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it must be the worn jockey wheels then :/ my chain snapped the other day while riding on my alpine and now it jumps in 8th and 9th gear, kinda pulls the bottom jockey wheel forward so i assumed it was the chain being twisted or something so i bought a new chain and still the same problem? the mech is a saint and is still perfectly inline but the jockeys look slightly worn so im assuming it needs new ones- reason why the chain wants to pull to the left if not over the mech just on the arm and hits the sides :/ anyone else had a prob like this and can tell me how to sort the issue ta!
Just to rule it out... Have you checked there's nothing jammed in the cassette in-between the sprockets? Or that the teeth aren't bent? Or just worn out?
Your chainrings and/or cassette will also be worn...putting a new chain will just make it worse until it beds into the worn shape of the existing rings...
its strange how it only does it in 8th and 9th gear but when i go to middle ring in 8th or 9th gear it seems a little better :/
the top gears have far less teeth, so the chain jumps on those first. In general when a drive train is worn its block, chain and inner ring that all need doing at once.....
so you think thats whats pulling the bottom jockey wheel over making the chain want to move away from it?
Worn jockey wheels won't cause a chain to skip - they're just guiding the chain, they're not part of the drive system as it were. You could use smooth circles, and as long as you could keep the chain on laterally they'd be fine.
If it's skipping I'd look at cassette - as above, smallest sprockets wear fastest.
New chain usually requires a new cassette unless you change it really frequently.