Originally Posted by cplager

I like this line of reasoning you provided. But I don't it really works to cause a messier situation.

I agree that in principle, mud stuck on a 20" wheel is more likely to be flung off a small wheel than a large wheel following this argument.

But (a) once it's flung off, it will be flung off at the linear speed of the wheel (and that will be the same speed regardless of radius) and (2) a wheel will saturate with mud relatively quickly and any new mud that gets on the wheel will come off regardless.

The only other factor that I' haven't already mentioned that I can see making a real difference is if you run the 20" tire at a lower pressure so it has a wider contact patch than the larger tire.

I've ridden larger tires in the rain at lower speeds and been quite covered with mud specks. Fenders are a good thing regardless of tire size.

Cheers,
Charles