For 1977 Shimano tried a new approach to indexing - ditching the twin cable and introducing a single core ‘piano wire’ cable that was flexible enough to negotiate the route from lever to derailleur, but rigid enough to ‘push’ as well as ‘pull’. This cable came with special outer cable that was highly incompressible and even resisted tension. The derailleur itself was a Skylark adapted to include a built-in indexing system based on a sprung ball bearing running over series of notches. The lever was also indexed. The final pieces of the jigsaw were the 5 speed Uniglide freewheel with twisted teeth (and an occassional ‘low’ tooth), and the Uniglide chain with bulged outer links.
The whole system represented a serious attempt to take indexing mainstream. The chain and freewheel were especially good. However I never liked the plastic levers or the piano wire cables with full length outer - ideal for filling with water - and I can’t say that I ever really trusted Positron II to work properly. This might partly be because I largely experienced it fitted to British made bikes, and British manufacturers at the time seemed incapable of resisting the temptation to fit a cheaper chain or mess with the specification in some other way. They were also spectacularly incompetent when it came to building frames that were straight - Raleigh frames in particular were often a good centimetre out of line - and Positron was extremely sensitive to the chainline.
A heroic failure, but lots of the incidental details - shaped chain links, shaped teeth, incompressible cable outer - lived to become integral parts of SIS, and what’s more it was precisely these incidental details that SunTour, with its focus on the derailleur itself, never mastered.
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