Rear derailleur for a bicycle

A derailleur for selectively shifting a driving chain among a plurality of sprockets coaxially mounted to a rear wheel of a bicycle, comprising:

• a transformable linkage mechanism including a base member, two linkage members and a movable member;
• a chain guide supported to the movable member and provided with a guide pulley for guiding the chain among the plurality of sprockets;
• wherein the linkage members are connected to the base member and to the movable member through connecting pins, wherein the connecting pins are slanted toward a plane perpendicular to an axis of the plurality of sprockets so that, when the linkage mechanism is transformed, the chain guide moves axially and radially of the plurality of sprockets;
• a return spring for constantly biasing the chain guide laterally inwardly with respect to the bicycle;
• a first horizontal shaft for swingably supporting the base member to a fixing member of the bicycle;
• a second horizontal shaft for swingably supporting the chain guide to the movable member;
• a first spring disposed at the base member for biasing the linkage mechanism in a direction of the guide pulley moving away from the axis of the sprocket assembly;
• a second spring disposed between the chain guide and the movable member for applying tension to the chain;
• wherein the transformable linkage mechanism and first and second springs cooperate together such that the position of the chain guide is forcibly shifted axially and radially of the sprockets by the transformable linkage mechanism and the shifted position of the chain guide set by the linkage mechanism is radially adjusted with respect to the plurality of sprockets by the balancing of the first and second springs.