It would be better to have a bit more clearance at the side, even for a rear tire, but the critical gap is at the top of the tire, and as long as this is close to 5 mm and as long as the tire has no knobs, I think it will be fine. I have slightly less than 5 mm clearance under the top of my rear fender over the Big One, and it has caused no problems. Of course, you'll want a much wider safety margin in front.
I have a steel frame that was designed around a 650x42 tire, I wanted to go a little wider fir more buoyancy.so I tried a pari moto 48. Is this enough clearance? the tight spot is in the crimped fender /chain stay area. What's good clearance? What's minimal clearance ?
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Patrick Moore
Alburquerque, Nuevo Mexico, Etats Unis d'Amerique, Orbis Terrarum