I can understand that smaller bikes have smaller wheels, so children’s bikes have little ones and adults’ bikes have big ones. All well and good. Why do they top out at the maximum diameter seen on both road bikes and mountain bikes? Why not use larger-diameter wheels for larger frames?
Well, standards. If all the grown-up bicycles have wheels the same size, everyone takes tires the same size and there’s no difficulty getting replacements. Though car wheels seem to have no such restrictions, I can accept that argument for the Teeming Two-Wheeler. But surely custom bicycles (such as those used in the Tour de France) can afford custom tires. Why do the Tour cycles have wheels no larger than my own?
Well, rules. We want to keep everything fair, right? I assume the International Committee Responsible for People Tearing Up the Road while in Tight Pants regulates the maximum diameter of a road racing wheel. How’d they come up with the number?
I can think of good reasons to have larger wheels: less rolling friction, smoother ride, what-have-you. I can think of good reasons to have smaller wheels: more structural integrity, less angular momentum. Has the current standard road wheel evolved to create a perfect balance of these factors? Or is it an arbitrary decision?
Larger wheels mean more rotating mass and that’s bad for a cyclist. A few ounces in wheel and tire weight makes a dramatic difference that even an amateur can feel. If anything you’ll sometimes see front wheels smaller than the standard 700c (very close to 27") wheel size.
Car wheels and tires are a poor comparison to bike wheels. You can easily use different wheel diameters on a car but get the same rolling diameter by using a different profile tire. Even if you change the rolling diameter a fair amount it won’t keep a car from funtioning. Bike tires have a very small profile compared to wheel diameter so the same thing is not possible. Also bikes typically use frame mounted caliper brakes which depend on a specific wheel size. If you change the wheel diameter all the geometry and measurements of the frame have to be changed to accomodate it.
John Harrison, thank you for an excellent link. It has a lot of information about wheels, but I can’t find a place where it directly addresses the issue of wheel diameter. It seems more concerned with the choice of spokes. It does list, under ‘Wheel Performance - Aero/Inertia Concepts’, that the inertia of a 650mm wheel is ~0.07 kg.m^2 compared to 700mm wheels at ~0.09 to 0.10 kg.m^2. By this argument, though, why use 700mm wheels at all? One presumes the rolling resistance is lower, but their comparison for rolling resistance (and drag) is between two 700mm wheels with different spokes. Is there a direct comparison of wheel diameters?
Padeye, by the same argument I gave John Harrison, if larger wheels mean more rotating mass why not use 200mm wheels rather than 700mm? The use of 700mm wheels suggests there is some benefit to larger wheels. In particular, why do most cyclists use the largest wheel allowed by the UCI, and only (potentially) go smaller on the front wheel? I’m not questioning smaller front wheels; rotational inertia affects handling as well as acceleration, so I can understand that. A lower front wheel also improves rider aerodynamics. I am questioning the lack of larger rear wheels. Without the UCI rules, would a larger rear wheel be desirable? If so, why does the UCI restrict its size?
Inertia is not always a negative thing. While it hampers your acceleration, it also gives you momentum once you get up to speed and is more stable at speed. Would not this effect, coupled with the lower rolling resistance, be beneficial for long-distance cycling?
Finally, given the thousands of dollars competetive cycling teams put into custom-built frames, I think the question of adjusting the frame to accommodate different wheel sizes is moot. Witness, as you say, 650mm front wheels on some bikes.
In pursuit riding on the track aero wheels are used, sometimes on the front too, and this helps smooth out the riders pedalling effort so they find it easier to stay on the optimum line on the track.
Going to smaller wheels can cause problems with handling, the bike can be way too sensitive to steering input, and climbing on a small front wheel machine makes the thing tend to wander side to side the road, especially on the real steep climbs.
Small wheels do not ride poor road surfaces well, the old ordinary with its very large front wheel was excellent for the roads of its day.
Trouble is that a large front wheel can mean excess rotating mass, which kills accelaration, and it is not as rigid as a smaller wheel so cornering is slowed and the machine has a tendency to run wide in tight fast turns.
The reason that the UCI set a limit on wheel size was not because of use on the road, pursuit riders were using larger and larger wheels to gain from the flywheel effect, but this meant that riders of average height were at a disadvantage as wheels became larger, and instead of a test of suppleness, and fitness and strength it was becoming more dependant on one dimension, that of rider height.
I remember Francesco Moser havingan attempt at the hour record on such a machine, but many regarded this as a false attempt as the record had been set by Eddy Mercx on a conventional machine, in fact there was a lot of dispute at the time that Moser had the use of a disc rear wheel too.
Many felt that this was not a fair comparison to the record ride of Mercx.
The world hour record is the blue riband of cycling honours and the genral idea is that to win it a rider must beat the previous best by using equipment virtualy the same as was used in setting the standard.
Moser did actually go on to take the record on a more conventional machine too so that kind of killed all the argument.
A small front wheel can be an aerodynamic advantage, particlarly on a pursuit or time trial bike where the rider takes an extreme down in front position that would be impractical in a road race. The bike is built with a shorter head tube and “bull horns” handlebars that put the rider’s hands where the drops would be on conventional handlebars. The back wheel has the front wheel, most of the frame, the rider’s legs and the cranks in front of it so there is less of an undisturbed airstream to take advantage of.
Well, OK, if that’s really what you want. Those wheels are used because they fit the bikes. The bikes are that size because those are the wheels available.