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On occasions if I ride over a particularly rough patch of Tarmac the chain will come slightly off the rear cogs on the derailer, make a nasty racket and create a bit of drag. A quick back pedal meshes it correctly and everything works fine.
This is the only time it happens, on shifts it's fine. Took me ages to work this out.
Chain isn't stretched, jockey wheels look okay. Could it be tension in the mech? Can you tighten it? Probably done around 5k miles
10spd or 11spd?
Would always look at chain and cassette wear first (and double check too), check that the gears are indexed correctly too. Have you left the chain a link or two too long per chance? Take the chain off, and check the jockey wheels are spinning fine too (just cos they "look" ok doesn't mean they necessarily are).
If it's 10spd the mechs didn't have as much tension in them as the 11spd mechs do, you can't officially increase the tension, but it's worth checking that you've got the B tension screw wound out enough (not too close that the jockey wheel hits the largest cog, but as close as you can get away with).
10 speed, chain has been replaced within 1k or so and made sure it was the same length. I'll try the b screw adjustment though nothing has been fiddled with and was fine for a long time. Maybe the jockeys are goosed.
Sounds a good excuse to upgrade....
Riding across rough tarmac I'd say....