Abstract: A rotable sprocket assembly transmits force from a chain into rotation of a bicycle wheel. 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 housings are stationary relative to each other and each is oriented in a plane which is normal to a central rotational axis. The sprocket segments are slidable relative to the housing in a 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. 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.