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I upgraded my rear-mech with a new cassette, chain and new cables a few weeks ago since then i've mangled two rear mechs!

Both times I've just been riding along; not in a big gear; not at the extremes of the cassette, the mech just seems to get caught on something (the chain?) and bends and snaps!

I've got to buy a 3rd mech and I'm wondering why it's happening, is it just bad luck?

The mech I replaced (an ageing XTR) I'd had for years without any miss-hap but now I've mangled two XT mechs, one was a Shadow and the one prior to that was a standard.

Any sugggestions?

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Is your derailer hanger bent? Does the derailer point straight vertically? Is it possible that it's shifting into the spokes? Did you try shifting through all the gears while it was up in a repair stand and seeing what the derailer does? –  freiheit Sep 8 '10 at 19:46
    
The hanger is fine.. The bike is steel and after the first mech went the hanger was bent but got it straightened and mech replaced at my LBS. Just to emphasises, the mech was nowhere near the spokes..! :o| –  Dog Ears Sep 8 '10 at 19:48
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@Dog Ears: Pictures might help folks diagnose this, particularly pics of the broken deraileurs if you have them. –  Neil Fein Sep 9 '10 at 4:06
    
For other Americans: "mech" is British for "derailer" (French, dérailleur). Sheldon on spelling this word: sheldonbrown.com/derailer.html –  Drew Stephens Sep 13 '10 at 14:46
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@neilfein, to me 'mechanical' implies a (mechanical) break-down. like a snapped chain or a cable or a broken mech/derailleru in this case. –  Dog Ears Sep 20 '10 at 13:39
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3 Answers

up vote 7 down vote accepted

Did you change the chain after the first derailleur broke? If the same chain was on and you're positive the derailleur didn't connect with the spokes, then the chain is a likely culprit. Another thing to watch out for is loose pannier straps (or anything else that could get caught in the chain and take out the derailleur).

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I think your right the chain was re-joined without using the correct replacement Shimano pin thingy. –  Dog Ears Oct 28 '10 at 21:56
    
@Dog Ears: Glad you found it! –  darkcanuck Oct 29 '10 at 4:01
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It turns out that this could have been caused by breaking and joining a Shimano chain without using the correct connector pin. The final time I got the derailer replaced along with the chain and it's been fine.

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If your chain is too long, and you are in the 'small-small' combination, it is possible for the derailleur to sort of fold up on itself. Flip your bike over and put it in the smallest gear up front and the smallest gear in the rear and make sure the chain isn't rubbing on the derailleur.

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