformed vehicle geometry. The solutions presented herein mirror those presented for impulse component decomposition for planar collision analysis (Brach and Brach, 2005). This is an expected result in that the impulse is simply the collision force weighted by the collision duration. This simple correction allows for the correct determination of the force components, in the LCS, when they are known in the NTCS.

The second issue addressed, in terms of showing its problematic nature, was the uniaxial nature of the CRASH3 structural response relationship. The use of a uniaxial modeling approach precludes the ability to explicitly model transverse deformations even within the simplified biaxial response case. It is for this reason that the modeled force in the region of induced damage is zero. A resultant from this fact is a predicted internal work, in the induced damage region, of zero and represents the basis for the general exclusion of the residual damage present in the induced damage region, from the internal work calculations. The problems arising from the uniaxial modeling approach are further exacerbated, in regards to the oblique impact case, when the load paths are taken as being aligned with respect to the relevant principle axis. The displacement and residual damage constraint that arises as a result of such an approach reduces the work-energy relationship to its corresponding non-oblique case. The use of uniaxial load paths arrayed in a radial manner, similar to the method employed within the Simulation Model of Automobile Collisions (SMAC) modeling approach, would allay the issue

38 Collision Magazine - Volume 10 Issue 1

of deflection and residual deformation being constrained along the relevant axis of the LCS but would not solve the issue of deformation developed within the induced damage region. The stiffness, for such a model, would lie along the radial load paths and the work-conjugate deflection and resultant residual damage would have to be determined axially, with respect to each load path. A number of potential solutions, while not explored in the subject work, provide theoretical solutions for both issues. The first is the development of an empirical relationship between the axial compressive force and the force in the transverse direction.

The second is the deterministic implementation of transverse load paths resulting in a model similar to those found within the context of soft body physics. The third method for consideration would be the implementation of a finite deformation biaxial plane strain model.

The third issue, unfortunately, is the most vexing and requires the provision of biaxial nodal residual damage measurements, which in turn would require a change in the reporting of controlled collision test results. Increasing levels of model complexity generally require an increasing set of model parameters, which in turn require quantification for utilization. One might posit the view that the longevity of extant residual damage based methods lies in the allure of the simplicity of use and of model parameter quantification. It is a relatively trivial task to use the information provided, for any given collision test, to determine the Campbell model coefficients and then the CRASH3 model coefficients. Depending upon the contractor, for certain frontal NCAP tests, the total change in the length of the direct damage region is provided. This does not hold for all contractors and does not hold for all collision test configurations. The total change in length of the direct damage region is useful but does not provide sufficient enough detail to map the displacement of individual boundary nodes from the undeformed to deformed configuration unless one makes certain assumptions regarding the splitting of the total transverse change in length. The change in length of the direct damage region may not be trivial, even for the case of full-width engagement frontal NCAP testing. As an example, NHTSA test No. v05278 reports a centerline residual damage depth of 453 mm and a change in the direct damage length of 44 mm, which corresponds to the latter representing 9.7 percent of the former.

R

eferences: 1. Brach R and R Brach. Vehicle Accident Analysis and Reconstruction Methods. Warrendale, PA: SAE International, 2005; pp. 116-125.

www.collisionpublishing.com

2. McHenry R and B McHenry. McHenry Accident Reconstruction. Cary, North Carolina, McHenry Software, 1998; pp. 52-54.

3. Noga T and T Oppenheim. CRASH3 User’s Guide and Technical Manual. Washington, DC: US Department of Transportation, National Highway Traffic Safety Administration, 1981.

4. Prasad A. CRASH3 damage algorithm reformulation for front and rear collisions. Society of Automotive Engineers, 1990. SAE Technical Paper No. 900098.

5. Prasad A. Missing vehicle algorithm (OLDMISS) reformulation. Society of Automotive Engineers, 1991a. SAE Technical Paper No. 910121.

6. Prasad A. Energy absorbed by vehicle structures in side-impacts. Society of Automotive Engineers, 1991b. SAE Technical Paper No. 910599.

7. Prasad A. Coefficient of restitution of vehicle structures and its use in estimating the total delta-V in automobile collisions. In: Proceedings of the Winter Annual Meeting of the American Society of Mechanical Engineers, Atlanta, Georgia: 1991c.

8. Sharma D, S Sterns, J Brophy and E Choi. An overview of NHTSA’s crash reconstruction software WinSMASH. In: Proceedings of the 20th International Conference on the Enhanced Safety of Vehicles, Lyon, France: 2007.

9. Singh, J. A fundamental reconsideration of the CRASH3 damage analysis algorithm: the case against uniform ubiquitous linearity between BEV, peak collision force magnitude and residual damage depth.

Traffic Injury Prevention 14(7), 2013, pp: 718-724.

10. Society of Automotive Engineers. Collision Deformation Classification (SAE J224 MAR80), 1980.

Appendix A Oblique rear impact: The vector diagrams for an oblique rear impact are shown in Figure A1.

The two options for defining the angle between the vectors are: (A1) Applying the difference formula to the first option:

(A2) Substitution of this result into equation (25) yields the following result:

(A3)

Figure A1: Vector diagrams for an oblique rear impact. www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 39

The Eabs on a LCS basis is: (A4)

Substitution of this result into equation (25) yields the following result:

The approach from the NTCS mirrors the derivation for the oblique front impact case.

Oblique left side impact: The vector diagrams for an oblique left side impact are shown in Figure A2. The directionality constraint regarding the residual damage vector follows the same approach as that for a front impact but with the constraint being along the y-axis.

The two options for defining the angle between the vectors are:

(A8) Where:

(A5) Applying the difference formula to the first option:

(A6) Applying the difference formula to the second option:

(A9) The Eabs on a LCS basis is: (A10)

(A7)

Figure A2: Vector diagrams for an oblique left side impact.

40 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

If the problem is approached from the perspective of the NTCS, the residual damage vector transformed from the LCS into the NTCS is:

(A14) Applying the difference formula to the first option:

(A15) Applying the difference formula to the second option:

(A11) (A16)

The magnitude of this vector is: (A12) The magnitude of the collision force vector, as determined from equation (23), is:

Substitution of this result into equation (25) yields the following result:

(A13) Applying the differential operator to equation (A12) and substituting the resultant and equation (A13) into the Eabs relationship reproduces the result of equation (A10).

Oblique right side impact: The vector diagrams for an oblique right side impact are shown in Figure A3.

(A17) The Eabs on a LCS basis is:

The two options for defining the angle between the vectors are:

(A18) The approach from the NTCS perspective mirrors that of the oblique left side impact.

Figure A3:Vector diagrams for an oblique right side impact. www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 41

From Crash to Crush Introducing the ultimate family of 3D crash solutions by FARO Technologies.

The Focus3D X330, Freestyle3D and FARO Reality, HD, Crash and Crime Zone softwares provide users the affordable and easy-to-use ability to scan as far as 330-meters or as close as 1.6 meters and deliver courtroom-ready presentations that close the case.

Scan. Analyze. Deliver.

For more information, visit our web site at www.FARO.com or call 800.736.0234 to request a live demo or speak to an expert. © 2015 FARO | FARO and the FARO Logo are registered trademarks of FARO Technologies Inc.

E

P

R EER

D

WE E I V

n i y t n i a t r s e e t c a n U m i d t n t s a E a s d n e e r e k o p a r S T r E g s n t i s v e i T r d i s De k e S n er e h m c c s i F . fro dent S C m a i i ill c W Ac

A

bstract

To make an estimate of initial speed from evidence gathered at the scene of an accident, an investigator must determine the path a crashed vehicle took, the resistance acting on the vehicle along that path, and the distance over which that resistance acted. Measuring distance is ordinarily a straight forward task, greatly aided by the use of precision survey instruments.

However, even where there is observable evidence by which to determine the path, estimating the resistance that a car developed during a pre-impact skid, or a chaotic post-impact slide or tumble to final rest, may introduce significant error.

A series of roadside experiments, consisting of several skid-to-stop tests, are often conducted at the scene in an attempt to determine how much 44 Collision Magazine - Volume 10 Issue 1

friction a fully braked vehicle can develop on that road surface. This paper demonstrates that the friction values obtained from such skid-tostop tests, and the speeds derived from them, are likely to be invalid or unreliable. Invalid for two reasons: first, because a test car cannot duplicate the deceleration rate of the accident car except by random chance, and second, the average deceleration value calculated by a skid computer does not isolate the quantity the investigator seeks to measure: specifically, the kinetic tire to road coefficient of friction, (µk).

The results are also unreliable because, as a statistical analysis of such empirical tests will demonstrate, the resulting range is so large as to be “little better than guessing”. 13

I

ntroduction

During each skid-to-stop test the deceleration rate is typically sampled and recorded as many as one-hundred times per second by a skid computer mounted on the windshield of whatever vehicle the investigator may be using. The raw data is critically damped to

www.collisionpublishing.com

Many accident reconstruction reports will involve at least one of these two equations. Eq.1 is often referred to as the speed equation, Eq. 2 as the critical speed yaw, or CSY equation.

eliminate extraneous signal noise and filtered to smooth out averaged values. These averaged values will then be adjusted by the computer for the slope of the road to produce a baseline drag factor. To calculate speed, the adjusted drag factor is then used in one of two equations, depending on whether the observed path of the crashed vehicle is linear or curvilinear: Eq. 1 Linear: d = length of observed skid marks f = adjusted drag factor

Relevant court opinions will be linked to these skid test procedures. Required test protocols of recognized engineering authorities will be specified. Variables affecting test results will be identified. Sources of independent testing will be reviewed. And the practical question of “What difference will all this make in a speed estimate?” will be answered.

D

efinitions

The definition of several terms is essential to framing this discussion.

Vehicle Axis System

The S.A.E. international convention defines traction forces, either acceleration or braking, as longitudinal (Fx), acting along the x-axis. Cornering traction forces are lateral (Fy) and aligned with the y-axis.

Eq. 2 Curvilinear: r = initial radius of observed yaw marks f = adjusted drag factor

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 45

Eq. 4:

Force Force is any influence which deforms an object or changes its motion. The imperial unit of force is the pound.

Gravity The force which causes any mass to be accelerated toward the earth at the rate of 32.2 ft./sec.2 (9.8 m./sec.2). This rate is defined as an acceleration of 1g.

Figure 1: Vehicle axis system Pitch is defined as rotation about the Y axis, roll as rotation about the X axis, and yaw as rotation about the Z axis.

Deceleration is negative acceleration. The deceleration rate achieved by a braking vehicle is often referred to as the drag factor, and identified by the symbol (f ). It can be expressed in units of 1g or as a decimal fraction of 1g.

Validity vs. Reliability and Uncertainty

Coefficient of Friction

Validity is a measure of fidelity. It requires that a measurement method be appropriate to the phenomena it seeks to quantify. For example, a spring scale is a valid instrument by which to measure weight, but not distance. Reliability is dependent upon the ability of a measurement device to give the same answers over time, across similar groups, irrespective of who uses it. Data are considered reliable when repeated testing yields consistent results. Reliability and uncertainty are opposite sides of the same coin. Both are a relative proposition that take the form of a bell shaped (Gaussian) curve for a statistically valid sample of a normal distribution.

By definition, the tire to road coefficient of kinetic friction on level ground is equal to the force of resistance developed by a sliding tire, divided by the weight on that tire.

Velocity Velocity is a change in position (distance) with respect to time which can be expressed in units of feet per second (ft./sec.). Velocity is a vector quantity defined by both the speed and direction of an object. The Greek symbol delta (∆) means ‘change in’.

Eq. 3:

Acceleration Acceleration is a change in velocity with respect to time (the second derivative of position) expressed in units of feet per second per second (ft./sec.2). Acceleration is also a vector quantity.

46 Collision Magazine - Volume 10 Issue 1

Eq. 5:

Where: Ff = the force of friction exerted by each surface on the other, parallel to the surface.

Fn = weight directed perpendicular (normal) to the surface.

For example, if a 4,000 pound car sliding at constant speed on a level asphalt road developed 2,800 pounds of drag, the coefficient of friction would be 2,800 # divided by 4,000 # or 7/10ths or 0.70 of 1 g. Since 1g is equal to 32.2 ft./sec.2, then 0.70 of 1g is equal to a deceleration rate of (- 22.54 ft./sec.2).

Just as a crashed car can be weighed with a scale, so too can the force of resistance acting against its continued motion. Drag sleds and skid trailers do just that; they directly measure the force that a braked tire develops when dragged across a road surface at constant speed. Because the peak of braking performance occurs just before a tire begins to slide, skid trailers can be set to emulate a car without antilock brakes, or set to release at a threshold to emulate a car with ABS. This equipment is expensive. It requires two op-

www.collisionpublishing.com

Tri-axial accelerometers measure acceleration simultaneously in three orthogonal axes: X, Y & Z. Adjustments are made to the computed drag value, both for the slope of the roadway on which the tests are being conducted, and for the forward pitch of the test vehicle’s suspension mounted chassis during hard braking. Some skid computers provide the option of either accepting a default pitch rate value or entering a specific number.

Figure 2: ASTM E274 tow vehicle and skid trailer erators with in-depth training, and is not ordinarily available to accident investigators.

The average deceleration developed during a skid-to-stop can be determined experimentally by several methods.

One is to directly measure the distance required to stop from a known speed, using equation 6.

T

Eq. 6:

wo interfaces During skid-to-stop tests the vehicle acts as the device for converting kinetic energy to heat, while the skid computer is the instrument which measures the efficiency of that device. Brake systems are composed of many parts, including brake lines, proportioning valves, pads, drums and rotors.

Where: Vi = initial velocity in ft./sec. and Ve = ending velocity in ft./sec. (usually 0) d = braking distance

S

kid computers Another method is to use a skid computer. These portable, less expensive, easily set-up units operated by a single driver, are widely used by accident investigators. Physical phenomena are often measured indirectly. Such is the case with skid computers which do not measure force. Rather, they measure the acceleration and elapsed time during a braking or cornering event with inputs from an accelerometer and a clock.

Typical accelerometers used in skid computers measure the difference in electrical capacitance of a suspended mass moving between two plates in response to a change in either speed or direction. The voltage difference is converted to an acceleration value. Acceleration is then integrated over time to calculate speed, and speed is then integrated again over time to calculate distance. Thus a small error in the acceleration value will result in an exponential error in the computed distance.

But pitch rates are not typically published for vehicles, and the suspension of both vehicles and accelerometers degrade over time. Skid computers, when calibrated, can be precise at calculating acceleration over short periods of time, but they do not measure the coefficient of friction between two surfaces. They measure acceleration, but only for the vehicle in which they are then mounted. Variations arise in the accuracy of different skid computers, the braking performance of the test vehicles, and the surfaces being tested.

Two friction interfaces are involved in braking: the primary, between brake pads and rotor or drum, and the secondary, between tire and road. They act sequentially, then simultaneously. To seize the rotation of a rubber tired wheel, the force of resistance exerted at the primary interface must exceed the force of resistance at the secondary interface. But an accelerometer measures a change in velocity, not force, and the coefficient of friction is the ratio of two forces.

Figure 3: Base brake system www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 47

Therefore, a skid computer can neither isolate nor measure the tire to road coefficient of friction during a braking event, any more than it can isolate and measure the brake pad to rotor coefficient of friction. It is the emergent sum of all the brake components acting in concert that will determine how quickly the device can complete three phases of braking:

wheel's rotational velocity by about 20%, "slip-friction" is encountered and presents the maximal values for friction observed throughout the skid. Some refer to this value as Peak Drag Factor. Development of Peak Drag Factor, (slipfriction) is the basis of ABS braking.10

“Mechanical Delay”

Sliding Skid is where fully locked tires may leave clearly visible skid marks while generating a slightly lower drag than ‘peak friction’. Spin-down cannot be instantaneous as the brake system must overpower the angular momentum of the vehicle’s tires and wheels. To eliminate the mechanical delay phase from the average drag value, the skid computer’s clock is normally triggered to start when the vehicle experiences a (-0.2 g) deceleration.

The phase of brake application beginning with pedal depression and ending with any resulting change in wheel rotational velocity. The vehicle does not appreciably slow during this phase, which lasts only about a tenth of a second. No tire marks are created during mechanical delay.

"Spin-Down” "Spin-down" begins at the end of mechanical delay (as previously defined) and ends with total four-wheel lockup.

This phase can last about 3-tenths of a second and includes tire marks ranging from light to dark in appearance. At the point at which the vehicle's forward velocity differs from a

“Sliding Skid”

Skid-to-Stop vs Skid-to-Crash The average deceleration generated during a skid-to-stop brake test does not replicate the average deceleration experienced during a pre-impact skid. If it did, there would be no accident to investigate. Pre-impact skids must termi-

Figure 4: Deceleration trace 48 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

nate in either a collision or a brake release. In either case the vehicle does not come to a stop until after the collision. The data in Appendix Table # 1 was used to generate Figure 4, a graph of time, acceleration, speed and distance recorded by an accelerometer during a typical skid-to-stop test. All data points are measured along the single axis of motion, defined as the longitudinal, or x-axis. According to the manufacturer, the deceleration rate is sampled one hundred times per second, with ten consecutive readings averaged for each 1/10th second data point.

The vertical red “Crash Line” is superimposed on the data to designate a hypothetical collision occurring one second after brake initiation. The computed average deceleration during the entire skid-to-stop event is (-0.813 g). This value actually occurs only at the one instant where time ≈ 0.63 seconds. Compare this to the interrupted “Running Average g” at one second, which is (-0.687 g).

This graph demonstrates that, even assuming identical test and crash vehicles, the average deceleration value computed from a skid-to-stop test will always be greater than the running average value generated during a braking event interrupted by a crash or brake release. So if the computed average value is used in Eq.1 for any event other than a skid-to-stop, the resulting speed estimate will be too high.

And regardless of how precise the skid computer results may be, speed estimates derived using Eq. 1 are predicated on uniformly accelerated motion, where the acceleration values are constant. Skid-to-stop tests produce non-uniformly accelerated motion.

travel without incident, the initial velocity is unknowable; only the length of the skid can be measured.

Spin down intervals will vary from vehicle to vehicle, depending on numerous factors. Exactly where spin down transitions to tire slip, and tire slip transitions to locked wheel slide, may be difficult to determine from the data recorded by a skid computer or from an examination of the skid marks, as demonstrated by Figure 5.

D

rag factor during post-impact motion Vehicles involved in an accident will also experience a change in velocity during collision and during post-impact motion (translation and/or rotation). Data gained from skid-to-stop tests may be of little value in estimating the drag factor during these events because:

1. There may be no mechanical delay or spin down interval associated with post-impact skidding.

2. The other half of friction, the surfaces in contact with the road, may now consist of one or more deflated tires, bare wheels, or plastic or metal parts, either sliding across or digging into the road surface, while weight may shift chaotically from one contact point to another during post-impact motion.

