addaduuuu

hey guys, i use this old bike to commute. rides well, is very practical so no complaints there, but it does have a shimano positron fh rear derailleur, and i was told that i'd be better off replacing the whole thing should it break. it still works fine, but if anyone has any suggestions for replacements in the event that it stops working, that'd be much appreciated. Also, i'm not really up on the technical lingo, so what are a couple key terms that'd help my in my search? tried to attach below a pic thinking it'd help but the website told me I cant since i just made my account lmao

JohnDThompson 

You can put pictures in a gallery associated with your user account. Let us know when you do, and other people can link to them here.

Andrew R Stewart 

Positron ders were pretty low grade in the Shimano playbook. Still they can and do work well for their design and grade of fabricating. They are subject to the same der damaging incidents like getting caught up in the spokes and stuff but by far the most common issue with them is cable related, or perhaps I should say rod related as some versions did away with the looped push/pull cable for a solid "rod" (contained in it's specific casing) that can both push and pull. These control cables/rods are few and far between these days and when they go bad replacing the der, cable and lever is the usual fix. There are bike shops with replacement Positron parts still on their shelves but good luck finding them. Remember that there were at least three different versions of Positron ders and that parts are not all interchangeable between the versions.

When replacing the rear der one needs to know how it is attached to the frame, what the max and min cog tooth counts it will work under, any lever or cog set compatibility (as is the case with modern indexed systems) needs (which if all are bought at the same time the seller should be able to advise), how the control cable is routed and what casing is needed (again, more indexing specific). As long as the replacement der is compatible with the cog set (likely a freewheel on this bike) the old cogs and chain should still be useable (assuming they're not worn out of also damaged). Andy

addaduuuu

alright i just posted the pics i took on imgur, search up verbatim "for da guys on bike forum" and you should find it. username is the same as here but with 3 less u's on the end. if you'd rather me post it somewhere else or you'd like more just let me know

curbtender

https://www.bikeforums.net/g/picture/32859915

addaduuuu

oh yeah, by replace i forgot to specify that it'll just need a whole different derailleur all together, not necessarily one of the same type. sorry if that caused some confusion!

curbtender

A nice Suntour GT with a claw mount.

70sSanO

Or a Suntour Cyclone. There has to be millions out there. A pretty universal friction RD that is built like a tank.

John

Andrew R Stewart 

Personally I would chose to take advantage of 40+ years of development and patents expiring and get a Shimano Acera/Altus with a hanger plate (the old Positron's is welded to the shaft of the top pivot). Simple friction shifter cost pretty little and open up all kinds of interchangeability. If one really wanted indexed shifting to be retained then matching each part to the "standard" will be needed and very possibly more parts will get replaced in that process.

Thankfully this bike isn't also a FFS one Andy

KCT1986

Attached is a link to the instructions for the SL-P213, which appears to be the correct shifter.

SI-N-88-000-00-ENG_GER_FRA_DUT_ITA.pdf (shimano.com)

These early index shifters used a unique RD that controlled the movement of the RD. They also use a pull/push action of the shifter/cable to assist the RD springs.
To maintain index function, all components were required.

An excerpt from the instructions of the RD index.

Duragrouch

That derailleur is a claw mount. You can get another like it, or get just the claw part to fit under the axle nut and mount any "direct mount" derailleur. My guess is you are on a budget. Shimano Tourney is available with an integral claw mount for about $14 new and works amazingly well for the price. It has a longer cage (swinging arm, so capacity) than yours but you have plenty of room so would be fine. Works for 6/7 speed but would also work for a 5-speed freewheel:

https://www.amazon.com/Shimano-Tourn...dp/B0052KDG4U/

Amazon item indicates it is an SGS cage length, that would be long, but my personal experience is that the Tourney is GS length which is "mid". Your bike I think has a "short" cage.

The Tourney (nor ANY modern derailleur) has no indexing ("click-shifting") in the derailleur, all modern indexing is in the shift lever or grip-shift.

Last time I heard the word positron was when I had a brain PET scan, Positron Emission Tomography.

addaduuuu

alright thanks a bunch guys! hopefully won't need to make use of this for a while, but it never hurts to be prepared haha