All other things being equal (hub bearings, tires and tire pressure), a larger wheel “rolls better” than a smaller wheel. The maximum common wheel size is 27″ or 700c. One could make the case that a 20″ wheel has a smaller frontal area than a 27″ wheel and would be just as fast. I don’t know.
Bearing friction on bike wheels is pretty darned low. To get to that better happy place, try replacing the grease with oil, use high quality ball bearings (do caged bearings really have lower friction than loose ones?), adjust the cones, and if using cartridge bearings, cut away the rubber water seal.
The rolling resistance (this discussion limited to rubber tires on asphalt) is also affected by tire pressure and tire tread pattern. Higher pressures = lower rolling resistance. Another caveat, maybe “too” high a pressure will result in a rougher ride and a loss of horizontal velocity. Perhaps there is a sweet spot in tire pressure, high enough to reduce rolling resistance, but not as high as to eliminate shock absorbing. No tread (slicks) are faster than treaded. Narrow tires faster than wide.
The cross-sectional shape of the wheel rim in a spoked wheel has a large effect on the air resistance of the wheel. A box section rim will have a higher resistance than a deeper and tear dropped shape rim. Fewer spokes will also mean less air resistance. Disc wheels, especially lenticular cross section ones, have the lowest air resistance (see links under “blogroll”) Look at time trial and triathalon bikes to see what the current thinking is. Do we all get the hints here?
Top of the line disc wheels or those fancy reduced spoke wheels are pretty expensive, and if bought new would blow the Canary budget limit. In my next post I will talk about inexpensive ways to make your wheels as fast as they can be.