3. Vertical motion may cause a decrease in the normal force, or the vehicles may leave the ground entirely.

Therefore the speed loss during an interrupted braking event cannot be accurately determined using the average drag value from a skid-to-stop test. Common sense confirms this. If a vehicle brakes hard enough to leave a skid mark on a roadway without stopping, and then resumes

4. If the car remains in contact with the road, any post-impact rotation will produce lateral acceleration, except in the unlikely event that the axis of rotation happens to pass directly through the accelerometer. As previously noted, skid-to-stop tests are intended to measure only longitudinal

Figure 5: Accident investigators pondering skid length

Figure 6: Post-impact damage

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 49

T

acceleration as a change in displacement along the x-axis.

echnical Guidance for conducting skid tests At least six recognized authorities: The Daubert Court, the Downing Court, the American Society for Testing and Materials, the American National Standards Institute, the American Society of Mechanical Engineers, and the Society of Automotive Engineers give specific guidance on how scientific testing generally, and test skids in particular, should be conducted, and how the range of the results should be evaluated.

DAUBERT One of the four Daubert criteria for evaluating the admissibility of expert testimony is the known or potential rate of error associated with the method.

ASTM E1337 The American Society for Testing and Materials defines a specific protocol for the skid testing of roadway surfaces to determine the friction limits. Selected excerpts follow: •

Less than 1 degree of pitch at maximum loading

•

Use an SRTT – Standard Reference Test Tire ASTM Spec E1136

•

Make at least 8 determinations of peak braking coefficient

ASTM STP 530

“

“All testing should be under statistical control. The number of tests for a given test site is determined by the accept-

The common theme underlying all of these protocols is the recognition that any estimate must incorporate a range of values obtained through a statistical analysis of empirically obtained results 50 Collision Magazine - Volume 10 Issue 1

able confidence interval and the standard deviation of the particular skid tester.” 9 SAE J2505 The Society of Automotive Engineers also details a specific protocol for skid tests. Selected excerpts follow: 4.3 INITIAL SPEED ”…if used for accident reconstruction purposes, speeds should simulate those of the subject conditions.” (The initial braking speed must be based upon the first level estimates of the investigator.)

6.1.2 MOUNTING LOCATION “Normally, acceleration transducer(s) should be located as close as feasible to the vehicle center of gravity.” (Skid computers are typically designed to mount on the windshield, which requires estimating the pitch rate compensation.)

7.4 DATA ANALYSIS “…In all cases the calculations should follow the procedures in 7.3 and the use of accepted methods from the field of statistical analysis… .A principal objective of conducting experiments is to estimate one or more physical quantities such as frictional drag, speed, distance, etc….”

S

tatistical inference The common theme underlying all of these protocols is the recognition that any estimate must incorporate a range of values obtained through a statistical analysis of empirically obtained results, as determined by the stated confidence interval. The method recommended by S.A.E. J2505 is as follows:

Eq. 7:

Where: S = standard deviation n = number of test trials for which data was collected Xi = the measured variable in test number “i” X = the mean value of measured test variables If the number of skid tests taken at accident scenes is less than thirty trials, the ‘student t’ distribution must be used to calculate the range.

www.collisionpublishing.com

limit should be when interpreting skid test data. “The confidence level generally used in engineering is 95%.” 6

Figure 7: Gaussian curve “The ANSI/ASME Standard on Measurement Uncertainty suggests that the value of t be given by the distribution with N-1 degrees of freedom, v, for N < 31 and by t = 2.0 for N > 31.” 6 “In 1908 William S. Gosset, an employee of the Guinness Brewing Company in Dublin, published a paper showing that when n is less than 30, though the normal distribution of the sample means cannot be used safely, this other distribution, dependent on the number of degrees of freedom, can be used.” 4 Three Sigma: Proof beyond a reasonable doubt The curve of a normally distributed population is shown in Figure 7. The Greek letter mu (µ) is used to represent both the coefficient of friction in accident reconstruction and the mean or average value in statistics.

Therefore, this author makes the following argument: The preponderance of evidence for the civil standard is met at ±1σ, (read as plus or minus one sigma), as it will encompass 68.2%, or substantially more than half of the population. Similarly, the engineering standard of 95% is met at ± 2σ. Finally, the criminal standard for proof beyond a reasonable doubt implies a confidence level at ± 3σ, which will encompass 99.74% of all possible values. Restated, the three sigma threshold translates to the possibility of a wrongful conviction in 26 of 10,000 prosecutions.

C

omputing Range from a series of skid-tostop tests When the number of skid-to-stop tests taken at accident scenes is small, the arbitrary selection of a confidence interval will have a marked effect on the range of computed speeds.

Example: A police officer takes three locked wheel skid-tostop tests (n = 3) at an accident scene and obtains the following values using a skid computer: (- 0.658 g), (-0.690 g), and (- 0.696 g). Compute the range at a confidence interval of 95%. Multiplying all values by (-1) eliminates working with negative numbers.

The standard deviation, represented by the Greek letter sigma (σ), shows the variation or dispersion from the average or mean value. A low standard deviation indicates that the data points are close to the average value at µ. Conversely, a high standard deviation indicates that the data points are spread out over a large range of values.

In civil litigation it is the plaintiff’s burden to establish a preponderance of evidence, while for criminal cases, it is the prosecution’s burden to establish proof beyond a reasonable doubt. The statistical limits of these two standards may appear somewhat vague, as legal concepts do not readily lend themselves to the strictures of scientific quantification.

However, consider that in statistics, the term ‘beyond’ implies a two sided test, where the upper and lower limits of a range of values are at some declared distance – or number of standard deviations - from the mean, or average value.

That stated distance is referred to as the confidence interval (CI), and it excludes only those areas in a normal distribution that are beyond that stated limit. None of the six authorities listed above have stated exactly what that

Then, assuming a speed limit of 55 MPH, calculate the required skid-to-stop distance using the average drag factor: Eq. 8:

Ranging the confidence interval

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 51

Avg G = 0.681 Two-Sided 50% 60% d.f.

2 0.816 1.061

70% 1.386

Low High

0.671 0.691

0.668 0.694

0.664 0.698

Low High

54.7 55.5

54.6 55.6

54.4 55.7

St Dev = 0.012 Distance = 148.5 ft 80% 90% 95% 98% 99% 99.50% t.100 t.050 t.025 t.010 t.005 t.0025 1.886 2.92 4.303 6.965 9.925 14.09 Range of Drag Values - G 0.658 0.646 0.629 0.597 0.562 0.512 0.704 0.716 0.733 0.765 0.800 0.850 Range of Speeds - MPH 54.2 53.6 53.0 51.6 50.0 47.8 56.0 56.5 57.1 58.4 59.7 61.5

99.80% 99.90% 22.33

31.6

0.413 0.949

0.302 1.060

42.9 65.0

36.7 68.7

Figure 8: Computed confidence intervals By holding that distance constant, the minimum and maximum speeds which result from the computed range of values across a span of confidence intervals can then be calculated, as per Figure 8.

Note: The first column notation (d.f.) refers to degrees of freedom where d.f.= (n-1) which for three skid tests is 2, for four skid tests is 3, etc.

Assigning the confidence interval A critical choice must be made at this point in the calculation process to define the acceptable degree of risk for a false conviction. If the engineering standard at ± 2σ is acceptable, then the possibility of a false conviction rate is 500 in 10,000, and the estimated speed range is from 53 to 57 mph. But if the acceptable false conviction rate is held at ± 3σ, then the speed range expands from 48 to 62 mph.

In such a case, a 55 mph speed limit becomes irrelevant.

Thus the prosecution may seek to minimize the confidence interval, while the defense may seek to maximize it.

It would seem that the decision of what is an acceptable confidence interval ought ultimately to rest with the triers of fact, the jury, who cannot be expected to make such a determination unless and until they become aware of its significance.

Two skid computers in the same vehicle The document in Figure 9 may be considered a Black Swan, as defined

by Taleb: a rare event of significant consequence.19 This (redacted) copy of handwritten notes records the results of three skid tests conducted with a Chevrolet Tahoe SUV police vehicle equipped simultaneously with two skid computers. These tests were taken about seven hours after a fatal crash in a 55 MPH zone. Not surprisingly, each unit produced a different value than the other for all three tests. These data were then used in the prosecution of a criminally negligent homicide to estimate the speed of two crashed vehicles, a Volvo sedan and a Ford SUV. The two computers produced respective average values of (- 0.681 g) and (- 0.652 g). Calculating the range between these two sample populations at an engineering standard

Figure 9: Skid test results using two accelerometers 52

Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

confidence interval of 95% yields a range of between (0.483 g) to (- 0.851 g).

3. No apparent economic or prosecutorial benefit is at risk.

Again using Eq. 8, with the average drag factor of (- 0.667 g), we calculate the required skid-to-stop distance from 55 MPH at 151.5 ft. Holding that distance constant, the minimum and maximum speeds which result from the computed range of speeds, when calculated at a 95% confidence interval, is between 47 and 62 MPH. Again, a 55 mph speed limit becomes irrelevant.

4. The tests are conducted with consistent scientific protocols.

E

xemplar vehicles “Historically, most individuals involved in accident investigation and analysis agree that, when done correctly, the most accurate way of determining the effective drag factor for a particular vehicle and road combination is by conducting skid tests at the accident site with the involved vehicle or as nearly an identical exemplar vehicle as possible.” 5

Simply stated, some vehicles accelerate, brake and corner better than others; the friction limits of any given vehicle cannot be inferred with confidence by testing the limits of a different vehicle.

5. The tests have the added imprimatur of being conducted under the aegis of the National Law Enforcement Corrections and Technology Center.

V

ariation among different police vehicles In 2005, the Michigan State Police skid tested 54 different prototype police vehicles from a mathematically corrected initial speed of 60 MPH. Corrected stopping distance = [(Target initial speed)²/(Actual initial test speed)²] x Actual stopping distance. These tests minimize pitch rate error by measuring stopping distances using an optical non-contact speed and distance sensor mounted near the middle of the test vehicle’s wheelbase. The average stopping distance for each vehicle varied from 129.5 feet to 180 feet, a range of 139%. That is to say, depending upon which vehicle was assigned to a responding investigator, the speed attributed to a crashed car at an accident scene could be subject to that same range of variation.

V I

Figure 10: Polize cars

ndependent tests Under the aegis of the National Law Enforcement and Corrections Technology Center, the Michigan State Police annually evaluate new model police vehicles and components. A complete description of their testing protocols can be reviewed at their website: https:// www.justnet.org/standards/Vehicle_Testing.html

Conforming to the standards set by the Downing Court (753F.2d 1224, (1985)) these independent NLECTC/ MSP tests are deemed credible because: 1. The tests are conducted strictly for evaluation of the vehicles, not skid test data per se.

ariation for a single police test vehicle In 2010 the NLECTC/MSP tested four different police prototype vehicles, each car equipped with different tire and brake pad combinations. Each of the twelve different vehicle/tire/brake combinations were subjected to twenty consecutive slide-to-stop skid tests from 60 MPH, in two sets of ten stops each, for a total of 240 skid tests. One of the most common police cars then in service in the United States, a 2009 Ford Crown Victoria Police Interceptor (VIN 2FABP7BV3AX125587) equipped with MaxStop Plus SM 98 Formulation brakes and Goodyear RS-A tires 235/55 R17 98W, produced twenty consecutive skid tests with stopping distances that ranged from 142.56 ft. to 198.23 ft. with an average of 163.23 ft. The corresponding deceleration rates are (0.844 g) to (- 0.607 g) with an average of (- 0.737 g).

At a 99.5% confidence interval, with n = 20, the computed deceleration rates range between (- 0.505 g) and (- 0.988 g). Stopping from 60 MPH at the average deceleration of (- 0.737 g) yields a distance of 163.29 feet. Using that distance, the deceleration limits result in a speed range of 50 to 69 MPH. This skid-to-stop test series demonstrates that: •

A single vehicle can produce widely divergent deceleration rates and speed estimates derived from them.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 53

2. The tests are not conducted in contemplation of litigation.

•

I

Three or four skid tests at an accident scene are insufficient to produce reliable results.

solating variation among brake pads In 2000 the NLECTC/MSP tested thirteen different brands of brake pads, each mounted on two commonly used police vehicles for panic stops. Figure 11 shows the results. Stopping distances were mathematically corrected from 70 MPH. The Chevrolet Impala stopping distances varied from 186.6 feet to 208.2 feet. The Ford CVPI stopping distances varied between 196 feet and 183.2 feet. Simply stated, brake pads alone make a statistically significant difference in stopping ability.

Isolating variation among tires - braking In 2001 the NLECTC/MSP tested five brands of tires on two commonly used police vehicles for panic stops on both dry and wet asphalt from 60 mph, as shown in Figure 12. Dry traction varied by 6% for a single car, wet traction by 13%.

Simply stated, the statistically significant difference which tires alone make in the stopping distance of a test vehicle will result in a commensurate difference in any speed estimate derived from such tests.

I

solating variation among tires – cornering Again, in the 2001 NLECTC/MSP tire tests, the same ABS equipped tire/car combinations were tested for steady state cornering limits on a 200 ft. radius skid pad. The results are summarized in Figure 13. To determine the lateral G attained, pi was multiplied by the diameter of the test circle and divided by the lap time. This quotient was then squared and divided by the radius of the circle, and then divided by 1 G. Dry traction varied by 9% for a single car, wet traction by 19%. Notably, the lateral cornering friction limit for every tire on both vehicles tested less than the corresponding longitudinal braking value.

This observation is bolstered by Car and Driver Magazine tests in 2005. That data demonstrates that 140 out of 144 cars had braking friction limits which exceeded their cornering friction limits. Simply stated, by far, most cars can brake harder than they can corner. Therefore, any speed estimate derived using Eq. 2 where the drag factor is obtained from skid tests will likely be biased on the high side.

O

ther Factors Affecting Tire/Road Friction Consider that the initial speed for skid tests is entirely at the discretion of the accident investigator, who, by those very tests, is attempting to determine the initial speed of an accident car. But as shown in Figure 14, the average coefficient of wet sliding friction is inversely dependent on both initial speed and tread depth. So the

NLECTC MSP 2000 Brake Pad Evaluations: Table 6 Panic Stop Projected Stopping Distance In Feet from 70 MPH *Average initial speed calculated from 12 decelerations with a 70 MPH target speed Chevrolet Imapla Ford Police Interceptor 2 Dist Ft. Ft./sec.

Brake Pad Maufacturer Dist Ft. g Ft./sec.2 g 187.50 28.12 0.874 Original Equipment 184.50 28.56 0.888 192.20 27.42 0.852 AC Delco 188.60 27.95 0.869 192.40 27.39 0.851 Advance Auto 185.80 28.37 0.882 194.80 27.05 0.841 Carquest 183.20 28.77 0.894 196.50 26.82 0.834 DANA B&C ceramic 183.90 28.67 0.891 186.60 28.25 0.878 Hawkhead 183.20 28.77 0.894 192.60 27.36 0.850 Honywell-Bendix 185.20 28.46 0.884 208.20 25.32 0.787 NAPA 196.00 26.88 0.836 192.00 27.45 0.853 Raybestos 183.30 28.76 0.894 188.00 28.03 0.871 Satisfied Brake 187.20 28.15 0.875 203.60 25.88 0.805 TMD Friction 188.00 28.04 0.871 Performance Friction 185.00 28.48 0.885 Wagner 186.10 28.31 0.880

Figure 11: NLECTC 2000 brake pad test results 54

Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

National Law Enforcement and Corrections Technology Center Bulletin Comprehensive Evaluation of 2001 Patrol Vehicle Tires Stopping Distance Tests from 60 MPH - straight, flat, granite-asphalt surface Tire Size: P225/60R16 Avg. drag (ft./sec)2

Dry Distance (feet)

Avg. drag (g force)

25.950 25.051 26.497

149.2 154.6 146.1

Goodyear Eagle RS-A 26.473 146.3 Brand X* 25.679 150.8 2001 Ford Police Interceptor BF Goodrich Touring TA VR4 26.753 144.7 Firestone Firehawk PV41 26.652 145.3 General XP-2000 V4 26.930 143.8 Goodyear Eagle RS-A 26.920 143.8 Brand X* 26.340 147.0 * typically designed for passenger car applications

2001 Chevrolet Impala BF Goodrich Touring TA VR4 Firestone Firehawk PV41 General XP-2000 V4

Avg. drag (ft./sec)2

Wet Distance (feet)

Avg. drag (g force)

0.806 0.778 0.823

23.021 22.063 22.857

168.2 175.5 169.4

0.716 0.686 0.710

0.822 0.797

24.393 21.536

158.7 179.8

0.758 0.669

0.831 0.828 0.836 0.836 0.818

21.898 23.700 23.440 25.598 21.043

176.8 163.4 165.2 151.3 184.0

0.681 0.737 0.729 0.796 0.654

Lateral g Calculated

Figure 12: NLECTC 2001 tire evaluation – stopping National Law Enforcement and Corrections Technology Center Bulletin Comprehensive Evaluation of 2001 Patrol Vehicle Tires Skid Pad: polished concrete, Diameter = 200 ft. - straight, flat, granite-asphalt surface Tire Size: P225/60R16 Time (sec.)

Dry Speed (mph) Calculated

Lateral g Calculated

Time (sec.)

Wet Speed (mph) Calculated

12.725 12.743 12.589

33.66 33.62 34.03

0.758 0.756 0.774

20.987 20.554 20.187

20.41 20.84 21.22

0.278 0.290 0.301

Goodyear Eagle RS-A 12.583 34.04 Brand X* 12.871 33.28 2001 Ford Police Interceptor BF Goodrich Touring TA VR4 12.922 33.15 Firestone Firehawk PV41 12.879 33.26 General XP-2000 V4 12.724 33.67 Goodyear Eagle RS-A 12.843 33.36 Brand X* 13.270 32.28 * typically designed for passenger car applications

0.775 0.741

20.248 21.948

21.16 19.52

0.299 0.255

0.735 0.740 0.758 0.744 0.697

21.022 20.569 20.667 20.345 22.209

20.38 20.83 20.73 21.06 19.289

0.277 0.290 0.287 0.296 0.249

2001 Chevrolet Impala BF Goodrich Touring TA VR4 Firestone Firehawk PV41 General XP-2000 V4

Figure 13: NLECTC 2001 tire evaluation – cornering www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 55

Figure 14: Treaded tire vs smooth tire, wet conditions.

Used with permission from the Michigan Department of Transportation investigator is faced with making a “chicken or egg” estimate as to the initial speed of the crashed car in order to determine the initial speed for the skid tests. And if the crashed car displays one or more smooth tires, and skid tests are conducted using a vehicle with fully treaded tires, the results will be invalid.

Changes in tire/road friction can change, especially when wet surfaces are exposed to wind or a change in temperature. Consider that the worn and polished surface of parallel wheel tracks may accumulate the most moisture, but then be the first to dry due to passing traffic. Further, tire under-inflation can significantly reduce wet tire traction at higher speeds.

“Under-inflated tires allow the center of the tire tread to collapse and become very concave, resulting in the restriction of drainage channels within the tire tread and a reduction in contact pressure... “Some studies (Henry, 1983) have reported a decrease in wet friction of 45 to 70 percent for fully worn tires, compared to new ones... A very small amount of water can significantly reduce pavement friction. Test results from an FHWA-sponsored study (Harwood, 1987) indicate that as little as 0.002 in (0.05 mm) of water on the pavement surface can reduce the coefficient of friction by 20 to 30 percent of the dry coefficient of friction.” 8

56 Collision Magazine - Volume 10 Issue 1

So a road surface described in an accident report as ‘damp’ at the time of the skid test may have been wet at the time of the accident, Verbal description is entirely insufficient to quantify this crucial variable. Numerous other variables affect stopping distance and brake force development.

