grinderbob

I just saw a hairline crack on my rear d body. It is right above the mounting screw. This might have been there for a while, just never noticed it.

How safe it is to ride like this? Will the separation grow? Should I sip some superglue as a band-aid?

thanks.

greyghost_6

The good thing about a rear derailleur is that it isn't under that much pressure, and if the crack went all the way through it would likely just fail to shift, or the tension spring go slack. PERSONALLY I would epoxy it and ride it for a little while longer until you find a new one. It may have just been installed WAY too tight, cracking that part trying to separate under the force of the bolt. Those gashes on the top are suspicious too, has it been wrecked? Rear derailleurs are cheap enough on ebay not to brake the bank.

operator

Or it'll fail and go into your rear wheel.

greyghost_6

Hmm, the derailleur body would have to shatter at the fixing bolt. Every time I see a derailleur get into someone's wheel its the hanger that brakes off. I think that the hanger would brake before the derailleur would brake even with that crack, but I am just going by the picture. But as I think dr.operator is trying to say is that he would rather be safe than sorry.

operator

Correct.

It's break by the way.

Mondoman

I recently had an older Shimano RD break clean through at the fixing bolt; it got caught in the chain & wheel and ended up bending the derailleur hanger. Fortunately, I was only going about 2mph at the time.

grinderbob

In recent rides, I did notice new clucking noise coming from somewhere, not sure if it is related though.

24tracktape

If that's a mid 90's Record or Chorus (8-speed) I have to say... Welcome to the club!

Many of them failed spontaneously like that, if you look on ebay, there always seems to be an 8 speed RD with a crack in the same place.

Although I upgraded to an early 9 speed model (which appears to be a bit different in its mounting bolt) I also fixed the RD's crack using an aluminium brazing rod and a Mapgas torch. I laid a strip of aluminium over the cracked area. Not the prettiest, but it will make a good spare...

bkaapcke

Derailleurs aren't that expensive. It's time to start looking for one. bk

wrk101

+1 Replacement time. Why take the chance?

jccaclimber

That's where you went wrong imo. That derailer is going to break. When it does it may well do bad things to the wheel and rider. That crack (good picture btw) is pretty far along. Be glad you found it, and be glad that a derailer is cheaper than a derailer+rear wheel.
What's with people that say "It's only a little tiny broken spot, and it doesn't take any load. Do I need to fix it?" If it didn't have any load going through it, it wouldn't be cracked in the first place. Do you go to your wife and say "My bone only has a little crack in it, and it's only in my arm, not my legs so I can still walk. Do I need to go see a doctor?

greyghost_6

Have you thought that it could have taken the damage when it was installed (over tightening the bolt)? What if it was cracked before it was installed? Have you thought that it might have gotten banged around during some point in its life? I just don't think that specific crack was developed over time by being under load (which rear derailleur take very little) and probably has been there for a while and you may have just now noticed it.

jccaclimber

If that derailer is built how I think it is, given the dozens I've taken apart of all sorts, that crack isn't going to be the result of over tightening that bolt.
I'm hoping that nobody in their right mind would install a derailer with a crack like that. Additionally, there still needs to be a way to make that crack even if it happened somewhere else.
Yep. Not in a bin, maybe in a crash. However, an impact with enough energy to make that crack is going to:
1) Likely (not guaranteed) mark up the body more than that one is.
2) bend the derailer in such a way that it isn't going to wore that well to begin with.
3) Do more at that joint than a crack like that (ie yield something more)

I suspect there was a crash that initiated that crack. However, I suspect the crack growth to the point where it is now happened over a longer time period than a single event.
I do agree that it has likely been there a while, but I disagree about the details of its formation.