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So I've been building up a late 80's steel frame to a modern 11 speed, but still using downtube friction shifters. After doing a few runs up and down my street all felt OK so I decided to take it out on it's maiden voyage.

A couple of miles down the road however something didn't feel right, after pulling in to a cul de sac to try and see what was wrong I was testing going up and down the gears a few times when the pedals jammed. Luckily I was going quite slow and managed to jump off. Anyway it appears one of the jockey wheels became jammed between the spokes. I'm guessing I may have offset the H limit screw too far or something?

Anyway the damage is it's cracked the housing of the rear derailleur hanging bolt (see below). Is this safe to use or should I get a new one?

enter image description here

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    It's hard to tell how severe the damage is from that photo. But it looks bad enough that I'd say the unit should be replaced sooner rather than later. – Daniel R Hicks Nov 23 '15 at 16:29
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Define safe. Are you just worried it will fail or that you will get hurt?

It is going to fail all the way and sooner rather than later.

If it jams into the rear wheel it might damage other stuff (like spokes).

The safe bet is to just replace it now.

If it jams into the rear wheel it might cause you to get hurt. If pedals jammed might cause you to fall then yes in that respect you could get hurt.

Save it for parts.

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    The cost of a new 105 derailleur is in no relationship to the trouble this one will cause you when it fails, sooner or later. Anyway as far as I can judge from the picture the pre-tension screw seems to have broken off. – Carel Nov 23 '15 at 13:56
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    Theres a decent chance it will go into the wheel, and that can easily lead to getting hurt. – Batman Nov 23 '15 at 15:19
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Coming at it from a Murphy/cost point of view - the cost of a new part now is roughly the same cost as a new one in a month or a year.

The damage incurred by replacing it now is zero.

The damage incurred by having it break later might be zero, but the worst case could be as bad as "unit snaps clean off, catching in chain, flinging around the cassette, catching in rear wheel spokes, locking the back wheel, bending your rear axle, and giving you an unexpected rear-wheel skid."

My personal experience is that derailerueueurs let go when most stressed, which is during hill climbs.

Saint Murphy says this will happen when you're alone, a hundred kilometres from home, and you have to walk in road cleats or bare feet because your phone is flat. All this assumes you avoided injury to yourself.

TL,DR; change it as soon as convenient.

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