Load: “As a tire is loaded, the surface area grows larger increasing the total friction force but lowering the coefficient of friction.” 18 “Load also decreases the peak coefficient of friction that can be achieved in cornering. Over the range of 50% to 125% of rated load peak friction level may decrease by more than 20% 7 Aspect ratio: “By far the greatest influence on tire cornering properties is the aspect ratio – the ratio of section height to section width. 7 Temperature: “As long ago as 1934, Moyer (5) showed that wet pavement-tire friction increased with decreasing temperatures” 14 “Automotive tires are visco-elastic materials, and their properties can be significantly affected by changes in temperature and other thermal properties such as thermal conductivity and specific heat.” 8 What difference does time make? It has long been known that the time between the crash and skid tests may also be important. Environmental conditions existing at the time of the accident may not be duplicated later. Figure

www.collisionpublishing.com

Figure 15: Coefficient of friction range as a function of time and date 15 shows that ASTM E274 skid test protocols used on the same dry asphalt surface produced a coefficient of friction of between (- 0.65 g) and (- 0.95 g) on different days over the course of one year. 15

M

inimum test information required At a minimum, information for both the test vehicle and the crashed vehicle should include the following: vehicle weight, tire brand and model, tire pressure, aspect ratio and tread depth, brake pad brand and wear, odometer, road temp, moisture and wheel paths.

Skid test results are inconsistent with equations of rectilinear motion A series of twelve equations, derived from Newton’s laws of motion, relate five parameters of rectilinear motion: Initial Velocity, End Velocity, Acceleration, Time and Distance.

When any three of these variables are known, the other two can be computed.

Although frequently used in accident reconstruction, these equations yield accurate results only when the acceleration rate is constant. And as has been demonstrated in Figure 4, the acceleration rate generated in a skid test is not constant. Therefore the values calculated by a skid computer will be different than the values calculated when using equations of rectilinear motion.

“4.4.2 Since those [negative acceleration] values are rarely the same across consecutive time intervals, we cannot use the [rectilinear motion] equations [developed] in the previous [4.3] section.” 3 Therein lies the intrinsic difference between a skid computer and a drag sled. Because the drag sled maintains a constant speed during its sampling of the tire to road friction interface, the measured skid resistance remains relatively constant during that process.

The difference between skid test results and rectilinear motion equation solutions is easily demonstrated, as shown in Figure 16. Take for example the earlier graph in Figure 4, depicting the slide-to-stop test from an initial speed of

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 57

Basic Equations Of Rectilinear Motion Speed (start)

47.7798

mph

Speed(end)

0.0000

mph

Vel.(start)

70.0770

fps

Vel.(end)

0.0000

fps

Drag factor Distance

0.8274 92.2400

G's ft

Acceleration Time

-26.6208 2.6300

fss sec

To find: Acceleartion (a)

When given: t vi ve

RM-1

Equation to use: a=(ve-vi)/t

Result -26.645

fss

t vi d

RM-2

a=(2d-2vit)/t2

-26.620

fss

vi ve d

RM-3

a=(ve -vi )/2d

-26.620

fss

t a ve

RM-4

vi=ve-at

70.01

fps

tad

RM-5

vi=(d/t)-(at/2)

70.08

fps

a ve d

RM-6

vi=sqrt(ve2-2ad)

70.08

fps

End Velocity (ve)

t a vi

RM-7

ve=vi+at

0.06

fps

Distance (d)

a vi d t a vi

RM-8 RM-9

ve=sqrt(vi2+2ad) d=vit+at2/2

92.24

fps ft

a vi ve

RM-10

d=(ve2-vi2)/2a

92.24

ft

t vi ve

RM-11

d=t(vi+ve)/2

92.15

ft

a vi ve

RM-12

t=(ve-vi)/a

2.632

sec

Initial Veloctiy (vi)

Time (t)

2

2

Figure 16: Equations of rectilinear Motion 47.78 mph, with a time interval of 2.63 seconds, producing an average drag value of (- 0.813 g), and a computed distance of 98.009 feet. Using rectilinear motion equations when given the same initial speed of 47.78 mph, end speed of zero and a time interval of 2.63 seconds, the derived drag factor is (-0.827 g) and the distance is 92.24 feet. Note that Eq.6 is identical to RM-3 and Eq. 8 is identical to RM-10.

Figure 17 shows a car experiencing uniform acceleration during steady state cornering. Since the speed remains constant, the length of the velocity vector (red arrow) will remain constant as it rotates while the car moves from A1 to A2 along a circular path during some time interval (∆t).

Thus the resultant acceleration (α), is directed centripetally (toward the center) along the y-axis, resulting from a change in direction, not from a change in speed.

ongitudinal skid tests do not measure lateral traction limits This brings us back to an examination of Eq.2, sometimes referred to as the critical speed yaw or CSY equation, and why skid-to-stop tests cannot provide valid data for determining the friction limits of cornering vehicles. This can be demonstrated both conceptually and empirically. Eq.2 is developed from the more general equation nine, balancing velocity with centripetal force.

teady state cornering limits Cornering limits can be determined empirically on a skid pad. A car follows a circular path of fixed radius while slowly increasing speed until its inertial force exceeds the limit of lateral traction (Fy), and the car slips off the path. Skid computers can also measure and record acceleration along the y-axis, but road widths normally prevent this procedure from being used at accident scenes. Thus, by default, the average acceleration value developed during a longitudinal skid test is then substituted into Equation 2 for the required steady state lateral friction limit.

L

Eq. 9 :

Where: r = radius of the curve g = acceleration due to the force of gravity µ = the lateral limit of the tire to roadway friction 58 Collision Magazine - Volume 10 Issue 1

S

Recall that, as per Figure 1, either tractive or braking forces, (Fx), are aligned with the x-axis, whereas lateral cornering forces (Fy), are aligned with the y-axis. When a www.collisionpublishing.com

•

•

Figure 17: Steady state lateral acceleration skidding test car remains in line, deceleration occurs only along the x-axis. There is simply no y-axis data produced by the accelerometer to use in the CSY Equation! The difference between an informed estimate and speculation is the analysis of relevant data. The x-axis data produced by a longitudinal skid test of one vehicle provides no y-axis data by which to estimate the lateral cornering capabilities of any other vehicle. A skid test is no more comparable to a skid pad test than an appendectomy is to a tonsillectomy.

They are two different procedures. Tires are locked during a longitudinal skid, while tires continue to roll at the friction limit of a steady state cornering maneuver.

And yet, based on this author’s experience, it is common practice in accident reconstruction that the (Fx) limit resulting from a skid test is routinely substituted for the (Fy) cornering limit of any vehicle which loses lateral traction at an accident scene. The fact that this substitution is routinely employed does not make the practice legitimate.

C •

• • • •

onclusions • A skid-to-stop test measures the braking efficiency of only that vehicle in which it is mounted under those instant environmental influences; it does not measure the tire to road coefficient of friction.

Skid-to-stop tests produce non-uniform deceleration; rectilinear motion equations assume uniform acceleration when applied to the path of travel of a vehicle.

Speed loss during an interrupted braking event cannot be determined from the average deceleration value developed by a skid-to-stop test.

Different cars may display different braking and cornering limits.

During successive skid-to-stop tests, the same vehicle may produce widely divergent deceleration values.

Two different skid computers returned different results during the same skid test when mounted in the same vehicle.

•

•

R

2.

3.

4.

5.

6.

7.

8.

Because no two instruments should be expected to produce the exact same values under the same test conditions, skid test results should be subjected to a statistical analysis to provide a range of values at a stated confidence interval. Where this condition has not been met, a re-evaluation of felony convictions based solely on speed related evidence from skid test results may be warranted.

Skid-to-stop test results cannot be equated to the resistance generated on the same surface by the post-impact motion of severely damaged vehicles when tires, wheels or brakes have been compromised.

When substituting the average longitudinal braking value of a single test car into the CSY equation for the maximum lateral cornering value of any crashed car, an assumption is made that a single point on the x-axis is equivalent to the entire range of values along the y-axis.

The results of skid tests at accident scenes will vary due to differences in testing protocols, human factors, instrumentation, vehicle components, and the environment.

eferences 1. Abernathy, R.B., Benedict, R.P., Dowdell, R.B., ASME MEAUREMENT UNCERTAINTY, Copple, F. and Luce, P.T., Determination and Improvement of Relevant Pavement Skid Coefficients, Michigan Department of Transportation, January 1977 Daily, Shigemura, Daily, Fundamentals of Traffic Crash Reconstruction, Volume 2, 5th printing, IPTM, Jacksonville, Fl. Ellis, Richard B., Statistical Inference Basic Concepts, 1975 by Prentice Hall, Inc. Eubanks J.J., Haight, W.R. Malmsbury,R.N. Casteel,D.A., A Comparison of Devices Used to Measure Vehicle Braking Deceleration, S.A.E. 930665.

Experimental Errors and Uncertainty Analysis, http:// mechanical.illinois.edu/media/uploads/course_websites/experimental_errors_and_uncertainty_analysis.2 0110110.4d2b917d39e926.91488777.pdf Gillespie, Thomas D., Fundamentals of Vehicle Dynamics, Hall, J.W., Smith, K.L., Smith, L., Web-Only Document 108: Guide for Pavement Friction, National Cooperative Highway Research Program, Titus-Glover Applied Research Associates, Inc. Champaign, IL ,J.C. Wambold, CDRM, Inc., State College, PA, T.J. Yager, NASA Langley Research Center, Hampton, VA, Z.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 59

Rado, Pennsylvania Transportation Institute, University Park, PA 9. Hegmon, R.R., Gilespie, T.D., and Meyer, W.E., Measurement Principles Applied to Skid Testing, Skid resistance of Highway Pavement, ASTM STP 530, 1973 10. Livesay, Ed, Understanding The Vericom VC-2000 Accelerometer- Pitfalls for the Operator and Making Comparisons to Other Drag Factor Determination Methods, http://www.tarorigin.com/art/Elivesay/ 11. Mahle, Stephen, 1999, The Impact of Daubert v. Merrell Dow Pharmaceuticals, Inc., on Expert Testimony, http://www.daubertexpert.com/basics_daubert-vmerrell-dow.html 12. Marks, Christopher C. O’N., Pavement Skid Resistance Measurement and Analysis in the Forensic Context, http://www.nzta.govt.nz/resources/surfacefriction-conference-2005/6/docs/pavement-skid-resistance-measurement-analysis-forensic-oontext.pdf 13. Masory, Oren and Wright, Bill, Error Analysis for Three Different Methods of Determining Drag Fac-

tor - http://www.tarorigin.com/art/Omasory/Dragfactor/.

14. Moyer, R.A., “Skidding Characteristics of Automobile Tires on Roadway Surfaces and Their Relation to Highway Safety,” Iowa Engineering Experiment Station Bulletin 120, 1934 15. Murphy, R.W, et.al., NHTSA Final Report 1971, Bus, truck, Tractor-Trailer Braking System Performance, Vol. 2 16. National Law Enforcement and Corrections Technology Center, Equipment Performance Report 2001 Patrol Vehicle Tires, 2002 17. SAE Recommended Practice J2505, Measurement of vehicle-Roadway Frictional Drag, June 2003.

18. Smith, Nicholas D., Understanding Parameters Influencing Tire Modeling , http://www.engr.colostate. edu/pts/Job/Understanding%20Parameters%20Influencing%20Tire%20Modeling.pdf 19. Taleb, Nassim Nicholas (2010) [2007], The Black Swan: the impact of the highly improbable

Note: 1 Mahle, Stephen, The Impact of Daubert v. Merrill Dow Pharmaceutical, Inc. on Expert Testimony, April 1999. (http:// www.daubertexpert.com/basics_kumho-tire-v-carmichael.html) . “Of course there are numerous odd propositions for which there exists some collection of individuals who will assert that they comprise a relevant scientific community and that the proposition is generally accepted within their community. The court discussed briefly the characteristics of a ‘relevant scientific community,’ citing the analysis of United States vs. Downing, which says that the inquiry should focus on “the non-judicial uses to which the scientific techniques are put”. The Downing Court’s elaboration of that point noted that the absence of non-litigation uses for a scientific technique is taken as evidence of a lack of reliability while the existence of non-litigation uses for a technique is taken as evidence of the reliability of the technique. The inescapable conclusion is that the relevant scientific community within which the technique finds acceptance must be the community of real world scientists who pursue science for non-litigation purposes and that finding general acceptance within the community of forensic scientists does not constitute general acceptance in the relevant scientific community.”

Appendix: Slide To Stop Table Time (sec)

Long. Accel. g (fxb)

Speed (mph)

Dist (ft)

Running Avg (g)

Avg (g)

0

0

47.78

0

0.000

-0.813

0.1

-0.438

47.09

6.965

-0.219

-0.813

0.2

-0.611

45.91

13.789

-0.350

-0.813

0.3

-0.709

44.44

20.416

-0.440

-0.813

0.4

-0.744

42.82

26.817

-0.500

-0.813

0.5

-0.771

41.16

32.977

-0.546

-0.813

0.6

-0.793

39.44

38.888

-0.581

-0.813

0.7

-0.864

37.63

44.542

-0.616

-0.813

0.8

-0.882

35.72

49.921

-0.646

-0.813

0.9

-0.886

33.77

55.018

-0.670

-0.813

1

-0.855

31.88

59.831

-0.687

-0.813

1.1

-0.878

29.96

64.367

-0.703

-0.813

1.2

-0.881

28.07

68.623

-0.716

-0.813

1.3

-0.883

26.1

72.594

-0.728

-0.813

60 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

1.4

-0.896

24.14

76.279

-0.739

-0.813

1.5

-0.902

22.17

79.675

-0.750

-0.813

1.6

-0.894

20.2

82.782

-0.758

-0.813

1.7

-0.89

18.23

85.6

-0.765

-0.813

1.8

-0.908

16.26

88.131

-0.773

-0.813

1.9

-0.905

14.3

90.373

-0.780

-0.813

2

-0.91

12.31

92.324

-0.786

-0.813

2.1

-0.921

10.28

93.98

-0.792

-0.813

2.2

-0.872

8.33

95.344

-0.795

-0.813

2.3

-0.858

6.43

96.427

-0.798

-0.813

2.4

-0.876

4.56

97.234

-0.801

-0.813

2.5

-0.903

2.61

97.76

-0.805

-0.813

2.6

-0.917

0.6

97.995

-0.809

-0.813

2.63

-0.905

0

98.009

-0.813

-0.813

Table # 1 - Vericom skid test data for Time, Longitudinal Acceleration, Speed and Distance. Note: the columns of Running Avg G and Avg G have been derived from and added to the published values.

One Sided 75%

80%

85%

90%

95%

97.5%

99%

99.5%

99.75% 99.9%

99.95%

Two Sided d.f.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

60%

70%

80% t.100 3.078 1.886 1.638 1.533 1.476 1.44 1.415 1.397 1.383 1.372 1.363 1.356 1.35 1.345 1.341 1.337 1.333 1.33 1.328 1.325

90% t.050 6.314 2.92 2.353 2.132 2.015 1.943 1.895 1.86 1.833 1.812 1.796 1.782 1.771 1.761 1.753 1.746 1.74 1.734 1.729 1.725

95% t.025 12.71 4.303 3.182 2.776 2.571 2.447 2.365 2.306 2.262 2.228 2.201 2.179 2.16 2.145 2.131 2.12 2.11 2.101 2.093 2.086

98% t.010 31.82 6.965 4.541 3.747 3.365 3.143 2.998 2.896 2.821 2.764 2.718 2.681 2.65 2.624 2.602 2.583 2.567 2.552 2.539 2.528

99% t.005 63.66 9.925 5.841 4.604 4.032 3.707 3.499 3.355 3.25 3.169 3.106 3.055 3.012 2.977 2.947 2.921 2.898 2.878 2.861 2.845

99.50% t.0025 127.3 14.09 7.453 5.598 4.773 4.317 4.029 3.833 3.69 3.581 3.497 3.428 3.372 3.326 3.286 3.252 3.222 3.197 3.174 3.153

99.90%

50% 1 0.816 0.765 0.741 0.727 0.718 0.711 0.706 0.703 0.7 0.697 0.695 0.694 0.692 0.691 0.69 0.689 0.688 0.688 0.687

1.376 1.061 0.978 0.941 0.92 0.906 0.896 0.889 0.883 0.879 0.876 0.873 0.87 0.868 0.866 0.865 0.863 0.862 0.861 0.86

1.963 1.386 1.25 1.19 1.156 1.134 1.119 1.108 1.1 1.093 1.088 1.083 1.079 1.076 1.074 1.071 1.069 1.067 1.066 1.064

99.8% 318.3 22.33 10.21 7.173 5.893 5.208 4.785 4.501 4.297 4.144 4.025 3.93 3.852 3.787 3.733 3.686 3.646 3.61 3.579 3.552

636.6 31.6 12.92 8.61 6.869 5.959 5.408 5.041 4.781 4.587 4.437 4.318 4.221 4.14 4.073 4.015 3.965 3.922 3.883 3.85

d.f.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Table # 2: The Student t Distribution Note: The first column notation (d.f.) refers to degrees of freedom where d.f.= (n-1) which for three skid tests is 2, for four tests is 3, and etc. www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 61

2005 Michigan State Police Vehicle Tests NLECTC: http://www.nlectc.org/testing/vehicles.html

Gas, 0-60 (Seconds) Accel, Gs (Fxd 0-60) Brake, ft (60-0 mph) Brake G (Fxb 60-0)

Camaro 5.7L SFI Automatic

Chevrolet

2000

5.80

0.471

135.70

0.886

Camaro 5.7L SFI 6-speed manual

Chevrolet

2000

5.77

0.474

135.70

0.886

Impala 3.8L SFI

Chevrolet

2000

8.99

0.304

142.70

0.843

Jeep Cherokee 2WD

Daimler Chrysler

2000

9.52

0.287

151.40

0.794

Jeep Cherokee 4WD

Daimler Chrysler

2000

10.24

0.267

153.50

0.783

Police Interceptor 4.6L

Ford

2000

8.75

0.312

143.30

0.839

Expedition 5.4L SFI 4WD

Ford

2000

10.93

0.250

167.80

0.717

Explorer 4.0L 4WD

Ford

2000

11.55

0.237

172.80

0.696

S-70 T5 Sedan 2.3L PFI Turbo

Volvo

2000

7.48

0.365

132.30

0.909

V-70 T5 Wagon 2.3L PFI Turbo

Volvo

2000

7.75

0.353

129.50

0.929

Police

Hummer HMCS 6.5L Turbo-Diesel

AM General

2001

18.55

0.147

171.60

0.701

Police

Camaro 5.7L Automatic

Chevrolet

2001

5.87

0.466

134.40

0.895

Police

Camaro 5.7L 6-speed manual

Chevrolet

2002

5.77

0.474

134.40

0.895

Special Service

Impala 3.8L

Chevrolet

2001

9.20

0.297

140.80

0.854

Special Service

Tahoe 4.8L SFI 2WD

Chevrolet

2001

9.77

0.280

151.20

0.795

Police

Tahoe 4.8L SFI 4WD

Chevrolet

2001

10.12

0.270

159.10

0.756

Police

Jeep Cherokee 4.0L PFI 2WD

Daimler Chrysler

2001

9.18

0.298

151.50

0.794

Police

Jeep Cherokee 4.0L PFI 4WD

Daimler Chrysler

2001

9.71

0.281

151.30

0.795

Police

Police Interceptor 4.6L SFI

Ford

2001

8.70

0.314

145.40

0.827

Special Service

Police Interceptor 4.6L SFI (CNG)

