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Is there a reason why it's considered ok to put a 23mm tire on a 23mm rim but unsafe to put a 32mm tire on a 31mm rim?
Internal width is the critical dimension. As rims get wider, the internal width is a greater proportion of external width. In your example, assuming the hook accounts for 3mm per side in both cases, the ratio of internal width is 17/23=0.74 in the first case and 25/32=0.78 in the second. It's a marginal difference, and there isn't really a hard cut-off, but >0.75 is something you want to approach with caution, just as <0.5 is. I have run at 0.84 and 0.41 without succumbing to fiery death, but in both cases I'd have been better off closer to the mid range. All of the above relates to road tyres. For MTB tyres, the manufacturer will usually have designed around lower ratio, probably somewhere between 0.3 and 0.6
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gbj_tester
Is there a reason why it's considered ok to put a 23mm tire on a 23mm rim but unsafe to put a 32mm tire on a 31mm rim?