Standardization is one of the hallmarks of our times, and a people’s favourite, since we have so many standards. Just look at bicycle tires, which abound in so many different formulas to express their size that they aren’t anything short of a foreign language. You have millimeters, centimeters, inches, widths, lengths etc. Basically, all these are ingredients that can spoil the fun you can have on your bike, so in this article we explain what’s the deal with them.
Initially, the tire size simply reflected the equivalent of its outer diameter, expressed in inches or millimeters (e.g. 26”, 27”, 700mm). Issues started appearing when rims having different widths were introduced on the market, because these rims required tires with a bigger baloon, that subsequently influenced their size. Even if the inner diameter remained unchanged and the tires fitted the wheels, not all of the tires were actually the same size as declared (for instance, in many cases, 26 inch mountain bike tires didn’t really have a 26-inch outer diameter).
Then, like now, if we reffer to the size expressed in inches, this was written as the wheel’s size x tire’s width. This means that a tire designed for a 26-inch wheel that had a width of 1.75 inches had a size of 26×1.75. Very logical and simple up to here, but some manufacturers started coding the sizes using decimals (like in the earlier example), while others used fractions (like 1-¾ inches). Mathematically, the two sizes are equal, but in reality, the two tires are not the same size.
What to do about that? Meet ISO (International Organization for Standardization) and ETRTO (European Tyre and Rim Technical Organisation) which (some would disagree) saved the day. These two organisms proposed another codification system which goes like this: the first number represents the tire’s width, while the second the rim’s diameter measured at the lowest point of its braking surface. Therefore, a 20-622 tire is a tire 20mm wide that fits on a rim with 622mm in diameter (typically a road bike rim or a 29er one). Also, the rim’s diameter expressed in this manner coincides with the inner diameter of the tire.
There is also the French system that measures the sizes we’re talking about – 700x20C. The first number reffers to the tire’s outer diameter in millimeters, the second number the tire’s width in millimeters, while the letter stands for a certain rim width, A being the narrowest, and D the widest. Currently, this system is used less and less since it became possible to mount tires on rims of virtually any width. However, it was brought up with the arrival of the 27.5 inch mountain bike, which used „B”-width rims.
Below we prepared a tabel with rim sizes of 4 different bicycle types and their equivalents in inch, ETRTO and French standards.
Tips regarding bicycle tire and rim sizes:
- Generally speaking, the width of the tire should remain within the interval obtained by multiplying the inner width of the rim with 1.45 and, respectively, 2.
- If you squeeze a tire so that its lateral ends completely stretch apart, the distance between those 2 ends should be equal to the width of the tire expressed in the ETRTO standard multiplied by 2.5.
- If the tire you mounted is too narrow for the rim, then you risk damaging the 2 components, as well as yourself.
- If the tire you mounted is too wide for the rim, then you risk damaging the tire’s side walls if you have V-brakes, or you risk the tire simply coming off the wheel in stressful conditions.
- For extreme mountainbiking, we recommend tires at least 2.25-inch wide, for XC 1.90-inch tire, while for road cycling, 23mm is the optimal size, since it reaches a great balance between comfort and low rolling resistance.