Ford

2001

12.14

0.225

149.30

0.805

Special Service

Excursion 6.8L SFI 4WD

Ford

2001

11.27

0.242

180.00

0.668

Special Service

Expedition 5.4L SFI 4WD

Ford

2001

10.04

0.272

162.30

0.741

Police

Explorer 4.0L PFI 4WD

Ford

2001

10.48

0.261

143.20

0.840

Police

Hummer 6.5L Turbo

AM General

2002

20.03

0.136

173.80

0.692

Police

Camaro 5.7L SPFI

Chevrolet

2002

5.69

0.480

140.80

0.854

Special Service

Impala 3.8L SPFI

Chevrolet

2002

9.21

0.297

148.60

0.809

Special Service

Tahoe 2WD SPFI

Chevrolet

2002

8.62

0.317

157.50

0.763

Police

Tahoe 4WD SPFI

Chevrolet

2002

8.73

0.313

158.10

0.761

Special Service

Intrepid 3.5L SPFI

Dodge

2002

8.72

0.313

149.40

0.805

Special Service

Durango SLT 2WD SMPI

Dodge

2002

9.23

0.296

154.60

0.778

Police

Durango SLT 4WD SMPI

Dodge

2002

10.80

0.253

160.60

0.749

Police

Police Interceptor 4.6L SPFI

Ford

2002

8.42

0.325

139.90

0.860

Special Service

Police Interceptor 4.6L SPFI

Ford

2003

8.84

0.309

154.70

0.777

Special Service

Excursion 4WD 6.8L MPFI

Ford

2002

10.31

0.265

164.80

0.730

Special Service

Excursion 4WD 5.4L SMPI

Ford

2002

10.58

0.258

164.30

0.732

Special Service

Excursion 4WD 4.6L SFI

Ford

2002

11.07

0.247

140.90

0.853

Police

Crown Victoria LWB SPFI

Ford

2002

9.90

0.276

141.70

0.849

Police

Hummer HMCS

AM General

2003

20.65

0.132

170.10

0.707

Special Service

Impala 3.8L SPFI

Chevrolet

2003

9.25

0.295

133.00

0.904

Special Service

Tahoe 2WD 5.3L SPFI

Chevrolet

2003

9.34

0.293

150.70

0.798

Police

Tahoe 4WD 5.3L SPFI

Chevrolet

2003

9.31

0.294

153.00

0.786

Police

Intrepid 3.5L SPFI

Dodge

2003

9.14

0.299

145.10

0.829

Special Service

Police Interceptor 4.6L SPFI

Ford

2003

9.14

0.299

148.60

0.809

Special Service

Expedition 2WD 5.4L SMPI

Ford

2003

12.30

0.222

152.70

0.787

Special Service

Expedition 4WD 5.4L SMPI

Ford

2003

11.27

0.242

156.40

0.769

Police

Explorer 2WD 4.6L SPFI

Ford

2003

11.75

0.233

145.00

0.829

Special Service

Impala 3.8L SPFI

Chevrolet

2004

9.00

0.304

134.20

0.896

Special Service

Tahoe 4WD 5.3L SPFI

Chevrolet

2004

8.58

0.319

142.40

0.844

Police

Tahoe 2WD 5.3L SPFI

Chevrolet

2004

9.20

0.297

145.50

0.826

Police

Intrepid 3.5L SPFI

Dodge

2004

8.56

0.319

137.40

0.875

Police

Police Interceptor 4.6L (3.55 axle) SPFI

Ford

2004

8.44

0.324

149.60

0.804

Special Service

Police Interceptor 4.6L (3.27 axle) SPFI

Ford

2004

8.25

0.331

150.40

0.800

Special Service

Expedition 2WD 5.4L SMPI

Ford

2004

10.11

0.270

142.90

0.841

Special Service

Explorer 2WD 4.6L SPFI

Ford

2004

8.90

0.307

141.80

0.848

Table # 3: Brake Test Results of 54 police vehicle prototypes 62

Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 63

at n ur t el e My h w the

A DIRTY TRICK BY A LAW OF PHYSICS, OR WHAT? Erik Carlsson This article is about a full-scale vehicle crash test that seems to have been made for no other reason than to teach a lesson to an automotive engineer who had the nerve to criticize a vehicle design that was subject to litigation. Be that as it may, the crash test backfired.

What brought the issue to court was the extreme face and head injuries the driver of a car sustained in an offset head-on collision with a pickup truck. The collision, which was severe, happened in daylight on a busy two-lane two-way dry country road. Neither of the involved drivers was after the accident able to tell what happened 1. However, the accident was witnessed by two other drivers, one traveling behind the car, the other behind the pickup truck. Those drivers testified at the police investigation that followed that the traffic was moving at about 40 mph. They both had a clear view of the collision from their positions a short distance behind the accident vehicles. They saw the car drifting into the lane of the oncoming pickup truck a moment before the impact.

Both accident vehicles rotated counterclockwise at the impact, the car about 150 degrees, and the heavier pickup truck about 120 degrees. Both vehicles deviated to the right following the impact, coming to rest a short distance from the point of impact.

Although no one doubted that the accident was caused by the car driver failing to pay attention to the road, the maker of the car nevertheless ended up in court as a defendant. The issue raised by the plaintiff's legal team was the design of the steering wheel hub and column in the accident car. The car was of a model that for a period of time was one of the bestselling compact cars before airbags became mandatory. The car had a small steering wheel, with a centerpiece about the size of the palm of a hand that served as a signal horn button. The visible surface of the centerpiece was made of soft plastic, and looked rather 64 Collision Magazine - Volume 10 Issue 1

collision-friendly. However, directly underneath the thin centerpiece was the upper end of the solid steering wheel shaft. Due to the severe crush of the car's front end at the impact, the rack and pinion steering gear and the firewall were displaced a substantial distance rearward, pushing the steering wheel rearward and upward, striking the face of the small driver.

[It is of interest to note that the defendant carmaker's legal team did not raise the issue as to whether the driver was wearing her seatbelt, which was of a type equipped with a tension reliever. This type of belt had at the time been heavily criticized by the NTSB because those belts had been found to permit excessive forward movement of the restrained occupant's upper torso in frontal collisions. Note: This is the type of seatbelt that was described in "Think of a Number, Then ….," Fall 2013 issue of Collision.]

The defendant carmaker retained as its expert witness an engineer who had previously been a designer at the company, and who in fact was said to be the designer of the steering wheel and column in the accident vehicle model. He testified at his deposition that the car's steering system met the FMVSS 203 requirement, which is that the resulting impact force must not exceed 2,500 pounds when the steering wheel is struck with a block of wood simulating a human chest, with the block moving at a speed of 15 mph.

He also made the rather surprising statement that "a couple of inches of supersoft [sic] foam would not have made a difference" when he was shown several different steering wheels removed from other vehicles that all had deeply recessed energy-absorbing hubs.

The engineer retained by the plaintiff's legal team addressed in his report the danger of a vehicle design lacking an effective energy-absorbing steering wheel hub and a fully collapsible steering shaft and column.

www.collisionpublishing.com

He further made a comment in his report that the car driver swerved to the right before the accident, though too late to avoid the collision. The badly bent steering gear rack was jammed in a position corresponding to half a turn to the right of the steering wheel from its straight ahead position. Half a turn is just about how much a driver with both hands on the steering wheel can be expected to turn it with a rapid movement of the hands, he wrote in the report.

While the comment about the car driver swerving to the right just before the impact was not addressed as being of significance in the engineer's report, the defendant carmaker's own expert nevertheless challenged the statement, claiming the car driver could not have steered to the right before the impact. If she had, he claimed, the pickup truck would have deviated to the left, not right, and continued front end first following the impact. His explanation for the position in which the steering rack was jammed was that the large rearward displacement of the left front wheel and suspension displaced the steering rack in such a way that the right front wheel turned to the right.

Computer simulation is always a useful tool, or is it?

To show that the car driver could not have swerved to the right before the impact, the defense expert included in his report the result of a computer simulation that showed the pickup truck continuing front end first and deviating to the left following the impact.

The computer simulation was naturally based on the law of conservation of linear momentum. The reasoning therefore was that had the car driver steered to the right before the impact, the car would have gained some momentum towards the side of the road that was to the right of the car and thus to the left relative to the pickup truck's direction of travel. At the impact, the car lost almost all its speed, thereby also almost all of its momentum. The lost momentum was all transferred to the pickup truck, which thus gained the momentum towards the side of the road that the car lost. The pickup truck therefore had to deviate to its left following the impact. (At least in theory!)

In the computer simulation both vehicles moved in straight lines towards each other, with the two lines of movement intersection each other at an angle of 10 degrees (or, if one so prefers, 170 degrees). Hence the

car was shown as initially being well into the pickup truck's path while gradually moving to its right so as to simulate the car driver's attempt to get out of the way for the oncoming pickup truck. The simulated vehicles collided with a small overlap and with the car's rear end intruding much farther into the pickup truck's path than its front end. Thus, if the simulation correctly depicted the accident, the car driver could not have avoided a collision or a sideswipe anyway by swerving to the right unless she had managed to get the car's front bumper to clear the pickup truck's bumper by some two feet. This seems to contradict the statements by the two drivers who saw the accident. They saw the car drifting into the lane of the oncoming pickup truck just before the collision, not that the car went far into that lane and then started to move back towards its correct lane before the impact. Nor can the comment by the plaintiff's engineer that the car driver swerved to the right just before the impact reasonably be interpreted as meaning that the car's rear end in such a case had to be much farther into the wrong lane than its front end.

Garbage In, Garbage Out [GIGO.] The perhaps most interesting aspect of this case is that the plaintiff's legal team did not raise the issue of the defense expert in his computer simulation very clearly misrepresenting the accident scenario by placing the car's rear end, and thereby also its Cg, farther into the pickup truck's lane than what can reasonably be deduced to have been the case at the accident. Not even the ten-degree angle with which the two directions of lines of travel intersected each other in the computer simulation could be said to truly represent the accident scenario. This angle was according to the carmaker's engineer based on a sketch that depicted the accident vehicles at the point of impact, but the proportions of the vehicles in the sketch were such that it was obvious that the sketch had not been drawn to scale! [The actual angle between the directions of movement by the two accident vehicles at the impact was never established.]

Instead of challenging the input data used in the computations, the plaintiff's legal team dismissed the defense engineer's computer simulation as merely GIGO, a term often used in the early days of computing.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 65

The carmaker and its legal defense team were evidently very offended by the characterization of their expert's computer simulation as garbage. In the apparent belief that the computer simulation showed what really happened, the defendant decided to conduct a full-scale crash test with two vehicles of the same make and models as the accident vehicles. For this purpose, the defendant retained an independent testing laboratory that was well known within the trial attorney community for its ability to conduct tests that showed the desired result. When the testing laboratory had obtained two vehicles of the actual makes and models and was preparing the crash test, the plaintiff's legal team was advised about place and time for the test. They retained their engineer to observe the test.

Photo 1

The crash test backfired!

The full-scale offset frontal collision crash test was set up in the same way as in the computer simulation, with each test vehicle moving in a straight line, and with the two lines of travel intersecting each other at 10 degrees. The stated test speed was 40 mph, for a closing speed of 80 mph. At the impact, both test vehicles span around counterclockwise and slid a short distance to their right following the impact, just as happened at the collision that was the issue of the case. Thus the test showed exactly the opposite of what it was supposed to show. This also includes the right front wheel of the car, which was not found to be turned to the right after the impact, as it was on the accident car, and which according to the defense expert was caused by the damage to the car at the collision.

Photo 2

I include three photos that are derived from a video recorded with an overhead camera facing down over the impact point at the crash test. Photo #1 shows the test vehicles less than 100 milliseconds before impact; photo #2 shows the vehicles at the moment of impact; and photo #3 a fraction of a second later, just before the vehicles disappear from camera view.

It appears in the photos as the pickup truck is the vehicle that crosses into the path of the car, not the other way around as at the accident at issue, but that is the way the testing laboratory had set up the crash test. The relative movements between the vehicles would in any case be the same, and therefore it makes no difference that the test had not been set up with the car being the one that approached the track at ten degrees angle from the top in the photos (unless of course the track was intended to affect the 66 Collision Magazine - Volume 10 Issue 1

Photo 3 vehicle's post-impact movement in a particular way, in addition to giving the illusion that the pickup truck's Cg deviated to its left following the impact. Note: The truck's Cg approached the track somewhat after the impact, but at angle that was less than the 10 degrees approach angle it had before the impact.)

What happened to the law of conservation of linear momentums? www.collisionpublishing.com

A question that certainly may be asked is why the pickup truck did not support the defense expert's computer simulation by obeying the law of physics and deviate to the left following the impact? The answer is that the law is not quite as simple and straightforward as the term "conservation" may imply. A vehicle moving in a straight line has momentum only in the direction in which it is moving, but momentum in a different direction may be created in a collision. The perhaps most illustrative example of this is where a speeding car hits a utility pole head-on, with the point of impact being well to the side if the car's centerline.

If the impact force is high enough, the car will spin around and pass to the side of the utility pole. Thus the car moves laterally and therefore gains momentum in a direction in which there was no momentum before the impact. The same is true in an offset head-on collision between any two vehicles when the collision force is sufficient to cause the vehicles to spin around. In such a case, both vehicles will gain lateral momentum. These momentums are equal, although in opposite directions.

A little while later the defense team returned to the testing track and told the waiting engineer that the test vehicles would be repositioned so as to increase the overlap from 18 to 24 inches. When this had been done, the engineer was asked to confirm that the overlap was now 24 inches, which he did. The impact site was then cleared and the crash test run, with the described result, and as illustrated by photo #3.

How could the defense team be so sure about the outcome of the test that they invited the plaintiff's legal team and engineer to watch?

The defendant naturally had to supply the plaintiff's legal team with copies of the videos of the crash test taken by the testing laboratory. This led to the observation that can be made in photo #2, which is that the overlap between the vehicles is not 24 inches! It is in fact not more than 20 inches. The observation that the crash test had been done with four inches less overlap than shown before the test and which the plaintiff's engineer had been asked to confirm prompted the plaintiff's legal team to allege that the testing laboratory on behalf of the defendant carmaker tried to affect the outcome of the test. The testing laboratory denied that such was the case. The reason for the fourinch reduced overlap is that there was a tolerance level of plus or minus six inches in the execution of the test, the laboratory claimed! The testing laboratory had therefore successfully executed the required crash test as the fourinch reduction in the overlap was within the established tolerance level.

The most likely answer to this question is that the testing laboratory had already conducted a similar test, and that the pickup truck at that test continued front end first and deviated to the left. This would naturally explain why the defense team wanted the plaintiff's team to watch the test.

The reason the test I have described in this article came out differently is due to what happened at the test site prior to the test. When the plaintiff's engineer arrived at the testing facilities, he was shown the test site and asked to confirm that the two test vehicles were of the same makes and models as the accident vehicles, and had not been tampered with in such a was as to effect the outcome of the test. The vehicles were at that time positioned at the intended point of impact, with their front bumpers in contact in the same way as shown in photo #2.

When the engineer saw the small overlap between the test vehicles, he asked why the test had been set up with such a small overlap. He was told that this was because he at his deposition testified that the overlap between the vehicles at the accident was two feet. The engineer then measured the overlap and saw it was only 18 inches. He pointed out that two feet equals 24 inches, not 18 inches, and that this difference would have a large effect on the collision dynamics. His attention to the small overlap led to a grave consternation within the defense team, which retreated to the office building.

There is no doubt that had the overlap between the crashing vehicles been six inches less than seen in photo #2, as it would have been had the vehicles not been repositioned before the test, the collision force could not reasonably have been so high as to cause such a rotation of the pickuptruck as can be seen in photo #3. The pickup truck would instead most certainly have continued front end first, and so would also the car. The result would have been that the vehicle impact that began as an offset frontal collision proceeded into a sideswipe.

Not the end of the story -yet.

No explanation was given for the claimed tolerance level of plus or minus 25% of the specified overlap, nor was there any mentioning of this tolerance level until after the plaintiff's legal team discovered the discrepancy and alleged that the defendant tried to affect the outcome of the test.

Anyone who has some questions about what I have described in this article is welcome to contact me. My e-mail address is erikcarlsson@live.com The car driver remained in coma for a long period of time after the accident.

1

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 67

ACCIDENT RECONSTRUCTION AND FORENSIC MAPPING SOLUTIONS DESKTOP MAPPING & ANIMATION SOFTWARE | EVIDENCE COLLECTION SOFTWARE & HARDWARE | TRAINING

WHEN YOU’RE LOOKING FOR THE BEST ACCIDENT RECONSTRUCTION SOFTWARE, YOU NEED TO LOOK AT

EVIDENCE RECORDER

FORENSIC CAD

Evidence collection software built specifically for the professional reconstructionist.

Law enforcement and accident investigators use MapScenes Forensic CAD.

Using a Windows Mobile or tablet device, Evidence Recorder allows you to experience the power, precision and productivity gains associated with being able to visually verify your ongoing field evidence collection – on-the-spot, right in the palm of your hand. From there, it can seamlessly integrate with the MapScenes Forensic CAD desktop software.

MapScenes Forensic CAD is the most precise desktop forensic mapping software available in the world. This powerful and feature-rich application provides law enforcement professionals and accident reconstruction specialists, like you, with confidence that the evidence they present is precise, compelling, and irrefutable.

LET’S GET BACK TO BASICS How quickly and easily can you deliver a simple 2D scene for court?

Let’s face it, most of the drawings prepared for case files or courtroom presentations are simple, straightforward 2D diagrams.

The most critical and stressful time spent is at the scene. The reconstructionist needs the best scene evidence collection software and the best diagramming software. That’s it.

So how does it work? The process begins with Evidence Recorder (EVR).

With the MapScenes system, your diagramming begins in the field.

When using EVR to take measurements, you’re actually building the framework for the diagram in real-time. The user interface is crafted to work very specifically with your workflow to ensure a brisk pace when taking measurements via total station or GNSS receiver.

The data transfer between Evidence Recorder and Forensic CAD is unmatched. The diagram you can see in Evidence Recorder appears in MapScenes.

The evidence (actual measurements made by the instrument) is available and verifiable. The drafting commands are powerful and flexible. Your final deliverable is both credible and reliable.

When you do end up working on a scene that requires more visual appeal, MapScenes still has you covered.

With add-on modules, you can bring your diagrams to life with animation and simulations, as well as the ability to view the point cloud data, take measurements, and create your diagram right within the point cloud.

WHAT IS THE

SYSTEM?

EVIDENCE COLLECTION SOFTWARE & HARDWARE

DESKTOP MAPPING SOFTWARE

WORLD-CLASS, CERTIFIED TRAINING

Reduce errors, save time, and eliminate frustration.

When accuracy and precision are non-negotiable.

Training by professionals, for professionals.

Evidence Recorder is supported with more total stations, GNSS receivers, data collectors, and PC tablets than ever.

Not only is Forensic CAD the best choice for forensic mapping, but it is also the best-priced desktop software package in this industry.

