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I'm worried about my derailleur snapping. The chain isn't running around the bottom pulley, it's running next to it, scraping against the bottom pulley and the frame. When I change gears it makes a horrific noise and the bike has ground to a halt without me touching my brakes. This has happened 3 times.

I am doing the London to Brighton charity bike ride in 29 hours. I don't have time to get it into a shop and I have a 54 mile ride to do and I am worried about my derailleur snapping.

What should I do?

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Pictures would be helpful –  Johnny Jun 15 '13 at 0:19
    
Pictures or a bit more description of the condition of your bike. Is it newish? Has it been abused? To me, this sounds like you may have bent the derailleur tab or the derailleur tension arm (either will need to be tended to.) –  WTHarper Jun 15 '13 at 0:37
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When you say it's scraping against the frame, do you mean the bike frame or the frame of the derailleur (commonly called the cage)? Scraping against the bottom of the pulley and the actual bike frame would be weird. Also, as others have said, pictures would help a lot. –  jimirings Jun 15 '13 at 1:10
    
This sounds suspiciously like a problem I had awhile ago. If your problem is the same as mine, your derailleur cage has become misaligned, and you will need to replace the whole derailleur. –  Stephen Touset Jun 15 '13 at 2:02
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You do have time - 20 hours to be precise. I have removed "London" tag since really the focus of the question should be your dodgy chain. –  PeteH Jun 15 '13 at 7:58
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1 Answer

I wouldn't recommend riding your bike at all until you have a professional look at it. Otherwise you're taking the risk of physically injuring yourself and the people around you, especially on a big ride.

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+1 for emphasizing the importance of safety over doing a ride. You can't quite ride if you are bloodied up on the side of the road. –  Zeus A. Jun 15 '13 at 0:24
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Not to mention, if the chain/derailleur snap and go into the wheel, you are going down and possibly taking many others with you. Going into a large ride with a known dodgy bike is reckless at best. –  JohnP Jun 15 '13 at 14:50
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