In the mid 1970s the Japanese bicycle industry developed an obsession with designing an effective indexed rear derailleur. Shimano was the key driver, taking out a blizzard of patents, starting in 1974. European manufacturers arrogantly mocked these efforts, particularly the early Shimano Positron models - only to be totally, and deservedly, destroyed when Shimano finally got it right with the SIS system in the mid 1980s.
The Bridgestone Synchro Memory Shift, patented in 1975 and manufactured by SunTour, was a fascinating response to the slightly dodgy engineering of Shimano’s first (1974) Positron designs. Bridgestone decided to go for Rolls-Royce engineering and technical over-kill.
A single cable winds around a massive (36mm diameter) sprung cylinder under the derailleur, and, as it is pulled, turns the cylinder, turning a cam that drives a roller on an arm that manoeuvres the (sprung) parallelogram into the correct position. As the roller ride over the ‘lumps’ on the cam there is even a tiny amount of over-shift before the roller settles back into the ‘dip’ that corresponds to the gear. The Bridgestone Synchro Memory Shift has a cam set up for five speeds.
The whole contraption is extremely solid and accurately made. However, for a mass market product, it is very complex and, therefore, very expensive to manufacture. And it is very, very, heavy for the cyclist to drag up hills!
However, other than the fantastic indexing mechanism, the design of the Bridgestone Synchro Memory Shift is very basic. It has only one sprung pivot, the parallelogram is not slanted and there are no high and low gear limit screws. The pulley cage is extravagantly offset - but this is the only concession to managing chain gap. These features are very untypical of SunTour designs of the time, and are all included in the Bridgestone patent - so I think this was very much a Bridgestone project.