a derailleur cage is a C-shaped channel.  Pulling the chain in either 
direction puts opening loads on the sides and especially inside corners of 
the channel.  You would hope it's not going to fatigue, but that's the 
failure mode, slow crack growth.  

On Monday, August 18, 2014 9:58:11 AM UTC-5, lungimsam wrote:
>
> That's bizarre. You would think a derailer wouldn't break like that at 
> that place because any pressure/ movement would make the derailer move, not 
> resist and break.
> Does the chain  sometimes jam between the big ring and the derailer when 
> shifting into the large ring? That's the only thing I can think of that 
> would stress the cage to make it break like that. But I am no expert. Chain 
> suck maybe? Old age?
> Mine jams sometimes. Operator error I think. Shifting too far too fast.

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