Abstract: A rotatable sprocket assembly transmits force from a chain into rotation of a wheel on a bicycle. A plurality of sprocket segments having peripheral teeth are mounted on a pair of sprocket housings so that the combined periphery of the sprocket segments is substantially circular. The sprocket segments are slidable relative to the housing in a substantially radial direction so as to vary the diameter of the combined sprocket segments. Each sprocket segment is formed from a plurality of hinged subsegments, with each subsegment having a single tooth thereon. The sprocket segments of one housing overlap the adjacent sprocket segments on the other housing so that the pairs of overlapping teeth combine to effectively form a single tooth. The sprocket segments are movable angularly relative to the sprocket housings, so that in combination with the radial displacement of the sprocket segments, the total number of effective teeth on the sprocket assembly can be varied in increments of one tooth.