An auxiliary power system for a bicycle having a motor (10) attached to the bicycle flame (22) by thin clamps (30), which can be arranged in a variety of positions to avoid disturbing the control cables. Power is delivered on demand via a sprocket driving a single freewheel (180) attached to a modified left crank (200). The gearbox (20) may be attached to the frame by means of a quick-release system. A bushing (600), (700), (820) may be used instead of a modified as a means of mounting the freewheel to the crank axle. (720), (610), (850b). To prevent accidental simultaneous braking and triggering, triggering the system is accomplished by placing the trigger switch (430) so that the rider's fingers must disengage the brake in order to engage the motor. Accidental triggering may be prevented by placing a trigger switch on each handle and wiring them in series so that both switches must be engaged to obtain power.