Chain jumping off lower jockey wheel?

JesseD

I have just replaced the cassette, chain, outer chainring and rear mech on my bike, they were all Shimano 5800 and have been replaced with the same, apart from the rear mech was a medium cage and is now a short cage one.

I was setting up the indexing on the rear mech and noticed the chain was not sitting properly on the lower jockey wheel and was sitting on the outside of the wheel slightly, this is happening despite the indexing being spot on, the problem also seems worse in the big ring but still happens whilst in the little ring?

Any ideas what may be causing it?

sungod

is the wheel fully seated in the dropouts? (loosen qr, put weight over bike, tighten qr)

is the chain length correct? (follow shimano instructions for the exact model of rear mech)

is the mech hanger loose/bent? (ideally use alignment gauge)

JesseD

sungod wrote:

is the wheel fully seated in the dropouts? (loosen qr, put weight over bike, tighten qr)

is the chain length correct? (follow shimano instructions for the exact model of rear mech)

is the mech hanger loose/bent? (ideally use alignment gauge)

Hi Sungod,

I have double checked the wheel alignment and it is sitting properly, I have checked the chain length as well and that is also good. Upon looking at the mech from the back I fear it maybe a little twisted which leads me to thing either the cage is bent or the mech hanger is bent, and given its a new mech then my money is on the hanger.

I am taking it into my LBS this morning to get the alignment checked as I don't have the tools and hopefully it is and they can sort it easily as I am racing tomorrow morning!

JesseD

My bet was on the rear mech hanger, took it to the shop and he pulled out a big adjustable wrench and bent the cage a bit and voila perfect shifting!

Apparently its a well know issue with Shimano rear mechs!