A mechanism for adjusting spring tension at a derailleur head mountable to a bicycle frame having a limiting stop, includes an orienting washer rotatably mounted to the bicycle frame and having a tail engaging the limiting stop. A coil spring has a first end fixed to the derailleur head and a second end rotatably mounted relative to the orienting washer. An adjusting sleeve in the head rotates the second end of the coil spring relative to the orienting washer, to adjust tension of the spring. A retaining arrangement releasably retains the second end of the spring at a desired angular position after the second end of the spring has been rotated. A bolt secures together the orienting washer, the spring, the sleeve and the retaining arrangement to assemble the mechanism and for mounting the assembled mechanism to the bicycle frame and the derailleur head.