The information on the size of your bicycle tyre is located on its sidewall. In general, all the information is on one side of the tyre, so if you don’t see anything, check out the other side! The information is usually engraved on the sidewall, without any color, so make sure to be in a well lit area in order to be able to read it.
The following information is provided :
Michelin, Hutchinson, Maxxis, Schwalbe, etc
PRO4, Python, High Roller, Rocket Ron, etc.
Size in inches or mm
This information is provided in a format such as «diameter x section»
- In inches (for MTB and BMX tyres) : for example 26 x 2.20 means that the tyre diameter is 26 inches and its section 2.20 inches
- In mm (for road and city trekking tyres) : for example 700 x 23C means that the tyre diameter is 700mm and its section is 23mm wide. The letter « C » is an indication of the matching rim’s width.
- Note that the diameter considered here is the outside diameter, or « overall » diameter, i.e the diameter including the « height » of the tyre, in other words the distance between the bead and the tread. This diameter is thus higher than that of the rim.
- This information is not subject to official standards, and thus leaves room to interpretation or to specific marketing positionning tactics. An MTB tyre with a section of 2.40 in one brand can actually have a smaller section than a 2.35 tyre of another brand. As for shoes, some brands size small, and others, large.
This is the standardized and most rigourous way of indicating a tyre size, as imposed by the « European Tyres and Rim Technical Organisation ». The format is of type XX – XXX, for example 23-622.
- The first 2 digits correspond to the tyre section in mm
- The last 3 digits correspond to the inside diameter of the tyre in mm. This is the diameter of the tyre bead, the part which is in contact with the rim. It therefore matches your rim’s diameter.
- This information is the only one reliable to ensure that a) the tyre diameter matches that of your rim (or your existing tyre) b) the tyre section or width corresponds to your existing tyre, and is compatible with your rim and frame.
Relationship between ETRTO and tyre size in mm or inches
Note that whilst it is possible to deduct the tyre size in mm or inches starting from the ETRTO data, the reverse is not true. It is therefore vital to check the ETRTO of the tyre you want to replace, or that of the rim if the tyre date is not available (tyre too old, unreadable or missing). You otherwise run the risk of ending up with a tyre loo large or too small for your rim:
- Road bike or city trekking tyres in « 700 » have an ETRTO diameter of 622. This is also the ETRTO diameter of MTB 29″ tyres.
- Road bike tyres in « 650 » have an ETRTO diameter of 571. A common error in cyclocross is to take tyres in 26″ (instead of 650), which are in fact in 559…(instead of 571) and are thus too small.
- « 26 inch » tyres include tyres with a wide range of inner/ETRTO diameters including 559, 571, 584 ou 590. The most common is 559, which is the standard MTB 26″ size.
- « 24 inch » tyres include tyres with a wide range of inner/ETRTO diameters including 507, 520, 531, 534, 540, 541.
- « 20 pouces » include tyres with a wide range of inner/ETRTO diameters including 400, 406, 428, 432, 438, 440, 451. BMX freestyle tyres are in 406. BMX race tyres include 406 (most common) and 451 (for younger riders and thinner sections) ETRTO diameters
- New « 27.5 » ou « 650B » MTB tyres have an ETRTO diameter of 584.
Indication of the recommended as well as the maximum tyre pressure, expressed in bars and psi.
This indication is there to legally protect the manufacturer. In moountain biking, actual pressures used by riders are much lower.
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