skyrider

Out of curiousity does anybody have any info on Shimano Positron deraillers? I pick a old no name bike with one last week also seen one today with 2 cables going into it . The one bike shifts flawlessly the second I didnt buy yet. Thinking of adding it to my collection. Are they hard to come by, I like to have them for historical reasons I guess owning a bit of history of shimano deraillers. Any input would be appreciated, thanks for your time.
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thurstonboise

I've only seen them on department store bikes. One of the early attempts at indexed shifting. I rode a Free Spirit 10 speed with that setup when I grew out of BMX. It worked alright, but there was something special about the cable and housing which I can't recall.

crazyb

i have a 1981 Schwinn World Tourist with a 10 speed Positron. The chain ring freewheels, not the rear cog. You can shift while not pedaling as the chain is always moving. I ride this bike daily to work and back, and it works wonderfully, but it does shift better if you dont pedal while shifting. As easy to shift or easier than anything from that era, including the best Campy.

skyrider

The free spirit I was looking at today had indicator marks on the derailler with the two cables running into it. Looks way ahead of its time.

crazyb

My rear der. has only one cable to it. Have no idea what a second cable would be for, or where it came from.
If this thing is for cheap, buy it and take pics.

thurstonboise

mine had the indicator marks, but only once cable and didn't have the front freewheel.
The cable might have been solid and worked as a push-pull.

Sheldon Brown has a little bit of info here

number6

First generation Pos-I-Tron used a solid wire, not a cable, special housing. The Nishiki Junior Sport 24" wheels came with it first in the Nishiki line. The front freewheels are a wee bit fragile, most had hefty guards on the rear mechanism to protect it in case of a fall.

At the time it was rumored that Shimano was prototyping a Dura-Ace version providing the advantage of shifting while coasting through a corner to be able to resume in the perfect gear, too much drive train friction to make that possible advantage worthwhile.

T-Mar

The first generation Positron used a dual cable, push-pull system because it had no return spring, so as not to overide the ball detents of the indexing. The second generation was PPS (Positron Pre-Select) and used the special, single, push-pull wire, in conjunction with the FFS (Front Freewheel System). There were still later versions called Positron II & III that were really 3rd and 4th generations. Confusing, isn't it? Most people consider Positron a disaster and short-lived but it worked well and survived into the early 1980s.

Push-pull is actually a bit of a misnomer for dual cable systems which had been used by several other manufacturers, decades prior to Positron. Each cable pulls the derailleur, but in opposite directions, so it's actually a pull-pull or double-pull system. For the system to work properly, the cables tension has to balanced, but when working properly they are very nice as there is no need for friction plates on the lever to balance a derailleur return spring. Consequently the shifts are very smooth and require little effort. In the case of Positron, these advantages are minimized due to the initial force required to overcome movement out of the index detents.

skyrider

Thank you all. Again there is always something interesting out there to learn about. I was in this old mans garage he is 95 years old, and has about 100 bikes all mostly boat anchors lots of 3 speeds etc. He charges extremely high prices for what there worth. But it was an interesting hour spent there. Anyway thanks all.

number6

Thanks T-Mar, I am glad I did not encounter the first generation. Forgot one thing to mention, the rear cluster did not freewheel but did have a clutch of sorts that would allow the cassette to move in case of mechanical disaster.

T-Mar

Good point. That was the Friction Freewheel, which was only used with the FFS. Most people familiar with the FFS think they are fixed cogs, but like you say, they will freewheel. They were designed so that the force requiring them to freewheel was higher than that of the crankset freewheel, forcing the crankset to freewheel during normal coasting conditions. If something abnormal happened, like the chain becoming derailed and caught between the chainwheel and chainstay, then the resistance would cause the rear cogs to freewheel. Many people that stumble across them erroneously think that they have a siezed freewheel.

99Super

I have a 79 Schwinn Cruiser 5 w/ the second generation Positron 5-speed. The system actually works pretty well, but every part is unique to this sytem. This bike was defintely NOT a deprtment store bike...