tt bike aerodynamics

[flickwet]

I have always been under the impression that all things being equal a lower profile provides a lower coefficient of drag due to reduced frontal area. Having said that I have always thought that 650 wheels were outlawed in USAC and UCI events, but apparently its only as long as the wheels are different in diameter. Thus why are not more TT bikes on 650 wheels? I would think the reduction in height while not significant is not inconsequential.

[rugbysecondrow]

Somebody will correct me if I am wrong, but when the wheels are reduce, wouldn't the rest of the bike have to be increased some to compensate? That would seem to make the frontal area net out.

Also, in the triathlon world, I thought the 650 wheel had more to do with spinning up more quickly and less to do with aerodynamics?

I might be incorrect.

[Mark McM]

It is true that 650 wheels do have less aerodynamic drag, weigh less, and have less rotational inertia -

However, they also have more rolling resistance (which largely, if not entirely, cancel out their aerodynamic savings), and less shock absorption. Rotational inertia only matters during accelerations/decelerations, and since and time trials (and triathlons) have fewer accelerations, the lower rotational inertia matters little.

All in all, 650 wheels offer little in pure performance gains over 700 wheels, so most riders and manufacturers stick with the more common and standardized 700 wheels.