Bianca Broadbent MCSP
Boardman Performance Centre, Evesham, UK
@thecyclephysio
Barney Wainwright PhD
Boardman Performance Centre, Evesham, UK
Leeds Beckett University, Leeds, UK
@bgwainwright
Introduction
Importance of drag for performance
It is 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 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.
Clinical reasoning and application
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
History
A 30-year-old competitive female cyclist presented to the Boardman Performance Centre for optimisation of her track cycling position for individual pursuit (IP) and team pursuit (TP). No specific injuries or discomfort was reported; therefore, attention could be focused on improving efficiency and aerodynamic profiling which following an analysis of her performance needs and competition results was the primary area for attention (Figure 2).
Figure 2. For Case Study A it was determined that the most important factor for her was reducing aerodynamic drag, with increases in power secondary to this.
Baseline Assessment and Biomechanics
The athlete possessed good flexibility through the hamstrings, hip flexors and shoulder range of motion. There was a clear imbalance between anterior and posterior leg muscle strength, with a preference to utilise the quadriceps over the hamstrings and gluteal muscles regardless of hip angle. Angling the saddle from a level to a slight nose down position improved saddle pressure mapping. Opening the hip angle by raising the cockpit up by 40mm resulted in a significant increase in power due to improvements in effective force production. It was also noted that the arm rests were suboptimal (low cup and limited adjustment), therefore it was suggested that alternative pads may offer a proprioceptive benefit. As the position was already on the limit of UCI regulations, no further adjustments to saddle fore-aft nor cockpit angle could be made. These biomechanical changes were recommended for investigation within the wind tunnel (Figure 3.).
Figure 3. Clinical reasoning of potential areas of prioritisation within biomechanics for Case Study A.
Aerodynamic Intervention
Baseline CDA was 0.1998 at 52.1 km.h-1 with an estimated power requirement of 412W.
Table 1. Changes in power required and predicted time savings in response to the key biomechanical and aerodynamic interventions for Case Study A. Negative values denote reductions in power requirement due to reductions in aerodynamic drag.
Set up | Description | Power required compared to baseline | Predicted time saving in IP | Comments |
Set up 1 | Baseline | - | - | Sustainable race position.
|
Set up 2
| “Forced”/tucked position | -30W
| 5.4s | This position is currently not sustainable in racing. |
Set up 4 | Cockpit raised 40mm | -33W | 5.9s | Note that this was adopting a sustainable racing position. |
Set up 5 | Aerocoach Align Wing arm rests | -36W | 6.6s | Swapped in for initial pads. Facilitated more relaxed shoulders. |
Set up 6 | Hands together | -41W | 7.6s | Extensions rotated inwards.
|
Set up 7 | Saddle angled down
| -44W
| 8.0s
| Improved comfort and altered the lower back profile. |
Set up 8 | Medium Kask Mistral helmet | -37W | 6.7s | Medium size helmet created a 7W increase in power required. |
Case Study B
History
An Ironman distance triathlete presented to the Boardman Performance Centre for optimisation of his triathlon position. The main complaint was shoulder and neck discomfort after holding his aero position for a relatively short period of time (< 1 hour). This resulted in a much more relaxed and high head position being held for the majority of the bike leg in competitions, resulting in slower race speeds than desired. These biomechanical issues were recommended for investigation within the wind tunnel. (Figure 5).
Figure 4. For case study B finding a position that was comfortable and sustainable was the priority. Due to the relatively low speeds of an Ironman bike leg, reductions in aerodynamic drag were a lower priority than prioritising power output, but incidentally would be achieved by improving comfort.
Baseline Assessment and Biomechanics
Assessment revealed anatomically broad shoulders with a bony restriction into shoulder adduction/flexion, as well as restrictions in the length of the posterior shoulder musculature including the latissimus dorsi. Flexibility to the spine and hips was reasonably good, however the saddle height was deemed to be too high causing overreaching and an increase in pressure at the saddle interface. As a result, there were several key positional changes that needed to be made to improve comfort and function, to improve position sustainability and power, regardless of their outcome aerodynamically. Comparisons across set ups allowed us to investigate the resulting changes in aerodynamic drag (Table 2.).
Figure 5. Clinical reasoning of potential areas of prioritisation within biomechanics for Case Study B.
Aerodynamic Intervention
Baseline CDA was 0.2772 at 38.9km.h-1 with an estimated power requirement of 268W.
Setup | Description | Power required compared to baseline | Predicted time saving in Ironman bike leg | Comments |
Setup 1 | Baseline | - | - | Unsustainable race position. |
Setup 2 | Saddle lowered 5mm | -1W | 15s | Behind saddle bottles removed to facilitate lower saddle position. |
Setup 3
| Saddle lowered 5mm | 1W | 17s | Total of 10mm lower. Increased comfort for little aerodynamic loss. |
Setup 4
| Pads 20mm wider and flared | -17W | 214s | Improved shoulder comfort and facilitated a lower head position. |
Setup 5 | Pads 20mm wider | -14W | 166s | A small increase in power requirement (+3W), but a further increase in comfort. |
Setup 6
| Higher hands | -17W | 212s | Improved aerodynamics, increased comfort. 10 degree under pad wedges added. |
Setup 7 | Hands and elbows 10mm further forwards | -25W | 306s | No change in comfort, but an 8W reduction in power required. |
Setup 8
| Xlab nutrition pack removed | -19W | 239s | Removing the Xlab nutrition pack actually increased the drag in this case. So ideally keep it on for both aerodynamics and fuelling. |
Table 2. Changes in power required and predicted time savings in response to the key biomechanical and aerodynamic interventions for Case Study B. Negative values denote reductions in power requirement due to reductions in aerodynamic drag. Positive values denote increases in drag.
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.
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