User:Ccrrccrr/Bicycle Tire History

Size standardization and designation.
In the very early 20th Century, the British standardized tire sizes, based on inch sizes. The 1920 list included 22, 24, 26 and 28 inch diameters, which were outside diameters of the tire (OD), not the rim diameter, and included 1 3/8", 1 1/2", and 1 3/4" widths, though not all of those widths in all of those diameters.

These also have metric designations. An exact description would be something like 711.2 x 44.4, but it was decided, perhaps by the French, to designate them by a nice round number, like 700 x 44. But the so called "700 x 44" was actually made to 711.2 x 44.4 dimensions to be interchangeable with the British standard tires. This system used a set of nominal OD values including 550, 600, 650, and 700, and widths including 32, 38, and 44, corresponding to the diameters and widths listed above. These widths came to be designated as A, B, and C.

Under this system, a 700C tire would have and outside diameter of 711 mm (28 inches), and a bead seat diameter of 711 mm (28") minus what a C tire adds, a C tire being approximately 44 mm, but exactly 1.75 inches.  Hence the standard bead seat diameter of  24.5 inches, or 622 mm for a 700C tire.

Given a rim with a bead seat diameter of 622 mm, ...