Hi, me again....
When my chain is in the largest sprocket at the back and if I pedal backwards the chain drops down to the smaller cogs. Obviously you're not meant to pedal a bike backwards but this happens when I get off and have to push/carry etc. Only does it when it's in the big cog at the back. Is this normal?
If that doesn't work I would clean and grease the jockey wheel bearings.
The extreme chain angle doesn't help in solving this. Some cassettes like Shimano and cheaper Sram ones are prone to this but the newer higher end cassettes and chains are much better at retaining the chain pedaling backwards. Some people use a 12 speed chain on an 11 speed block and this can solve the problem without affecting shifting.  Pedaling backwards means you are pushing the chain which results in a little slack and pedaling forwards creates tension which helps retain the chain. No amount of fiddling with the mech B tension screws will solve this satisfactorily so you just need to live with it I'm afraid.Â
