An automatically and smoothly continuously variable transmission, particularly for a bicycle, for transmitting input mechanical energy from a crank to the drive wheel. A ratchet, having an oscillatory, alternating motion member and a continuous rotary motion member, connected to the driven wheel, has a drive spring drivingly linked between the ratchet alternating member and the crank for driving the ratchet alternating member in one direction of motion. A return spring is linked to the ratchet alternating member for applying a return force upon the ratchet alternating member to return the alternating member in the opposite direction.