www.sheldonbrown.com/tire_sizing.html
WTB's Global Measuring System image Which Size Tire Fits Which Size Rim? To make matters worse, in the early days of cycling, every country that manufactured bicycles developed its own system of marking the sizes. These different national sizing schemes created a situation in which the same size tire would be known by different numbers in different countries. Even worse, different-sized tires that were not interchangeable with one another were often marked with the same numbers! Traditional Sizing Systems The traditional sizing systems are based on a measurement of the outside diameter of a tire. Unfortunately, evolution of tires and rims has made these measurements lose contact with reality. Here's how it works: Let's start with the 26 x 2125 size that became popular on heavyweight "balloon tire" bikes in the late '30's and still remains common on "beach cruiser" bikes. This size tire is very close to 26 inches in actual diameter. Some riders, however were dissatisfied with these tires, and wanted something a bit lighter and faster. The industry responded by making "middleweight" tires, marked 26 x 175 to fit the same rims. Although they are still called "26 inch", these tires are actually 25 5/8", not 26". This same rim size was adopted by the early pioneers of west-coast "klunkers", and became the standard for mountain bikes. Due to the appetite of the market, you can get tires as narrow as 25mm to fit these rims, so you wind up with a "26 inch" tire that is more like 24 7/8" in actual diameter! A second number or letter code would indicate the width of the tire.
Note that even some tire manufacturers get confused about this. In particular, some Continental models are mis-marked, using a fractional designation where they should be using a decimal. Dishonesty in Sizing Competitive pressures have often led to inaccuracy in width measurement. Here's how it works: Suppose you are in the market for a high performance 700 x 25 tire; you might reasonably investigate catalogues and advertisements to try to find the lightest 700-25 available. If the Pepsi Tire Company and the Coke Tire Company had tires of equal quality and technology, but the Pepsi 700-25 was actually a 700-24 marked as a 25, the Pepsi tire would be lighter than the accurately-marked Coke 700-25. In self defense, Coke would retaliate by marketing an even lighter 700-23 labeled as a 700-25. The situation got so out-of-hand that cooler heads have prevailed, and there is a strong (but not universal) trend toward accurate width measurements.
The width difference between these sizes would make them less-than ideal replacements for one another, but any rim that could fit one of them would work after a fashion with the other. The following is a partial listing of traditional tire sizes that are sometimes seen in the US, with their ISO bead seat equivalents.
Schwinn Juvenile 12 1/2 x anything 203 mm Juvenile, scooters 10 x 2 152 mm Wheelchair 8 x 1 1/4 137 mm Wheelchair Traditionally, fractional sizes are made for straight-sided rims. High-performance sizes (571 mm /26 x 1 & 630 mm /27") have evolved toward hook-edged rims.
recumbents image French sizes: In the French system, the first number is the nominal diameter in mm, followed by a letter code for the width: "A" is narrow, "D" is wide. The letter codes no longer correspond to the tire width, since narrow tires are often made for rim sizes that originally took wide tires; for example, 700 C was originally a wide size, but now is available in very narrow widths, with actual diameters as small as 660 mm. French Size ISO Applications 700 C 622 mm Road bikes, hybrids.
John Allen for Sutherland's Handbook For Bicycle Mechanics, the bible of bicycle technology. Sutherland's has a more detailed, more thorough version of this chart. Width Considerations Although you can use practically any tire/rim combination that shares the same bead seat diameter, it is unwise to use widely disparate sizes. If you use a very narrow tire on a wide rim, you risk pinch flats and rim damage from road hazards. If you use a very wide tire on a narrow rim, you risk sidewall or rim failure. This combination causes very sloppy handling at low speeds. Unfortunately, current mountain-bike fashion pushes the edge of this. In the interest of weight saving, most current mountain bikes have excessively narrow rims. but that defeats the purpose of wide tires, and puts undue stress on the rim sidewalls.
WTB Website: image GMS Global Measuring System The current industry standard for specifying the actual inflated size of a bicycle tire does not account for subtle variation in tread and casing size. To address this problem and provide you with more information for comparing tires, WTB has introduced the Global Measuring System (GMS) for tire measurement. The GMS uses a two-number system: the first number is the width of the casing, and the second number is the width of the tread, both in millimeters. These measurements are taken on a rim which is 20mm wide at the bead-capturing point, with a tire inflated to 60psi and maintained for 24 hours. In addition to being able to accurately size a tire, knowing the actual casing size and tread width provides an indication of air volume, tread characteristics and tread contact area; all of which provide you with a more concise idea of what ride characteristics to expect from each of WTB's tires.
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