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Not sure if this is happening when out riding or not, but it is definitely happening when checking it out inside.
When in the granny ring and the outer most cassette sprocket (the smallest one) the chain will fall off the lower jockey wheel and kind of runs between the jockey wheel and the cage.
The jockey wheels look fine.
Any ideas on what might be causing this? and therefore how to fix it?
Cheers
dont use that gear? bit of an extreme gear to use
Does it always fall to the same side?
Anyway- check chainlength, gear hanger and see if you have any stiff links. Also check rear mech tension.
And, as above- its a poor combination to use that is repeated using the middle ring and thus causes a less extreme deviation to the chainline.
Erase the question Mark and then seanodavs response is spot on. Don't use that gear.
This, or the opposite (big ring, big sprocket), is strictly forbidden.
Chain runs diagonally - at worst it falls off, at best it's excessive wear to drivetrain.
You can use the full cassette in middle ring though.
Yea always comes off to the same side.
Point taken about not using that gear, but I'm pretty sure that what is happening never used to.
Cheers
New bike time
As above,caused by an extreme chainline. If it's only just started doing it, then it's probably down to the jockey's teeth being worn /less pronounced, and less able the limit sideways movement of the chain(same reason a worn jockey doesn't usually shift as accurately as a new one).
As above, what do the teeth on the jockey wheel look like? Big & square-ish or small & pointy? If it's the latter, u need new jockey wheels.
I came across a guy in the Peak cursing his bike for a similar problem. On closer inspection, I pointed out the inner cage plate was on upside down.
As has been said chain length is 1st point to check. Wrap round largest front and back ignore the mech and add a link. Don't forget that the Speed link will be half a link as well. Next thing is to make sure that the mech is on the hanger properly. I have seen them where they have not been rotated and located far enough back so do do put enough tension in the chain. Had I not seen it I would not believed it possible to do.
If still no joy then it looks like the tension spring in the mech is to slack. Not a replaceable item.
Bad gear to use and probably chain too long so that the rear mech can't take up the slack so the chain is loose.
But the simple answer is don't use that gear...