Your bicycle\u2019s tires might not be something you think about very often, but they have crucial implications for every aspect of your ride. Better understanding how your mountain, gravel, and road bike tire size is measured can help you fine-tune your equipment, and learning how tire size relates to performance can make you faster when it counts. What are the basics of bike tire size?<\/p>\n\n\n\n
Bike tires are typically measured in two dimensions\u2014 diameter and width. The diameter measurement is an approximation of the tire\u2019s total outside diameter including treads, and the width is a measurement of the approximate total width of the tire when mounted and inflated. For mountain bike tires these dimensions are expressed in inches, while a millimeter-based system called French sizing is used for road, gravel, and track. For example, a 29 x 2.25 mountain bike tire is about 29\u201d in diameter and about 2.25\u201d wide, while a 700c x 25 road tire is approximately 700mm in diameter and 25mm wide. <\/p>\n\n\n\n
This makes it pretty straightforward to fit a modern tire to a modern rim\u2014a 700c tire will almost definitely fit a 700c road rim (we\u2019ll explain that \u201cc\u201d later), and a 29\u201d tire will likely fit a 29\u201d mountain bike rim. But some obsolete or unusual sizes can be misleadingly labeled, and any tire\u2019s nominal measurements (especially width) are really just approximations. Rim width and tire pressure can significantly influence the size of a tire when mounted and inflated, and tires often measure a bit larger or smaller when installed than the printed dimension would suggest. <\/p>\n\n\n\n
To reduce confusion, most tires are also labeled with a second system of measurements called ISO (formerly known as ETRTO). The ISO measurement displays the tire\u2019s nominal width in millimeters, followed by the diameter of the tire\u2019s bead (the surface that actually attaches to the rim) in millimeters (ex: 25 x 622 is a common road tire). This measurement can help resolve any ambiguity about whether a tire will fit a particular rim, but as with other systems, the ISO measurement of a tire\u2019s width is an approximation and may be impacted by pressure and rim width.<\/p>\n\n\n\n
Nearly all modern road bikes use 700c wheels and tires. It used to be widely accepted that narrower tires were faster and 23mm was the standard width. But recent research has proven wider tires to be faster and more comfortable in most situations. As a result, 700c x 25mm and 700c x 28mm are now the most common road tire sizes; many riders prefer even wider widths of 30mm or 32mm. The limiting factor is usually the bike itself, with some frames unable to accommodate tires beyond a certain width. Most new road frames can at least fit up to 28mm tires but double-check your frame\u2019s allowance before sizing up.<\/p>\n\n\n\n
A few other less common wheel and tire sizes exist for road bikes. 650b (ISO 584) and the rarer 650c (ISO 571) are two examples, both sometimes used on bikes for smaller riders. The letter that follows the diameter measurement in French tire sizes originally delineated width, but it\u2019s now mostly just useful to differentiate between similarly-named but incompatible sizes. For instance, a 650b tire will not fit on a 650c rim.<\/p>\n\n\n\n
It\u2019s also important to understand the different types of mutually-incompatible road tires. Clinchers are most common; these are the familiar tires that seat into a walled rim around an inner tube. Certain clincher rims can also be used with tubeless tires, which use a liquid sealant in place of an inner tube. Finally, tubular tires are permanently sewn closed around an internal tube and are glued into a specially-made rim. All 3 of these tire types use the same sizing standards and terminology but are generally not interchangeable.<\/p>\n\n\n\n
Tires are printed with a manufacturer\u2019s recommended pressure range, and road riders used to think inflating their tires to the highest possible pressure was fastest. But with the move to wider tires has also come a trend towards lower pressure. The science of tire pressure is complicated, but wider tires require less pressure for the same volume of air than narrower tires, allowing for a more comfortable ride. Additionally, wider tires at lower pressures reduce bouncing and are actually faster on most surfaces than smaller, harder tires.<\/p>\n\n\n\n
It\u2019s tough to make a generalized recommendation for pressure\u2014riding conditions, your weight, and the tire\u2019s size all play a part. But generally, the larger the tire and the rougher the surface, the lower the optimal pressure. With each 3mm increase in tire width, you can usually reduce pressure by 1 Bar (~14 psi). Also, tubeless tires can generally be ridden at lower pressures than tubed tires of the same size. Some tire and rim manufacturers have calculators on their websites that make personalized recommendations for pressure; these are a great starting resource to make your ride faster and more efficient.<\/p>\n\n\n\n