Weird RD issue

[Mike_P]

This has me foxed, the RD is changing perfectly bar the chain slips on the second largest cogs and when on the largest cogs will not shift to the second largest but will shift to the third largest when so activated.

 

[roubaixtuesday]

What do you mean by "slips"?

Does the chain stay in the same gear but slip over the rear cog or front chainring?

Or does it try to change into another gear, and if so, which one?

Do you see this on the stand, or under load, and does it get worse with more load?

 

[Mike_P]

Is okay on the stand, it's when changed to under load it the chain seems to be trying to stay on the third. It's as if the indexing is out for the second largest cog but okay for the rest.

 

[roubaixtuesday]

Maybe chain wear if it's got a bit slack, or jockey wheel wear? I find changing these can drastically improve shifting.

Alternatively, bent hanger?

 

[Ming the Merciless]

Is okay on the stand, it's when changed to under load it the chain seems to be trying to stay on the third. It's as if the indexing is out for the second largest cog but okay for the rest.

Do you have a barrel adjuster? The cable tension and alignment may not be right and the error appears when you get to the left hand extreme of the cassette. So try changing with barrel adjuster first to see if you can sort it. Changing down reluctance can be a sticky cable or not enough tension in the derailleur springs. If old derailleur that can be down to gunk and it just needs a clean.

 

[Mike_P]

Have tried tweaking the barrel adjuster, will give it another turn. Chain has been changed.

 

[Cycleops]

The reluctance to shift sounds like a cable issue. If a bit old replace the inner and outer.
The slipping is probably a worn cassette/chain. How long since you changed them?

 

[Mike_P]

The cassette is about 18 months old, the chain a month and the RD 15 months. Could be the cable, the outers are probably of some age.

 

[boydj]

New chain, old cassette - not a good combo.

 

[Milkfloat]

The inability to get the indexing right throughout the whole range of the cassette can often be due to a misaligned hanger. I would start by aligning the hanger and also make sure the cassette/ freehub does not wobble.

 

[Mike_P]

New chain, old cassette - not a good combo.

The old chain was nearing 0.75 wear; changing a chain at such at point normally causes no issues and will extend the life of the cassette.

 

[Mike_P]

Changed the derailleur hanger (although the one on looked okay), new cable and outer and cassette tightened but still the issue persisted. Decided to fit a new derailleur and problem resolved.

 

[mickle]

I had similar symptoms recently. I removed the derailleur to clean the jockey wheels, reinstalled it and it didn't index properly across the cassette. Turns out, in my haste I'd not captured the cable properly under the pinch bolt, which had altered the pull geometry/ratio.

 

[Mike_P]

In this case there is something mighty strange with shape of the cage on the RD that has been removed. It bent soon after originally being fitted and got straightened out but compared to the new one is now slightly twisted, not enough to eyeball on its own but plainly noticeable placed side by side with the new one.