650b

650b is a bicycle wheel and tire size that has been widely used on various types of bikes, particularly mountain bikes and gravel bikes. In modern specifications, 650b refers to a bead seat diameter (BSD) of 584 millimeters. When a tire is mounted, the overall diameter varies with tire width, but the size is commonly marketed and perceived as 27.5 inches in diameter.

Origins and naming of the size trace back to the traditional French wheel-sizes system, where numbers roughly indicated the wheel’s diameter and letters signified a family of widths. The 650b designation originated from those early conventions and was later standardized in contemporary terms as ISO 584 mm BSD. Today, the label 650b is often used interchangeably with “27.5-inch” wheels, though the exact outer diameter depends on the tire mounted.

Usage and suitability have evolved over time. 650b gained prominence in the 2010s as a middle ground between the smaller 26-inch wheels and the larger 29-inch wheels, offering a balance of roll-over capability, acceleration, and weight. It remains common on many gravel and mountain bikes, and is also seen on some touring and non-traditional road setups. Tire widths for 650b bikes range from narrow road-like tires to wide, inflated off-road tires, including 2.0 to 2.8 inches or more in some plus-size configurations.

Compatibility and considerations include frame and brake compatibility, since disc brakes and rim brakes can be used depending on the frame design. Fork and rear triangle clearance must accommodate the chosen tire width and the bike’s geometry.

See also: wheel sizes, 27.5-inch wheels, ISO bead seat diameter, disc brakes, gravel bikes.