Combine with your existing hardware, or choose from one of our hardware bundles. Evidence Recorder is included on all hardware configurations and offers the absolute best in value.

Starting at: $995 per user.

Training is essential in all facets of law enforcement. MapScenes acknowledges its responsibility to the industry by ensuring that all trainers and educational courses are kept up to date through a base educational standard, preserving quality and integrity.

At any time, you can add more functionality with our add-on modules–Animation module and Point Cloud module–for a truly custom approach to your exact requirements.

We have set an industry benchmark by developing a comprehensive framework that provides investigators with the knowledge and skills required to use the MapScenes system easily, efficiently, and expertly.

TO LEARN MORE, OR TO OBTAIN A FREE TRIAL VERSION, CALL US OR VISIT OUR WEBSITE: 1.800.668.3312

www.mapscenes.com

® MapScenes is registered in the U.S. Patent and Trademark Office by MicroSurvey Software Inc. MicroSurvey is part of Hexagon.

ED W E I EV R l ia r o t Edi

Analysis and Performance of a Booster Seated 6 Year Old HIII Anthropomorphic Test Device When Utilizing Seat Belt Cinching For Lap Slack Control Michael T. Vecchio SafetyWorks – Safety Services

74 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

A

bstract

ing children in booster seats and whether there was There is a paucity of data in regard to how seat any safety benefit for this kind of application. From belts behave in actual seating and real world our perspective this clearly addressed a way that assists automotive environments. The vast majority being in-position without being intrusive. of current understanding is derived from test protoBesides providing a way to stabilize children sitting in cols that do not accurately reflect real world seatbelt booster seats, CG-Lock based lap belt cinching techuse. While there is much data to substantiate the efnology also seemd to consistently allow the occupant fectiveness of seatbelts, there are many instances, for to sit comfortably in crash-position at all times, as if instance, where people ligitmately "wearing a seatyou had just donned your seatbelt. bellt" are ejected from the vehicle. As a result of this, there must be other effects when wearing seatbelts All testing was performed comparatively, using curthat can occur. Current seatbelts are not designed to rent design seat belts with buckle pretensioners. The keep the occupant in position. They are designed to specific test components can be found in Appendix C. be donned by an occupant in-position. Whether the occupant stays in-position is usually not examined In practical terms, the utilization of this restraint in certfication testing. This can be especially true for strategy allows complete freedom for the child in the upper body torso loop during normal driving and child occupants. use, while preventing lap belt slack from developing A series of tests were conducted using a CG-Lock lap- around the pelvis. During a crash event the pelvis is belt cinching device to determine the effect of being pre-restrained while the torso loop becomes locked as in-position on a booster seat compared to being out of it normally would by the inertial retractor. The effects position when seated on the same booster seat. Data of this restraint strategy in terms of slack removal and was collected for a standard mid-size seating environ- occupant response are examined. This paper examment using FMVSS 213 parameters and is examined ines the utility and possibilities of this concept both and analyzed for both in-position and out-of-position for crash safety and child stability. When installed situations. The testing quantified this difference for properly, it has been reported that children using the the 6 Year Old HIII ATD when seated in a booster "CG-LockTM” based technology do feel more comseat. fortable because they ride more stably since there is no tendency for the cinched seat belt system to loosen The tests demonstrated that there is a statistically and the seat to tip during vehicle maneuvering and relevant and significant difference in terms of injury cornering. probability between in-position and out of position.

The data presented in this paper were developed in Possibilities for a restraint strategy that addresses this the pursuit of making the current continuous loop issue are explored in this article. slip tongue seat belt systems more effective in terms of improving booster seated child passenger stability. ntroduction TM A device based on “CG-Lock ” seat belt cinch- A system was proposed and patented which would aling technology is utilized in this study as well as low the lap loop of a continuous loop system to be the second generation CG-Lock lapbelt cinching “locked” at a comfortable level of snugness, while the device. This device and how it attaches to a seatbelt “torso loop” remained free to “reel” from an inertial buckle is shown in Appendix C. The reason the CG- locking retractor and as a result would perform as Lock, a performance driving add-on, was chosen is that originally designed. Also, this eliminated the need it had been getting so many testimonials from adult to engage the ALR, auto locking retractor, sometimes drivers in regard to its injury preventing attributes in utilized in the rear seat which most children feel is too vehicle crash events that the question emerged as to restrictive, as it restricts the complete seatbelt torso the effectiveness of such an add-on device for stabiliz- and lap paths.

I

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 75

G

eneral Method Of Assessment In order to more clearly understand the utility of lap belt cinching technology as it relates to in-position testing, a test series was devised which investigated the difference between in-position and out of position when restraining a 6 Year Old HIII ATD seated on a booster seat. Prior side by side sled testing of the CG-lock lap belt cinching technology with 50th percentile HIII male anthropomorphic test devices had indicated no reduction in the performance of a standard mid-size seat belt system. In-position and in-position utilizing a CG-Lock device produced similar results. Another purpose of that test series was to assess any interactions with the restraint system of a mid-size vehicle. Because the CG-Lock did not inhibit the performance of the standard seat belt system utilized, it can safely be assumed that a comparison between a booster seated 6 YO HIII test device in-position utilizing a standard seat belt system and in-position utilizing a standard seat belt fitted with a CG-Lock Technology deviceTM would produce similar values. Also, to assess this hypothesis during testing, various “snugness” levels were utilized by adjusting lap tension with the CG-LockTM device. No demonstrable differences could be assessed in the examination of that data. That is, the benefits derived from lack of slack, not specific level of tightness. Meaning that specifically once all slack was removed and the seatbelt "snugged" to the FMVSS 208 levels of tightness no further benefit was realized.

Because the scope of this test series included testing a 50 percentile male HIII device, as well as the HIII 6 YO ATD on a booster seat, it was decided to utilize a right front seat environment for all testing.

A DN101 1996-1999 Taurus occupant front seating passenger environment was chosen as the test bed for assessment. The specific dimensional relations of this vehicle were transferred to a rigidized sled environment which utilized the appropriate standard right front Taurus seat and ‘B’ Pillar relationship.

A standard FMVSS 213 sled test pulse was utilized along with a commercially available mid-priced FMVSS 213 approved consumer booster seat. The specific test components used are detailed in Appendix C. Also of note is that while this testing utilized the FVSS 213 sled pulse, it is not an FMVSS 213 test because that testing requires a simulated rear bench seat with specific CLD, compression load deflection, foam specifications.

76

Collision Magazine - Volume 10 Issue 1

T

est Methodology Objective The objective was to determine whether lap belt cinching with the CG-Lock devices provided any discernable safety benefit for a booster seated 6 YO anthropomorphic test device with the underlying assumption that in-position with the standard seat belt system and in-position utilizing a lap belt cinching device would yield the same result. The difference being that the lap belt cinching device, because it is locked, preventing slack formation, would keep the child consistently in-position compared to a more random position that might be exhibited by the child when only belted in by the standard inertially locking single loop seat belt. It has been noted that because single loop seat belt systems do not lock, movements of the vehicle and occupant tend to adjust seating position and relationship of occupant to the seatbelt and seating system.

T

est Matrix Development An in-position and out-of-position matrix was developed which included testing with the back attached to the booster seat and with the back removed from the booster seat. Also, lap belt snugness levels were adjusted from the 15 lbs specified in the FMVSS 213 standard to about 1 lb. In-position tests were conducted utilizing initial production samples of the CG-LockTM version V1a and Version 2 which are functionally equivalent.

T

est Procedures Testing was conducted on a VIA sled (see Figs. 1 -3) utilizing the standard FMVSS 213 child seat pulse shown in Appendix B. Side and overhead video cameras were used as well.

The sled was fitted with the right hand front Taurus environment previously noted. The “D ring” height was set for the 50 percentile male occupant. It was felt this adequately addressed the needs for the 6 Year Old HIII ATD when seated on the booster seat in the right front seating environment. The installed ATD can be seen in Figs. 1-3. The 6 YO ATD was fitted with triaxial head, chest and pelvis accelerometers.

The booster seat was installed according to the manufacturer’s directions. Once fitted to the Taurus seating system, the 6 YO HIII ATD was seated in the booster seat and positioned utilizing FMVSS 213 protocols for in-position testing. While this ‘real world’ Taurus environment is somewhat physically different than the FMVSS 213 environment, positioning procedures were kept as similar as possible to FMVSS 213. It is one of the reasons the 15 lb belt tension was chosen as a comparator for two of the runs as well. It should be noted that the 15 lb tension level specified in FMVSS 213 is not easily achievable in the field

www.collisionpublishing.com

in a "real world" installation in today's cars. Technicians sometimes use both feet pressed against the back of the FMVSS 213 test bench to achieve it.

OOP positioning was accomplished by utilizing the same procedure as in-position seating of the ATD, but then simulating what the end position of dummy/vehicle movements might produce. For this, the ATD was folded forward, then its mid-sagittal plane rocked side to side 20 degrees and finally the legs were gently pulled horizontally until the H-Point of the pelvis was moved 3 inches forward. Fig. 4 shows the space created at the rear of the ATD pelvis.

While there is no direct statistical basis for this positioning, in part it was determined from observations of the children of family members of the test group when sitting in booster seats. Also, to the author’s knowledge, OOP testing typically has not been performed specifically because statistical data is not available for general OOP positions.

At the same time, lack of statistical data does not obviate the need for testing and understanding of this kind of data. To that end, the OOP positioning described here was developed. It is the author's belief that it will be validated.

T

est Results A total of eleven tests were conducted. Of those, ten were used for statistical analysis of the data.

The test data with booster seat back was compared to that without booster seat back. Also, various levels of lap belt cinching tension were examined and compared.

Based on this review of the data it was decided to collect all in-position data together and all OOP data together using 3 ms cum duration resultant pelvis acceleration and 3ms cum duration chest accelerations as comparators, as well as HIC36. This provided 3 potential injury variables as test comparators, as well as a sufficient statistical sample size for each. Except for the 3ms cum dur plevis data, these comparators are from FMVSS 213.

Besides the examination of the data, a test of “reasonableness” was applied to the assumptions. Was it “reasonable” that back or no back should be similar and is degree of slack an influencing variable? After some discussion it was decided that since the dummy was always adjusted to OOP from in-position and that the OOP position adjustment process was always the same and derived from in-position as a start, the seatbelt contacted the dummy in a way that was similar to in-position. As a result, relationship of the belt to the 6 YO HIII ATD is the same for in-position vs, OOP. What was different was the space behind the dummy H-Point as shown in Appendix A Fig 4. In a similar manner, once all the slack is removed from the seatbelt sys-

tem and the ATD is positioned, his relation to the seating and belt systems is always the same. Another way to put that is, “No slack is no slack.” Once the seatbelt is snuged down, the degree of tightness does not seem to influence occupant position. It was judged that any confounding in these variables is sufficiently minor so as not to mask any major effects, particularly in-position vs. OOP. This was why a "formal" procedure was established to generate an OOP condition.

It must be emphasized that these variables were included in the matrix in an effort to both assess their effects and support the assumption of minimal confounding. A careful examination of the data seems to support these assumptions.

T

est Data Analysis & Results The test data for resultant pelvis 3ms cumulative duration, resultant chest 3 ms cumulative duration and HIC36 were collected and analyzed for the inposition runs and out-of-position runs. A total of 10 data points for each of the 3 injury probability criteria were analyzed. Five runs were collected for in-position and 5 runs for OOP.

A T-test was applied to each of the sample sets of 5 samples each to determine if they were from the same populations.

In both the pelvis resultant 3ms cumulative duration and chest 3 ms cumulative duration data the hypothesis that these came from the same population was rejected at the 95% confidence level. The sample set for in-position and the sample set for OOP are clearly statistically different.

The averages for each of the data sets were calculated and can be seen in Table 1.

Comparison of In-Position with Out-of-Position Chest 3ms Pelvis 3ms HIC36 Avg (g) Avg (g) Avg In-Position 44.66 51.68 431.2 OOP 52.33 63.32 501.6 T-tst P= 0.022 0.021 0.188 Reduction -7.67 -11.67 -70.40

Table 1: Test data results (averages) Note the improvement in both 3 ms cumulative duration resultant pelvis and chest acceleration for the ATD inposition. For the HIC36 it was not possible to reject the hypothesis that both samples were from the same population. In part, this may be caused by the fact that the head is under the least control of the seat belt system. A range of curves are overlayed in Appendix B to provide a sense of

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 77

Appendix A: Test Setup

the variability in the data. The chest resultants from which the 3 ms cum dur is derived are overlayed and are typical of chest performance at the upper end of improvement, while the two resultant pelvis curve overlays give a sense of the ranges produced in the overall pelvis readings.

C

onclusion While there was variation in the data for the booster seated 6 YO HIII ATD for both chest and pelvis it is clear that being in-position on a booster seat does produce reduced injury probability data values.

For in-position, a reduction of 7.67 g’s was calculated for chest and 11.67 g’s for pelvis versus OOP. This represents a 14.65 % reduction in chest g’s and an 18.43 % reduction in pelvis g’s compared to being out of position.

Fig. 1: 6YO HIII ATD In-Position Side View

Having observed children riding in booster seats, the assumption of typically being out of position seems reasonable. Lap belt cinching also seems to increase riding comfort and child/booster seat stability while enhancing the possibility of the child being in position should a crash event occur.

In addition, as with adults, in the videos we reviewed, reducing the slack in the lap loop of a standard seat belt system allows improved pelvis control and also reduced chest g’s. Lap loop slack reduction has also been shown to improve driver retention in lower speed collision frontal events, as well as lateral events.

Fig. 2: 6YO HIII ATD In-Position Front View

Kinematic analysis suggests that the lap belt tends to “ride up” over the abdomen in the out-of-position tests, replicating the well-known abdominal injury issues seen with booster seats. The lap belt stays over the iliac crests of the hips when CG-Lock TechnologyTM devices are utilized.

Anecdotal data from lap belt cinching CG-LockTM adult users involved in rollovers and secondary collisions suggests additional benefits not studied in this testing, but relevant to it as well.

Fig. 3: 6YO HIII ATD In-Position Oblique View

Further, rollover testing may also indicate a reduction in the possibility of being ejected and a reduction of headto-roof contact.

A

cknowledgments The author would like to acknowledge Lap Belt Cinch, Inc. for releasing this data and allowing its publication. I would also like to acknowledge the work performed by John H. Young in both managing the testing and assisting in the development of the test matrix.

Fig. 4: 6YO HIII ATD OOP 3” Forward 78 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

Appendix A: Post Test Photos

Fig. 5: In-Position (Post) Test

Fig. 6: Out of Position (Post) Test

Appendix B: Sample Test Data

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 79

Appendix C: Components Tested Front Restraint Hardware - Standard hardware: Right hand front retractor, belt and tongue assy. For 1999 Taurus: Part No. 2F1Z-54611B08-AAB. The standard buckle assembly is attached to the right front seat assy: Part No. 2F1Z5461202-AAB. Lap Belt Cinch: CG-Lock Verison 1a assy October 2008, SeatSnug Version 2.

Child Booster Seat - Cosco Pronto El-Dorado Belt-Positioning Booster Car Seat no. 22491AOL. Purchased from local Walmart 5058 New Hudson, MI. CG-Lock - Schematic of CG-Lock seatbelt cinching add-on device:

80

Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

E

P

R EER

D

WE E I V

A Method to Establish Delta-V and Collision Force in a Two-Vehicle Collinear Collision where Stiffness Data for One of the Vehicles is not Available Jai Singh

Biomechanical Engineering Analysis & Research, Inc. 82 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

A

bstract

The subject work focuses on the development of the deterministic equations for quantifying the initially unknown closure and separation phase stiffnesses for one collision partner, in a two collision partner system, in which each collision partner is modeled using the single degree of freedom approach coupled with a uniaxial linear-linear load-unload structural response model and for which the stiffnesses for one collision partner are known and the coefficient of restitution is estimated. Also presented, as functions of the initially known and estimated parameters, are the closed form solutions for the closing velocity, separation velocity, the change in velocity for each collision partner, the duration of both closure and separation, and the time-parametric kinematic response relationships. A worked example is also provided.

I

ntroduction

The treatment of the structural response of a motor vehicle subject to collision loading as being uniaxial represents a well-established modeling methodology in the field of motor vehicle accident reconstruction.

This approach is perhaps most clearly evidenced when it is coupled with both a single degree of freedom model for each collision partner and colinearity. When contextually apt (e.g. in-line head-on or front-to-rear impacts), this modeling methodology greatly simplifies the analytic endeavor. The inertial frame of reference, if parameterized by a rectangular Cartesian coordinate reference, can be chosen to have its principal axes aligned in a time-invariant manner, with respect to the collision, along the aligned axes of the collision partners. This mitigates the necessity for the explicit employment of coordinate transformations when working between the inertial frame of reference and the local frame of reference associated with each collision partner.

If the structural response ‘element’ of each collision partner is taken as being aligned along the relevant principal axis of each collision partner, the same argument holds for the transformation of the load bearing response to the local frame of reference as it did for the local frame of reference to the inertial frame.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 83

For the approach described in the previous paragraph, the following points hold in regard to extant residual damage based modeling methodologies for quantifying collision severity. The first is that the traditional Campbell model is predicated upon empirical linearity between equivalent barrier speed (EBS) and the residual damage depth present following a collision. The second is that the Calspan Reconstruction of Accident Speeds on the Highway (CRASH3) model is analytically linear in regards to the relationship between collision force magnitude, normalized to the undeformed collision partner direct damage length, and the dynamic deflection present during the closure phase of a collision. Thirdly, this linear relationship reduces to a linear relationship between peak collision force magnitude and the depth of residual damage when viewed empirically or when the separation phase, analytically, is treated as exhibiting no recovery of internal work absorbed during closure (i.e. treating the coefficient of restitution as being zero). Finally, the reformulation of the CRASH3 model leaves the underlying model unchanged secondary to the direct relationship between the model parameters.

From the dynamic perspective and in a manner consistent with Newtonian mechanics and thermodynamics, a linear relationship between EBS and residual damage depth as well as between peak collision force magnitude and residual damage depth can only be achieved analytically if the closure and separation phase responses are linear in a uniform and ubiquitous manner. From the perspective of the force-deflection response, excluding the use of infinite or zero valued regions of response, which are unrealistic, even from the modeling perspective, the linear responses must have finite slope. For a two collision partner system, in the ideal case, the stiffness of each on both loading and unloading would be known quantities. For impacts involving the front of a motor vehicle, the linear loading and unloading modeled stiffnesses can be determined, albeit with care, using the total barrier force generated from the appropriate New Car Assessment Program (NCAP) test. Simply, the properly filtered total barrier force is normalized by the mass of the test vehicle to generate the dynamic center of mass acceleration. This signal is numerically integrated twice to yield the dynamic center of mass displacement.

