Improvement to a device for varying the length of the pedal crank of a bicycle by using a rail (7) intended to guide an extensible pedal crank characterized in that a sleeve (3) is fixed to the drive axle of the crank gear and has a recessed portion throughout its length wherein can slide the crank arm (9) going beyond the two extremities of the sleeve and supporting at one end the pedal (5) and at the other end the length variation rail guide-roller differential (6). An other improvement is characterized in that the sleeve (3) has a very elongate oval shape provided with a rectilinear slot (12) wherein can slide a carriage (11) supporting on the circular rail side the guide-roller differential (6) and on the other side the pedal (5).
