Abstract: Additional propelling power for a bicycle is provided by mounting the bicycle seat to reciprocate in an up and down direction and converting this motion into rotary motion by a ratchet gear. Rotation of this ratchet gear takes place when the seat moves down under the weight of the bicycle rider, a compression spring returning the seat to its up position when the rider lifts his weight from the seat. The motion of the ratchet gear is imparted to the rear wheel of the bicycle through a gear train which amplifies the rotary motion and further which may be provided with a gear shift mechanism for varying the number of rear wheel rotations imparted for one down stroke of the bicycle seat. The auxiliary seat drive is provided in addition to the conventional pedal drive sprocket and chain arrangement normally provided on bicycles.