The boundary conditions imposed by the fixed, rigid, massive barrier (FRMB) result in zero displacement of the barrier and zero (in the ideal of the modeling approach) deflection of the common collision interface, which is the impacted face of the FRMB. As a result, the displacement of the dynamic center of mass is equal to the structural deflection experienced by the test vehicle. The time-parametric relationship between the total barrier force and the displacement of the dynamic center of mass of the test vehicle yields the force-deflection response. The problem be84

Collision Magazine - Volume 10 Issue 1

comes more difficult, however, when one collision partner has unknown stiffness values and for which there exists no controlled collision test data from which to quantify the dynamic stiffness parameters. This is a relatively uncommon occurrence but not a rare occurrence when it comes to front impact data. It is more of an issue when seeking rear-impact data. Until recently the Vehicle Crash Test Database (VCTB) administered by the National Highway Traffic Safety Administration (NHTSA) contained Federal Motor Vehicle Safety Standard (FMVSS) 301R test data only through the 2005 model year. The handful of newer tests, in regards to availability, consist of 70 percent overlap impact tests.

A typical situation that is framed by the context of the preceding paragraph, is one of having to reconstruct a collision in which the loading and unloading stiffnesses are known for one collision partner and the residual damage depths are known for both collision partners. When the stiffness parameters are known for both collision partners, the coefficient of restitution becomes fixed and is directly quantifiable. For the typical situation, however, a ranged valuation is generally utilized for quantifying the coefficient of restitution. Presented in the subject work is a method for utilizing the coefficient of restitution for developing a range for the stiffness parameter values for the collision partner for which they are not known a priori.

T

heory: Force-deflection based relationships The details of the derivation of the equations of motion and the work-energy relationships for the linear-linear load-unload model for the single degree of freedom collinear collision have been presented elsewhere.

Figure 1 depicts a plot of the relationship between the collision force magnitude and the structural deflection experienced by each collision partner for this modeling approach.

We denote the time-parametric collision force as Fc(t) and the dynamic deflection experienced by the collision partners as δ1(t) and δ2(t). We define the closure phase of the collision as commencing at t = 0 and reaching a terminus, contemporaneous with the start of the separation phase, at t = tc. The loading stiffness (i.e. structural stiffness during closure) is denoted as k1l and k2l, respectively, for each collision partner. Because of the monotonic nature of the linear model, the peak deflection occurs at the terminus of closure and is denoted as δ1(tc) = δ1d and δ1(tc) = δ1d, respectively, for each collision partner. The separation phase initiates at the terminus of closure and continues until the collision force magnitude returns to zero value at t = ts.

The unloading stiffness (i.e. the structural stiffness during separation) is denoted as k1u and k2u, respectively, for each collision partner. The residual damage depth is dented as δ1(ts) = δ1s and δ1(ts) = d1s, respectively, for each collision

www.collisionpublishing.com

Similarly, the IWR for each collision partner is: (6) (7) The total IWA is the sum of the individual components as is the total IWR. The coefficient of restitution is typically defined as the ratio of the separation velocity to the closing velocity. An alternative definition comes from the ratio of the total IWR to the total IWA. (8)

Figure 1: Collision force magnitude versus structural deflection for two collision partners, each following the linear-linear load-unload modeling paradigm. The triangles at the bottom of the figure represent, at half scale, the internal work absorbed (IWA) and internal work recovered (IWR) by each collision partner. partner. Implicit in the theoretical development is delimitation of exceeding the elastic limit for each collision partner resulting in the presence of non-zero valued residual damage depths.

At the terminus of closure, the collision force for the model attains a peak value Fc(tc) = Fcp. Newton’s Third Law of Motion can be used to relate the loading stiffnesses and the peak deflection.

We consider the following set of terms known {k1l, k1u, δ1s. δ2s}. The coefficient of restitution is subject to a banded estimate. The following set of terms are initially unknown {k2l, k2u, δ1d, δ2d, |Fcp|} and require determination in terms of the known terms and the coefficient of restitution.

The first step in the solution process is to solve for δ1d in terms of the known terms. This is done by eliminating the peak collision force magnitude from equations (1) and (2).

(9) The second step, back substitution of this solution into equation (1), defines the peak collision force magnitude in terms of the known terms: (10) The third step is to solve equation (8) for the peak structural deflection of the second collision partner.

(1) The definition of the unloading stiffness allows for relating the same to the peak collision force magnitude, the peak deflection and the residual damage depth.

(11) The fourth step is solving equation (1) for the loading stiffness of the second collision partner.

(12)

(2) (3)

The fifth step is equating the solution for the peak collision force magnitude from equation (1), in terms of the first collision partner, to the solution from equation (3) and solving for the unloading stiffness of the second collision partner.

The IWA for each collision partner is:

(13) (4) (5)

The last three equations may readily be written in terms of the system known parameters by the substitution of the solution of peak deflection of the first collision partner from equation (9). For the peak structural deflection of the second collision partner, substitution of the solution from equation (9) into equation (11) yields the following:

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 85

(14) Substitution of the solutions from equation (9) and (14) into equation (12) followed by algebraic simplification yields the solution for the loading stiffness of the second collision partner in terms of the known parameters.

(15) Similarly, for the unloading stiffness of the second collision partner: (16)

K

inematic response relationships The basic kinematic response relationships for the two collision partner linear-linear load-unload model have been previously derived (Singh and Carpenter, 2013). These relationships are partially derived, here, again, as a foundation for showing the terminus relevant solutions. Defining the referenced to the inertial frame of reference displacement of the first collision partner as u1(t), the second collision partner as u2(t) and the common collision interface as uIV(t), and using the standard dot notation for denoting time derivatives, the application of Newton’s second and third laws of motion for the two collision partners yields the following pair of second order differential equations of motion for the closure phase.

Equations (9), (10) and (14) - (16) represent the solution set. In examining this solution set, it is desirable to obtain, where possible, forms of each equation that aid in understanding the manner in which each dependent parameter changes as a function of the independent parameters. This is readily done for equation (9) by factoring out the unloading stiffness from both the numerator and denominator from the term on the right side of the equality.

(17)

(22)

(23) Algebraic rearrangement yields the following result:

Taking a similar approach to equation (10) yields the following result: (18)

(24) From the force balance for the collision force:

For the peak dynamic deflection of the second collision partner, algebraic manipulation of equation (14) yields the following solution:

(25)

(19)

Substitution of this solution into both equations of (24) allows for the elimination of the displacement of the common collision interface.

The solutions for the loading and unloading stiffness for the second collision partner are not easily reduced but the following solutions are readily attainable, in terms of the residual damage depths of the two collision partners.

(20)

(26) (21)

86 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

Defining the effective stiffness during loading as:

(27) The solution for the effective stiffness during loading, in terms of the known system parameters, can be obtained by substituting the solution for k2l from equation (20) into equation (27). The time-domain displacement solution for equation (26), under zero initial displacement, is (Singh, 2013a):

(33) Where:

(34) The unloading circular frequency in terms of the input parameters is: (35)

Where:

(28)

The solutions for the velocity and acceleration during the separation phase are:

(29) In (28), the o in the subscript denotes the initial value (i.e. at the start of closure). The velocity and acceleration solutions are obtained by simple time differentiation.

(36)

(30)

(31)

(37)

The time at which closure terminates can be found from the velocity solution.

The solution for the time at which the separation phase terminates is derived from the acceleration solution (i.e. solving for τ when the collision force drops to zero).

(32)

(38)

The separation phase analysis is slightly more complicated.

We introduce the time variable τ = t – tc such that τ = 0 denotes the start of the separation phase. The start of separation conditions are not the start of closure conditions and rather are the terminus of closure collisions. Finally, the collision force at the start of separation is peak, rather than zero. Using the subscript c to denote the terminus of closure values, the solution for the time-domain displacement is:

Finally, the change in velocity for each collision partner, the closing velocity and separation velocity are defined in a standard manner (Carpenter and Welcher, 2001).

www.collisionpublishing.com

(39) (40)

Collision Magazine - Volume 10 Issue 1 87

Equation (40) can be written for the jth {j: j = 1, 2} collision partner as (where the subscript k denotes the opposing collision partner):

Also:

(50) (41)

F

urther simplifications In order to further simplify the forms of the solutions, we first employ the fact that the unloading stiffness of the first collision partner can be written as a simple real, not necessarily integer, constant of the loading stiffness of the same first collision partner. Denoting this constant value as α, equation (17) may be rewritten as:

(42)

Solving equation (50) at t = tc and substitution of the result in equation (49) yields the following result: (51) The left side of the equality can be rewritten fully in terms of the established input parameters by substitution for the peak deflection of the first collision partner from equation (42) and for the peak deflection of the second collision partner from equation (44).

Similarly, for the peak collision force magnitude: (43) Similarly, we may write the residual damage depth of the second collision partner as a real, not necessarily constant multiple, β, of the residual damage depth of the first collision partner. This allows equation (8) to be rewritten as: (44) Rewriting the loading and unloading stiffnesses for the second collision partner:

(52) Substitution of this result into equation (51) yields the following solution:

(45)

(46) The effective loading and unloading stiffnesses are: (47)

(53) Substitution for the loading circular frequency from equation (29) followed by algebraic simplification yields the following solution for the closing velocity: (54)

(48) For the closing velocity, we start with equation (28) and substitute t = tc. The operand of the sine function results in the value of the sine function being unity. The first term on the left side of each equality in the two equations is the same. As a result, we can generate a single equation by subtracting the second equation from the first.

The solution for the separation velocity is obtained by simply multiplying the closing velocity by the coefficient of restitution. The solution for the change in velocity of the jth collision partner is obtained by substitution of equation (54) into equation (41) followed by simplification.

(49)

88 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

(55)

W

orked example: Consider the simple case in which the following are known:

k1l = 2.41511 . 106 N/m k1u = 1.42848 . 107 N/m m1 = 1500 kg m2 = 950 kg δ1s = 1.25 . 10-1 m δ2s = 8.75 . 10-2 m The following can readily be calculated: α = 5.91476, β= 0.7, δ1d = 1.50 . 10-1 m, |Fcp| = 3.63 . 105 N and meff = 582 kg. The remaining parameters of interest are dependent upon the choice of ε. For this example, we consider the cases of ε = 0.3, 0.4, 0.5 and 0.6. This range of values is provided solely for the purpose of allowing the reader to visualize the changes in the output values. By no means is the use of such a range, herein, a suggestion that such a range should be employed for any specific reconstructive analysis. The results are shown in Tables 1-3. ε 0.3 0.4 0.5 0.6

δ2d (10-1 m) 1.06 1.26 1.56 2.04

k2l (105 N/m) 2.89 2.44 1.97 1.50

k2u (105 N/m) 16.3 8.01 4.47 2.62

Table 1: Peak deflection and stiffness parameters for the second collision partner. ε

0.3 0.4 0.5 0.6

keffl (105 N/m) 1.32 1.21 1.08 0.925

keffu (105 N/m) 14.6 7.59 4.33 2.57

ωl (sec-1)

ωlu (sec-1)

15.0 14.4 13.7 12.7

50.1 36.1 27.3 21.0

tc (10-1 sec) 1.04 1.09 1.15 1.24

τs (10-2 sec) 3.13 4.35 5.75 7.47

The relatively large magnitude of the unload stiffness, when compared to the loading stiffness, for the first collision partner, coupled with the same trend in the result for the second collision partner, manifests itself in the substantially greater, in magnitude, unloading phase circular frequency when compared to the loading phase circular frequency. In turn, this is manifested in the substantially shorter duration for the separation phase when compared with closure. If the initial velocity, with respect to the inertial frame of reference, for one of the collision partners is known, the time-parametric kinematic response of both collision partners can readily be ascertained. Setting the initial velocity of the second collision partner equal to zero and substituting the results in equations (28), (30), (31), (33), (36) and (37), for the case of ε = 0.3, results in the solutions shown in Figures 2-4.

Figure 2: Displacement-time response for the subject example with ε = 0.3. The closure phase solutions are shown in green (first collision partner) and orange (second collision partner).

The separation phase solutions are shown in red (first collision partner) and blue (second collision partner).

Table 2: Effective stiffnesses, circular frequencies, closure duration and separation duration. ε 0.3 0.4 0.5 0.6

vc (m/sec) 3.51 3.65 3.87 4.19

vs (m/sec) 1.05 1.46 1.93 4.19

∆v1 (m/sec) 1.77 1.98 2.25 2.60

∆v2 (m/sec) 2.79 3.13 3.55 4.10

Table 3: Closing velocity, separation velocity and change in velocity for each collision partner.

Figure 3: Velocity-time response for the subject example with ε = 0.3. The closure phase solutions are shown in green (first collision partner) and orange (second collision partner). The separation phase solutions are shown in red (first collision partner) and blue (second collision partner).

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 89

Figure 4: Acceleration-time response for the subject example with ε = 0.3. The closure phase solutions are shown in green (first collision partner) and orange (second collision partner).

The separation phase solutions are shown in red (first collision partner) and blue (second collision partner).

The shape of each response for the other cases mirrors that which is shown in Figures 2-4.

D

iscussion The accident reconstruction literature, especially the segment focused upon the determination of collision severity based upon the residual damage present to a motor vehicle following a collision, is replete with the treatment of each collision partner as a single degree of freedom coupled with a uniaxial load path. Clearly, methods of greater complexity, ranging from multiple degree of freedom (MDOF) modeling approaches to full scale finite element modeling implementations, exist and provide potentially increasing granularity depending upon the objectives of the analytic endeavor. Increasing model complexity, however, generally requires an increase in the number of system parameters that need to be quantified prior to implementation for any case under evaluation. It is certainly a plausible explanation that the typical quantities of interest such as the closing velocity and the change in velocity incurred by each collision partner, being limited in scope and determinable by less complex methods, drive the general usage towards simplicity.

Emori (1968), in one of the earlier papers published in the field of motor vehicle accident reconstruction, employed the SDOF approach coupled with structural load path responses that were uniaxial and linear during closure. The separation phase model, however, employed an initial linear region, in regards to the force-deflection response, which was infinite in slope. Physically, because of the time parametric nature of both the collision force and the structural deflection, the interpretation of this analytic approach is of a decreasing magnitude of collision force while 90 Collision Magazine - Volume 10 Issue 1

the structural deflection remains at its peak value. While this approach has been adopted by others (McHenry and McHenry, 1986; Prasad, 1991; McHenry and McHenry, 1997; Neptune, 1998), there exists no physical basis that serves as an explanatory mechanism for such a modeling approach. The unload response is indeed stiffer than the load phase response but cannot be infinite in magnitude. From the perspective of extant residual damage based methods, Campbell’s linear relationship between the depth of residual damage and the EBS is empirical (Campbell, 1974) while the CRASH3 model utilizes the linear modeling approach, but only considers the closure phase (National Highway Traffic Safety Administration, 1983).

The reformulation of the CRASH3 model (Prasad,1990) represents a simple change in the model coefficients, which can be directly equated to the coefficients of the Campbell and original CRASH3 models (McHenry and McHenry, 1999), and therefore retains the original CRASH3 modeling approach (Sharma et al., 2007). In order to ensure that the underlying dynamic model generates the linearity between EBS and residual damage depth, as per the Campbell model, or between peak collision force magnitude and residual damage depth, as per the CRASH3 model, the dynamic model must be ubiquitously and uniformly linear in regards to the closure phase response and in regards to the separation phase response, for collisions that exceed the elastic limit (Singh, 2013b). The linear-linear load-unload model clearly has its limitations (Singh, 2013c) but can be and is appropriate under the conditions in which the underlying actual response is linear.

A number of previously published works have developed much of the underlying theoretical framework in regards to the implementation of the uniaxial linear-linear loadunload structural response model for the SDOF approximation. The subject study, to the author’s knowledge, however, is the first in which the relevant equations are developed in a form that that is based upon the typical set of known, unknown and estimated parameters typically encountered within the context of the field accident reconstruction endeavor. That the system unknown parameters can be readily written in terms of the system known and estimated parameters is of little surprise and in this regard the derived equations idiomatically speak for themselves.

There are, however, a number of points of interest, worthy of note, as related to certain extant practices. The first is that the residual damage depth and peak deflection are related but differing quantities. A resultant of this fact, noted previously (Singh, 2013b), is that the time-parametric force-deflection response is not a shifted equivalent of the relationship between peak collision force magnitude as a function of residual damage depth. The former, for any given collision test, maps to a single point in regards to the latter. An associated result is that the determination of the

www.collisionpublishing.com

IWA for a collision partner with initially unknown stiffness parameters by the following relationship is thermodynamically incorrect (the B parameters are in reference to the CRASH3 model and the IWA is often referenced as the energy absorbed, damage energy or crush energy).

(56) The second point of note is that for this modeling approach, when the stiffness parameters of one collision partner and the coefficient of restitution are unknown, the estimation of one (i.e. both the loading and unloading stiffness or the coefficient of restitution) deterministically determines the other. Generally, estimates of an appropriate range of values for the coefficient of restitution, predicated upon the structural composition of the collision partners involved and the collision configuration (e.g. bumper-to bumper, underride, etc.) are easier to make and more defensible than estimates of the loading and unloading stiffness. Irrespective of this, however, is the inappropriate analytical approach of presupposing independence of both estimates.

In a similar vein, it is inappropriate to independently estimate the collision duration as both the duration of the closure and separation phases are deterministically related to the system parameters.

The final point is the consideration of the limitations of the subject work. The inherent limitations of the SDOF modeling approach coupled with the uniaxial linear-linear load-unload model hold for this work. Secondly, and deriving from the first, the equations derived are appropriate when the underlying modeling tenets hold and when both collision partners exhibit non-zero magnitude residual damage depths. For collisions in which the elastic limit of a collision partner is not exceeded, the unload response for that collision partner is non-linear when the loading response is linear. Thirdly, the equations derived do not account for net unbalanced externally applied forces such as effective braking. Finally, left for subsequent work, are the development of a set of guidelines for determining the appropriate methodology for determining the bounds of linearity in the structural response based upon controlled collision test data.

R

eferences: 1. Campbell K (1974) Energy basis for collision severity.

2.

Society of Automotive Engineers Technical Paper No. 740565.

Carpenter NJ and J Welcher (2001) Stiffness and crush energy analysis for vehicle collision and its relationship to barrier equivalent velocity (BEV). Society of Automotive Engineers Technical Paper No. 2001-01-0500.

3.

Emori R (1968) Analytical approach to automobile collisions. Society of Automotive Engineers Technical Paper No. 68016.

4. McHenry RR and BG McHenry (1986) A revised damage analysis procedure for the CRASH computer program. Society of Automotive Engineers Technical Paper No. 861894.

5. McHenry RR and BG McHenry (1997) Effects of restitution in the application of crush coefficients. Society of Automotive Engineers Technical Paper No. 970960.

6. McHenry RR and BG McHenry (1999) McHenry Accident Reconstruction (Rev. 2). Cary, North Carolina: McHenry Software, Inc. Pp. 71-72.

7. National Highway Traffic Safety Administration (1983) CRASH3 User’s Guide and Technical Manual. United States Department of Transportation. DOT HS 805 732.

NTIS Publication No. PB83112201.

8. Neptune JA (1998) Crush stiffness coefficients, restitution constants and a revision of CRASH3 & SMAC. Society of Automotive Engineers Technical Paper No. 980029.

9. Prasad A (1990) CRASH3 damage algorithm reformulation for front and rear collisions. Society of Automotive Engineers Technical Paper No. 900098.

10. Prasad A (1991) Coefficient of restitution of vehicle structures and its use in estimating the total Delta-V in automobile collisions. Crashworthiness and Occupant Protection in Transportation Systems, 1991: presented at the Winter Annual Meeting of the American Society of Mechanical Engineers, Atlanta, Georgia, December 1-6, 1991. AMD-Vol. 126/BED-Vol. 19, pp. 217-246.

