Problem ID help - chain jumping

[HumpTdumpty]

Hi all - my chain jumps in exactly the same spot “clicking” once in every cassette rotation over the bottom jockey wheel - Q) Is this a new derailleur required or a new chain ? Or both any suggestions most helpful - I recall stamping down on the pedals in the bottom gear “like a idiot” and an almighty clunk sound never been the same since - apologies for my lack of technical terminology

 

[Witzend]

Turn the bike upside down and rotate the cranks like you're backpedalling. Watch where it skips in the jockey wheels and check the links around that part of the chain - you may have a stiff link. I've put enough of a twist in my chain to do this by putting too much power down.
If you don't have a chain tool to loosen it, you can often do it by hand with gentle persuasion.
Edit - if it is every CASSETTE rotation, check for any bent/broken teeth - I've done that, too.

 

[Ajax Bay]

my chain jumps in exactly the same spot “clicking” once in every cassette rotation over the bottom jockey wheel

If this is once per cassette rotation then this is not a chain issue or a pulley wheel issue: it's a cassette or freehub issue.
Does this happen whichever sprocket the chain is on?

 

[sevenfourate]

Easiest way to free up a link is to manipulate / gently massage / bend it against the direction of normal movement. Ie: Flex the joint at 90 degrees to the plates….to give the roller / plates a chance to be able to move freely……

https://www.google.co.uk/search?q=f...ate=ive&vld=cid:b2674219,vid:T0ycgdX35yY,st:0

 

[Dogtrousers]

As a @Ajax Bay says, the question of which sprocket is in use is important.

If it happens only in the biggest sprocket then it could be the derailleur B screw needs tightening and the mech is sitting too close to the sprocket. Although that wouldn't really explain the "once per cassette rotation" part of it. But might be worth checking anyway.

 

[HumpTdumpty]

If this is once per cassette rotation then this is not a chain issue or a pulley wheel issue: it's a cassette or freehub issue.
Does this happen whichever sprocket the chain is on?

Yes it does fairly consistently

 

[presta]

Your description doesn't make sense, what's the connection between different sprockets with different peripheries, and the passage of the chain over the tensioning pulley? If it's chain issue it should be once every revolution of the chain not the sprocket, if it's once every revolution of the sprocket regardless of which sprocket, then that suggests the freehub.

I broke a chain once stepping hard on the pedal as I pulled away from a standstill, that then tinkled each time it passed through the derailleur, but once per revolution of the chain.

I wouldn't be replacing anything until I'd got to the bottom of the cause, and established that it isn't just maladjusted.

 

[Ajax Bay]

The tension pulley's 'periodic' jerk is the negative canary (sign of issue), not the cause.
If you have one, try another wheel in there (doesn't need to be 11sp) and see if the symptom's still there.
If it is, temporarily replace the cassette with an old one (again, doesn't need to be 11sp).

 

[roubaixtuesday]

...

Yes it does fairly consistently

On all sprockets?

 

[raleighnut]

My money would be on a damaged chain, check that a pin hasn't come adrift from the plate and it's not got a 'stiff link', either could cause it