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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10 responses

5 02 2011
Keith

Very nice write up !

If you really wanted to have a bike that was a dinosaur you would need a positron with Shimano’s FFS (front freewheel system).

These were both solutions that were looking for a problem… :)

6 02 2011
evillerider

Thanks!

When I first heard about the FFS, I thought of that damn Autobike, and wondered if there would be any chance of compatibility in the unlikely event I encounter both, but then I thought, “Why bother? Surely there are better bikes to work on!”

5 07 2011
Al Kine

I have the same type of dérailleur on a mens Sears and Roebuck “Free Spirit”.The bike was giving to me by a friend.Don`t know much about it.But I have the problem of it Moving the chain between low gear and the spokes.Any idea`s as to how I can solve this problem.Other than that, the bike rides quite well.I`m 65 years old and I`m not really up on these 10 speed bikes.But I can follow instructions, so any help at all would be much appreciated.
Thank You
Al Kine

6 07 2011
evillerider

First thing you want to check is if the derailleur has been bent towards the wheel. The derailleur cage and jockey wheels should lie in a parallel plane to the rim of the wheel, and if they’re not, you’ll have to bend it back. No tricks to doing this, just grab it & bend.
If that isn’t the problem, try making an adjustment on the wire. Position the derailleur so it sits comfortably in the lowest gear then release the pinch bolt on the wire. Then, pull the shifter down as low as it will go and then tighten the pinch bolt on the wire. This will prevent you over-shifting and pulling the chain into the wheel.

Hope this helps!

7 07 2011
Al Kine

All Right! Got it to work properly! I had to back off on the limit screw a little,after adjusting the wire.But it runs the full range of gears,both up and down without any problems now.I Thank You for your help.
Al Kine

22 10 2012
derek chadbourne

There is a distributor in canada that still has positron cables

2 06 2013
Sid

Do you know who the distributor is for the positron cables?
Thanks, Sid

3 06 2013
Derek

Babac

3 06 2013
evillerider

Check out Babac Cycles (though I have a feeling that you’ll really have to search their website, as I don’t think they’re organized where you would think you’d find them).

1 07 2014
Karl

Just for your information there are plenty of late 70:s – early 80:s bikes using positron gearing systems in Sweden, mostly 400 and FH. In case you ever need a replacement direly, have a look there (online auction sites etc.)! I know some other Scandinavian and Finish bikes came with them as well, and probably German too. I wish the bike the best of luck! (Adjustment and installation instructions in Swedish: http://www.cykelhobby.com/positron.html)

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