I've got a set of Crud mkII's on my road bike. Clearances are pretty tight (old Dynatech 700 running 23mm Michelin Lithion 2 tyres on Open 4CD rims), and last year I was very aware of the guards rubbing slightly - as soon as the tyres got wet and picked up any grit you could hear it, which, if nothing else wasn't very helpful psychologically. I've recently re-fitted them for this winter, and must have been much more careful, as so far they seem to be rub free. The stays will hold the guards off the wheels pretty effectively if you pull the guard into a more extreme position than is required, tighten the screws, then allow the guard to settle into back. The length of the trailing end of both front and rear guards is good, and the chainset area is protected by an extended moulding. Only the beginning of their 2nd winter, but so far, I've been very happy.
Richard