11. Sharma D, S Stern, J Brophy and E-H Choi (2007) An overview of NHTSA’s crash reconstruction software WinSMASH. In: Proceedings, 20th International Technical Conference on the Enhanced Safety of Vehicles. Lyon, France, June 18-21, 2007. Paper No. 07-0211.

12. Singh J (2013a) Laplace domain analysis of the collinear vehicle-to-vehicle collision under linear force-deflection constraints coupled with net externally applied loads. In: Proceedings, 23rd International Conference on the Enhanced Safety of Vehicles. Seoul, Republic of Korea, May 27-30, 2013. Paper No. 13-0016.

13. Singh J (2013b) A fundamental reconsideration of the CRASH3 damage analysis algorithm: the case against uniform ubiquitous linearity between BEV, peak collision force magnitude and residual damage depth. Traffic Injury Prevention 14(7): 718-724.

14. Singh J (2013c) Further developments regarding the dynamic modeling of motor vehicle collision response using the SDOF approach. Collision 8(1): 10-31.

15. Singh J and NJ Carpenter (2013) Work-energy relationships for the collinear single degree of freedom impact model under the case of net unbalanced externally applied forces. Society of Automotive Engineers Technical Paper No. 2013-01-0794.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 91

ED W E I EV R l ia r o t Edi

d e s a B � o e d i V t n e d i n c o i c t A c u r t s n o a i c s e A R s n a r T of ight 235 Fl Adam Cybanski Gyro Flight and Safety Analysis

92 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

ideo cameras are everywhere in the modern world. Although sales of camcorders and photographic cameras (which include the ability to record video) have been decreasing, there have been massive increases in the number of other devices that can record video. GoPro camera sales have doubled every year since they were first released, and the number of cell phones and tablets (all with video recording capability) in the US has exceeded 300 million, greater than the population of the country. Car dashboard cameras are selling well, and their popularity continues to rise as price decreases. Consumers install these portable video cameras in their vehicles to record accidents, monitor their vehicles while away, capture road trips, and document employee driving.

The proliferation of these cameras is illustrated by the fact that the fatal crash of TransAsia Flight 235 was caught on three separate automobile dash cameras.

V

Many people do not realize the amount of useful data that can be derived from a short ten second video. With identifiable features, a video analyst can determine the speed, location, heading, acceleration / deceleration, rate of turn of the camera (and that of the vehicle it is mounted in) at a rate of up to 29 times per second. In addition, the same data can be obtained on other vehicles within the field of view of the camera. The accuracy of the derived data depends on the distance from the camera to objects of interest, but can match or exceed GPS accuracy. Whether captured from a camera mounted in a car, a drone flying overhead, or a pedestrian holding a cell phone, analysis of video can produce detailed and useful information for an accident reconstruction.

Video motion analysis is based on proven fundamentals of photogrammetry, matchmoving, geolocation, and timedistance speed measurement. Photogrammetry is the science of making measurements from photographs, and is employed to first determine the location and orientation of the camera based on references visible in the image, then to determine the location and orientation of other objects in the image, given the camera position. Matchmoving, also known as camera tracking, is the process of analyzing a video clip to determine camera motion in three dimensions, and that of moving visible objects in the scene.

Time-distance speed measurement has been used by law enforcement for decades, initially using reference objects a fixed distance apart, then using systems like the Visual Average Speed Computer And Recorder, and using aerial surveillance of ground features. Geolocation is the determination of real-world geographic coordinates of objects, and is conducted for all identifiable features in a video.

In recent years, new technologies have improved capabilities for analyzing video. Geographic Information Systems like Google Earth and Streetview make it quick and easy to identify landmarks from a video, then determine a pre-

cise latitude, longitude, and altitude for each point. The amount of cameras has dramatically increased the chances that an event will be caught on video. Software has been developed for the motion picture industry that facilitates the precise analysis of captured video, and computer hardware continues to improve, so that the large number of calculations required for photogrammetric video analysis can be carried out quickly and efficiently.

In recent years, this type of analysis has been used in aviation accident investigations. A Canadian F18 fighter aircraft crashed at an air show in Lethbridge, Alberta, and the video of the accident was analyzed in order to help determine the cause of the crash. Footage from pole-mounted ramp security cameras and onboard dash/cockpit cameras has been analyzed to reconstruct aircraft flight paths in support of several international accident investigations.

Recently, analysis of a handheld video in a helicopter accident revealed pertinent data for the investigation, including distances, speeds, accelerations, and other parameters to determine the detailed accident sequence, and was employed in a 3D visualization of the accident. The visualization matched the original captured video, further validating the analysis conducted.

In the crash of TransAsia 235, dash cam video from several automobiles was analyzed in order to determine the velocities, tracks and orientation of air and land vehicles.

Prominent features such as road markings, lamp posts, and corners of buildings were tracked in matchmoving software in order to calculate camera motion. These features were positively identified in Google Earth / Streetview, and their precise 3D coordinates were determined. This survey data was imported into the software, then the camera solution was oriented and scaled, producing a geolocated 3D point cloud of the surrounding environment. Prominent features of the aircraft and visible ground vehicles were also tracked and oriented/scaled from engineering measurements so that their motion was also part of the 3D scene. The calculated camera and object motions were analyzed to determine the actual positions, orientations and velocities of the vehicles. The derived data was validated by comparing it against vehicle performance information, through cross checks against reference vehicles, and by visualization, and showed that this analysis methodology can support or prove equivalent to current analysis methods using traditional data recorders..

Video analysis represents a new tool with significant potential for the modern traffic accident reconstructionist. It can provide precise, detailed information in cases where an EDR is not available, or can be used to augment EDR data. Video footage has traditionally been qualitative in nature, but the methodology described herein shows that it is also a good source of quantitative data for an investigation, making it invaluable for accident reconstruction.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 93

e Cas

on

ti olu

S

Three car, in-line crash analysis with CDR Data Rusty Haight Collision Safety Institute

Foundation for the Analysis Having reviewed the photos of the involved vehicles, it should be clear that the F-150 sustained front-to-rear damage, the Corolla sustained rear-to-front as well as front-to-rear damage and the RAV4 sustained rear-tofront damage. The severity of the rear-to-front damage on the Corolla is significantly greater (without assigning a specific magnitude to it at this point) than its frontto-rear damage. The damage to the rear of the RAV4 suggests something of an underride such that the front of the striking vehicle (we may conclude it is the Corolla not the F-150) went somewhat under the rear of the RAV4 and that there was an offset to the relative left side of the lane/involved vehicles. Correspondingly, the center and right front (from the driver’s perspective) of the Corolla exhibits what would be consistent with it having underrode the RAV4.

The damage observed on the front of the F-150 is largely to the center and right front of the truck at and above the bumper level. The damage to the rear of the Corolla is widely distributed across the width of the entire rear end and induced damage extends essentially to the rear doors.

The damage patterns, generally, are consistent with the most severe impact in the sequence being between the front of the F-150 and the rear of the Corolla and then the relatively less severe impact being between the front

of the Corolla underriding the rear of the RAV4. The damage is generally consistent with the description offered by the involved drivers in terms of their description of their order in the road in the relevant direction of travel. The first issue which might then be addressed then is: what is the order or sequence of the impacts and “who hit whom first?” To that end, the CDR Tool report might prove useful.

While a party or witness might have perceived or offered the recollection of a single or of multiple impacts, their observations or subjective impressions/beliefs may or may not fit with the objective information found on/ from the vehicles as a function of damage or in the CDR Tool report. Here, for this case study, the focus is on application of the CDR Tool report in a crash analysis and while not excluding a further analysis of the damage which might be made by the reader by review of additional photos or as a function of a hands-on vehicle exam, we turn our attention to the CDR Tool report.

CDR Tool Report Going into any analysis using CDR Tool retrieved/reported data, one of the first things one has to do - as it relates to the CDR data - is to figure out if all or part of the data is or is not associated with the crash/event being examined. Leading into that, of course, one would want to review the CDR Tool report’s Data Limitations text keeping in mind that the Data Limitations may not

Figure 1: CDR File Information 94 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

be only the source of information one might need in an evaluation. For example, for some systems, the CDR Program Help file might provide additional insight into nuances of a given system.

Having first contemplated the contents of the Data Limitations text found in the CDR report for the Corolla involved in this crash, we turn to the CDR File information data table on page 1 of the report and find that the report contains information associated with three “Front/Rear” events (Figure 1). In that regard, in the Data Limitations text we find these passages: “...The airbag ECU records data for all or some of the following accident types: frontal crash, rear crash, side crash, and rollover events. ... The airbag ECU has the following recording pages (memory maps) for each accident type to store event data: three pages for frontal or rear crash, ...”

There are other similar passages in the Data Limitations text consistent with these but, for the moment, by inference, we can deduce that the system is capable of differentiating between frontal and rear crashes and that the listing in the File Information data table indicates that the three recorded events are individually frontal or rear events and the term “front/rear” is not used to specify a direction in that context. That is to say, the designation “front/rear” should not be inferred to mean, for example, “front-to-rear” or “front-and-rear.” In this context, the use is indicating that the aribag control module’s EDR subcomponent has recorded event which are “front or rear” events and may then be recorded separately in each of the three memory pages available to the Event Data Recorder (EDR) subcomponent.

Next, we might look to the “Front/Rear Event Record Summary at Retrieval” data table (Figure 2) which lists the currently recorded and recovered events and identifies the number of times the module has experienced a “trigger (TRG)” threshold. A trigger threshold is analogous to what CDR Data Analysts have seen in CDR Tool reports for GM vehicles as “algorithm enable” or the point where the airbag control module (ACM) “wakes up” and begins running the deployment decision making process. As outlined in the 2013 CDR User’s

Summit presentation “Toyota EDR Accuracy - deltaV, Speed, etc.” (by Bob Anderson and Rusty Haight), the “trigger threshold” for Toyota systems such as that found in the Corolla here is 2g along the longitudinal axis.

While there has been some discussion within the community about a frontal negative longitudinal bias associated with Toyota modules, testing discussed in that presentation and again in a data review and presentation at the 2015 CDR User’s summit (same authors) shows that the observed 2g trigger threshold holds true for Toyota systems and that a calibration-based bias on individual systems may impact the reported delta-V (particularly in lower delta-V events) when compared to instrumentation which may be used in a crash test. Later research by others has also identified this same trend.

Specific to this instance, we find that the Toyota ACM has recorded three events in total (triggers or TRGs1, 2 and 3) and all are, consistent with the naming convention of the data table, identified as “front/rear” events however specific directionality/polarity (from the front as opposed to from the rear) isn’t found in Figure 2.

Even with the backdrop of an understanding of the damage to the involved vehicles in mind, those entries then don’t directly address the first question in the case problem: 1. What is the order of the impact(s)? Or, which vehicle hit which vehicle first? What information supports your conclusion(s)?

For this Toyota airbag control module (ACM), the order of the recorded events is identified by two data elements: (1) trigger (TRG) count and (2) at least partly by way of a chronological naming convention descriptor (i.e.: “Most Recent” or “First Prior” event). This is also addressed in the Data Limitations text in the passage: “... The data recorded by the airbag ECU in the event of a frontal/rear crash includes information that indicates the sequence and interval of each previously-occurring frontal/rear crash event.

Figure 2: Front/Rear Event Record Summary at Retrieval www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 95

- Time from Previous TRG - TRG Count In this particular report, we find at Figure 2, there were three recorded “front/rear” events retrieved during imaging. They are identified as identified as the “Most Recent” (TRG 3), preceded by the “First Prior” (TRG 2) and a “Prior” event (TRG 1). In Figure 2 we also see a time between events indicated simply as “Time” but then in the “System Status at Event...” data table for each event (example as seen in Figure 3 for TRG3), we find the parameter “Time From Previous TRG (msec).”

This latter parameter, as described in the Data Limitations text, tells us that for the parameter: “... “Time from Previous TRG”, the recording trigger of side crash and rollover is not considered. The upper limit for the recorded value is 5000 [ms] or 5120 [ms] for ECUs with 1.024 data sampling intervals. ...”

Given the information found in Figure 2 and the corresponding values in each record’s “System Status at Event...” data table (like that seen as Figure 3) we can say that the time between TRG2 and TRG3 was 1,120 (msec) which is also found as -1120 (ms) in the “Front/ Rear Event Record Summary at Retrieval” data table (Figure 2).

At this point, we can conclude, for a variety of noted reasons, that TRG2 preceded TRG3 by 1.12sec but this doesn’t necessarily address TRG1 and it’s time relativity to TRG 2. For this analysis, we might turn to the “System Status at Event (1st Prior Frontal/Rear Event, TRG 2)” data table and specifically the parameter “Time From Previous TRG (msec)” as well as a description of that element from the Data Limitations text which reads, as previously noted: “... For “Time from Previous TRG”, the recording trigger of side crash and rollover is not considered. The upper limit for the recorded value is 5000 [ms] or 5120 [ms] for ECUs with 1.024 data

sampling intervals. ...” and add to that this passage from the Data Limitations text: “... “Prior Event” is the event that occurred before the “1st Prior Event” that reached the greatest MAX Delta-V. Therefore, “Prior Event” is not always the prior event of “1st Prior Event”. ...”

Undeniably, the latter passage isn’t, at first blush, the most artfully worded section to be found in the Data Limitations text and the inherent ambiguity has been abused elsewhere by way of interpretation or misrepresentation; however, as a function of testing conducted prior to and during the 2015 CDR User’s Summit, and as part of the crash test data review conducted at the CDR User’s Summit by the author (and co-author Bob Anderson to be published in an upcoming issue of Collision after review), a “Prior” event is not necessarily the last event with the greatest absolute value delta-V. That presentation and testing from the Summit demonstrated that a “Prior” event is never the immediate “First Prior” event regardless of severity. It may be a greater or lesser severity magnitude event (in terms of the absolute value of the delta-V) which would/may have overwritten a previously recorded event by virtue of polarity along the longitudinal axis being a more significant factor than simple the magnitude of the delta-V. Then, when we include the time between recorded events, here specifically the “First Prior” and “Prior” event, and see it is greater than, in this case 5000ms (5sec), we find that “5000 or greater” is displayed for the parameter “Time From Previous TRG (msec).”

Since there are no other TRGs to be accounted for (we see that TRGs1, 2 and 3 are all recorded and reported), the logical conclusion would be that TRG1, IF it occurred on the current/same ignition cycle as TRGs2 and 3, it occurred more than 5 seconds (5000ms) before TRG2.

Figure 3: System Status at Event (TRG3) 96 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

In summary at this point, we know that there have been three TRGs and that TRG1 precedes TRG2 by more than 5seconds. We know that TRG2 precedes TRG3 by 1.12sec. To attempt to associate any or all of these events with the crash series currently being analyzed, we might next look at the polarity of the events.

Referring back to the combined data pages from the original CDR Tool report found earlier in this issue of Collision, TRG1 was a product of the vehicle/module reaching the trigger (“wake up”) threshold and recording a -5.3mph (-8.6km/h) delta-V. This then was a longitudinal front-to-rear event and we recognize the deltaV might be somewhat/slightly under reported given its magnitude as previously discussed.

TRG2 was a product of the vehicle/module reaching the trigger (“wake up” threshold) and “observing” a +22.0mph (+35.4km/h) delta-V. This was a longitudinal rear-to-front event. Then 1.12sec later, TRG3 was a product of the vehicle/module reaching the trigger (“wake up”) threshold and “observing” a -7.9mph (-12.7km/h) delta-V. This then was a longitudinal front-to-rear event.

At first blush, one might now conclude that the Corolla first rearended the RAV4 (TRG1) then was rearended by the F-150 (TRG2) and pushed back into the RAV4 (TRG3). If we discount the accounts of the events given by the drivers of one impact by the F-150 to the rear of the Corolla followed by one impact between the front of the Corolla and rear of the RAV4, that begs two

questions: (1) is the observed damage to the involved vehicles consistent with this scenario and (2) is the remainder of the data found in the CDR Tool report consistent with this scenario?

The damage to the front of the Corolla and rear of the RAV4 could be “overlapping” such that the slightly less severe event (TRG1) could have preceded a second impact with the RAV4 (TRG3) after the Corolla was hit by the F-150 (TRG2); however, that would only fit in a scenario where there was more than 5 seconds between TRG1 and TRG2. That potential then begs a comparison of the pre-crash data recorded for the recorded events (TRG1 versus TRG2 and TRG3) and perhaps the potential closing speed necessary for TRGs1 and 3 to have occurred in that way; meaning are the delta-Vs for TRGs1 and 3 (the front-to-rear events which might be attributed to hitting the RAV4) consistent given the relative weights of the Corolla and RAV4 in light of, for example, the pre-crash data?

The pre-crash data for all three TRGs may be reviewed in the larger offering of the CDR Tool report found earlier in this issue of Collision; however, for this part of the analysis, we’ll examine the pre-crash data for TRG1 as Figure 5, and that data for TRGs2 and 3 as Figures 6 and 7 respectively.

In Figure 5 (TRG1) we see the Corolla at a stop (speed=0) about 4+ seconds before the TRG1 “time zero.” We see the Corolla, in that event, accelerating to a speed of about 12.4mph (20km/h) very close to “time

Figure 4: System Status at Event (TRG2)

Figure 5: Pre-Crash Data (TRG1) www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 97

0.” If TRG1 were to be an initial contact between the Corolla where it rearended the RAV4 at about 12mph and sustained then a -5.3mph delta-V, using the equation for closing velocity in an in-line collision (Eq 1), the Corolla would have to be overtaking the RAV4 by about 8mph (13km/h). At this point, we can see that for the pre-crash data from TRG1 to be a product of the Corolla rearending the RAV4, the RAV4 would then have to be moving forward at about 4mph (4mph where the overtaking speed is 8mph for an impact speed of about 12mph).

(Eq 1) This clearly then might be a possibility. It would suggest then that the RAV4 and Corolla were stopped in traffic and then the RAV4 pulled out followed by the Corolla which accelerated to roughly 12mph then rearended the RAV4 which was moving at about 4mph.

But then there’s the time between events. Keeping in mind that the time between TRG1 and 2 is greater than 5sec, we might consider what’s reflected in the pre-crash data for undeniably close in time related TRGs2 and 3 in light of what we find in TRG1. In the pre-crash data for TRG2 (Figure 6), we see the Corolla slowing from 19.9mph (32km/h) to about 2.5 mph (4km/h) when, as we know from the delta-V, it is rearended by the F-150 and a +22.0mph (+35.4km/h) delta-V is recorded.

Relative then to TRG2, the overlapping and extended pre-crash data seen for TRG3 in Figure 7 shows the Corolla slowing to 2.5mph as seen in Figure 6 then it being accelerated to at least 16mph (26km/h) to “time 0” in TRG3 which, given the delta-V of -7.9mph (-12.7km/ h) is where the Corolla rearends the RAV4. Notably, the brake switch in between TRG2 and TRG3 shows “ON.” Given what we’re seeing in the pre-crash data, for TRG1 to have been the Corolla hitting the RAV4 first, that collision would have had to have taken place and then the drivers would have to resume traveling along the road for greater than 5 seconds before the Corolla was

rearended by the F-150 and pushed into the RAV4. In theory, of course, this is possible.

