How to deal when a rear derailleur is worn?
 

I'm in the process of fixing up my mountain bike. The shifting seems a little dodgy, the cables were recently replaced so I don't think it's those. The rear derailleur is Saint (9-speed, M810).

There are a few weird things.

1) It doesn't always shift (either up or down).

2) For a few of the larger cogs, the chain winds up riding up the cog, like the chain edge is stuck on cog, rather than meshed. Really weird.

Could the pulleys be worn? They aren't in great shape. I got the bike used, it was well maintained but ridden hard (ex-pro demo bike).

Any other ideas?

Sure the pulleys could be worn. Your house could also be blue but we can't know that either without either being there or looking at a good photo.


Other reasons of poor shifting are (in no specific order or even all possibilities included)- bent hanger or bent cage, gummy pivots (cage, B pivot or even parallelogram links), bent cog teeth, loose cogs on body, mismatch of cog/der version/shifter, cable (even new cables can have issues), generally bad adjustment, der parallelogram return spring broken or weak, chain mismatch with cogs, chain link bent/twisted, and more.


One way to quickly narrow down things is to ask a shop for an assessment of your drivetrain. This kind of wild goose chase is hard to do long distance. Andy.

What area are you looking for in a picture? The hanger is straight, the drivetrain is in good condition (cogs don't show wear or bent, not loose, chain not stretched or twisted).

The lower pulley has a small chunk missing between two teeth. Both pulleys feel a little stiff, I'm not sure the upper bushing slides as smoothly as one might like. The springs feel stiff.

How does one go about diagnosing a worn derailleur?

The first suspect when a problem arises is the last thing that was changed beforehand. So the fact that the cables were changed recently is a reason to consider them, not to reject them. If the cable housing was not properly cut or capped, is way too long or short, or if there is some other source of excess friction you can experience the above symptoms.

Pulleys can be removed and cleaned up then lubed. But i'd consider replacing the cracked one, as they usually are sold in pairs the top one would also be replaced.


Ders develop slop in the pivots of the mount, the cage and the parallelogram links. Grabbing the bottom of the cage and wiggling side to side then doing so with a new one will give you some idea of the slop differences.


But I agree with CB in looking at the cable install first. Andy.

Were the cable housings changed at the same time the cables were changed?

It used to be that all cable housings were the same, but there are now shifter specific cable housings (longitudinal support wires instead of spiral wound).

It is possible that something is broken with the spring in your derailleur, but in that case it would not shift at all.

The"B-Screw" should be adjusted so that the upper jockey wheel is as close to the cassette/freewheel as possible without rubbing in all gear combinations.