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Tyre and Wheel Sizes

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wheels
workshop

The naming conventions of wheel and tyre sizes follow a logic that is not obvious but is consistent.
Standard sizes developed originally as result of ergonomics but the actual size varied slightly from country to country. Most descriptions referred to the outside diameter of the tyre with a second part describing the thickness of the tyre. The rim size would be determined by deducting one from the other, so that a 26 x 1 1/4 is a larger rim than a 26 x 1 3/8 and is therefore not interchangable. We try to use the new ISO descriptions as these accurately describe the rim and tyre size, but an understanding of the old descriptions is useful and throws some light on their origins. 

If you still can't find what you are looking for, there is a more comprehensive list here.

ISO No. Conventional Name Origin Use
635 28 x 1 1/2 UK Large frame traditional Roadster bikes.
630 27 x 1 1/4 UK The old British touring size; now almost obsolete
622 700c France Continental racing size, used by almost all modern tourers and hybrids. In northern Europe they call it 28 x 1 5/8 just to confuse us.
597 26 x 1 1/4 UK Old racing size, now very uncommon.
590 26 x 1 3/8 UK Standard British utility bike size, found on old single and three speed bicycles. Some decent tyres now available.
584 650B France This size became almost obsolete but is now becoming fashionable again in the US. Why???
571 650A France Racing size for many time trial and triathlon bikes with a small front wheel.
559 26 x 1.75 USA Mountain bike out of Beach Cruiser; fatter tyre on smaller rim gives plenty of cushioning and mud clearance, now available with tyres as narrow as 1".
540/541 24 x 1 3/8 UK/France Two sizes are interchangable, sometimes found on kids racing bikes, old recumbent rear wheels.
520 24 x 1 UK/France Some small framed racing bikes, low-pro front wheels and Airnimals.
507 24 x 1.75 USA Kids and small wheeled mountain bikes. 24"cruisers. A few plump touring tyres are now available.
451 20 x 1 3/8 UK Old British shopper size, sometimes called 20 x 1 1/8 as a racing recumbent tyre.
406 20 x 1.75 US Originally for BMX's and balloon tyred shopper bikes, now becoming the standard for quality small wheeled bikes with lots of good tyres and rims available.
349 16 x 1 3/8 UK Bromptons and some other folding bikes, looked as if it was going out of fashion for a while but the success of the Brompton has lead to a bit of a revival, with good quality tyres now available.
305 16 x 1.75 US Mini BMX's and front wheels on some recumbents, there are a couple of good quality tyres.
204 12 x 2 US Mostly kids bikes and baby buggys.