Help needed - rear derailleur failure!

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2015-07-06 11:20 AM

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Subject: Help needed - rear derailleur failure!
What would be the reason for the rear derailleur hanger to fail?
I just replaced my rear shifter cable on a Shimano 105 derailleur / Ultergra 10 speed cassette (12-30t).
Everything seemed to be shifting fine on the work stand.
Took the bike out for a ride, and the first time I shifted into the 30t gear, BANG, the whole assembly sheared off at the mounting hanger.

What did I set up wrong? The cable tension may have been a little high, as it was hesitating a tiny bit going into the smaller gears, but it seemed pretty smooth.

2015-07-07 6:05 PM
in reply to: coreyasaur

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Subject: RE: Help needed - rear derailleur failure!
Did the chain kink at one of the pulleys? That or one of the kids knocked over Dad's bike and superglued the hanger back on for you!
2015-07-13 11:38 AM
in reply to: coreyasaur

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Subject: RE: Help needed - rear derailleur failure!

Lots of torque being applied with a 30T sprocket.  I'd also suspect a hung chain causing the hanger to break.

Mark

2015-07-17 3:57 AM
in reply to: coreyasaur

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Subject: RE: Help needed - rear derailleur failure!
When you get such a large cog, you add lots of torque and maximize (or maybe even exceed) the max chain length the derailleur can handle (you'll see the max tooth the derailleur can handle, which is the large chain ring minus the small chain ring, plus the difference between the largest cog and the smallest cog of the cassette). When you get to the extremes, you also need to take wheel flex and frame flex into account, two factors that you can't see on a work stand. Basically, the derailleur will be really close to the spokes (minimal clearance), but once you add the torque on the derailleur, the flex from the wheels and the flex from the frames, you may very well loose that complete clearance you had on the work stand...
2015-07-20 1:14 AM
in reply to: coreyasaur

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Subject: RE: Help needed - rear derailleur failure!
How old is the 105 derailleur.

Is it actually compatible with 30 tooth cogs?

Plenty can only go to 28, especially a few years old.
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