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 example is the long cage, ‘GT’ version. It has one of the most interesting pulley cage designs in this collection. The guide pulley pivots around the normal front, sprung pivot in a fairly normal way, although it has a large (37mm) offset from this pivot. The tension pulley, however, is mounted in a separately pivoting piece of pulley cage, sprung by the same spring, but with a softer action so that it articulates before the guide pulley. This means that for the higher gears the guide pulley stays in the same place relative to the pivot, but for lower gears it starts to rotate around the pivot, moving away from the wheel axle. This could be a very clever design, maintaining chain gap quite well on wide ratio freewheels which tend to have top cogs that are relatively similarly sized, but the bottom cogs that are very different sizes from each other.
Smart, but immediately consigned to the dustbin of history.
Browse associated documents.