Yesterday while I was at work someone stole the rear derailleur and hanger off my bike. Everything else left alone.

Why would someone steal a derailleur? Seriously. Of all the parts.

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    Because they needed one, I suspect. – Daniel R Hicks Aug 16 '19 at 21:01
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    Perhaps they intended to strip more off but someone came along and scared them away? – Affe Aug 16 '19 at 21:07
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    I've seen the same at the station where I park. No definitive answer but theories: It's the only way to get a hanger to fit - but anyone who knows what a derailleur hanger is is probably too much of a cyclist to wreck someone else's bike; Someone's stealing bits to replace those damaged or left behind when they stole another bike; They want the derailleur and it's easier to take the hanger as well; – Chris H Aug 16 '19 at 21:36
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    Is it a particularly high end bike/groupset? – Matt Holland Aug 17 '19 at 0:37
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    We have no idea what their motivation was, and even if we did, knowing is unlikely to make any practical difference to your life. – David Richerby Aug 17 '19 at 12:04

Some thieves will intentionally disable a bike so that it can’t be ridden home. Then they’ll come later in the day or evening with the appropriate tools to finish the job.

Often they’ll steal a wheel or your seat or handlebars. But if you had those secured with security bolts then the derailleur might be the next easiest thing to steal that would totally disable your bike — and on many bikes you just need a hex wrench to take it off. And you can put it back on just as easily when you’ve taken the rest of the bike.

Otherwise, no clue. It may have been done by the same thieves who stole an inner tube from a bike here on SE while leaving the tire and rim.

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    You need a hex wrench to take it off but also you need a tool to break the chain unless there was a quick link! – Carel Aug 17 '19 at 10:29
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    @Carel not if you take the cage apart. Often that’s a single screw. – RoboKaren Aug 17 '19 at 14:19
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    A pithier version of this answer might be: some people just want to watch the world burn. – Weiwen Ng Aug 17 '19 at 15:11
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    @DanielRHicks I was actually thinking through that, though. If the handlebars, wheels, and seat have pitlocks, what would be an easy way to disable the bike in a way that it could be made rideable again? Messing with the brakes doesn’t render it unridable. If the chain had a quick link then maybe that. Pedals need a longish wrench and can be very hard to get off. There aren’t that many options. Got any good ones? – RoboKaren Aug 18 '19 at 0:45
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    @DanielRHicks the point of stealing a bike is to retain the value. That doesn’t happen if you break things. – RoboKaren Aug 18 '19 at 2:40

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