Abstract: In combination with a bicycle having a pedal crank shaft housing, a pedal crank shaft journaled in said housing, a frame having a front diagonal bar, an upwardly extending bar forming a seat support, a rearwardly extending fork, a forwardly extending fork, a wheel carried by said forward fork, a rear wheel carried by said rear fork and a sprocket assembly on the rear wheel, an auxiliary motor unit attachment comprising, a motor have a power output member, a power input member mounted on pedal crank shaft on one side of pedal crank shaft housing for transmitting power output from motor to the pedal crank shaft, a sprocket assembly mounted on the pedal crank shaft on the other side of the pedal crank shaft housing, a chain connecting the sprocket assembly with the rear wheel sprocket assembly, and left and right pedal cranks extending from the ends of the pedal crank shaft, for engaging the pedal crank shaft to impart its movement to the shaft and to disengage the shaft when power is transmitted from the power