Abstract: A spring-assist drive for a pedal-operated rider-propelled vehicle such as a bicycle which makes use of a wind-up coil spring mounted within the frame of the bicycle as auxiliary power. The spring is selectively wound by the rider operating the pedals to turn a spindle on which is mounted a sprocket wheel. A chain from the sprocket wheel drives a pivoting gear assembly comprising a ratcheting sprocket and attached drive gear. In a preferred embodiment, the tension on the chain will be adjustable by a chain tension adjustment mechanism. The drive gear releasably engages a power gear mounted on a common hub with the spring. A brake band and drum assembly holds the energy in the spring until the brake is released by the rider, whereupon the spring unwinds to drive a sprocket wheel mounted on the common hub, which sprocket wheel is connected to a sprocket wheel mounted on the spindle, turning the rear wheel drive sprocket wheel and the rear wheel.