By Bruce Wright, March 5, 2005
A while back we had a display at the McLean Health and Beauty Expo. It was a small show, and since the weather was mild and the sun was out, most people seemed to have other things to do than attend the expo. Since I rarely leave home without some reading material, I had time to delve into the complexities of rim and tire sizes.
Tire size consists of tire width and tire diameter. At one time, tire diameter directly related to the diameter of the tire tread when mounted and inflated. The outside width of the mounted and inflated tire was used for the tire width. Since that convention was originated, tire shape and proportion have changed. Higher performance tires may actually have a different outside tread diameter than that labeled on the tire sidewall. A 26 inch tire may actually be less than 25 inches in diameter.To make matters worse, different countries use different sizes for the same tires. The inch-based size convention uses either decimals or fractions for tire width, which are not the same. The 26x1½ inch tire has an ISO diameter of 584 mm while the 26x1.5 inch tire has an ISO diameter of 559 mm.
Rim measurements have also changed. Traditional rim sizes were based on the size of the tire that would fit the rim. A 26 inch rim actually has a much small diameter, closer to 22 inches. A 20 inch rim can be used to reference two very different rims that have a diameter of either 451 mm (17.8 inches) or 406 mm (16 inches).
To solve this problem, the International Standards Organization, ISO, created a system of classifying tires and rims that uses the actual physical measurement. The bead of the tire, which is the part of the tire that contacts the rim, is used for measuring the tire diameter. Where the bead of the tire meets the rim is used for measuring the rim diameter, so that a ISO 406 tire has the same diameter as an ISO 406 rim. To ensure that the ISO system is not confused with the traditional measuring system, the width is listed first and the diameter second: 35-406 (ISO) vs. 20" x 1.35" (traditional).
So...the bottom line is, when trying to match tires with rims, it's best to always use the ISO system to ensure that the tire diameter matches the rim diameter. The rim width used by ISO is the inside distance between the rim walls, where the tire bead meets the rim. ISO tire widths are measured differently, and the ISO width will not always match the rim width. If the ISO rim and tire diameters match, the tire will likely fit the rim. However, very wide tires are not recommended for narrow rims, even if their diameters match. Sheldon Brown provides a very good chart showing the appropriate range of tire widths for various rim widths.
The above chart is a section of a lengthy discourse on tire sizing systems by Sheldon Brown. Another good source of information is Chapter 19 of Barnett's Manual, which is used as the bicycle mechanic training manual by the Barnett Bicycle Institute.