650C

650C is a historical bicycle wheel size designation used to describe a wheel of about 650 millimetres in overall diameter. It belonged to an older, non-ISO family of wheel sizes that included 650A, 650B, and 650C, a nomenclature used before standardized bead-seat diameters (BSD) were adopted. The "C" indicated a particular class of rim, but there was no single universal BSD for 650C; sizes varied by manufacturer and era. As a result, 650C rims and tires were not generally interchangeable with other modern sizes.

In practice, 650C wheels were most common on smaller-framed road bikes and some early triathlon or time-trial designs, particularly during the 1970s and 1980s when frame geometry and clearances sometimes favored smaller-wheel configurations. The widespread shift to ISO 622 mm BSD, marketed as 700C, and the later adoption of 584 mm BSD for 650B wheels contributed to a decline in 650C usage. Today, 650C is largely obsolete, appearing mainly on vintage bikes, restorations, or specialist builds.

Parts availability for 650C—rims, tires, tubes, and spokes—remains limited compared with modern wheel sizes, which can complicate restoration or maintenance. Contemporary cyclists generally encounter 650C only when dealing with older frames or collectors’ bikes. For most current cycling needs, the related sizes are 700C (ISO 622) and 650B (ISO 584). See also: 650B, 700C, bicycle wheel sizes.