bluehair

How can I tell the size/length of a Shimano XT deraillieur by looking at it? It is a M750 9Sp with no other markings that I can readily see. The reason I am asking is that I built a touring bike years ago which I only use when I'm on vacation in a very hilly area. The rear gearing is 11-32 and the front is something like 28-48 (I may have miscounted), but I have never been able to shift smoothly through all the gears without jamming or rubbing somewhere. Since I'm usually pumping hard or coasting fast so it hasn't been a huge issue, but I'd like to get it right. I bought the rear deraillieur long before I knew various lengths existed. I thought a mountain rear d. was a mountain rear d. The front d. is specific to the chainrings, that I am certain of. Thanks to anyone. P.

Retro Grouch 

You almost certainly have the right one. There is actually a short cage version of the XT derailleur, but they're rare.

Rear derailleurs have two significant capacities: large cog and chain wrap up. They are different things. The large cog capacity of your derailleur is 34 and you need only 32. The chain wrap capacity is 43 and you need only 41. You'll be good.

HillRider

You problem may be with chain length. If it is too long, the chain will hang slack in the granny chainring and the smallest cogs. If it's too short (which is worse) it may not allow the big-big chainring-cog combination, which you shouldn't use but the chain should permit it for mechanical safety.

bluehair

It occurred to me to just measure my mountain bike rear derailleur and compare the two cage lengths. It turns out, believe it or not, that there is over a 2.0 cm difference in the cage lengths. I know the mountain bike is a long cage; so I guess I wound up with the rare short cage XT! I know the chain length is good (although it won't be if I change derailleurs). I have never read anywhere what the specific symptoms are of a rear d. that's too short, but it looks like mine may well be. What seems to happen is that the shifting just jams; I can go down a gear and then back up and I'm usually OK. I initially thought it was the used 105 brifters, but I don't know...I don't know...I don't know. Well, thanks for trying to help. Great forum isn't it. P.

bluehair

Well, the shifting is now fine on the workstand, but I've been here before. Now I see what a long cage does...it takes up chain for those gear spreads, but does it do anything else? It's my impession that the guide pulley does the work. I'm about to get a new derailleur so any quick reply will be helpful.

Retro Grouch 

You've got it. The guide pulley does all of the work and all that the cage does is to take up the excess chain slack. Longer cages take up more slack.