Creaform’s GDPL cycling team: The bike positioning debate

We are finally ready to share the results of the CFD analysis of our cyclists! To refresh your memory,

Martin and Francois were having an argument while riding at the Grand Defi Pierre Lavoie (GDPL) for?Creaform’s?team: Martin, with his regular road bike and upright position, estimated that he was delivering twice the power to keep up to speed with Francois on his triathlon bike and using a more horizontal position. Francois was saying that the extra power could not represent more than 20%...?

This is the kind of matter we take seriously at Creaform!??

As soon as they got back from their 1000 km ride (yes they rode night and day for 60 hours), they gathered a few of us to get to the bottom of this.?We first started by scanning and acquiring the geometries of the two cyclists all geared up on their respective bikes.

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From the scans, we could figure the frontal area, which we know to be quite important for the aerodynamics of a bluff body such as a cyclist. Fran?ois, on his triathlon bike, has a total frontal area of 0.395 m2. Martin, on his regular bike, exposes a larger part of his body and his total frontal area is 0.498 m2, which is 26% larger than Francois'.??

?That information is a very important metric, and one could argue that it is enough to answer the question. So, Martin's drag would be about 26% larger than Francois'.?

However, it is not fun to have gone that far and not analyze this further! Philippe, our CFD coordinator, took the scans and performed unsteady simulations of the 2 cyclists to evaluate their drag coefficient (Cd) using Simcenter Star-CCM+ software. Simulations are performed independently, considering that Martin and Francois are too busy arguing side-by-side and thus not drafting.?

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Don't worry they are not on fire! They are just surrounded by eddies resolved using advanced turbulence modeling.?

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The moment you were waiting for: The final results!

Francois'?CdA = 0.241 m2?

Martin's?CdA = 0.322 m2?

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What is?CdA?? It’s the product of the drag coefficient?Cd?and the frontal area?A, and it contains all the information about aerodynamics that we require to settle the debate…??

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Briefly, the drag area relates the drag force?D?and the dynamic pressure from formula?D=CdA ?rU2, with?r?the air density and?U?the velocity. The power to overcome drag?P?adds another dependence on?U, making the relation to the third (P?∝?U3). Needless to say, increasing the velocity has a heavy cost on the cyclist power demand, and it is recognized that at high speed, about 90% of the power supplied by the cyclist is used to overcome drag.??

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So, this being said, Martin's drag area is 34% larger than Fran?ois', so to maintain the same speed, Martin must deliver 34% more power. Martin's guess was quite high at 100%, and Francois' was a little more precise but not quite enough at 20%. Now, the analysis does not account for Francois' neck pain to maintain his position and the extra brain power required to stay focused!?

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Extra trivia question:?

Given that the 2 cyclists have the same weight and maintain their static position in hill descent (and thus keep their respective?CdA ),?what would be their speed difference at equilibrium???