The conventional rear derailleur for shifting the driving chain has hitherto been well-known, which ordinarily comprises a fixing member, a linkage mechanism provided with a base member, two linkage members and a movable member, and a chain guide provided with a guide pulley, a tension pulley and a pulley plate provided with an urging plate which extends toward the path of the chain travelling from the guide pulley to each sprocket at a multistage sprocket assembly and is opposite to the chain. The linkage mechanism, which fixes the base member to the fixing member, is mainly swingable by a first horizontal shaft, interposes between the linkage mechanism and the fixing member a first spring which biases the linkage mechanism to move away from the center of the multistage sprocket assembly, pivotally supports the chain guide to the movable member in relation of being swingable through a second horizontal shaft, and interposes between the chain guide and the movable member a second spring which applies tension to the driving chain, so that the first and second springs balance in tension with each other to control the position of the guide pulley with respect to each sprocket at the multistage sprocket assembly.
The rear derailleur employing the aforesaid first and second springs is disclosed in, for example, the Japanese Patent Laid-Open Gazette No. Sho 58-149,882, which is so constructed that the first and second springs apply the predetermined tension to the chain and balance in tension with each other to set the chain guide in position, in detail, the guide pulley at the chain guide in position with respect to each sprocket at the multistage rear sprocket assembly. Also, the chain is shifted from a smaller diameter sprocket to a larger diameter one at the multistage sprocket assembly through the urging surface at the pulley plate coming into contact with the chain to forcibly urge the chain toward the larger diameter rear sprocket. In addition, generally an operating wire is fixed to one linkage member and an outer sheath for guiding the operating wire is supported to the base member.
In the driving stage that the chain is stretched between the smaller diameter sprockets at the multistage rear sprocket assembly and at a multistage front sprocket assembly, when the chain is shifted from the smaller diameter front sprocket to a larger diameter front sprocket by operating a front derailleur, a linkage mechanism L, as schematically shown in FIG. 9, swings counterclockwise around a first horizontal shaft P.sub.1 as shown by the arrow in FIG. 9 and along the plane perpendicular to a hub shaft and the chain guide CG swings in the same manner as the linkage mechanism, both the linkage mechanism L and chain guide CG shifting from the positions shown by the dotted lines to that shown by the dot-and-dash lines respectively. In this case, the outer sheath O supporting position on the linkage mechanism is displaced from the point X.sub.1 to X.sub.2 in FIG. 9 as the linkage mechanism L swings, so that a curvature of a curving portion of the outer sheath O at the multistage rear sprocket assembly side is reduced from that shown by the solid line to that shown by the dot-and-dash line. Accompanied with this, the tension of operating wire increases to act on the linkage mechanism L to an extent of the tension increase, whereby the linkage mechanism L, even if the operating lever is not operated, deforms automatically axially inwardly of the multistage rear sprocket assembly.
The conventional rear derailleur provided with the first and secnod horizontal shafts parallel to each other is constructed to make the path of swinging motion of the linkage mechanism parallel to that of the chain guide, so that when the chain is shifted from the smaller diameter front sprocket to the larger diameter one, the guide pulley GP at the chain guide CG shifts radially along the plane perpendicular to the hub shaft HS at the multistage rear sprocket assembly S as the linkage mechanism L and chain guide CG swing around the first and second horizontal shafts P.sub.1 and P.sub.2 respectively. The displacement increases the tension of the operating wire so as to deform the linkage mechanism L axially inwardly of the multistage rear sprocket assembly S, whereby the guide pulley GP is displaced from the position shown by the solid line to that shown by the dot-and-dash line in FIG. 15. In other words, the guide pulley GP is displaced toward a second rear sprocket S.sub.2 of a larger diameter and adjacent to a first rear sprocket S.sub.1 carrying the chain. As a result, the chain guided by the guide pulley GP to the first rear sprocket S.sub.1 is slanted between the guide pulley GP and the first rear sprocket S.sub.1 to come into contact with the urging surface at a pulley plate of the chain guide CG to generate a sound. In order to prevent the soundmaking, there is the trouble that an operating lever for the rear derailleur must intentionally be restored. Especially, when a positoning mechanism is provided at the rear derailleur or the operating lever therefor, the conventional derailleur has a large defect.