![]() If the rim is not marked with its bead diameter you can read this from the tyre – it’s the one dimension they have in common. Subtract 6mm from overall width to estimate the rim section, or simply measure the distance between the flanges. But some manufacturers give the overall width instead. Some rims are also now marked according to ISO with their bead diameter and internal section (width where the tyre fits) 622-17 for the size usually fitted to touring and hybrid bikes, for instance. Look for the ISO numbers and you always know exactly where you are. For example, 650B and 26×1½ are both 38-584 and should be marked accordingly. Two tyres with the same ISO markings are completely interchangeable, even if one seems to be metric and the other is also marked in inches. For example, between 18-622 and 25-622 there are tyres of 19, 20, 22 and 23mm section. Where you see a block of two or more compatible sizes, having the same bead diameter, you can assume that intermediate sizes exist that will also fit the same rim. The table below lists most of the sizes of tyre that are likely to be found in the UK. Note that American- and Japanese-branded road tyres tend to be smaller than described and often lack any ISO markings. The American column is for tyre sizes originating in the USA and where US markings may confuse. The first two headings need no explanation. Refer to the columns headed Imperial, Metric or American in order to work out what actual (ISO) size you have. The former is ‘fractional’, the latter ‘decimal’, and as a rule decimal and fractional tyre sizes that look the same are not.Īs far as possible, then, use these columns to convert to ISO dimensions and then have nothing more to do with those old markings! It might be supposed that these two tyres are both notionally 26" diameter and one and a half inches in width, but 26×1½ is a whole inch bigger than 26×1.5. Tyres of the same size may come with lots of different markings: 28×1¼×15/8, 28×1¾×1¼ or 700×32C, for example, which are all 32-622.Įven worse, the same or very similar markings can sometimes be found on completely different sizes of tyre: 26×1½ and 26x1.5. Although they’re familiar they can be confusing. ![]() Sometimes you can’t find any ISO size on a tyre, only the old-school markings. Switching from 630 to 622, for example, needs another 4mm of brake reach. By comparing this dimension you can see if the brakes will reach when a different size rim is fitted to your bike. ![]() Half of the bead diameter (that is, the bead radius) corresponds with the distance from the centre of the wheel to the centre of a brake block. These figures are useful for setting your cycle-computer and precisely calculating gears. That’s how the diameter and circumference (circ – equals distance rolled in one turn of the wheel) are calculated in the Tyre size table. But the height of the tyre usually corresponds quite closely to tyre section, so overall diameter approximately equals the bead diameter plus twice the section. Its actual width is normally less, but varies according to how the tyre is designed and the width of rim to which it is fitted. The initial two-digit number is the tyre section and roughly indicates the tyre width. If you’re ever in desperate need of a tyre and can find one with the same bead diameter, it will probably fit even if every other thing about it is different any tyre marked xx-622 will, at a pinch, fit any 622mm rim. This is the bead diameter at which the tyre fits onto the rim.Ĭheck the bead diameter of the tyres on your bike. The three-digit number after the dash is the more important. The ISO size comprises two numbers separated by a dash. To bring some order to the situation, most tyres are now marked according to the International Standards Organisation (ISO) in addition to more familiar markings. For example, some tyres marked 28" are actually much less than 27 inches in diameter, while 26×1.5 and 26×1½ are two completely different sizes. One way to define it, popular in times past, was to measure the tyre’s outside diameter and its maximum width when inflated, but this can be misleading. It’s worth noting that these markings have the potential to cause confusion, not least because a tyre’s ‘size’ can be hard to pin down. If so, then you’ll need to check the markings to be found on the side of the tyre. Of course, this isn’t much help if identical tyres aren’t readily available, if you fancy something different or if you’re looking for replacements in unusual and hard-to-find sizes. How do you know what size tyres will fit your bike? It should be easy: buy exactly the same model of tyre as those already on the bike.
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