Middleweight Tire Sizes
According to SheldonBrown.com, middleweight tire sizes are common light and fast tires. They replaced the popular heavyweight tires, or "balloon tires," that were seen often in the 1930s. Middleweights that are 26 inches in size actuality measure out at 25 5/8 inches. A 25-inch tire could actually be a bit smaller, measuring around 24 7/8 inches.
Tubular Tire Sizes
Racing tires are tubular and have no beads. Beads are steel cable hoops that make the edge of a tire. Tubulars range in sizes from 26 inches for motorpacing bikes and the larger, more common 28-inch size for other kinds of racing bikes. All tubulars fit the rim snugly with a cement binding that holds them together.
French Sizes
Tires measured in French are shown with a letter designation after the tire size is listed in millimeters. For example, 400 "A" is a smaller-rimmed tire created for a juvenile or child rider. The "A" stands for narrow; "D" is for wide. However, currently there are many different types of sizes that may not necessarily correspond with the letter listed.
Rim Measurements Count
Sizing the appropriate tire to fit the rim is important for safety reasons. Too wide of a tire can cause road hazards as the result of pinch flats, when an object gets in between the rim and the tire during riding. Rim damage can also occur. For the best tire fit, wide rims require wide tires and narrow rims need narrow tires.
ISO Size Conversions
The International Organization of Standardization (ISO) has developed a system by which the correct bicycle tire size can always be known. This conversion system is based on millimeters. You can use the ISO Cross Reference chart to measure the bead seat diameter and determine the correct tire size needed, regardless of whether the tire is originally sized fractionally or measured in decimals, French sizes or other designations (see Resources).