Imagine a situation where the Corolla rearends the slower moving RAV4 at, for example, 8:30am. The drivers stop to exchange insurance information and then resume driving and the Corolla is then hit from behind, for example at 9:00am, by the F-150 and we see TRG2 and 3 recorded. The first problem with this scenario; however, is that none of the involved drivers assert this scenario took place. Neither of the drivers of the Corolla or RAV4 report or claim a crash occurred before the Corolla was hit from behind by the F-150 (editorially, we might wonder if their interviews may have taken place before they learned how the crash “really happened” as might be explained to them later by a ‘counselor’ of some sort...).

But one has to wonder what more information might be available to the analyst looking to more fully understand the crash and move beyond the threshold question: does this data belong to this crash? Here, we have to establish if TRG1 is related to the crash sequence at hand...or not.

There are sources available to public and private analysts alike including such on-line services as CarFax (both the public and law enforcement versions) and Autocheck which might provide insight regarding a vehicle’s history particularly including previous reported crashes.

For insurance professionals, checking claims/claimant’s records with services such as Insurance Services Office (ISO) is a commonplace investigative step. Here, as it turns out, the Corolla in this instance was actually involved in a wholly separate crash some two and a half years before the crash which serves as the basis for this case study. In that case, the Corolla driver pulled from a stop after the car in front of it left the stop sign but then rearended the car which had left the stop ahead of the Corolla after it suddenly stopped a second time: a description consistent with the reported -5.3mph deltaV for TRG1.

Surely this detail might have been disclosed to the reader earlier in this analysis; however, from a practical application perspective and given the type and severity

Figure 6: Pre-Crash Data (TRG2) 98 Collision Magazine - Volume 10 Issue 1

www.collisionpublishing.com

of the previous crash TRG1 (not to mention the value of the observation to some that the original event had remained recorded in this module/vehicle for some 30 months since it occurred) and its potential for leading one to at least theorize TRG1 was in some way associated to the crash which serves as the basis for this analysis, not offering that earlier in this analysis reinforces the analytical process as outlined which would/should be applied in virtually all such instances/cases.

So now, in answer to the questions posed (1. What is the order of the impact(s)? Or, which vehicle hit which vehicle first? What information supports your conclusion(s)?), we have ascertained that...

The event observed in the report as TRG1 is unrelated and the order of the impacts associated with the three vehicles described in this case study are TRG2 (F-150 into the rear of the Corolla) and then TRG3 (Corolla into the rear of the RAV4). This, of course, also addresses the core question for every application of CDR Tool report data regardless of vehicle manufacturer: “is the data I’m looking at from the crash I’m analyzing?” Here, we can say that two of the recorded events are from the current crash sequence but the third is not. This allows us to move to the remaining questions posed: 2. When the F-150 hits the Corolla, what is the closing speed of the F-150?

Using the same equation (Eq 1) suggested for the previous analysis and the weights of the F-150 and Corolla as adopted for this case study, the closing or overtaking speed of the F-150 would have been about 31mph (49.5km/h) (where restitution was assumed to be 0.1).

Given the damage observed to the rear of the Corolla, the restitution might be lower (say, for example 0.5) which would mean the closing speed could be as high as 32mph (52km/h).

3. When the F-150 hits the Corolla, what is the speed of the Corolla?

Using the Toyota’s pre-crash data for TRG2 (Figure 6), the Toyota was moving slowly at the time the F-150 rearended it. The data closes to “time zero” for TRG2 suggests the Corolla was moving at approximately 2-3mph

(4km/h). That is also then consistent with the pre-crash data seen for TRG3 (Figure 7) at the “-2 count” point.

Notably here, we find another attribute of the data we might stop and consider. In Figures 5 and 6, we see the pre-crash data associated with time increments relative to “TRG” or “time 0.” In Figure 7, instead of “relative time,” we see an indication of a “count.” In the CDR Tool report Data Limitations, we find this passage relative to pre-crash data: “... If 2 or more frontal/rear events occur successively within a period of 5000ms (or 5120ms for ECUs with 1.024 data sampling intervals), the actual sample time before the trigger is not displayed for subsequent events.

The sample time before trigger will only be displayed for the first event of the successive events. For subsequent events (i.e second event or later events), the pre-crash “Time (sec)” data is replaced by integers -5 through -1 and the heading “Time (sec)” is replaced with “Sample Count.” The time between “Sample Count” integers (-5 through -1) cannot be determined. The time between the last integer and TRG cannot be determined. ...”

On one hand, this is consistent with the previous observation that TRG1 is unrelated to TRGs2 and 3 because time is displayed for TRG2’s pre-crash data instead of a count (and we know there was more than 5000ms between TRG1 and TRG2 anyway). On the other hand, it also explains why the report form element for the precrash data seen in Figures 6 and 7 are different for the time/count parameters.

In a review of information relative to this characteristic of Toyota data from the Collision Safety Institute CDR Data Analyst’s course, we find that a Toyota ACM of this vintage is essentially buffering pre-crash data at about 0.1sec (100ms) intervals. The most recently observed pre-crash samples before the module “feels” an event which meets trigger threshold is, when recorded, then labeled as the “0 (TRG)” sample (memory page) indicating it was the last observed within the last 0.1sec. The next older sample is one which was observed less than 1 second before trigger threshold was reached but some time older than the one which is labeled “0(TRG).”

Figure 7: Pre-Crash Data (TRG3) www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 99

For example, in Figure 5, the first sample in the precrash data older than the one at “0(TRG)” is reported as “-0.1” which means it is greater then 100 but less than 200ms older than the sample offered as the “0(TRG)” sample. When that next older sample (the one in Figure 5 identified as “-0.1”) “ages” to 1000ms (1sec), it becomes the sample then 1 second older than the next 100ms sample identified as the one older than the sample at “0(TRG).”

So, in Figure 6, we see a sample set for pre-crash data labeled “0(TRG)” for “Time(sec)” as described in the Data Limitations text. Then we see a set of data marked as the set at “-0.8” (seconds). That data is 800 but not 900ms older than the observed data identified as at “0(TRG).” Older than that, we see a set of data which is identified as “-1.8(sec).” that data is one second older than the “-0.8” data set. Moving “back in time” in the data table, each next older set is one second (1000ms) older.

What the Data Limitations text is then telling us is that the ACM’s EDR component can determine/report the relative time of the sample older than the one at “0(TRG)” in the first occurring event in a sequence where more than one event occurs in a 5second period and then the relative age of each older sample where each older sample is then 1second older. It can’t determine, for sequential events occurring after that first event, what the relative time was for the next older sample set. For that reason, the parameter “time” as seen in TRG2 (Figure 6) becomes “sample count” in TRG3 (Figure 7).

with an understanding that, leading into TRG3, there appears to have been some braking.

4. When the F-150 hits the Corolla, what is the impact speed of the F-150?

Notice this is a different question than #2 above which addressed closing speed. To get to impact speed here (having addressed the pre-crash data in the previous question), we need to have first addressed closing speed.

Inasmuch as the pre-crash speed of the Corolla when it was rearended was about 2.5mph and the previously calculated closing speed of the F-150 was about 31mph, the over-the-ground or “impact” speed of the truck would have been about 33 to 34mph (31+2.5mph=33 to 34mph). It could even be slightly higher given the range applied for restitution as seen above.

It “would have been nice” to have the data from the F-150’s Powertrain Control Module (PCM) for comparison; however, as noted, the truck was left running after the crash where there were no deployments for the F-150 and no one, on the scene, had a CDR Tool with the PCM adapter assuming the truck were to have been shut off immediately after the crash sequence.

5. What is the delta-V for the Corolla when it is hit by the F-150?

The CDR Tool report tells us in the “Longitudinal Crash Pulse (1st Prior Frontal/Rear Event, TRG 2 - table 1 of 2)” that the longitudinal delta-V experienced by the Toyota being rearended by the F-150 was +22mph (+35.4km/h).

Given the function of the ACM as seen in Figure 6 and described in the Data Limitations text, we can deduce that the time between the “counts” identified in Figure 7 as “-1 count” and “-2 count” (ah ah ah...) would be one second and the time between “-1count” and “0(TRG)” in TRG3 is the undetermined value but we can conclude it is less than 1000ms (1sec).

The notion that there is a “bias” in the accelerometers or calibration of a Toyota module is effectively irrelevant here given the magnitude of the delta-V. In the final analysis, if the Corolla was moving at about 2-3mph when hit by the F-150, it left that crash at 24-25mph heading for the RAV4 which leads us to the next questions...

Since TRG2 would start the clock for the time between TRG2 and TRG3, we can conclude that the data observed at “0(TRG)” for TRG3 is captured about 1.21.3sec after the values observed at “0(TRG)” for TRG2 (with a little rounding given the parameters found in the Data Limitations text and allowing for event duration). That would be consistent with the “-1count” data for TRG3 to be captured a second after that captured at “-2count” and for the time between these events to fit (at about 1.12sec).

6. When the F-150 hits the Corolla, approximately how far is the Corolla away from (behind) the RAV4?

In short, there isn’t an inconsistency with the pre-crash data lacking a reported time increment for the pre-crash data in TRG3 and given prior testing by many in the industry, we can rely on the speeds reported by this ACM 100 Collision Magazine - Volume 10 Issue 1

And...

7. What is closing speed of the Corolla relative to the RAV4?

From a practical perspective, these two questions feed into one another taken together with question #5 above.

We know the time between the events (1.12sec) and now we want to know the distance between them. If we may assume there was a constant speed for that period, we could simply calculate the distance as speed times time = distance. But here, we have several things going

www.collisionpublishing.com

on which describe, in one way or another, the speed of the Corolla between the F-150 rearender and the time it hits the RAV4. They are: (1) the initial speed and (plus) delta-V for the Corolla, (2) the reported pre-crash speed from TRG3 and speed reported at “0(TRG)” and (3) a calculated closing speed given the delta-V experienced hitting the RAV4.

39feet behind the RAV4 when struck by the F-150. It is more likely that, at 2-3mph, the Corolla was about 2125ft behind the RAV4 and the RAv4 was moving along at about the same speed while the F-150 was overtaking these slower moving cars at some 33-34mph.

If we adopt the initial speed of the Corolla at 2-3mph and then the delta-V of about 22mph, it means that, immediately after being rearended, the Corolla was moving at about 24-25mph (question #5). Assuming that remains constant for the 1.12sec, that means the Corolla was as much as 39 feet (11.8m) behind the RAV4 when the F-150 hit it. This would be, given the remaining data available, the furthest the Corolla would have been behind the RAV4.

Where, in a given crash, there may be limited at-scene data or vehicle documentation beyond photos for the Analyst to work from, the contents of a CDR Tool Report may serve as an excellent source of information to “fill in the blanks” and ultimately help the Analyst better evaluate the crash in a more complete way.

If; however, we adopt the observed speed closest to “time 0” in TRG3, where it suggests that the Corolla was moving at about 16.2mph (26km/h) at that point and assume that as a constant speed for the 1.12sec between events, that means that the Corolla was about 25ft (7.9m) behind the RAV4 when it was rearended while moving at 2-3mph.

On the other hand, if we take the relative weights of the vehicles, delta-V for the Corolla as a function of the impact with the RAV4 (-8mph or -13km/h) and assume a 0.1 restitution, that would put the closing speed for the Corolla as it rearended the RAV4 at something on the order of 13mph (21km/h). That would mean, again adopting the 13mph is constant for the time between events, that the distance between the rearender from the F-150 and the RAV4 would have been about 21ft (6.4m). This would represent the least distance between the Corolla moving at 2.5mph and the RAV4 in front of it.

The distance calculations thus far assume that the RAV4 is stopped and we’re “measuring” to that fixed point but we shouldn’t lose sight of the idea that the closing speed calculation is not a calculation of an impact speed where the target vehicle is moving at impact - in this case likely moving away from the Corolla.

If we adopt, for the sake of the discussion, that the RAV4 was moving at a speed which roughly corresponded to that of the Corolla when it was struck by the F-150 (say 2-3mph) and the calculated closing speed of the Corolla at the RAV4 would be about 13mph, then the Corolla’s speed over the ground - its impact speed - would have been closer to 16mph as reported for the “0(TRG)” value in TRG3.

At the end of the day, given the data available it would be unreasonable to conclude that the Corolla was some

Conclusions

This case study should be seen to reinforce certain related concepts including...

1. Data elements found in a CDR Tool report are often not “stand alone” elements; often an element or parameter found in one part of a report should be considered in light of a corresponding, similar or complimentary element found elsewhere in a report. Context is everything in these reports and that eliminates working in a vacuum; don’t focus on one element or parameter as the answer to all your questions.

2. The information found in the Data Limitations text can be invaluable in terms of fully understanding the content of and meaning of data elements/parameters found in the report. While the importance of the Data Limitations can’t be understated, careful/thoughtful interpretation of passages is often required against the backdrop of other published studies to fully grasp its meaning.

3. The Analyst has to start by asking the question: “is this data from the crash I’m working on?” Sometimes the answer is straightforward but there are times, such as in this example, that additional analysis or even some out side sources of information may be necessary to fully address the question.

4. Competently evaluating a crash - even with the inclusions of CDR Tool report data - is more than getting the impact speed. There’s more to these reports and a thorough, competent analysis has to take into account more than “just” how fast the one driver might have been going. Appropriately using CDR Tool report data isn’t always as simple as looking at the “pre-crash data” and stopping there. The notion that we can somehow decide early on “what’s most important to me” in an analysis and be restrained by that early and often limiting decision is based on flawed logic. One must consider every reported aspect of the data available whether it’s from the Human, Vehicle and Environment or from the CDR Tool report to properly evaluate a collision.

www.collisionpublishing.com

Collision Magazine - Volume 10 Issue 1 101

Crash site evidenCe

A Peterbilt 2-axle tractor pulling double belly-dump trailers uphill over a narrow bridge crossed the centerline in front of an approaching Ford sedan. Both drivers reacted, but impact occurred between the left, rear tractor drive wheels and the left, rear corner of the Ford sedan.

Expert AutoStats®

4N6XPRT BioMeknx™

Expert AutoStats® is a computer program containing over 42,000 vehicles cars, vans, utility vehicles, and light pickups. 1945-2012 model years are represented.

This program can be used alone or with our Expert Qwic Calcs® and Expert VIN DeCoder® programs. Also accessible from within REC-TEC.

(Lap top & D esktop W indow s 3 2 & 64 -bit O S C om p atible)

Enter:

M anufacturer and Year

Select:

M odel from pop-up window

Output:

Wheelbase Overall Length Height Wt. Distrib

Curb Weight Width Drive Wheels Front/Rear Track

M id-60's to present output also includes (when available): Front Overhang Rear Overhang Bumper Heights Hood Height Turning circle Wheel radius Bumper-to-Hood Bumper-to-Window

For a full reconstruction of this 8 second crash sequence, visit www.edccorp.com/wheelimpact

HVE directly answers the following questions using Newton’s laws of motion:

Questions

Answers

Impact Speed

Peterbilt: 54 – 59 mph Ford: 50 – 57 mph

Available Reaction Time Before Impact

Peterbilt: > 4 sec Ford: 2.3 sec

Peak Impact Force

Total Fx = -36,100 lb Fy = 44,000 lb Fz = 12,700 lb At Wheel Fx = -13,400 lb Fy = 7,300 lb Fz = 6,900 lb

Peak Impact Acceleration

Peterbilt: Acc Total = 3 – 4 g Ford: Acc Total = 13–18 g

5. Information was obtained by review of over 65 scientific texts, several of which are now "out of print"

www.4n6xprt.com/4n6bm.htm

PROGRAM ORDER FORM (Please Print)

Required Technologies

SIMON 3-D Vehicle Dynamics Simulation Model DyMESH 3-D Collision Model, Version 3 HVE 3-D Terrain Model

Expert AutoStats® 4N6XPRT BioM eknx™ Expert Qwic Calcs® 4N6XPRT Ped & Bike Calcs® 4N6XPRT StifCalcs® Expert VIN DeCoder® (pkg) Expert TireStuf® Order Processing

- $ 625.00 ea.

- $ 495.00 ea.

- $ 275.00 ea.

- $ 375.00 ea.

- $ 650.00 ea.

- $ 550.00 ea.

- $ 85.00 ea.

= ________ = ________ = ________ = ________ = ________ = ________ = ________

- Check = $5.00 / Credit Card = $10.00 / G ovt. P.O . = $15.00 = _______ Notarized Affidavit Requirement $25.00 per required Notarized Signature = _______

Q Deliver via electronic download (Email address Required) Q Ship on USB - $35.00 per disk per program

Other software may claim to answer these questions. Have you closely evaluated their claims and the basis for their results?

California Sales Tax Enclosed is:

- 8.50%

TOTAL (U.S. Dollars)

= 0.00 = ______

= ________

= ________

Visa ___ MasterCard ___ Am. Ex. ___ Exp ____/____ - Check ___ - Gvt PO ___

Card Number: Name on Card: Signature:

_______________________________________________ _______________________________________________ ________________________________________________

Please M ake Checks*/Purchase Orders Payable to:

4N6XPRT Systems®

To learn more about HVE and all of the available tools for crash investigators, call 888-768-6216 or visit www.edccorp.com. Our professional sales staff will help you get started using HVE today!

8387 University Ave - La M esa, CA 91942-9342 - U.S.A. FAX: (619) 464-2206 Phone: (619) 464-3478 Web: http://www.4n6xprt.com

Knowledge = Power COL-1112

www.4n6xprt.com/4n6as.htm

The program includes: 1. Over 65 printable illustrations and charts 2. 10 Separate categories or sections of information related to forensic biomechanics 3. Information on walking speeds of people from 17 months to > 65 years 4. Calculation modules for: - segment length from total body height, - segment weights and Centers of M ass (CoM ) based on total body weight, - Predicted M aximum Walking Speed based on total height

Contact Name: ________________________________________________ Company: ____________________________________________________ Street: ______________________________________________________ City:____________________ State:____ Zip:__ __ Country: __________ Phone: (_____) _______________ Fax: (_____) ___________________ E-M ail: ______________________________________________________

Peterbilt: Total Fx = - 43,000 lb Fy = 39,400 lb Fz = - 400 lb At Wheel Fx = -23,000 lb Fy = 20,900 lb Fz = -6,200 lb Ford:

Expert AutoStats® is currently owned by over 700 private individuals and 300 Law Enforcement agencies in the U.S.A., Canada, and South America. Additional information, brochure, and order from can be downloaded from:

4N6XPRT BioMeknx is designed as a collection of the Biomechanical data of importance to the Accident Investigator into one easily accessible reference location. It is designed as a program to be useful to ALL accident investigators, not just the Biomechanical expert.

Engineering Dynamics Corporation • 8625 SW Cascade Blvd., Suite 200 Beaverton, OR 97008 USA • Phone 888.768.6216 • www.edccorp.com

C hecks * M U ST * be draw n on a U.S. bank. O rders are shipp ed within 10 working days of receipt.

Prices subject to cha ng e without notice.

M ultip le orders and Package discounts availab le. Please call for details.

2015 CDR SUMMIT DVD

C

2015 Vol.10 Iss.1

ollision C The International Compendium for Crash Research

Volume 10 Issue 1

Collision: The International Compendium for Crash Research Volume 10, Issue 1 - SPRING 2015

collisionmagazine.com

Spring 2015