Originally Posted by
loubapache
There is a lot of truth to that and it can be explained with physics.
Assuming everything being equal (mainly the friction/adhesion force of the mud on the tire but this is a complicated function of rubber compound, tread design, mud wetness and even mud composition).
Assume the big wheel and the small wheel are traveling with the same linear speed, v, (such as 10 mph).
Mud can only stay on the tire if there is enough centripetal force, which is provided by the friction/adhesion force.
For the same linear speed, the mud on the smaller wheel needs to have a higher centripetal force to stay on because
This was the same argument that was was presented already. Both the big wheel and the small wheel will quickly become saturated so that any new mud quickly flies off. Once you reach this point, big wheel, small wheel, doesn't make any difference.
Wider/lower pressure tires will fling up more crap because of wider contact area. And folding bikes have the rider closer to the ground and that probably amplifies the effect, too.
This is probably getting somewhat silly. If you're concerned, get fenders.