| 摘要 |
Each of pedal cranks (4) and (4) of a bicycle, comprising a main crank (41) of which base end is fixed to a crank pin (31) which passes through the center of a large gear (3) around which a chain (32) is passed and an auxiliary crank (42) which is close-fitted longitudinally slidably to the main crank (41) and provided with a pedal (5) at its tip part, wherein a cam roller (43) is installed projectedly from the side surface of the auxiliary crank (42), a cam (6) guiding the cam roller (43) is installed on a bicycle frame (1) in parallel with the pedal crank (4) rotating plane, and the cam (6) guides the cam roller (43) so as to extend the auxiliary crank (42) gradually as the pedal crank (4) rotates from a just upright direction to a forward direction and retract the auxiliary crank (42) gradually as it rotates from a forward direction to a downward direction.
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