[MENTION=288568]Nickfrogger[/MENTION], here's how the 45mm Bluemels mounted up. The front mudflap hangs pretty low, but I could make it even lower if I move the bracket behind the fork. A person could squeeze a 53mm fender in, there should be room under the seatstay bridge and it could probably flex to fit between the fork blades, and it would give better coverage for a wider tire. But on my Multitrack, the chainstay bridge is the limiting factor. I don't know whether all Multitracks have one or not, but it's what keeps mine from having room for a 38mm tire plus fender. The tires pictured measure 35mm wide and 32mm high on my rims. If you've never installed SKS fenders before, some people say they're a chore, but I've put them on my wife's bike and another bike of mine and they're not bad. It took me about 30 minutes to have them installed, but I spent a couple hours fiddling to see if I could squeeze a 38 in w/o success. Leave the ends of the fender stays uncut for a while, they'll shift and bend a little over time until they settle in.
Damn, my bike is filthy.