rjeffb wrote:

I, on the other hand, am an engineer and the answer is...it depends.

The *mass* of the wheel has no direct impact. The *moment*, however, does, and the mass is one factor that goes into the moment (the other is how that mass is distributed around the axis). All things being equal, a large diameter but small sidewall tire can have a greater moment than an equally large but bigger sidewall tire, because moment also involves how much of the mass is furthest away from the axis (that's why you don't see bikes with solid wheels like a car; putting mass close to the axis adds useless weight but not useful moment).

All things being equal - same sidewall size, same tire weight per unit volume - the moment is approximately equal to mass times the square of the radius. Double the diameter of the wheel and you quadruple the moment. So a bigger wheel has four times the moment of a smaller wheel.

BUT, the force that's keeping the bike up is the moment times the centripetal acceleration, which is (v^2)/r. Obviously v stays the same regardless of the size of the wheel (if you're going 10mph the outside of the wheel is going 10mph no matter what it's size is). When you double the diameter of the wheel, you cut the centripetal acceleration in half. Net result, four times the moment times half the acceleration equals twice the stabilizing force.

So, again stressing "all other things being equal," a wheel twice as big provides twice the stabilizing force. But that's only true of a bike that has just flown off the end of a ramp and is airborne. The majority of the stabilization comes from the contact patch (which is mostly a function of tire width and inflation) and especially rake - a cruiser and a sports bike can have the same tires but they sure do have different stability profiles because of the different rakes. I believe I recall that rake itself has several components having to do with not only the angle of the front fork but the fork's placement relative to the handlebar turning axis, but I'm fuzzy on that.

The center of gravity thing is misleading, because it is important to have a low center of gravity (small wheels win) but it's also important to have a center of gravity that is a smaller percentage of the radius (big wheels win - a true sports car with a center of gravity below the axles cannot flip over).

As Raybur correctly points out, a larger wheel also handles all but the largest potholes better by riding over them rather than through them, and handles changes in road surfaces better by smoothly leveraging itself over the obstacle rather than bouncing over it - to picture this just imaging a tire a hundred feet tall rolling over a pothole.

Isn't it pretty amazing that people can stay upright on rollerblade wheels?

Thanks for the explanation. Just curious, though: did you really mean "centripetal" force, or should it be "centrifugal" force?