One More Metre
In a 200-metre sprint finish, is it better to be on a Cervélo R5 or S5?
Sprints are chaotic, so it’s hard to point to any individual effect, such as bike frame aerodynamics, as a significant aspect of the analysis of a pro road-racing season. But no individual sprint win is a statistic – each sprinter wants the best chance to win.
As engineers we like to measure, so we have a lot of data. By taking our measured data of the aero drag and weight of two Cervélo bikes, the R5 and S5, and combining it with estimates about the typical weight, power and speed of a 200-metre dash to the line by Edvald Boasson Hagen of Team Dimension Data, this question can be addressed.
Jim Martin’s seminal paper “Validation of a Mathematical Model of Cycling Power” captures the fundamental physics of powering a bike. This work is the basis of most math models of cycling, including much of the current work being done with field testing, both the traditional regression method and the newer virtual-elevation method, and now with Alphamantis Technologies’ Aerostick, including the measurement of ambient wind.
One convenient application using Jim’s model is Tom Compton’s excellent website, analyticcycling.com. A straight-up sprint comparison is simple enough to model, so we used his page simulating the final sprint. Tom set up this page to compare two identical riders on different wheels, but with the flexibility he’s built in to the input fields we easily used it to compare two identical riders on different frames.
To make the comparison, we put Tyler as the “Standard Rider” on a Cervélo R5 and Tyler as the “Test Rider” on a Cervélo S5. Since the rider is the same for both cases, the only differences are aero drag and weight between the two Cervélo models.
We know from our wind tunnel tests that a particular sprinter has an aerodynamic drag area (CdA) of about 0.333 metres squared. We also know that the Cervélo S5 with rider has about 0.013 m2 less drag area than an R5. So as inputs for our analysis we used 0.333 m2 for the R5 and 0.320 m2 for the S5.
Since the question is in reference to professional riders in a UCI sanctioned event, the R5 has been ballasted up to the UCI minimum of 6.8kg. The S5 in its heaviest team build is typically no more than 7kg. So the difference in weight is 200 grams.
The remaining parameters were set to identical values for both cases (same 74kg rider, same wheels, etc.). The results: The rider on the S5 would cross the finish line 0.059 seconds, or 1.127 metres, ahead of the rider on the R5.
Of course, bike racing’s not a math problem – the chaos of a field sprint is impossible to model completely, and there are many strategic factors in a race that have more influence on the result than the one metre difference between these two bikes. Caught behind a slowing rider? On the windward side of the bunch? Forced to avoid a spectator’s camera in the turn? Naturally, a good sprinter maximizes his chances by paying attention to all factors. And all else being equal, the Cervélo S5 is a factor you can rely on to give a surprisingly large advantage that’s real, always there, and independent of other strategic factors in the race, even in a sprint.