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I got a cross bike from another rider and swapped out the 23c tires for some 32c ones. That got me thinking, how exactly can you tell what tires can fit on a given rim? If I want to go wider, any way to find out?
The received wisdom on tire and rim width can be found by scrolling down near the bottom of this page by Sheldon Brown. (http://sheldonbrown.com/tire-sizing.html)Note the rim width you need is not the commonly quoted outside width but the internal width between the bead hooks so you would have to pull the tire to measure it, unless you can find the measurement on the rim maker's web site. Of course most bike manufacturers use rims that the rim manufacturers don't own up to making so that can be hard, it is easier to measure them! Also note that you can go two or three sizes outside of the indicated limits, they are pretty conservative. For example at least two rim manufacturers are now claiming that 18 mm rims are perfect for 23 mm tires and that is one or two sizes below the recommended limit for 18 mm rims, if Sheldon's chart listed 18 mm rims. If your rims came with 23 mm tires they most likely are 13/14/15 mm rims and 32 mm tires should be fine. If you want to go to 38 mm they should be fine on 15 mm rims, ok on 14's, and they might work on 13's but that would be pushing it.