jnbrown

I still have the original XTR rear der that came on my Santana tandem.
Its probably about 15 years old and I have never lubed it.
Lately its shifting like cR@p. Sometimes it won't shift up, sometime it won't shift down.
sometimes its in between and makes ugly noises.
I lubed the cables and made sure the cable was not getting hung up.
I have bar end shifters, The cassette is only a few years old and does not have that many miles on it.
The der feels like it has a lot of play in it compared to my road bikes.
I am looking at replacing it with a RD-M771-GS (medium cage, capacity 33T)
I think the medium cage will work with my 30/42/52 front and 12/30 rear.
When I am on the 30 in front I never use below the 4th cog (21T) in the back.
Any ideas?

Thanks

JiveTurkey

The SGS is generally meant for triple crank. It will shift the same as the GS, so there's no real advantage (except a tiny weight saving) by going with the GS.

Retro Grouch 

Since you say that your derailleur has a lot of play I suspect that you're right. If it was my bike, however, I'd have the derailleur hanger alignment checked before I spent money on a new derailleur.

PunctualAlex

Try overhauling the rear derailer first. Unless something is obviously broken (bent cage, something snapped, worn pivots) you should be able to fix everything. Try this: take the derailer off the bike. If it's the series of XTR I'm thinking of, it should have a massive return spring inside the parallelogram- try hitting that with some solvent and working the thing back and forth. Blast it out with compressed air and try again. It should move smoothly and there should be no play in the cage, pivots, or parallelogram. You can check Park Tool for more info on how to re-build a derailer- you can probably salvage this one by cleaning the spring or overhauling the pulleys.

jnbrown

What is GS for then?

Jeff Wills

Saving a couple grams.

FWIW: I had a Deore XT of similar vintage as your XTR. It survived 3 bikes, but eventually started shifting like poop. It tossed the chain into the spokes, despite being adjusted properly. When I wiggled the cage, I could feel & see lots of play in the parallelogram. I replaced it with a new Deore XT and everything worked perfectly again.

Wordbiker

Any derailleur can benefit from a good cleaning, but there's no real fix for worn pivots. The pins are pressed into place and peened.

Derailleurs of that era sometimes had replaceable parallelogram springs, though you'll have a real chore finding the part. I'd say 15 years is a respectable amount of time for any part to last. Give it a decent burial.

The GS should work fine with your stated gearing. Using the shorter cage (in theory) will have less leverage against the knuckle spring, and therefore more spring pressure. The shifts will be snappier and chain tension higher...or so they say. I used to set my bikes up like that for racing, and even if the benefit is psychological rather than actual...it's still a benefit, eh?

JiveTurkey

In general/theory,
Road SS = double crank, 27T or smaller cassette
Road GS = triple crank, 27T or smaller cassette
MTB GS = double crank, 34T or smaller cassette
MTB SGS = triple crank, 34T or smaller cassette

HillRider

For MTB use (real off-road, not trail or road use) I wonder if the shorter GS cage would be less vulnerable to damage from rocks, logs, etc since it doesn't hang down so far. Otherwise the advantages have to be insignificant.

tellyho

I find that the joint between the parallelogram and the cage wears out after a while. If you have play there, I don't think there's any solution.

kenhill3

In my own experience the pivot bolt is the first thing to wear out, other than perhaps jockey wheels.

Put your finger on the pivot bolt head where it shows on the der. body, wiggle the der. side to side, and you will feel the slop if it's there.

Combine this with some wear on the parallelogram bushings and you have a derailleur that can't be adjusted to shift/index BOTH up and down the cogs.

JohnDThompson 

I've resurrected my 1978 edition Huret Duopar several times by re-peening the rivets with a hammer and punch. Might be worth a try on the OP's Shimano.