With the SMS Max SunTour (working with Bridgestone) decided to respond to the slightly dodgy engineering of Shimano’s Positron design with technical over-kill.
The cable winds around the 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 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 SMS Max 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 very, very heavy for the cyclist to drag up hills!
Other than the fantastic indexing mechanism, the design is a single sprung pivot slant parallelogram with adjustment screws on the back in the SunTour traditional style.
This short cage example, however, has the pulley cage massively offset from the bottom pivot, echoing the design of the double pivot SunTour Trimec. SunTour was obviously wrestling with the issue of chain gap!