Jockey pulley (upper jockey wheel) suddenly not engaging with the chain when in the three largest (easiest) cogs on the big ring only The gears work fine across the whole cassette on the inner ring but all of a sudden the chain has stopped engaging with the upper jockey wheel when in the big ring and three largest (easiest) sprockets of the rear derailleur. The jockey pulley looks out of position when this happens (almost horizontal to the lower tension pulley jockey wheel - the chain then simply isn't reaching the upper jockey wheel) and the chain makes an unholy racket with the drive train eventually seizing up entirely until I shift into smaller (harder) sprockets. This seemed to occur straight after I removed then replaced the rear wheel, catching it a little on the brake pads as I did, which resulted in a bit of a clunky removal. Is this likely to be the result of a bent, misaligned rear derailleur?

Man handling the rear derailleur I discovered the jockey wheels were indeed in the wrong position as a result of the rear mechs pivot point having seized up. I can push it into the correct position and the chain engages the jockey wheel and runs normally but the mech isn't moving freely by itself. This means it isn't extending fully to run in the big ring - larger cog gear combinations. Given the difficulty in servicing the internals of the rear mech I'm planning to replace the whole unit.

As an interim measure in the short term I've used a generous dose of penetrating oil around the pivot and it's freed things up enough to get things working for now. I wouldn't give it too long before a new mech is needed though.