Joe_Hoffmann

My bike is a 1981 10 speed schwinn world tourist, with the pre-select gears, (freewheel ratchet is in the front by pedals, not on rear wheel)

The shift cable going to the back derailleur has frayed in half finally, so I cant shift anymore.

For those not familiar with the positron cable, it is solid steel wire that allows the derailleur to be both pushed or pulled into the correct gear. There is no spring to pull the cable back.

Now from what I hear, people tend to avoid these shift assemblies if possible, because the parts are not easy to find....and they were cheap in the first place (well...the cables look cheap, but everything else is solid)

What would be the best way to get my bike going again? I really like the positron because it gives a solid and smooth feeling to the shifts (due to there being an actual lubricated metal-on-metal mechanism that holds the gear setting, unlike a plastic twist grip with cheap plastic), and its easy to adjust!


Can I use a hardware store throttle cable as a replacement? I have a MIG welder if I need to use one.

Jeff Wills

What exactly is broken? The inner wire? The outer housing?

You might be able to replace the inner wire with a piece of piano wire, but the outer housing is unique to Positron II. I doubt you could make a replacement.

There's a couple available on Ebay right now:
Shimano Positron Shifter Derailleur Cable 1190mm Black | eBay
Measure your housing length. There were several lengths available.

Joe_Hoffmann

the outer housing has broken through. the inner cable is good. Im not sure if thats the correct length for a replacement, and I dont know how to find out what length I need, besides measuring.

But then what am I measuring? the housing or the inside cable?

wheelreason

This is what I hear;

"My harpoon tip is broken, and I really like the way it pierced the thick whale skin, oh and where can I get a good wick for my lamp"...

Joe_Hoffmann

Get out of here you unintelligible boob. If you have nothing to contribute to solving the problem, you are part of the problem.
If you can't grasp what is going on, then I suggest you put down the marijuana joint and step away from the computer.

AnkleWork

Few posters here actually seek answers.

wheelreason

Easy there Captain Ahab, do you know what unintelligible means? Cause for someone who doesn't get it, you seem upset...

Now to your question. You like Positron II, I can't help you with that, some folks like midget hookers, so there is no accounting for taste. The inner cable (wire actually), can be fashioned from steel wire (check tackle shops in the shark section) The outer casing could be fashioned from a variety of things (really stiff SIS will work) as will soft copper tubing and the like. Of course for those who enjoy the company of normal sized financially compensated women, a new drivetrain is the answer...

AnkleWork

Good idea as they probably offer stainless steel. Piano wire from a hardware store should also work.

fietsbob

Positrons dont use a cable , to push as well, a pull they used a stiff solid wire.

Took a long time to break , they gave up that idea a long time ago ..

prepared to Fake it? have a shade tree mechanic's spirit?

get another rear derailleur, operated with a cable, which only pulls, will be pulled back with a return spring .. and a friction thumb shifter ..

AnkleWork

Hilarious, in re. your question.

fietsbob

OR.. take the bike to a pier , and see if it floats. go buy a new bike ..

dedhed

I might have some positron stuff in a junk box at work. I'll have to look next week.

Delmarva

Good advice to replace the rear derailleur and shifter. Although that means learning to position a derailleur manually. Bit of a step backwards for Joe.

Joe indicated the freewheel ratchet is in the front by the pedals? Does that mean the cassette is connected solid to the rear wheel and the chainwheels freewheel? I may be confused....

edit: I did a little googling and found a good explanation of Positron and freewheeling chainwheels on Disraeligears.com As an early attempt at making shifting less difficult it strikes me as an early step in the right direction. Must be quite a sight to see the entire gear train spinning away while the pedals are stationary!

fietsbob

yea the front freewheel lets people stop pedaling and shift , while coasting ..


part of Shimano's lowering the skill level required to buy and ride a derailleur bike..

index shifting is the part that stuck ..

wheelreason

Yes, this is why we are suggesting piano or fishing wire. Both are single strand tempered steel...

Delmarva

Hadn't thought about stopping pedaling to shift but that's what millions of riders were taught to do with igh. Positron just delayed the inevitable by making a derailleur feel like an igh when shifting.

fietsbob

You play the piano? piano strings are not very thick, something like .020" before they start being wound, to make the lower notes ..

Jeff Wills

Geez... you go away for a bit and things go to H-E-double-hockey-sticks.

You definitely need to replace both the inner wire and outer housing. I'm pretty sure the measurement is the length of the housing from one end to the other. Find a flexible measuring tape and find out how long your existing one is. You can replace it with a longer one (although it looks goofy), but a shorter one will break pretty quickly.

If you can't find a replacement, you're going to have to replace everything: shifter, cable, and derailleur. It can be done, but you'll be spending much more than the bike is worth.

fietsbob

It's for good reason the whole positron kit was let Die , not long after it was made ..