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Old 03-11-09, 06:12 PM   #1
tbinmd
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How did my rear derailleur end up in my rear wheel?

Riding my bike ('06 Madone) last night, my rear wheel suddenly locked up. I jumped off and found my rear derailleur had completely detached from the frame, and was entangled in the spokes of my rear wheel.

At the time I was riding at perhaps 18 mph, sitting down, and hadn't shifted for at least a minute or so. I was probably in the second or third largest rear gear, and the big gear up front. I had experienced no problems before this.

I've never crashed this bike, nor have I bumped it into anything hard, or laid it down on that side roughly.

The aftermath - the derailleur detached from the hanger completely. It stripped the threads inside the hanger, and left the hanger bent outwards. The hanger (apologies if that isn't the appropriate term) is part of the carbon frame. Once in the spokes, the derailleur took out two spokes.

This happened directly outside of a bike shop. I brought it in there - they had no guesses on how this happened. I then took it to the Trek dealer that I bought it from. Same deal - they had seen this happen on Mountain bikes, but never on a roadie.

I'm still waiting to hear from them as to what will happen to it. I guess they have to assess the situation, then potentially contact Trek for warranty repair/replacement.

Any theories as to how this could have happened? Has anyone experienced anything similar? What is reasonable to expect from Trek and/or the LBS?
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Old 03-11-09, 06:15 PM   #2
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If you hadn't shifted, did you run over anything? Twig? Debris? If not then it might've failed just on it's own.
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Old 03-11-09, 06:18 PM   #3
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I didn't notice running over anything, but it certainly is possible.
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Old 03-11-09, 06:34 PM   #4
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A stick probably got kicked up into the chain and thus through the derailleur and ripping it off
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Old 03-11-09, 06:37 PM   #5
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If you hadn't shifted, did you run over anything? Twig? Debris? If not then it might've failed just on it's own.
A twig or something in the lower chain run is the typical way that happens. When it catches in the lower derailleur pulley it jams the chain and the easiest things to bend are the derailleur cage and hanger.

Like the bike shop guys said, that's a lot more likely to happen mountain biking than road riding.
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Old 03-11-09, 07:18 PM   #6
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It is actually pretty common, unfortunately. As others have said, it only takes a little stick or something to really foul things up.
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Old 03-11-09, 07:21 PM   #7
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Thanks to all. Tough luck on my part I guess. Any thoughts on the likely repair/warranty options?
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Old 03-11-09, 07:24 PM   #8
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Did the entangled derailleur break or bend the hangar (or it's screw) clean off, or did the screw loosen which caused the derailleur to fall off?
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Old 03-11-09, 07:26 PM   #9
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Go to your LBS and get the RD hanger checked as the first thing. Someone may have kicked it for fun. It's easy to fix, but few have the tool at home.
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Old 03-11-09, 07:32 PM   #10
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I've seen this happen when a tight link in the chain fails to go through the rear derailleur. This is more common on a new chain or a chain with no lubrication.

Al

Last edited by Al1943; 03-11-09 at 07:47 PM.
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Old 03-11-09, 07:36 PM   #11
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Did the entangled derailleur break or bend the hangar (or it's screw) clean off, or did the screw loosen which caused the derailleur to fall off?
Again, I may be getting the terms wrong here, but at the apex of the joint of the seat and chain stay is a metal tab with a threaded hole in it. It's that part that I've been calling the hanger. The derailleur screwed into that.

The incident pulled the derailleur out of the hole in the hanger, stripping the threading in the hanger and bending it. So to fix it I guess one would either have to bend the hanger, then somehow deal with the stripped threading, or replace the entire hanger. Is that possible, given that the stays are carbon?
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Old 03-11-09, 07:38 PM   #12
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Go to your LBS and get the RD hanger checked as the first thing. Someone may have kicked it for fun. It's easy to fix, but few have the tool at home.
Thanks. It's sitting lonely in the shop now, awaiting its fate, so hopefully they will be able to fix it. I wasn't too hopeful after the "I've never seen this happen before" reaction when I brought it in, but the head mechanic wasn't there; hopefully he can take care of it without too much trouble.
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Old 03-11-09, 07:39 PM   #13
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^^ Yes, modern bikes have replaceable hangers for this very reason.

