Derailleur cage jumping - Why??

[teeonethousand]

Yes it's a quick link chain, but it doesn't jump at one point. Rather, it drags the derailleur cage whenever I put power down. It's something to do with the interface of the new chain and the rest of the drivetrain. I just put back on the dirty old chain and the issue goes away!

You've answered it in the last sentence...it's the incorrect chain.

 

[almostvegancyclist]

Sounds like chain suck on a worn front ring?

I think this is the answer. I can't confirm as I don't have a new chainring right now. I'll have to get a new one and see if it goes away.

 

[almostvegancyclist]

You've answered it in the last sentence...it's the incorrect chain.

I assure you it's the same model as the old one...Sram PC1110..

 

[Ian H]

I assure you it's the same model as the old one...Sram PC1110..

Where did you buy it? Could it be a pirate copy?

 

[almostvegancyclist]

Where did you buy it? Could it be a pirate copy?

I doubt it. I bought a few from Hopkins or Tweeks a while ago, can't remember which. Anyhow they look genuine. I'm now inclined to think the chainring must be worn and it's pulling the chain due to this. I'll have to fit a new chainring and see if it goes away.

 

[Ajax Bay]

an unusual setup, with a Deore M4120 RD & GRX 600 2x cranks. Rear cassette is 11-42 & chainrings are 46-30. So this goes over the official RD capacity which is 41t [(42-11)+(46-30)=47t]. However, I ignore the largest sprocket when sizing the chain, so treat the 36t sprocket (which is the next one down) as the largest. This gives me [(36-11)+(46-30)=41t] which is within the derailleur capacity. And I avoid going big-big.

https://bike.shimano.com/products/components/pdp.P-RD-M4120-SGS.html
Max. front difference = 10t (OP has 16t) "I avoid going big-big" Ha ha!
The reason this happens on the small ring only is because the RD is having to wrap the extra 16t worth (46t large - 30t small).
This means the return section of the chain is under inadequate tension: this makes chain suck more likely to occur.
If it's not the chain/RD/cassette causing this (new chain, right speed, correct length(ish), running through RD, pulley wheels OK, and new cassette). This RD has not got a clutch (the 5120 has).

https://www.cyclechat.net/threads/rear-wheel-lock-up-mystery.288675/
"I just put back on the dirty old chain and the issue goes away!"

 

[almostvegancyclist]

https://bike.shimano.com/products/components/pdp.P-RD-M4120-SGS.html
Max. front difference = 10t (OP has 16t) "I avoid going big-big" Ha ha!
The reason this happens on the small ring only is because the RD is having to wrap the extra 16t worth (46t large - 30t small).
This means the return section of the chain is under inadequate tension: this makes chain suck more likely to occur.
If it's not the chain/RD/cassette causing this (new chain, right speed, correct length(ish), running through RD, pulley wheels OK, and new cassette). This RD has not got a clutch (the 5120 has).

View attachment 775345

https://www.cyclechat.net/threads/rear-wheel-lock-up-mystery.288675/
"I just put back on the dirty old chain and the issue goes away!"

Yeh I'm aware of what it says on the Shimano website about 10t max difference but I call bullshit on that one as I've been using this set up for a while with no issues and so have many others. I think it's just to force people to use their chainsets which has a 10t difference.

By the "return section", do you mean the bottom side or the top side? The chain seems to be tensioned adequately on both sides but I could be wrong.

I've just ordered new chainrings so I'll report back!

 

[Ajax Bay]

I suggest you'll only need to change the small ring.
When you're on small small, does the return section of the chain foul the RD cage (and by noise tell you to shift at the front)?
Tension section (top) providing the drive; return section (bottom) returning the chain to the cassette.
What I'm saying ref demanding the RD operate out of spec (capacity) is that the return section's low tension makes the sucking worse. You are lucky it's just 'jumping'. Try cycling up Wrynose with this: you are feared the chain will suck and lock the cranks: result = a zero speed clipped in fall (or trying to get up with a 40" gear - middle ring).

 

[Person]

I've had that problem a few times when putting on a new cassette and chain and it was because the small chain ring was slightly worn. The large chain ring was no problem because I assume of the greater tension. You can either wait until the chain stretches slightly or replace your chain rings.
I can say 100% that was my problem.
I use shimano 105 r7000 and found it to be cheaper to buy new cranks rather than just the chain rings - £85 from Merlin.

 

[almostvegancyclist]

I've had that problem a few times when putting on a new cassette and chain and it was because the small chain ring was slightly worn. The large chain ring was no problem because I assume of the greater tension. You can either wait until the chain stretches slightly or replace your chain rings.
I can say 100% that was my problem.
I use shimano 105 r7000 and found it to be cheaper to buy new cranks rather than just the chain rings - £85 from Merlin.

Same here with ordering cranks rather than chainrings... £45 for a GRX crankset but £70 for two chainrings! Madness.