Abstract: A bicycle includes a pedal-activated spindle, a sprocket-mounting sleeve and a front sprocket sleeved fixedly on the sleeve. A planetary gear train interconnects the spindle and the sleeve, thereby rotating the sleeve at a speed larger than that of the spindle. When a pedaling torque applied to the bicycle is above a predetermined torque, a ring gear of the train rotates in a casing. The ring gear is limited to rotate between a non-pushing position and a greatest-displacement position, and is biased by a spring to the non-pushing position. When the ring gear rotates from the non-pushing position by a predetermined angle, a sensing unit activates a motor unit so as to drive an auxiliary electrical transmission, thereby transferring a torque from the auxiliary electrical transmission to the sleeve. A first one-way clutch interconnects the train and the sleeve so that the output of the train can be transferred to the sleeve while preventing the rotation of the sleeve from being transferred to the train.