Rear derailleur not picking up slack in chain

fermion

When I went out riding the other day my chain was slipping badly. I noticed that when I went into the small ring or cogs there was a lot of slack in the chain and it was visibly hanging much too close to the ground.

I was planning on replacing the chain soon anyway so fitted a new one to see if this would fix the problem. I then realised the problem was my rear derailleur (Shimano Ultegra 6700) which does not seem to be properly moving back to keep the chain taut when I shift into higher gears.

The bike is only 3 months old so I phoned the manufacturer (Planet X) and spoke to one of their mechanics. He was convinced the problem is simply to do with the the derailleur cable having stretched with use and it should be an easy fix. I've never had a problem with the gear indexing however and the derailleur shifts into all the cogs as it should - it just doesn't pick up the slack in the chain when moving into the small rings.

Any advice would be great. Is the Planet X guy right and the cable tension just needs to be set-up correctly? If anything's unclear I can upload some pictures this evening to illustrate the problem.

shmooster

Cable tension is for the sideways movement, not the chain tensioning, so that sounds wrong. If it's only 3 months old might be a warranty fix unless it's been damaged.

arlowood

shmooster wrote:

Cable tension is for the sideways movement, not the chain tensioning, so that sounds wrong. If it's only 3 months old might be a warranty fix unless it's been damaged.

+1 The Planet X mechanic is full of *****. The chain tension is managed by the cage/jockey wheels which have a totally separate spring mechanism built into the mech body. Fiddling with the cable tension will not cure the problem you describe.

Any chance the mech has slipped from it's normal mounting position. Also if you drop the chain off the chainset at the front you should be able to feel if there is any resistance in the cage. It has a pretty strong spring so if the spring has broken or is damaged in some way it will be easy to tell

dee4life2005

I had exactly what you describe just one/two weekends ago on a shimano ultegra 6700 rear derailleur. New chain and cassette and the chain was slipping badly, and I had a tell-tale stripe of oil over the chainstay which indicates lack of chain tension. It was then I noticed the rear derailleur wasn't taking up the slack, especially in the smaller cogs on the rear.

In my case it was just that I had been riding in a lot of wet weather and the derailleur pivots had got clogged with crap (particularly the one that the jockey wheel cage is attached to), and the spring wasn't strong enough to over come this. I disconnected the cable, took the derailleur off the bike, and spent the best part of 2 hours giving it the clean of its life (that was a bit OCD though, 30mins - 1 hour probably would have done ), spraying GT85 into all the pivots and working the derailleur back and forwards so that it flushed all the muck out. It's like new again, and the problem with the slipping chain went away immediately.

I suspect it's just needing a good clean, it should be obvious when you take it off the bike if this is the case.

fermion

Thank you all for the responses.

I have been out in the bad weather recently and although the derailleur looks clean from the outside perhaps its gunked up on the inside. I'll take it off and give it a clean over the weekend, if that doesn't sort it I'll have to phone Planet X and insist they take it back for a warranty replacement...

smidsy

Odds on that it is gunked up.

Take off, clean, lube and re-fit and I bet it is fine.

JayKosta

After cleaning (and don't try to remove the cage from the deraileur body), if the jockey wheel cage rotates easily on its pivot that means the round coil spring that is hidden within the pivot has either broken or has slipped out of its proper position. That spring should have strong tension to pull the jockey cage rearward for chain take-up.

Jay Kosta
Endwell NY USA

pitchshifter

Could the chain be too long?

dee4life2005

you can get similar symptoms if the chain is too long, but in the case of the OP they say they've had the bike 3 months and the wording suggests it's only just happened - so unlikely i would have thought.

+1 for note from JayKosta above, don't try to remove the jockey wheel cage from the derailleur body!)

badgermilk

Yep, check the chain length. Just take a link out if too long.

fermion

I'm pretty sure the chain length is about right because if I made it any shorter I wouldn't be able to use low gears in the big ring.

I can feel that the jockey wheel cage is very stiff and does not rotate easily around the pivot - I guess this must be the problem. Going to take it off and give it a clean, will report back.

bondurant

Same thing exactly on my 6700 rear mech last year. Be surprised if it's not good as new after a clean.

tomhammersley

I've had a similar experience with RD-6700. It was full of gunk, but when I stripped it down and took the whole cage off, I have my suspicions that the spring inside the knuckle got dislodged from the retaining hole; it was an easy mistake to make when reassembling it. And once it was dislodged, the cage became very very lazy.

I put a lot of mileage on that mech and yours is new. Perhaps something to consider?

Monty Dog

Check that the mech is correctly engaged on the frame mount otherwise there is nothing for the spring tension to work against - once had the lug shear-off on a mech which created the problems described.

dee4life2005

did you have any luck with your derailleur ?