Ranzak

Ok I have a 2003 Fisher Marlin, with a 6 month old 8 spd Shimano LX derailleur (replaced original that was ripped off in accident), a brand new cassette and rear wheel and chain and chainrings installed by LBS. Ever since I picked up the bike under a climbing load the rear derailleur shifts by itself.
I called my Bike shop they told me to turn adjuster in 1/4 turns to adjust tension until the problem went away. I just moved the problem up and down the cassete. [originally it jumped around between the 3 and 4th largest cogs then the 2nd and 3rd] I never got it to go away.
On a ride the next week out of town I stopped in another bike shop and asked them to look at it. 1 minute later the mechanic was done and said he did all he could do because the derailleur was "worn out".
Out on the trail I found out that the problem had moved to the 4th and 5th biggest cogs.

So several questions: 1) Any ideas on what to try?
2) On a bike that is cleaned and lubed weekly if not more and that is shifted
gently, what is the service life of a rear derailleur?

powers2b

Are the cassete and shifters Shimano also?

Enjoy

Ranzak

Shifters Shimano, cassette is SRAM model that was on the bike OEM. Chain is SRAM which ive used since original chain, and has never been a problem.

sydney

You probably couldn't 'wear out'a lx RD in 6 months if you tried. Did the bike ever shift right after ripping off the RD? If not, then the hanger may be bent. Your LBS should have had you bring it in rather than guessing at the problem, and the fix.

Ranzak

Yes, the RD worked just fine for 5 months until the new chain, new cassette, and new chainwheels ...

Retro Grouch 

Once again, I'm going to side with sydney.

A bicycle is a pretty simple machine. Whenever the normal tuneing process doesn't seem to be getting the job done, the first thing that I do is to check to see that everything is in the default mode. With rear derailleurs, expecially on a mountain bike that had it's derailleur trashed in an accident, a bent derailleur hanger is a real likely suspect.

Some time ago somebody started a post asking about your favorite bicycle tool. I said my derailleur hanger gauge because it frequently solves shifting problems that have stumped lots of other mechanics and makes people think that I'm smarter than I really am.

Ranzak

To defend the LBS they want me to bring it in, its just my LBS is 35 miles away and my riding time is limited and I just haven't gotten it back to them.

Bikewer

I'm with Retro Grouch on this one. The impact that broke your previous DR may well have bent your hanger.

I've seen several of these on department bikes, and some hangers so soft you can bend em' by hand.

Ranzak

Hmm the hanger was replaced when the new RD was installed and it worked fine for 5 months ...

Wheel Doctor

Bent deraileur hanger! It will drive the novice wacky.

hangman

There's something here that doesn't sync up with me. A bent hanger and poor cable adjustment will move a problem up and down the cassette, but it won't make the bike shift by itself. Couple of things that can do that:
Incorrect cassette and shifter compatibility. Thread shows new dang near everything. SRAM sells Shimano compatible and proprietary cassettes (meaning they only work with SRAM shifters). Check the box it came in to be sure. Could be the wrong one. You won't determine this by looking at it alone.

I've also seen 9sp shifters sold as 8sp. That'll make your head spin. Check the Shimano spec data on Shimano's site or packaging instructions. There's a model number stamped on every shifter. It will work on extremes but not in the middle cogs.

I've seen cable inner wire heads not mounted/routed/seated correctly into shifters cause a problem. They get jammed in the motion of the shifter.

I've seen non-index compatible cable housing used where it shouldn't be. The housing just compresses and doesn't make a clean shift. Also related, I've seen index housing used without proper cable ferrules (the little metal or plastic caps pressed onto the ends of housing pieces). The cable housing continues to pull through the cable stop 'cause there's no ferrule to stop it's drawing through.

Lastly, I've seen cables routed so short that simply turning the handlebars effects a partial shift.

I've also seen rear frames bent to where it messes up chainline, i.e. the chain doesn't lie parallel to the intended forward line of the frame, causing the entire shifting mech to always be pulling to one side.

I've noticed in the thread it said new chainwheels. Shimano compatible? Bottom bracket get replaced? Insalled correctly? I've seen people replace crank sets and/or bottom bracket spindles with incompatible or improper length spindles which can throw out chain line, causing shifts but at the front not rear derailleur.

9speed chain on an 8sp cassette or beefy chainwheels won't let the chain sit down fully onto the sprockets and could cause a shift as the chain is floating. Chain slap from riding offroad can make it bounce to the next sprocket. Also having the chain too long can cause that. Was the new chain cut to the proper length upon replacement?

One key question I'd have is does the bike shift into easier pedaling gears or harder pedaling gears? This can indicate where a problem may be.

A bent derailleur hanger simply will cause a chain to not get fully into the next gear when shifted, but not cause a "shift by itself" symptom.

Just thought I'd bring up some other things to look at if the derailleur hanger replace didn't work.

I second the "bring it to the LBS" idea.