higher RPMs means more centripetal force imparted into the particles of water and mud that spin off the wheel. a 20" wheel traveling at ~18mph will fling mud 4 ft. or more into the air. the last time i rode my folder without fenders in the rain, i was caked with mud specks, even my helmet. crap got EVERYWHERE. and it was a 20x1.25 slick tire (not particularly wide).