The Aerodynamics of Body Hair

There’s a kerfuffle in the cycling world right now thanks to a lighthearted experiment kicked off by triathlete Jesse Thomas while he was doing some wind-tunnel testing on his bike earlier this year. You can read his full account here; the short version is that he forgot to shave before heading to the wind tunnel, so he shaved halfway through the day. The result, which no one expected, was that shaving saved a remarkable 15 watts of power due to reduced drag, which according to the wind tunnel experts is equivalent to over a minute in a 40km time trial and was much more significant than any of the other tweaks he made to his helmet, riding position, and compression suit. The scientists followed up by testing five more riders with varying amounts of leg hair, and found remarkably consistent results. For cyclists, shaving really does matter.

Of course, top cyclists already shave, so this shouldn’t be a surprise. But the effect is much larger than anyone expected, and larger than an earlier study from the 1980s found, which has raised questions. Wind tunnel time is generally too expensive to “waste” on amusing experiments like this, and leg hair is too complex to accurately model with computer simulations, which is why there’s so little data available. A question that occurs to me: is it possible that body hair might matter for running too? I look at the photos of the magnificent beard U.S. mid-distance runner Will Leer is sporting this year as he notches the best times of his career, and I wonder… If he’s planning to shave it anytime soon, I hope someone puts him in a wind tunnel for a before-and-after comparison.

As far as the cycling result goes, some people won’t believe it until it’s replicated – which is an interesting problem in science, because generally no one wants to bother confirming the results of experiments that others have already done. There’s no glory (or publication or tenure) in that. As it happens, there’s currently a major controversy among psychologists over a special issue of the journal Social Psychology devoted to attempts to replicate 27 “important findings in social psychology.” Ten of them failed to be replicated. As Michelle Meyer and Christopher Chabris note in Slate, the issues raised in the controversy apply to many, if not most, branches of science, including exercise science. The cooler and less probable the result, the more we should suspend our judgment until others can verify it. In this case, the attention generated and the outlandishly large drag savings suggest to me that there’s now a reasonably good incentive for someone to put together a proper, peer-reviewed wind tunnel study of the effects of body hair. I hope they’ll include a few runners… just in case.


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