Positron-ic!

1 02 2011

In the late 70’s, Shimano introduced its first mass market attempt at indexed derailleur shifting, Positron. There were several versions of it, starting with a dual cable version, and none of them stuck around for very long. The consensus of most of the research I’ve done on it is “good riddance.”

As I’ve been fixing up more bikes to add to the ECOS bike library, I came across one with this evolutionary dead end of a gearing system (version II to be exact). It was a cute little cruiser with a quirky shifter, and in the rush to get it stripped down and painted green, I initially didn’t realize how much of an oddball I had on my hands.

The Positron shifter. Chunky but still pretty cute, eh?

The more I examined it, the stranger it got. My second clue that I wasn’t dealing with anything like I’d ever repaired before came when I saw the “cable,” which technically isn’t a cable but a thick wire. On the derailleur, there is only one limit screw, and no springs. In the shifter, there’s neither friction nor a clicking mechanism to keep it in place. I also found that the freewheel was quite stiff, so I ran a whole bunch of oil through it and let it work through while I hit the books (er, interweb)  to try to figure out how to make it all function.

The Positron II derailleur. Notice the single limit screw and the solid wire instead of cable.

Positron is different than modern derailleurs that use a spring to return the derailleur to “normal” (high gear, except on low-normal derailleurs) when tension is relaxed on the cable. There is no tension on the Positron wire, instead it works by directly pushing and pulling the derailleur back and forth. There are five notches in the derailleur, which hold it in gear instead of springing back into high when the tension is released.

The Positron derailleur from the back. Note the saw toothed indexing mechanism that clicks each gear into place. In modern systems, the indexing action is in the shifter.

And the freewheel? It’s supposed to be stiff (though this bike may now have the loosest one around). If you’ve ever had a stiff freewheel, you know how annoying it can be as it either shifts itself into higher gears as you’re slowing down or just throws off the chain. But this doesn’t seem to be an issue with the Positron because of the lack of derailleur spring.

This springlessness also changes the strategy for adjusting the gears. After making sure the frame and derailleur were properly aligned (and bending both back into shape), I set the wire “tension” by moving the derailleur into the lowest gear and pulling the shifter down as low as it would go and tightening the pinch bolt. Then, I set the limit screw, which corresponds to the highest gear. Once I quit trying to set it up like conventional gearing, the shifter worked beautifully.

ECOS bike #38, ready to roll.

When I first met this bike, it was a sad little rusty brown Venture with a misaligned frame. Now, after a whole lot of elbow grease, it has been reborn as ECOS bike #38, ready to be loaned out and ridden again by a U of A student this spring. I took her for a little test ride in the basement corridors of SUB, and it made me glow inside when two different people randomly came up to me and said “nice bike,” (I wish I had a “before” picture). And it’s such a sweet ride – both peppy and comfortably upright, it reminded me of Marjory.

I’ll worry about it, though. The chances of finding a replacement Positron part, should I ever need one, are slim to none, and it would be easier to replace the shifter, cable, derailleur and freewheel with something more common (an old SunTour friction setup would be nice) in that situation. None of the sources on the net I looked at had any love for Positron, and even though it’s working well now, it will still need to stand up to the (often surprising) rigors of an existence in a bike share program. In the meantime, I hope many people will have the chance to enjoy this authentic 70’s ride, and I hope that Positron won’t live up to its reputation.

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