Steplessly-variable ratchet gearing for a chain drive

A chain or belt drive system which automatically alters the gear ratio according to load requirements comprises a driving wheel (15), a driven wheel having a driving connection (teeth 29) with a chain or belt, and means (23, 25, 27) providing a driving connection between the driving wheel and the driven wheel. The axis about which the driven wheel rotates is movable relative to the axis about which the driving wheel rotates. The movement gives rise to a change in the effective gear ratio and is opposed by a resilient biasing force. The driving connection between the driving wheel and the driven wheel comprises a plurally of driving elements (23) carried slidably by the driving wheel and cooperable sequentially through ratchet teeth with the driven wheel. The sliding connection allows relative movement of the driven wheel with respect to the driving wheel whilst maintaining the driving connection. A cam controls sliding movement of the driving element. The drive system may be used in a bicycle.