There is described a bicycle (1) in which the rear wheel (3) is provided with a planetary hub gearbox (17) and is mounted on a single rear arm (13) positioned to one side of the rear wheel. The planetary gearbox comprises an outer casing (19) having an external bearing surface to engage a bearing mounted to the rear arm (13) of the bicycle, and releaseable mounting means for attaching the rear wheel (3) of the bicycle to the outer casing (19) of the planetary hub gearbox (17). The rear arm is of tubular or hollow construction, with the drive chain (15) or belt running enclosed within the rear arm (13).