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Yesterday, I replaced my cassette and chain, having ridden the previous ones through the winter, and re-indexed my rear derailleur and checked the limit screws. Today, I had ridden slightly less than a kilometer on that when I noticed the pedaling got stiff in a manner slightly unlike being in a too-high gear, so I shifted to a smaller gear (i.e. larger sprocket), rode about ten meters and heard a snap and a ping.

The snap had been the rear derailleur breaking and I suspect the ping was from the spring in the bit left attached swinging to the extreme position. The derailleur was pointing diagonally upwards, and the chain was on a sprocket near the middle of the cassette, intact.

The derailleur was a Campagnolo Centaur 11, the cassette was Shimano Ultegra CS-R8000, the same size as the previous one, and the chain a Shimano Dura-Ace CN-HG901-11. The brifters are also Campagnolo Centaur 11-speed.

How did my rear mech fail, and what should I pay attention to when installing and maintaining the replacement so as not to have this happen again?

Edit: I received the replacement mech and when I was installing it, I noticed the hanger is bent inwards. I'm not sure if this is a cause or effect.

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  • So, you replaced the cassette and chain, adjusted the rear derailleur, went for a ride and BAM - "the chain was around the middle of the cassette" - does that mean the chain was broken, a link broken or did a link come apart? Something pulled the derailleur into a diagonally upward position. Does it look like the chain is the culprit? Any pictures?
    – David D
    Mar 30, 2019 at 15:42
  • I didn't think to take any pictures, sorry. The chain was intact, but it was on the middle sprocket.
    – HAEM
    Mar 30, 2019 at 17:24
  • It sounds to me like the chain jammed somehow. A common cause of this is having the chain too long -- did you match the length of the new chain to the old one, link-for-link?
    – Daniel R Hicks
    Mar 30, 2019 at 18:56
  • @DanielRHicks I measured the chain for one full link's overlap on the large sprocket/chainring, the way I always do it. Too bad the old chain's in the bottom of the metal recycling bin right now so I can't check.
    – HAEM
    Mar 30, 2019 at 19:03
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    My bet would be on the chain jamming in the cage. The resistance you felt was maybe a idler starting to seize. Either that or the cycling gods smote you for mixing Campy and Shimano components.
    – Argenti Apparatus
    Mar 30, 2019 at 19:10

2 Answers 2

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From the information available I would advise you to make sure:

  • Chain is of correct length
  • Derailleur is not running too close to the cassette (B-screw adjustment)

In the workstand pay close attention to what happens with chain on the largest chainring and simultaneously shifting towards the largest sprockets, especially if shifting several gears at once.

I've seen cases where the chain and derailleur can get jammed with the cassette here when derailleur is setup too close to cassette, leading to situations such as this.

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  • Also, chain failure can snap the derailleur off. A loose sideplate or quick link or a pin working its way out can catch on the derailleur and snap it right off. The new chain in this case means that's a distinct possibility.
    – Andrew Henle
    Apr 5, 2019 at 9:48
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I recently had a similar mystery failure, and I remember noticing a similar issue with the pulleys back then. Namely, the pulleys had bad bearings, and I had noticed that and put them back on with the screws a bit loose to let them turn a bit. Bad idea; they ended up seizing and snapping my derailleur hanger off.

This time it was probably just the wear of being driven through rain, snow and springtime street dust. The last time I remember going overboard with degreaser to get the gunk off the otherwise healthy-looking pulleys.

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