After watching a recent stage of the Tour de France I started wondering; is there any reason from an aerodynamic and heat-transfer perspective, why cyclists shave their legs?

Lance Armstrong in the 2009 Tour de France: Smooth legs

Aerodynamics

The immediate thing that comes to mind is that cyclists shave their legs in an attempt to reduce drag and therefore gain a competitive advantage. However, given that human legs present a cylindrical cross-section relative to the oncoming airflow then we can assume a leg will cause the flow to separate behind it - as it will for all bluff bodies. Without changing the shape of our legs (i.e., make them more airfoil like) the best way to minimize drag is to minimize the size of the separation behind a leg. As with dimples on golf balls the separation can be minimized by inducing a turbulent boundary layer using a rough surface. A smooth surface on a bluff body at low speeds will typically have a laminar boundary layer and therefore higher drag. This then implies that a rough (dare I say hairy) leg is likely to encounter less drag than a freshly shaved, smooth leg.

Heat Transfer

Another concern for cyclists riding in mid-summer in the Tour de France is heat dissipation. The thinking supporting shaved legs is probably that direct contact with the air speeding past a cyclist unhindered by hair will achieve maximum heat transfer through convection. This is wrong, because the speeds achieved by cyclists are not high enough to cause a turbulent boundary layer (as I discussed earlier), which would maximize heat transfer to the surrounding fluid. So again, a rough (hairy) leg will dissipate heat more efficiently than a smooth one, offering a competitive advantage.

Conclusion

Given two strong scientific arguments for hairy legs - why do cyclists shave their legs? All I can say is that it's not motivated by optimizing heat-transfer or aerodynamics to gain a competitive advantage, so it must be something else.

The design of racing bikes has attempted to systematically eliminate drag at ever greater costs. I think it's about time cyclists looked in greater depth at the human component too - pushing tradition and dogma aside.

As an aside, I noticed that the official clothing for the 2008 Olympic US track and field team included leg and arm bands that were specially textured to reproduce the dimpled golf ball effect. So this additional clothing is well grounded in a good understanding of fluid flow. However, why compensate for shaving off our special textured (hair) coating that provides the same benefits as dimples do naturally?