zacster

I have an old Trek 7000 mtb that I bought on eBay. I'm finding the rear shifting doesn't stay in place, I'll put it in a gear and it will jump down when I stress it. If I adjust it back, sometimes it will jump up as I've now overshifted the other way. It is 7 speed Deore LX from 1990. I've tried adjusting it but it hasn't helped and actually made it worse at first, but I got it back. Which of the following would cause this?

Worn out shifter
Overstretched cable
improper length and routed housing (this doesn't look right compared to my Trek 8000 from the same period, the front/rear cross under the down tube.)
worn out chain
worn out cassette
worn out derailleur

I guess that's everything in the shifting path. Could it be a little bit of all of the above too? I changed the front shifter when it wouldn't work at all, and I have the matching rear but haven't tried it. I was told it may not work, but the ratchet seems to catch off the bike. I otherwise still have everything on it the way it came. I'm not looking to replace everything, but which is the likeliest suspect? My Trek 8000 is pristine and shifts perfectly by comparison. This 7000 looks like it got used and abused a lot.

bradtx

zacster, Crossed derailleur cables under the down tube isn't uncommon and usually done to improve the routing of the cable housing. I've found mostly cables and housing the cause of phantom shifting and to a lesser extent a derailleur or a shifter.

Brad

PHT

I would first check the derailuer hanger and derailuer itself for proper alignment.

Andrew R Stewart 

All of the items listed should be looked at. Add upper pulley float too worn (too much float), freewheel or cassette cogs are loose on the body, wrong type of cable casing (not SIS grade) and the deal breaker- a broken frame. Often this kind of problem on such an old bike is a combo of a few issues that collectively add up. Andy.

FBinNY 

Besides a bent or misaligned hanger, I'd suspect sticky cables. When the cables don't slide easily you have to over-compensate to get decent shifting in one direction, but it comes at the expense of sluggish shifting in the other.

Try this diagnostic. Adjust the trim as well as possible for good downshifting (to larger sprocket). Now downshift to any middle gear and then upshift one position. If it doesn't shift immediately, pluck the bare wire away from the downtube sharply like a guitar string. If that completes the shift, it means that the cable didn't return all the way before and the housings need to be oiled or possible replaced.

Also, as Andrew S. stated, a worn or sloppy jockey wheel can have the same effect of forcing over compensation to complete shifts. One trick I use to confirm this is to switch the lower and upper pulleys and see if it makes shifting more positive. If so, you can leave it that way for now, but need to buy a new set at your earliest convenience.

zacster

Well, here's the kicker. I broke out my Park Cyclone Chain Cleaner that's been in the plastic unused for 2 years now, and cleaned the chain. It was of course very worn out, made obvious by the cleaning. But what I also discovered was that the frame DOES have a crack in it, on the chain stay. I had not noticed it before, but it looks like it was there for a while.

I'm not concerned about the crack, the frame is aluminum it doesn't seem to effect the ride. If it fails, it fails, I'm not very attached to this bike.

I'll try the suggested derailleur adjustments to see if it is the cables/housing, and I'll try swapping pulleys. I have another bike that needs derailleur cable housing, so maybe I'll buy enough for both, and a good tool to cut it. I've always wanted a good cable/housing cutter, but I've seen too many negatives about the Park and Pedros. The Shimano is never available anywhere.

FBinNY 

If the right chainstay is cracked it's entirely possible hat it's deflecting abnormally under load. Movement of the stay might be shortening it slightly, allowing a bit of extra chain slack and moving the RD outboard a hair, just enough to make a shift.

Depending or where the crack is, you might be able to improvise a splint using sheet steel and hose clamps. A decent job will last until a new unrelated crack elsewhere kills off the frame.

BTW- if you want a nice cable cutter, I have a bunch of New old stock, old jagwire cable cutters (blue handles, same as the Shimano of that era). I'll sell you one for $19.00 including postage within the USA.

zacster

I FINALLY rode this bike again. The weather here just sucks. All I did was clean the chain and re-lube it and now it stays in place. Sometimes the easy fix is all you need.

ksisler

Or you could just buy a branded set of complete cables with housings for the brakes and the shifters. They come precut and work great for bikes in the normal size ranges (up to about a 66cm frame). Nasbar carries Shimano and one other brand (which turns out to be a bit better than the Shimano ones, imho). Even though I have all the materials and tools on hand, I keep several sets of the complete kits as it speeds work and reduces the brain cells required to get a bike out the door.