Aero Tech #3 — Sprint Aero Position
The track cyclist’s biggest enemy isn’t always other riders while out on the boards. Aside from a necessary mental strength to power through, the greatest battle to overcome is air resistance. At a speed of 50 km/h (31mph), about 90% of the riders power output is used to overcome the force of air upon the rider.
Over a series of blog posts, I will break down track cycling aerodynamics and respond to my research with a series of innovative concepts and product designs.
This week we are talking about the aero body position in sprinting. How much benefit it can make? How long can a rider hold an effective aero position for without affecting their power output?. And what are the downsides to extreme aero?
The longest event for a sprint track cyclist (that doesn’t involve using pursuit aero bars) is the Keirin. The Keirin is typically 1.5 kilometers or 6 laps on a 250m velodrome. For half the race, typically 6-9 riders pace around the track drafting behind a motorcycle. The riders save valualable energy in the slipstream as they build up speed. The last 750m the pacer pulls off and it is all on!
The Kerin is regarded as one of the more difficult and prestigious sprint races to win. It takes witts, tactics, power and sometimes a bit luck. Top keirin riders can exceed 70kph before the end of the race. To achieve these speeds on a bicycle, a great contributing factor is the aero position.
Body position and drag
Earlier this year (February 2019) researchers Thijs van Druenen, Yasin Toparlar and Thomas Andrianne from the Netherlands and Belgium published a paper discussing how various sprint positions adopted in road cycling affected aerodynamics.
“A few riders have adopted a rather exceptional and more aerodynamic sprint position where the torso is held low and nearly horizontal and close to the handlebar to reduce the frontal area. The question arises how much aerodynamic benefit can be gained by such a position. This paper presents an aerodynamic analysis of both the regular and the low sprint position in comparison to three more common cycling positions.”
Among two other ‘out of the saddle’ sprint positions that were trialed in the research, below are the three seated position that were compared using wind tunnel and CFD analysis. Of the five positions tested in the research, these three are the most relevant in track sprinting. Each position illustrated has a varying hip angle from horizontal - 24.9°, 10.7° and -9.4° respectively.
Below illustrates a real world comparison between the ‘back upwards’ sprint position of Ethan Mitchell (New Zealand) and the ‘back horizontal’ sprint position of Mathew Glaetzer (Australia). The ‘back down’ position isn't commonly observed (except with a bit of comedy) thus I have not pictured an example of any professionals using the position in a sprint effort.
Image source: https://twitter.com/CyclingAus/status/884234914601644033
To analyse the results from the study, we first need to understand a couple of the terms used.
Frontal Area: The silhouetted surface area of the rider and bike pushing through the air. The larger the frontal area, the more air resistance there will be. (Source)
Drag coefficient: A quantity that is used to quantify the drag or resistance of an object through the air. It is used in the drag equation in which a lower drag coefficient indicates the object will have less aerodynamic drag. The drag coefficient is associated with the frontal area. (Source)
Drag Area: The drag area of an object represents the effective size of the object as it is "seen" by the air flow around it. (Source)
Based on the data supplied by Andrianne, Toparlar and van Druenen, in combination with a drag force calculator by Omnicalculator.com I have identified a reduction in drag from a standard ‘back upwards’ sprint position to a ‘horizontal back’ position by 17.0%, and a further 2.9% reduction to a negative back angle position (illustrated as the ‘back down’ position).
How does the extra 2.9% in the unconventional and uncomfortable looking ‘back down’ position effect the power output for a sprint?
Note: I have undertaken these calculations independently using the data supplied in the research paper and an online drag calculator. If you spot any errors in my calculations, please let me know, and I will do my best to correct them.
Body position and power output
In a second academic publication from May of this year (2019) by Austrailian researchers Luke Daly and Michael Kingsley, the duo have analysed how changes in back angles on the bike ranging from 70° to 110° from vertical affect the power output of sprint cyclists.
The study identified that any changes in hip joint angles did not affect the power output or cadence of a 6-second sprint. They also noted that despite changes in handlebar height and reach from the bikefit, the rider did not consistently maintain the hip angle that was intended for the sprint effort, but referred back to their natural sprint position. They noted that trained cyclists tend to adopt different body positions on the bike when higher work rates are required. I believe this may be a physiological response of the rider getting the most out of their body during the effort.
Based on the two studies, it is safe to assume that with some training, the body can be conditioned into a long and low position to maintain a comparable power output while reducing drag considerably.
Let us know your thoughts on the aero position. Have you experimented with seeing how low you can go? How did that affect your performance?