Aerodynamic efficiency is something that only applies to time trial athletes, right? From a performance perspective, absolutely not.
More and more, modern-day road racing is becoming more about speed and attention to detail. In the most basic sense, athletes need to be aware of the difference between the common riding positions and the scenarios where aero equipment is favourable. The point of this blog post is to provide the basic knowledge related to the former.
The basics of drag force in cycling
When speed is the key performance outcome measure, it’s power-to-drag that matters most. At speeds above 12.9 km/h, wind resistance exceeds rolling friction and by the time the athlete is travelling at 32.2 km/h, 90% of resistance to forward motion is in the form of aerodynamic drag (Lukes et al., 2005). That means both athlete power output and system coefficient of drag (bike plus rider) are vital performance indicators.
Of the total system coefficient of drag, between 31-39% of aerodynamic drag can be attributed to the bike, with the remaining 61-69% being attributed to the athlete (Lukes et al., 2005). Given the contribution of the athlete to drag force acting on the system, the position of the athlete has the most significant impact on aerodynamic efficiency.
“Above 12.9 km/h, wind resistance exceeds rolling friction and by the time the athlete is travelling at 32.2 km/h, 90% of resistance to forward motion is in the form of aerodynamic drag. Of this, up to 69% is attributed to airflow over the athlete and the remaining is attributed to airflow over the bike.”
Participant and equipment
Jordy Villani volunteered his time to this round of aero testing. Jordy stands at 186 cm, weighing in at 75 kg, with a system mass of 83 kg. For the testing, Jordy was wearing his basic Cuore skinsuit, Suplest aero shoes and Rule 28 AeroSox. Control wheels were used (HED GT3 and Stinger Disc), limiting the variables contributing to a change in system coefficient of drag. The same testing protocol was used that we used for time trial positioning and the subsequent data was analysed and post-processed in a custom built data analysis package, specific to the application of track aero testing. Jordy’s standard power2max unit was used to collect power data throughout the session, performed at Darebin International Sports Complex (DISC) indoor velodrome.
Image by: @aaronupson
Baseline Assessment
During the fit process, we often reinforce the primary contact point at the handlebars. Hands on the hoods is the primary contact and indeed, for approximately 90% of your time on the bike, this should be the contact point of choice.
Jimmy Whelan (InForm - Make) having his road position dialed in before his 2918 National Championships campaign.
Image by: @aaronupson
Motion capture data from STT Systems kinematic analysis software, showing the position used for baseline aero testing of road race positions. Trunk inclination angle of 37 degrees.
For our road position aero testing, we used this position as our baseline. For Jordy, we were able to determine that his baseline road position coefficient of drag (CdA) was 0.316 m^2.
Baseline CdA density plot for the regular road position. An average CdA of 0.316 m^2 was recorded.
hands in the Drops or Time Trial Style?
With the front end of a road bike, you have a few options for improving aerodynamic efficiency. The most obvious option is to place the hands in the drops, with the outcome of a reduced torso angle.
Motion capture data from STT Systems kinematic analysis software, showing the "hands in the drops" position. Trunk inclination angle has dropped to 25 degrees, while forearm tilt remains almost identical to the baseline position.
Indeed, when we look at the outcome measured on the track, we see a very substantial drop in CdA. For Jordy, there was a 6.3% improvement with an average CdA of 0.296 m^2 recorded.
CdA density plot for the regular road position (Variable 1) vs the hands in the drops riding position (Variable 2). The hands in the drops position results in a substantial improvement in aerodynamic efficiency.
Want to further improve your speed for a given power output? The other option a rider can take is to keep their hands on the hoods and flatten the forearms. The time trial style position keeps the torso at a similar angle, while reducing the contribution of the forearms to the frontal area of the athlete.
Motion capture data from STT Systems kinematic analysis software, showing the "time trial style" position, in road application. The trunk inclination angle remains almost identical to that of the "in the drops" riding position, while the forearm angle is substantially reduced.
So how much does it matter? Reducing the contribution of the forearms to the frontal area of the athlete has a very substantial impact on aerodynamic efficiency. This position sees a further 4.4% improvement in CdA with a value of 0.282 m^2 recorded.
CdA density plot for all three road positions tested (Variable 1 - hoods; Variable 2 - drop; Variable 3 - "time trial style"), each showing a distinct outcome for aerodynamic efficincy.
The Outcome
Hands in the drops provides an impressive improvement in aerodynamic efficiency, equivalent to 24 Watts. If performing a time trial over a 40 km distance, this improvement would translate to a time saving of 72 seconds at 375 Watts.
The use of the time trial style position sees a further improvement in aerodynamic efficiency, equivalent to an additional 16 Watts. While the margin of improvement is less than the initial change, the outcome is no less impressive. Over a 40 km time trial, this improvement would translate to a time saving of 124 seconds over the baseline position, or 52 seconds over the "in the drops" cycling position.
Equivalent power gain and time improvements, associated with a variation in the road cycling position.
Summary
We have provided aero data for the three primary road positions relevant to road cyclists. This information allows the athlete to make an informed decision about something that is incredibly easy to adopt. Equipment choice and position optimisation should reflect the goal of meeting the demands of road cycling. While this is most relevant for the competitive athlete, the concept remains relevant to all road cyclists, from elite to club level athlete and recreational cyclist.
The measured improvements in aerodynamic efficiency come down to a change in torso angle and a change in the angle of the forearms. Lowering the torso and reducing the contribution of the forearms to the frontal area of the athlete both have a significant impact on aerodynamic efficiency.
Special Thanks
The data analysis processes would not have been possible without the help of Scott Gigante, creator of the custom data analysis package we use.
References
Lukes. R.A, Chin. S.B & Haake, S.J 2005, The Understanding and Development of Cycling Aerodynamics, International Journal of Sports Engineering, vol. 8, pp. 59-74. Retrieved from: https://link.springer.com/article/10.1007/BF02844004