I am converting my road bike (Cannondale R500 Caad2) into a tour bike for some summer touring around Lake Erie (first tour). The old tires were flimsy slicks, and had proven less than able to take on even suburban roads given how broken and potholed my neighborhood is (not to mention a few old brick roads that make for interesting rides), though they were great for paved trails.

After some research, I found the Schwalbe Marathon Plus tires pretty recommended. I bought a couple of 700x28s, and after wrestling to get them on, I found out that the additional thickness and height caused the tire to rub against my frame (note: not the brake assembly, but the frame's mount for the brake assembly, which has a little bulb for the nut that sticks out just enough to catch the tire). I had accounted for the additional width (they are fine horizontally), but the lack of vertical clearance is the problem.

Some trial and error showed that if I mounted the wheel a millimeter or two shallow in the vertical dropout, the rubbing problem was solved with a little extra clearance. (Note: I have quick-release wheels) I have yet to take it for a test-ride, and in any case figure that this situation is not as secure as I could make it. Do you agree that it is necessary to somehow put something in the dropout like some rubber or metal to buffer and give stability to the situation? Is this an immediate problem, or can I get a few test rides in first? And any suggestions on what would work well for an insert?

(And, unless this is really a dangerous option, I would prefer to do this small modification rather than getting new tires or sanding down the frame's bulb.)

Thank you for your help.