a rotatable orienting washer;

a coil spring having a first end fixed relative to the derailleur head and having a second, opposite end rotatably mounted relative to the orienting washer;

an adjusting device for rotating said second end of the coil spring relative to the derailleur head, without contacting said orienting washer, to adjust tension of said coil spring, said adjusting device including:

an adjusting sleeve adapted to rotatably extend through the derailleur head and parallel to and at least substantially along the entire length of the coil spring, and connected one of directly and indirectly with the second end of the spring, and

an adjusting tool for engaging said adjusting sleeve to rotate said adjusting sleeve in said derailleur head;

a retaining arrangement for releasably retaining said second end of said coil spring at a desired angular position after said second end of the coil spring has been rotated, said retaining arrangement including:

a spring adjust collar rotatably mounted relative to said orienting washer, and said second end of said coil spring is fixed to said spring adjust collar,

gear teeth on said spring adjust collar, and

a holding device for engaging with said gear teeth to retain said spring adjust collar and said second end of said coil spring at a desired angular position after said spring adjust collar has been rotated; and

a securing and mounting arrangement for securing together said orienting washer, said coil spring, said adjusting device and said retaining arrangement to assemble said mechanism and for mounting said assembled mechanism to the derailleur head and one of a bicycle frame and a derailleur cage.