TheEnthusiast

Well I'm only 19 and have been into older 10 speeds from the 70s and 80s. Yesterday, I decided to take my trusty 1988 Schwinn Sprint for a cruise and as I shifted down, the gears shifted by themselves to the largest cog. then when I got home I took it apart to see what was wrong and now the derailleur body moves freely side to side and the chain is very loose. This derailleur never had any problems until I let a friend borrow my bike and he didn't even take it apart.

clydeosaur

It appears to still have cable tension. The derailleur seems to have bought the farm.

cny-bikeman

The gears cannot shift by themselves to the largest cog. If the shift lever pivot is loose the rear derailleur can shift to the smallest cog on it's own.

If it was indeed the largest cog you shifted to:
Judging by the fractured spoke guard your bike fell over on it's right side at some point (maybe while your friend had it, maybe not) bending the derailleur inward where it is attached to the frame. When you downshifted the derailleur went too far, hitting the spokes/guard. As a result the derailleur appears to be twisted (at least from this angle) and if so needs to be replaced. Need to also check the chain for any twisting or tight links. Be glad it did not get caught worse, as it can damage the dropout.

By taking the derailleur apart, which was not necessary in this case and very seldom necessary at all, you unwound the spring that tensions the pulley, but if the derailleur is bent that's irrelevent.

TheEnthusiast

I did take the little plastic cup off the spring and it flung of and I put it back on. Well there are 3 holes that the spring goes into but I cant seem to remember which hole. I put it back on but unfortunately nothing helps.

cny-bikeman

The middle hole should be OK, but that's not the problem. You need to wind up the spring - difficult to explain but easy for the right person to do. The plastic cover does nothing.

Google rear derailleur spring tension and look at a few results until you get it, or take to a bike co-op or knowledgeable friend.

bikeman715

you can try again to remove the cover and move the spring to the next hole and maybe one of the three will work , if not then it time for a new derailleur . as cny-bikeman said it does look bent .

TheEnthusiast

Thank you cny-bikeman you are great help as well as the rest of you! Ill just take it to the bikeshop tomorrow and see what he can do.

mack_turtle

if the derailleur is dead, it might be VRY difficult to get a new one that will work. FYI

cny-bikeman

Incorrect - friction shift, almost anything will work.

mack_turtle

correction: getting something that looks period-correct on that bike will be difficult. if you just need it to function, a Tourney will probably work. if you want something similar to what's on there, it will come from Ebay or your local bike co-op.

TheEnthusiast

****! I wish I didn't take it apart! All is does is move freely from the pivot arm.

fietsbob

Good time to add a bike repair book to your Library, .. or at least check one out from the Public Library...

SBinNYC

You have a Sun Tour V-Luxe derailleur. Here's a link to an exploded isometric of it.
https://www.velo-pages.com/main.php?g2_itemId=12562

The chances are the cage tension spring (21) is broken. It most likely one of the two tabs broke off. The inner tab fits into a hole in the derailleur body (1). The outer tab fits into the cage tension axle (22). It's possible to bend a new tab from the remaining spring. It takes a lot of strength and patience. Be sure to wear eye protection while bending the spring.

After it's replaced you remove the cage stop pin (24) and wind the cage back up so it has tension to wrap up chain. Then you replace the cage stop pin (24).

TheEnthusiast

I have a lot of maintainence books from the 70's but they don't mention any springs in the derailleur. :/

TheEnthusiast

SBinNYC thank you I would have never found that link. It was a big help!

cny-bikeman

That's because one*does not generally take them apart! Knowing that the pivot pin is removable means you can see if the spring is broken without disassembly. Remove the pin and, looking from the inside/back of the derailleur, rotate the cage clockwise until you pass the home for the pin. Hold on place and screw the pin back in.

All of the above is moot if the derailleur is bent - just replace it, and don't worry about period correct - it's not vintage.

goldfinch

You can get a Tourney derailleur with a claw for less than fifteen bucks. (A six/seven speed deraillleur should work with your five speeds on the rear). Be sure to properly set the limit screws when installing. Your book should tell you how. If not, the Park Tool website give good instructions.

SBinNYC

All derailleurs have at least 1 spring; almost all have 2 springs and many have 3 springs (but not Sun Tour).

All derailleurs must take up the chain slack. That's the first spring - the one that probably broke on your derailleur.

The second spring is used to counter the friction supplied by your shift cable. Remember most shift cables can only pull; they can't push. The second spring is in the derailleur body. It pushes the derailleur to its outer position. It's a replaceable part in some derailleurs. There have been derailleurs that did not use Bowden cables but used push-pull cables. These derailleurs did not need the second spring. Some pole derailleurs from the 1950's had the spring push the derailleur body inwards, with the Bowden cable pulling the derailleur cage outwards.

Many derailleurs also have a tension spring where the derailleur mounts onto the frame or claw. It automatically moves the derailleur body up and down to keep a constant clearance between the cage and the cog in use.