chain problem!
 

Hi guys, help required.
Symptoms... on granny ring the chain runs great down to the 14 tooth ring on the rear no probs there.
on the big ring the chain runs great up to the 30 tooth ring no probs there.
Alas on the middle ring it keeps jerking through the bottom jockey wheel,as if the jockey wheel is sticking, which it obviously isn't because it runs great on everything else.
Any ideas?

My guess is the middle ring has worn to the point the teeth are just slightly hooked, and you are getting borderline "chainsuck". i.e. the chain is trying to hang on to the ring, but there is just enough tension in the RD to pull it off of the teeth. Full blown chainsuck is when the chain stays attached to the ring and is rammed into the underside of the FD.

If the drivetrain is worn and this is just now beginning to happen some form of chainsuck is a likely cause, especially it you put lots of miles on the middle ring.

The other possibility (especially with Shimano derailleurs) is a mis-adjusted B screw which is allowing the upper pulley engage the sprockets through the chain. Buried in the last step of the Shimano instructions is a reminder to check pulley clearance in all gear combinations. This is because upper pulley height varies with changes in chainring size.

If the problem is uniform regardless of which rear sprocket then the chainring is the common denominator and some kind of chainsuck is more likely. If it's worse on larger rear sprockets then check the upper pulley clearance.

thanks for that, it makes good sense because recently i've been getting a bit of chain suck!!!!
new middle ring on the way.

cheers for now

new middle and inner rings fitted, went out yesterday and had a great day.
problem solved

thanks

Last year I put a new chain on one of my bikes, and immediately could not pedal 20 feet w/o sucking the chain up into the FD on the middle ring. So I put the bike back on the rack and ordered a new middle ring.

It arrived, I installed it...then promptly found the small ring was doing exactly what you described. ARGH!!! So I had to order a new small ring.

Next time I'll test *all* the rings when such a problem surfaces, so I don't need to disassemble the crankset twice.