I'm inclined to think that a tight or seized link may have caused such an occurence, as Al1943 suggested.
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Old 03-11-09, 08:33 PM   #14
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Perhaps the excessive chain angle, plus a terribly adjusted L limit screw could have caused the derailleur to catch your spokes.
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Old 03-11-09, 09:04 PM   #15
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Good thing it didn't have dork disk on the wheel, it would have almost certainly gotten scratched!
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Old 03-11-09, 09:12 PM   #16
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Perhaps the excessive chain angle, plus a terribly adjusted L limit screw could have caused the derailleur to catch your spokes.
Limit screw wouldnt have anything to to do with it if he were in the third largest cog. The chain angle was large but not extreme, although it might have helped a stiff link do its work.
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Old 03-12-09, 11:58 AM   #17
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Did it look like this?

I was about a 1/4 mile from my house the other day on the 2nd ride of the season. I switched to the large front cog and the rear wheel locked up. I think what happened is that the chain dropped off the front cogs all together and the chain got jammed in the rear derailleur. Bent my new Velocity wheel (about 30 miles on it) and the bike had just been tuned after it wouldn't shift correctly at the end of last season after the wheel install. When it locked up the tire blew within 2 seconds after the skid started. I took it all apart and I think the part of the frame that the derailleur mounts to is bent, but it's hard to tell.

Back to the spinning bike for now...
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Old 03-12-09, 01:16 PM   #18
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^Wow, I've just never heard or seen these things before. It seems like the chain (or a link) jams onto the tensioner arm of the derailleur and swings it up and over into the wheel or frame.

You and tbinmd might need to compare notes and see if there's any similarity in the components you're using, in case it has something to do with it (chain model, derailleur model). I can't make an equation in my mind that this is caused by faulty or bad drive-train tuning, because otherwise this would be happening to an awful lot of bikes out there.

For that matter, what method of chain "link-joint" had you guys used?

Yeah, like Al1943 said, it makes sense that a new chain with a stiff link-joint or and old un-lubed chain with seized links would do this.

Re-joining a chain with a chain/splitter tool always leaves the link stiff. I always make sure I un-seize this link before using the bike. I never assume that it would loosen up on its own.

.

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Old 03-12-09, 01:25 PM   #19
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I've done it twice myself - both times on my mountian bike. if you have a replaceable derailer hanger you should be fine.
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Old 03-12-09, 01:54 PM   #20
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Did it look like this?

I was about a 1/4 mile from my house the other day on the 2nd ride of the season. I switched to the large front cog and the rear wheel locked up. I think what happened is that the chain dropped off the front cogs all together and the chain got jammed in the rear derailleur. Bent my new Velocity wheel (about 30 miles on it) and the bike had just been tuned after it wouldn't shift correctly at the end of last season after the wheel install. When it locked up the tire blew within 2 seconds after the skid started. I took it all apart and I think the part of the frame that the derailleur mounts to is bent, but it's hard to tell.

Back to the spinning bike for now...
This is similar, though in my case the derailleur ripped completely out of the hanger so the chain was the only thing keeping it on the bike.
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Old 03-12-09, 04:10 PM   #21
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Did it look like this?

I was about a 1/4 mile from my house the other day on the 2nd ride of the season. I switched to the large front cog and the rear wheel locked up.
A chain that's 1 link too short will do that.
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Old 03-12-09, 05:11 PM   #22
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...This happened directly outside of a bike shop....
Am I just too cynical?
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Old 03-12-09, 05:24 PM   #23
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I think mine was a result of the shift to the top ring where the chain fell off when I switched back down to the small ring. It jumped off the cog and I think the "wave" from the slack in the chain caused it to jam the rear derailleur. I had 200 plus trouble free miles on the bike before this.

I don't think I'll take it back to the same shop for the repair.
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