Originally Posted by
energyandair
If you mean that a smaller circumference wheel will always be slower with the same power input, my understanding is that this is incorrect and that, if anything, the reverse is true, particularly at high speeds on smooth surfaces.
The reasons I believe that, all else being equal, the small tire is faster are:
- The frontal area of the tire and rim is smaller in the same proportion as the reduction in circumference (or outer diameter) of the tire.
- The frontal area of the spokes is smaller to a greater degree than the reduction in circumference (or outer diameter) of the tire. The primary reason for this is that as the wheel size reduces, spoke length reduces faster because the amount of the diameter occupied by hub, rim, and tire remains the same. There is a secondary smaller effect stemming from a slightly lower velocity at the outer tip of the spoke where the peak velocity is highest and thus matters the most (square law effect). A third potential reduction in drag is that the greater strength of the smaller rim makes it feasible to use fewer spokes.
A fourth potential reduction in drag, which I haven't seen suggested elsewhere, relates to the dynamics of the shape. It is known that if two protuberances are in a given air flow at the right distance, even if both are fixed to the same object, one can in a sense draft the other. The trailing one occupying or near the low pressure region, smoothing out a turbulent wake. With too much separation they're just two pieces hitting the wind. It is conceivable to me that the system of the leading edge, hub and trailing edge might behave in this manner, the smaller separation in the smaller wheel producing a lower drag. Unlikely but conceivable.
Originally Posted by
energyandair
As far as velocities are concerned, regardless of the wheel size.
- Where the rubber meets the road, its velocity is zero (unless the tire is skidding)
- The axle of the wheel moves forward at the speed of the bike. (for example 20 mph)
- The top of the tire moves forward at twice the speed of the bike (40 mph in this example)
Correct, the spoke speeds will be the same measured proportionally from the contact point to the center, where that section of spoke exists in both wheel configurations. So the proportional difference in drag due to spokes (given the same diameter, shape and number of spokes) is simplified for practical purposes to the proportional difference in spoke lengths.
IMO @
acidfast7 is correct that none of this matters for time spent in commuting, and it's a little beyond the scope of the initial question. But who's to say that going a tiny bit faster isn't desirable for other reasons than shaving a few seconds off the commute?