SLX derailleur breaking in exactly the same place - possible causes?

[Josh]

Hi Folks,

See attached pics.  The jockey wheel cage on my wife's bike has broken in exactly the same place as her first derailleur a year ago.

The bike was bought in 2021 - we would sometimes have issues with the chain slipping off the jockey wheel.  Minor issue, easy to reseat.  After about a year the first break happened (2022).  we replaced the derailleur, and continued to have issues with the chain slipping off the jockey, but far less frequently.  Exactly a year later the same break (this past weekend).

Any thoughts as to why this might be happening? 

 

 

[BaGearA]

How easy does that jockey spin , and does it have side to side play ? 

 

It seems like the chain coming off and putting pressure on that corner could cause it but the wheel itself not being 100% would be the place I start looking 

 

 

Yes jockey wheels ( well mostly the bearings ) wear out and replacing them often sorts out shifting quite nicely  

[Piernas Flacas]

The only issue I've ever had with a derailleur was because an incorrect B-screw setting. But in my case was only the hanger that broke.

[Josh]

Thanks for the insights and advice - very helpful! I’ll check on the jockey play and both, and post feedback later. 
 

grateful to know I can just replace the cage! On the first break I think we may have looked for one but probably not hard enough! 

[openmind]

Might be a b-screw issue - check that the top jockey wheel clears the top cog of the cassette properly when in that gear. May be that it hits the cog with enough force to bend it and derail the chain. Then with further pedalling force, the derailed chain rips off the top jockey wheel. If the top jockey wheel hits the top cog, turn the b-screw in to move (rotate in the plane of the bike) the jockey away from the cog. If it's 12-speed, this is especially important. There might be a mark on the inside of the cage that shows where the top cog should be in relation to the cage position (my 12sp XTR has this). 

This is speculation, may be wrong, but the b-screw is always a good place to start. 

This explains it in painful detail:

 

Edited November 27, 2023 by openmind

[PregoRoll]

Maybe chainline? 

Has the bike been converted to a 1x system? Maybe the front blade is offset to far to the right so when you head up the cassette near the top, the chain line is very skew. Putting unnecessary pressure on the upper jockey wheel? 

[Mook]

Bent RD hanger perhaps causing excess torque from the cassette straight onto the inner side of the top pulley wheel axle screw. That could explain the chain coming off at certain times.

 

My 2 cent's worth. Might be way off the target here, but with me 😝 it does happen sometimes.

[RobertWhitehead]

3 minutes ago, Mook said:

Bent RD hanger perhaps causing excess torque from the cassette straight onto the inner side of the top pulley wheel axle screw. That could explain the chain coming off at certain times.

 

My 2 cent's worth. Might be way off the target here, but with me 😝 it does happen sometimes.

I tend to agree with you on this one, I wish I stumbled across this thread 3 minutes earlier so that I could've posted it first 🤣.

Hear me out quickly: 

a) It is two different rear mechs on the same bike, therefore any setting or failure pertaining to the rear mechs comes from the bike. This leads me to eliminate any setting on the rear mech itself. 

b) If it was the b-screw the rear mech would catch the cassette making one hell-of-a noise going up. One would almost have to man-handle the mech to get it to stay in the top position

c) If it was related to the chainline the chain would continue dropping off the ring in the front. This is due to the spring in the rear mech pulling the chain to return to its resting / neutral position. 

 

A very vital clue is this:  we would sometimes have issues with the chain slipping off the jockey wheel.

 

This tells me that the hanger is bent and that the chain is causing the damage. If you fix the hanger alignment this will not happen again. 

[droo]

1 hour ago, RobertWhitehead said:

 

b) If it was the b-screw the rear mech would catch the cassette making one hell-of-a noise going up. One would almost have to man-handle the mech to get it to stay in the top position

You seriously overestimate the mechanical sympathy of some bike riders. I'm going with a chain that's too long, B screw adjustment, or a combination.

Bent hanger would cause shifting issues on half the cassette long before it broke anything.

[Mamil]

29 minutes ago, droo said:

You seriously overestimate the mechanical sympathy of some bike riders. I'm going with a chain that's too long, B screw adjustment, or a combination.

Bent hanger would cause shifting issues on half the cassette long before it broke anything.

Ha ha ha... Mechanical sympathy... Love that.  I dropped my bike at lbs this morning and knowing me as he does he raised an eyebrow and said "you didn't get a screwdriver out did you?".

[Josh]

Thanks folks - an update to some of your questions:

1. bike was not a conversion - made as a 1x, all factory components

2. No play on jockeys on either derailleur, jockeys spin freely. Jockey's on second derailleur replaced once.

3. Shifting smooth (apart from instances where chain has come off jockey).

Based on feedback it seems sensible that the cage has broken from the pressure created by the chain coming off the jockey, getting stuck in the cage and stressing the metal.

From your posts, possible causes are:

Incorrect B-Screw setting, Bent hangar, The wheel itself, the chainline being skew.