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Old 01-01-11, 06:46 AM   #1
conomac
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Bent Derailleur Cage

On a ride the other day, my derailleur hanger snapped in two and sent my rear derailleur into the spokes. Luckily I was going very slow when it happened so no frame damage or wheel damage...but my derailleur, a Shimano RD-7800 needs some TLC. The derailleur cage is bent. The tension and guide pulley teeth also have some chips and sharp metal burrs caused by the chain coming off track and binding in the pulleys.

Below is the tech doc and parts list for the RD-7800. I want to replace #'s 7, 9, and 10...but can't find a source for all the parts. I would have thought I could find the parts for $20 or so...I found one site that is charging $50 for a new pulley set. I can get a new derailleur ~$100. Am I screwed into buying a new derailleur or can anyone point me to a good source for parts?

techdocs.shimano.com/.../RD/EV-RD-7800-2254A_v1_m56577569830609123.pdf
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Old 01-01-11, 09:11 AM   #2
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It will be much cheaper to just buy a different Shimano derailleur. Shimano is very poor at supplying individual replacement parts. Roger
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Old 01-01-11, 12:01 PM   #3
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Sadly the days where things like replacement cages were readily available are behind us. You might find them, but the price might be prohibitive compared to replacing the RD. Before you buy, try straightening the damaged parts (unless they're carbon). It relatively if you have good hands. Remove and disassemble the cage and straighten the inner and outer plates as best you can, clean up the pulleys with a file or emery board then reassemble onto the RD. After remounting you might have to tweak the cage as a unit so it hangs vertically.

You may or may not get it good enough depending on the extent of the damage and you're hand skill, but you have nothing to lose by trying except a few minutes of your time.
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Old 01-01-11, 04:42 PM   #4
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I've bent several RD cages and bent them back no problems- pully wheels are easy to find many shops will have good used ones for cheap!
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Old 01-03-11, 12:42 PM   #5
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Thanks for all the advice. Yeah, I figured replacement parts were hard to come since I couldn't find much online.

I have straightened the cages as best I could - both were bent and twisted. I decided to replace the derailleur cables at the same time, so we'll see how it shifts once I get the new cables installed. Good advice on the taking a file to the burrs on the pulley wheels.

Worst case, I'll pick up a new Ultegra for $80 from performance.
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Old 01-03-11, 01:49 PM   #6
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This sounds like a 50-50 proposition. Many times when the derailer is traumatized, the two main (mounting and cage) pivots are no longer parallel, irrespective of any cage replacement.
In these cases, the derailer's indexing will be off as the cage would rotate off-axis relative to the axle and sprockets, thus making proper cable adjustment impossible, and also being affected by shifts at the front derailer since the cage rotates a lot whenever a front shift is made.

Hoping your repairs work on this one!
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Old 01-03-11, 02:46 PM   #7
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Many times when the derailer is traumatized, the two main (mounting and cage) pivots are no longer parallel, irrespective of any cage replacement.
In these cases, the derailer's indexing will be off as the cage would rotate off-axis relative to the axle and sprockets, thus making proper cable adjustment impossible, and also being affected by shifts at the front derailer since the cage rotates a lot whenever a front shift is made.
I'm worried about this...and that I've significantly weakened the cages by bending/straightening them. The more I have thought about it, the more sense it makes to just get a new derailleur.
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Old 01-03-11, 04:07 PM   #8
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I'm worried about this...and that I've significantly weakened the cages by bending/straightening them. The more I have thought about it, the more sense it makes to just get a new derailleur.
Unfortunately, thats how it usually works out.
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Old 01-03-11, 04:36 PM   #9
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I'm worried about this...and that I've significantly weakened the cages by bending/straightening them. The more I have thought about it, the more sense it makes to just get a new derailleur.
You could replace it with an Ultegra 6600 with only a slight weight penalty.
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Old 01-03-11, 06:32 PM   #10
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Originally Posted by conomac View Post
I'm worried about this...and that I've significantly weakened the cages by bending/straightening them. The more I have thought about it, the more sense it makes to just get a new derailleur.
The RD cage doesn't have to be all that strong.

It's on the slack side of the chain loop, so the normal load is limited by it's own return spring. There is greater stress on the upper section of some designs, where there's some side load during a shift. On many, however the jockey wheel is mounted directly to the lower body, and the cage us unstressed while shifting. With either design, if you got it reasonably straight so the pulleys are in the same plane, it'll probably last until your next crash.
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Old 01-03-11, 07:29 PM   #11
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+1 FB - if you got it straight enough, it should have plenty of miles left in it... as long as none of the pivots are too sloppy now.
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Old 01-03-11, 07:36 PM   #12
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Usually with derailers, if they're not straight enough, it's visible, as long as the hanger is known straight.
It's pretty easy, if the cogs are clean, to sight down the pullies along one of the bigger sprockets.
I check first looking down, with one pulley above the other, then rotate the cage to horizontal and sight everything down again.
A problem derailer always looks a little off, but sometimes it's just the cage plate and the shifting actually works fine in all the gears, a bent cage plate having relatively little effect on indexing.
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