SLX 12s rear mech with clutch turned on – I’m finding the chain jumping off the top jockey wheel and getting stuck between the jockey wheel and mech cage. It only happens now and again, seems to be happening on rough ground at speed.
I’ve never seen it before and no idea why it’s happening – anyone else experienced this?

Twisted chain link?

I cant tell you why it happens but I have warrantied a few mechs for sawing themselves in half due to this problem.

@benpinnick – that’s interesting as the bike it’s on is a 3 month old Aether 9a. Currently on its second rear mech after the original exploded spectacularly for no apparent reason.

I have the exact same issues with my SLX 12sp mech!! It literally drives me insane but I can’t seem to find a fix for it…

@bullandbladder – Deore by any chance? I had one do that exploded on a flat piece of bridleway, Slam69 in Gloucester told me it was “crash-damage” when I took it back despite my pointing out that the twisting damage was caused by the lower half of the mech getting wedged between the chain stay, wheel and cassette AFTER it had exploded unexpectedly.

Not going to be darkening their door and inconveniencing them with my money again.

SLX, not Deore. Honestly I’ve never seen a mech so comprehensively destroyed – even the jockey wheels were broken! I kind of assumed a stick from the pile I’d just ridden over had somehow been sucked in between the top jockey and the cassette and caused havoc.
I’m thinking that 12 cogs is one too many as I’m also now on my third chain in circa 3 months.

If I could figure out image posting here I’d put up a picture of mine, snapped and twisted describes it. Managed to take out the mech, a 4 week old chain and a 2 week old cassette as it bent 4 of the cogs.

This bike is my first long term experience of a shimano drivetrain and I’ve not been impressed with the mechs. I’m on my 3rd after a year, the original chain and cassette lasted admirably.

Yes started to happen to me on a second slx mech at BPW last week. First one replaced after really screwing itself up with a few increasingly resistant pedal strokes. Same symptoms this time as described by OP. Not impressed.

Does moving up to Xt solve the issue i.e. there’s a design flaw in the cheaper derailleur’s cage?

Same issue – luckily it only buggered the alloy inner plate on my cage (and top jockey) so I popped a Deore plate on which looks like it might be (tougher) steel. I also popped an alloy top jockey in which seems to leave a much smaller gap that the chain might not drop in – time will tell.

Some have mentioned bending cahe plate a little to close the gap up between inner plate and jockey.

I’ve had that in the past, was always due to a sticky jockey wheel (bearing or crap built up) but I reckon that’s the first thing you’d have checked?

My SLX mech has a very inconsistent clutch. It is switched on, but if I simulate the cage moving fast a few times the clutch works and then stops working on subsequent movements. I have to switch the clutch off then back on again for it to work correctly for a bit.

The only thing that fixed it was a new clutch assembly, it used to work fine, I think the link between the cage and clutch has worn (you get a new one with a new clutch)

I would hazard a guess its the clutch sticking and causing the chain to lose ‘adhesion’ to the top jockey, and then derail but I don’t know for sure. Sticking clutches is a feature of 12 speed shimano Im afraid, if its an Aether of course you can send us the mech and we’ll service and replace the clutch for you as well as any damaged jockey wheels.

Thanks @benpinnick but not sure I could ask you to do that – the mech now on the bike is not the original.

With the cable removed, there is a massive amount of play between the moving parts of the mech – as if all the pivots are worn, which I suspect might be something to do with it. I’ve always found Shimano drivetrain to be good quality, but this really is rubbish. I was beginning to question whether or not it was a genuine shimano part 😆

I’ve destroyed 2 XT 12 speed rear mechs in the last 12 months. The clutches die pretty quick.

So I’ve now got 2 x slx mechs with cracked plates and the replacements out of stock everywhere and a third mech which has started to drop the chain off the top jockey wheel and damage it’s cage.

Reading over above again where it’s mentioned the clutch may be a cause and googling that a bit this guy thinks backing off the clutch tension and keeping it greased up helps.

12spd clutch service