Tunnel Vision

In the first of a two-part feature, UK physiotherapist Bianca Broadbent and physiologist Barney Wainwright — who are both bike fitters — examine an individual approach to optimising aerodynamics.

It’s well understood that the speed, and performance, in cycle time trialling and the cycle leg of non-drafting triathlon is determined by the resistive drag forces experienced and the mechanical power output created by the cyclist.

For this reason, aerodynamic optimisation is becoming increasingly important within cycling, with technological advances targeting equipment and materials to reduce aerodynamic drag (El Helou et al., 2010).

The aerodynamic drag forces can account for up to 90% of the total resistive forces in the system depending upon the relative air speed and surface gradient, with rolling resistance and bearing resistance comprising the remaining resistive forces to overcome (Martin et al., 1998; Kyle & Burke, 1984).

Of the drag forces generated, the bike alone accounts for approximately 20% with the body responsible for the remaining 80% (Crouch et al., 2017).

Therefore, to maximise performance in most cycling events, attention should be given primarily to reducing the aerodynamic drag caused by the body itself.

The aerodynamic drag forces in cycling are determined by the frontal area of the cyclist and bike (A) as well as the drag coefficient (CD). CD represents the disruption to the air that is caused by an object travelling through it.

A slender classically tear-dropped object creating less air disruption (small CD) than a blunt object such as a flat plate (high CD).

In the case of cycling, both the A and CD can be manipulated through positional changes and changes to equipment such as clothing and helmets.

The predominant aerodynamic resistance in cycling is due to pressure drag (Crouch et al, 2017), and as a result positional changes focussed on reducing CD can be more effective than changes to A, especially at high speeds.

WIND TUNNELS

Wind tunnel measures of aerodynamic drag in cycling, normally represented as the coefficient of drag area (CDA), are widely accepted as the reference measurement method (Debraux et al., 2011), with modern cycling-specific wind tunnels offering the ability to measure pedalling cyclists over a wide range of speeds and yaw angles to match the event-specific air speed conditions.

Because wind tunnels take direct measures of drag forces with high precision sensors and provide well controlled environmental conditions, they are highly sensitive to small changes in both A and CD, (Defraeye et al., 2010) and therefore, offer considerable advantages, especially in terms of measurement precision and sensitivity, over systems that only measure changes in A.

Other methods are available to measure CDA (Debraux et al., 2011), including recent on-bike pitot tube devices, but these have significant disadvantages and limitations that may affect the efficacy of positional interventions and changes, or make positional adjustments impractical.

INDIVIDUAL OPTIMISATION

From our practical experience, it is clear that there are few strategies for reducing aerodynamic drag that apply equally to all cyclists. Individual differences in position, anthropometry, functional abilities, relative air directions and speed, and the interaction of various equipment with the body make optimising aerodynamic drag a highly individual process.

The overall aims of this article are to provide insight to bike fitters, biomechanists and human performance specialists alike to illustrate some of the considerations for aerodynamic testing.

The case study presented below attended the Boardman Performance Centre for an “AeroFit” which encompasses both biomechanical and aerodynamic optimisation.

The biomechanical assessment seeks to identify functional limitations to position and opportunities to optimise power output, as well as direct and inform the possibility of viable aerodynamic changes.

It should be noted that the case study is presented to provide a snapshot of the process and key changes made within the session, and does not describe all iterations and positions tested.

We usually recommend that the rider return if this is their first attempt at positional optimisation to facilitate sufficient positional adaptation.

CASE STUDY A

Gender: Female

Age group: 30-39 Event: Track Pursuit

Issues: None reported

Priorities: Aerodynamics and power

Shoulder flexibility: Good

Hip flexibility: Good

Hamstring flexibility: Good

Biomechanical intervention: Raising the front end by 40mm resulted in significant increase in power. Adjusting saddle to a slightly nose down position helped improve saddle pressure. Different arm rest pads could help improve proprioceptive feedback and stability. UCI regs restricted further changes to saddle or bar fore-aft.

Baseline CDA was 0.2772 at 38.9km.h-1 with an estimated power requirement of 268W.

Set up 1: Baseline

Sustainable race position

Set up 2: "Forced"/tucked position

Power required compared to baseline: -30w

Predicted time saving in IP: 5.4s

This position is currently not sustainable in racing

Set up 4: Cockpit raised 40mm

Power required compared to baseline: -33w

Predicted time saving in IP: 5.9s

Note that this was adopting a sustainable racing position

Set up 5: Aerocoach Align Wing arm rests

Power required compared to baseline: -36w

Predicted time saving in IP: 6.6s

Swapped in for initial pads. Facilitated more relaxed shoulders

Set up 6: Hands together

Power required compared to baseline: -41w

Predicted time saving in IP: 7.6s

Extensions rotated inwards.

Set up 7: Saddle aligned down

Power required compared to baseline: -44w

Predicted time saving in IP: 8.0s

Improved comfort and altered lower back profile.

Set up 8: Medium Kask Mistral helmet

Power required compared to baseline: -37w

Predicted time saving in IP: 6.7s

Medium size helmet created a 7w increase in power required.

The traditional approach to aerodynamics would not suggest that a 40mm increase in front end height would lead to a 33W saving over baseline. And a virtual wind tunnel calculating frontal area would suggest it was slower. But this change allowed the rider to sustain a tucked position more easily, which greatly changed the Cd component of drag. This had a large enough impact to more than make up for any increase in A.

CASE STUDY B

Gender: Male

Age group: 40-49

Event: Ironman triathlon

Issues: Shoulder and neck discomfort

Priorities: Comfort and Sustainability

Shoulder flexibility: Poor with some bony restriction

Hip flexibility: Good

Hamstring flexibility: Sufficient

Set up 1: Baseline

Unsustainable race position

Set up 2: Saddle lowered 5mm

Power compared to baseline: -1w

Predicted IM time saving: 15s

Behind saddle bottles removed to facilitate lower saddle position

Set up 3: Saddle lowered 5mm

Power required compared to baseline: 1W

Predicted time saving in IM bike leg: 17s

Total of 10mm lower. Increased comfort for little aerodynamic loss

Set up 4: Pads 20mm wider and flared

Power required compared to baseline: -17W

Predicted time saving in IM bike leg: 214s

Improved shoulder comfort and facilitated a lower head position

Set up 5: Pads 20mm wider

Power required compared to baseline: -14W

Predicted time saving in IM bike leg: 166s

A small increase in power requirement (+3W), but a further increase in comfort.

Set up 6: Higher hands

Power required compared to baseline: -17W

Predicted time saving in IM bike leg: 212s

Improved aerodynamics, increased comfort. 10 degree under pad wedges added.

Set up 7: Hands and elbows 10mm further forwards

Power required compared to baseline: -25W

Predicted time saving in IM bike leg: 306s

No change in comfort, but an 8W reduction in power required

Set up 8: Xlab nutrition pack removed

Power required compared to baseline: -19W

Predicted time saving in IM bike leg: 239s

Removing the Xlab nutrition pack actually increased the drag in this case. So ideally keep it on for both aerodynamics and fuelling.

DISCUSSION

These two case studies show how different interventions effect different people. In both cases, changes were made that would commonly be considered less aero – raised front end in case study 1 and wider pads in case study 2 - but helped improve performance. We’ll explore these variables in more detail in part two of this article in the next issue of IBF magazine.

Bianca @thecyclephysio

Barney @bgwainwright