Phoible

I took the rear derailleur off my bike, and replaced it with an ultegra 9-speed long cage derailleur. I have a 12-25 8 speed cassette in the back, and 53/39 in the front. I used the same chain that I was using with the old derailleur (a short cage dura ace 9-speed) - the length appears to still be correct. It will shift into all of the gears, but it clicks a lot in the gears on the big ring.

From what I can tell, it looks like the chain doesn't sit quite flat on the outer derailleur pully - its a bit off to the side. Adjusting the limit screws doesn't seem to help this. The derailleur cage doesn't seem to be bent.

Does anyone have ideas on what could be causing the derailleur to act like this - is it the long cage with a double, or could something be worn out (I got the derailleur used at a LBS - not sure about its history)?

Al1943

Have you tried adjusting the cable tension with the barrel adjustor?
Be sure you have enough chain so that the derailleur does not reach its limit when the chain is on the big chainring and biggest cassette cog.
Don't understand why you'd want to change to a long cage derailleur if the short cage was OK.

Al

cuda2k

Describe what you mean by "off to the side" a bit more. I'm having a hard time picturing it. And have you isolated exactly what is making the 'clicking' noise?

Phoible

Yes. If I tighten the barrel adjuster, the derailleur jumps to the next cog up.

Big/Big seems to work. It just makes noise.

I wanted the derailleur off of that bike for a bike that I was building up. I put on the long cage because it was lying around, and I figured that it should work.

The pulley should be centered above the chain. I think that it isn't - the pulley runs along one edge of the chain, and skips as it pops in and out of the grooves in the chain.

I may try adding a link to see whether that helps. I didn't think that the cage length should affect the required chain length.

Retro Grouch 

I'm wondering if your derailleur hanger is mis-aligned.

I'm thinking that it might work with a short cage rear derailleur but the longer cage will exagerate the misalignment and might make just enough difference to cause the problem that you're describing. Check your bike from the back to see if the derailleur arm looks like it's exactly parallel to the cassette cogs.

Phoible

I'll have to check. I crashed earlier this year, and bent my derailleur hanger. I paid a LBS to true it, but its possible that they screwed it up.

cuda2k

Yup, sounds like the LBS got it well enough for the short cage, but Retro is right, the longer cage will exagerate it.

Bob Pringle

I don't understand why the longer cage would exaggerate the shifting problem. The longer cage simply places the tensioner pulley farther away from the axle so it can take up more slack, but the guide pulley appears to have the same relationship to the axle (and the cassette cogs).

barba

The same angle but a longer arm would mean more deflection.

Bob Pringle

Right! But the longer arm(s) carry the TENSIONER pulley, which has practically nothing to do with shifting the chain laterally on the cassette cogs. It's the GUIDE pulley that does the shifting, and it's just as close to the cassette in a long cage derailleur as the guide pulley in a short cage derailleur.

Not trying to be a wise-(guy) here, but I don't understand the reasoning.

Retro Grouch 

Because the OP said the shifting was OK but the chain was off to the side of the outer pulley.

Phoible

It shifts fine, but makes noise. Looking at the hanger, it might be bent just a touch inward. Which would make sense given the symptoms. I may just put on a short cage derailleur and call it a day.