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Chain coming off pulley
#1
Hello, I have a m340 acera rear derailleur, and the chain comes off of the bottom of the lower pulley and sits between it and the cage. I can get it to go back on by back peddling, but it happens again. I am not sure if this happen when i shift or not.
On pavement it seems to work fine, but on dirt is when it acts up.
The pulleys seem to sit correctly under the sprockets. Any ideas?

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#2
Since it's just on dirt, I assume it is coming off when you're getting bounced around. It looks like that derailleur has a cage that wraps around the pulley fairly close to keep the chain on. It the cage or the derailleur in general bent at all? Do the two pulleys sit perfectly vertical with one right below the other?
Only other thing I can think of is if the chain was too long and had extra slack so it wasn't as tight on the pulleys as it should be. Changed your chain recently?
I have seen the teeth on pulley wheels wear down enough that the chain can slip off them. Are the teeth still pretty much there?

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#3
Thanks for the reply. I did have the chain replaced recently, and thats when things started to act funny. Is there a way to check if it is too long myself?
Also, while trying to see if I could spot the problem at home, it did happen while shifting to the largest cog with the back wheel lifted. I was able to remedy that by adjusting the barrel adjuster. This is with the chain on the middle ring upfront. I do this while riding also. Could that be causing the problem, should I only be on the small ring when shifting to the largest cog?
The teeth on the pulleys seem in good shape(from what I have found online) and they seem to sit vertical to each other, but honestly I am not too experienced, so maybe to my untrained eye, it looks ok.

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#4
Shimano advise chain length =
Fit chain round largest front chainring and largest rear sprocket + 1 link.
http://bicycletutor.com/calculate-chain-length/

[font=Trebuchet MS]Ride hard or ride home alone![/font]
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#5
I forgot I had the old chain, so I counted the links on the new chain and they were the same amount as the old chain, which I did not have this problem with.

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#6
Well the next question - is it the right chain.
8 speed / 9 speed?
Is there a number stamped on the chain links?
Is it the same make as your old chain?
Just maybe the chain being new makes it more supple and prone to "bouncing".
In which case the mech' has lost some of it's spring tension?
Years ago you could buy an extra spring that hooked from the deraileur to increase its tension. This was to keep the chain from jumping off the sprockets / chainrings when bouncing across rough ground.
On the other hand the new chain may not be as sideways supple and is resisting bending to the chainline?

[font=Trebuchet MS]Ride hard or ride home alone![/font]
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#7
7 speed cassette on bike and old chain was kmc that just says" narrow" on links. The new chain is a shimano cn-hg 73, which I see is a 9 speed chain. So maybe this is the problem?
Would any kind of adjustment to RD accommodate thinner chain to stay on pulley or just need another chain?

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#8
Sorry jrm but I have no experience of 9 speed chains. All my stuff is 8 speed.
If a shop fitted it or supplied it wrongly then maybe you could get the correct one from them?
Just a thought - the bottom jockey wheel should have no side play whereas the top one has (on Shimano sets). Check that this is so.
I suppose you could narrow the cage to guide the chain but I think the screws are a fixed length. You could put a washer under each screw head. The screws are the pivots / axles for the jockey wheels. Doing this though is likely to jam the jockeys?

[font=Trebuchet MS]Ride hard or ride home alone![/font]
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#9
yeah, I will check for play. The screws are fixed length. Maybe I'll remove the pulleys and clean em good and lube em.
Wish I could take the bike for a ride on some trails, to check if maybe re-adusting the barrel has made a difference, but we got at least 8 inches of snow the other day. boo.
Thanks for all your help.

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#10
i recently purchased a 10 speed bicycle and all of a sudden the chain is coming off of the sprocket every time i stop peddling or when i try to back peddle. Any suggestions to what the problem could be?
what can i do to fix this?

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#11
Chris,
Is this a new bike?
and -
When you say sprocket, do you mean the back gears?

[font=Trebuchet MS]Ride hard or ride home alone![/font]
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