Bicycle Mechanics - Rear derailleur tore off...

So I was riding today and my rear derailleur started dropping in gear so I figured it just needed to be adjusted, before I could pull over it tore off and wedged into the spokes of my back tire.

I would like to know what caused this? I think the derailleur jammed, but I can spin both the idler and jockey easily and all springs are good. Possibly caused by the chain?

Total damage:
Broken Chain
Two spokes need to be replaced
New derailleur cable and housing
New rear derailleur mount
Possibly new rear derailleur
New idler pulley (current one is cracked)

http://img801.imageshack.us/img801/1249/dscn0038small.jpg

http://img843.imageshack.us/img843/5938/dscn0046small.jpg

puppuplader, I'd like to hear from the gurus about this myself as this is the second such event I've read about lately. Does the chain have a special link to remove/install and if so is it still on the chain?

Brad

Hey Brad,

The chain does not have a quick link.

Devin

EDIT: Just noticed the Idler is cracked so that will need to be replaced also.

Something similar happened to me awhile back.
Rear derailer caught on a spoke.
http://farm6.static.flickr.com/5174/5458824030_3b60f4d776.jpg (http://www.flickr.com/photos/44332608@N03/5458824030/)
Jammed wheel (http://www.flickr.com/photos/44332608@N03/5458824030/) by 1nterceptor (http://www.flickr.com/people/44332608@N03/), on Flickr

When I got home took a closer look, I think the link that was
riveted to close the chain(KMC Z) opened up a bit. Causing havoc with my RD.
http://farm6.static.flickr.com/5094/5459451169_517028d283.jpg (http://www.flickr.com/photos/44332608@N03/5459451169/)
Failed chainlink (http://www.flickr.com/photos/44332608@N03/5459451169/) by 1nterceptor (http://www.flickr.com/people/44332608@N03/), on Flickr

So I replaced the bad link with a quick link(or powerlink, whatever it's called),
replaced the crooked spoke and trued the wheel. Evrything seems back to normal again.

Pupuplader, did you hit anything, say and expensive stick (the type which likes to go into the spokes / mech)? The mech hanger has broken, saving the frame, the mech has some damage which looks like impact on the side, but can't tell if this was due to the spokes or a fall; if was a fall, would sugest that this weakened the hanger, and this has eventually failed as designed (this is on the basis that when you said the chain was dropping, it didn't go into the wheel).

1nterceptor, yoiur jockey wheels are shot, they look like spurs, am thinking that this could well have contributed to your issues, would imagine you will need new jockey wheels, cassette, chain, and possibly chain rings

My own opinion on both of these is poor maintenaince and bent derailleur hangers.

A properly aligned rear derailleur hangs straight down and its impossible that it can accidently get caught in the spokes. In fact the normal motion of the wheel rotation would push it free.

So the logical conclusion to me is a chain that jammed for one reason or another and that the damage was a result of that. A broken hanger would certainly cause a chain to jam as well but I`m pretty sure I`m looking at a lot of sticky links in that second shot of the winter bike.

You can get away with straightening a steel hanger. An alloy hanger really should be replaced if bent. Straightening it just weakens it further and most sell for less than $15. A bent hanger is also the only way the lower cage in a a rear derailleur will even come close to the spokes in a rear wheel.

This often happens after a bike has suffered a blow to the right side, which bends the derailleur hanger and makes shifting into the spokes possible. This probably happens most often when the wheels are removed and the bike is put int he trunk of a car. It takes suprisingly little force to bend a derailleur hanger - they are usaually designed to be the weak link so when trouble brews it is not the derailleur or frame that fails, just the cheap and easily replaceable hanger.

Having a slightly bent hanger will make the shifting unreliable as well as able to smash into the spokes.

The number one reason RDs get torn off this way is because they overshifted into the spokes. The second reason is a damaged chain binds in the idler cage and pulls the whole thing back. The third reason is something like a twig of other debris gets caught up in it, or it hits a obstacle like a rock you passed too close.

You don't feel it happening until it's too late, because the force involved is so small compared to pedaling forces, or in the case of getting caught in the wheel there's simply too much momentum.

The third cause is blind luck, it happens especially on mountain bikes and those are the breaks. But the first two usually give many warning signs and can be easily be prevented.

The RD limits are usually set correctly when the bike is first set up, but over time the bike gets dropped, or other things happen and the RD hanger gets bent in a bit. This causes the indexing to go off a bit, and the owner corrects it with the trim adjuster, and goes on his way. But, the bending of the tabs also moves the entire RD along with the limits inward, so while you've corrected trim the limits are now off. So the first way to prevent losing the RD is to remember that RD cables never get tighter. If you find yourself adjusting the trim to bring the RD out a hair, it can only be because the hanger got bent in. Simply correcting the trim without rechecking the inner limit invites disaster later on.

The second gives plenty of warning, Chains don't get stiff links all of a sudden. It's a process that happens over time. If you find the chain skips or hear the RD clicking and pinging at intervals as you pedal, stop and check it. If the chain has stiff or damaged sections deal with it ASAP.

Thanks for the input everyone. I got a ride from a friend the day before this happened and I had to take off my back tire to get it to fit so I probably bent the hanger then.

I usually am very meticulous with cleaning but this last week was very busy at work and I neglected my bike. So probably a combo of the bent hanger and grimy chain.

Is there a specific spoke that I should be looking to replace with? I was looking at chainreactioncycles.com and am not sure which to get.

If you do not currently have butted spokes (the spokes are not noticably thicker at the ends than in the middle) then just go to the LBS and get a DT champion 14 ga. or whatever plain gauge spoke they have in the right length. It shouldn't cost you more than a dollar per spoke (probably less).

Thank goodness you didn't have a dork disk. It would probably have gotten scratched!