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Tyre sizing issue

I bought a Raleigh road bike for my 6 year old. It has 24 X 1 or 25-520 Kenda tyres on it. He's doing some cyclo-cross so I found some Kenda 24x1 3/8 or 37-520 tyres in the UK and bought them into Ireland.

Have received the tyres today and went to fit them but the inner diameter of the bead on the CX tyres seems to be at least 1 inch too long and the tyres won't fit. Am I stupid and missing something here as I assumed they would fit the existing rims no issues.


  • DeVlaeminckDeVlaeminck Posts: 8,688
    My understanding is if they are both labelled 520 then they should be the correct diameter.
    [Castle Donington Ladies FC - going up in '22]
  • veronese68veronese68 Posts: 26,940 Lives Here
    Yes, there can be differences in diameter of some tyres between standards, there are two seat diameters for 26" tyres for example. But as above, if they are both marked 520 they should be the same. Have you checked the markings on the tyres? Could the supplier have sent the wrong size? There is a good explanation on the Sheldon Brown website.
  • singletonsingleton Posts: 2,468
    Seems strange.

    As I understand it the 520 on a ETRTO sizing would mean that 520mm is the internal diameter of the tyre - perhaps you can measure both and see which one is the wrong size.
  • prcodyprcody Posts: 67
    Turns out it was a 37-540 I bought which is still listed as a 24" tyre but maybe I have to much of a road style 24" wheel to fit a tyre of this width.
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