Abstract: In a protective device to be incorporated in a bicycle rear derailleur for preventing a drive chain from being unexpectedly disengaged with the smallest sprocket wheel, the device has an internally threaded cylindrical portion and a substantially flat flange portion extending integrally from one end of said cylindrical portion, said cylindrical portion being screwable, in place of a lock nut, onto a standard mounting bolt which is employed in the known rear derailleurs for connecting a shifter body to a hook-shaped bracket; said flange portion being designed so that it extends, when incorporated with the rear derailleur, in parallel and slightly spaced with the opposite surface of the smallest sprocket wheel of a multi-stage freel wheel assembly; and by means of the smooth outer surface of said flange portion a desired chain fall prevention is attained.

Abstract: A bicycle sprocket wheel comprising a multi-speed free wheel assembly containing teeth thereof are set right and left alternatively for ensuring easier engagement with a running chain during gear change operation.