If you are in a bicycle shop looking to replace your bicycle tire tubes, leave your calculator at home. Simply converting millimeters to inches or vice versa won't expedite your purchase, get you back on the road or do anything to explain why bicycle tire sizing is so counter intuitive. For practical purposes, a discussion of tire tube sizing is actually a discussion of tire sizing, because the tubes are marked with the notation of the tire they are meant to fit.
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Seeds of a Patchwork System
When there are quantifiable mathematical relationships, such as a bicycle wheel’s diameter and rim width, it would make sense to use a standard set of notations to describe them. Unfortunately, for the bicycle’s first 100 years of history, that has not been the case. The French established a system of notation, the English scoffed, and Americans couldn’t quite decide whether fractional or decimal notation was more user friendly. The result was a patchwork system of tire tube sizing, and converting between them was at best cumbersome and at worst impossible.
The Chart That Explains Things
The European Tyre and Rim Technical Organisation in conjunction with the International Organization of Standardization developed a unified system of notation. To convert tire tube sizes from one notation to another, it is necessary to use the chart published by the ISO available at any reputable bike shop. The standard is called the ISO 5775, and if your shop doesn’t have the chart available, it is published at the websites of the ISO and the ETRTO.
Interpreting the Numbers
Tire sizes, and by extension the proper tubes to go in them, are denoted by two numbers. The first refers to the circumference or diameter of the wheel while the second roughly, but not perfectly, notes the width. For example, an American bicycle marked with the numbers 28 X 1 3/4 describes a tire that is 28 inches in circumference and 1 3/4 inches in width. A French notation that is similar but not quite equal, given in millimeters, is 700C, where “C” indicates an acceptable range of widths. The ISO standard notation for such a tire is slightly variable, given that several widths are appropriate for such a rim, but they range from 30-622 to 42-622 where the first number indicates the width in millimeters and the second number indicates the circumference in millimeters.
Different Measuring Points
Already, a system that sometimes used circumference and sometimes used diameter was in need of some serious reworking, but confusion was only one problem plaguing the industry. First, different companies or countries measured the wheel at different places. Some chose the outside circumference of the tire while others measured the bead seat circumference, the point at which the tire’s edge makes contact with the rim. Second, American tires with fractional notation are not interchangeable with tires using decimal notation, even if the numbers are mathematically equal. The ISO/ETRTO sought to change all that, but the difficulty in converting measures for older tires remains.