A reversible drive device for bicycles, comprising:
- a driven sprocket detachably and rotatably installed at one side of a bicycle's rear wheel at a position spaced apart from the rear wheel, said driven sprocket being connected to a drive sprocket of a pedal through a chain and rotating the rear wheel using the rotating force of the pedal, and being shaped into a ring gear with internal and external teeth;
- a hub shaft is fixed to the center of the rear wheel and rotatable fitted over a fixed shaft, thus rotatable supporting the rear wheel on the fixed shaft;
- a rear wheel gear detachably held on said hub shaft so as to be rotatable around the fixed shaft along with the hub shaft, said rear wheel gear being eccentrically arranged in a concentric cavity of the driven sprocket with an eccentric gap being formed between the rear wheel gear and the driven sprocket, said rear wheel gear thus transmitting the rotating force of the driven sprocket to the rear wheel;
- a fixed intermediate gear having a fixed shaft and arranged in said eccentric gap and always engaging with the external teeth of the rear wheel gear;
- a movable intermediate gear having a movable shaft and arranged in said eccentric gap in a way such that the movable intermediate gear always engages with the internal teeth of the driven sprocket and is movable between two positions:
- a first position where the movable intermediate gear engages with both the driven sprocket and the fixed intermediate gear so as to rotate the rear wheel gear in a normal direction, and a second position where the movable intermediate gear engages with both the driven sprocket and the rear wheel gear so as to rotate the rear wheel gear in the normal direction, said movable intermediate gear being placed at said first position when the driven sprocket is rotated in the normal direction due to a normal rotating force of the pedal, and being placed at the second position when the driven sprocket is rotated in a reversed direction due to a reversed rotating force of the pedal;
- and
- a fixed bracket having an arcuate guide slot on the inside surface and fixedly mounted to said fixed shaft of the rear wheel, and holding the rear wheel gear, the driven sprocket and the two intermediate gears, said guide slot being exposed to said eccentric gap and having the same radius of curvature as that of an internally toothed surface of said driven sprocket and movably holding the movable shaft of said movable intermediate gear, thus allowing the movable intermediate gear to be movable between the first and second positions in accordance with a rotating direction of the driven sprocket.