Originally Posted by
SteveInConverse
Ok, had the wife measure the area as Im at work and she reported 52mm of space at the rear chainstay. I'm guessing I could comfortably run a 42-45mm tire back there?
Run it, yes, comfortably, no.
The basic recommendation is to run 6mm of spacing to either side of the tire so the tire should ideally be 12mm narrower than the frame, running closer is possible but this number is meant to deal with the fact people don't true their rims the moment it comes slightly out of true, offer space if a spoke breaks, deal with road debris. On the upside, you may never run into a problem, on the down side you might slightly tweak a wheel on a gravel road and never notice that the dirt on the tire ate its way through the frame where the tire was ever so slightly rubbing. I've seen this happen with two road bikes, one aluminum and one very high end, thin wall steel. I personally have a bike with a slight groove in the stay from hitting a pothole on a downhill hard enough to break two spokes, the tire survived but the multi-mile ride home wasn't gentle on the frame but 4130 cromoly isn't weak or thin. Your fx might not be a worry but you should be aware the recommended space is for a reason.