Not from lots of personal experience, but fenders with knobby tires and certain types of mud are always going to be a bit of a gamble and certainly a compromise. That's another reason why I left so much space between tires and fenders, in case I had to ride through gucky stuff. I suspect I'd have had problems anyway if it was bad.....but slicks pick up a loooooot less mud than knobbies, and then of course there are knobbies and there are knobbies....