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Sure I've seen this on here before, I've posted at least once but it was not long after the group was launched (105/5800)
Every 12 months or so the bottom of the cage starts bouncing up and down with the chain miss-aligning on the gear, last weekend it involved the rear mech bouncing up and ripping itself off (through peddling force I admit....) and bending the rear hanger.
Checked all the usual, hanger straight, chain length, chain direction, indexing, everything you'd usually go through.
Is it a known fault?
Tempted to go Ultegra chain/cassette/RM this time around
Interesting as I had the same problem on my last frame....every so often the rear mech would jump up and down like a banshee when in the larger cogs. Never got the bottom of it as it happened quite infrequently. Since then I have swapped frames and cables and the problem seems to have gone but I wont rule it coming back as I havent ridden the new frame much. I wonder whether its related to chain alignment?
Sounds like a stiff link.
That's exactly how it behaves SR, I'm using a Shimano 105 chain installed by the pin (I very rarely find these stiff link if joined correctly) and checked after.
Correct ratio RM's been fitted as well