Rear Mech Sheared Off - Fatigued?

philclubman

Hi all,

Posting out of interest only as I built the bike myself so not looking to blame any shop or mechanic. Also the derailleur was Ultegra 6500 so possibly anywhere up to 20 years old.
I've just bought a Dura Ace 7800 to replace it.

I was commuting home last night and moving away from lights when I skidded to a halt - was in the middle of the cassette. Lifted the bike to the kerb and had a look. The rear derailleur had sheared off, the chain buckled and the cable inner ripped out of the shifter.

I've posted a pic below. I had crashed and scraped the mech in a crit last April and you can see some of the marks from that. I wonder if I have set something up badly - i don't think the chain caught and it definitely didn't over-shift into the wheel. The other thing I wonder is if the body of the mech could have failed from fatigue and from having been dropped before. The mech hanger is intact.

As I say, posting out of interest. A good excuse to buy a new bling Dura Ace derailleur and no injuries so all is good.

Perhaps it was poor setup on my part, but perhaps it snapped through fatigue and previous damage.

Interested in any thoughts.

ugo.santalucia

The fracture profile is not consistent with fatigue. That part is massively overbuilt for the job it has to do

Bobbinogs

I think the previous crash damage and the site of the fracture seem to be suspiciously close. BTW, I would be looking to proactively change the hanger as well whilst I was at it. I replaced one recently, cost £12 and took 5 mins. In my case, I had suffered from some shifting issues followed repeated spills on club rides and the replacement sorted that immediately (well, that and the new RD, cable and re-indexing ;-) )

keef66

Think he might struggle to replace that hanger; looks like it's part of the frame.

Which is OK if steel, but a bit risky if alu.

Ben6899

Looks like alu...

philclubman

Thanks guys. The frame is aluminium and the mech hanger is replaceable. I have a mech hanger alignment tool and will check it's aligned before I fit the new derailleur.

Any more thoughts or theories on why it might have failed where it did?


Edited to add,
I will replace the hanger as suggested.
Thanks

Matthewfalle

Looks like it's failed at the inside lower pivot and been dragged outwards by the spring no longer being held in straight tension snapping it at the highest leverage/weakest point on the outside.

Age, weakness following bash, number of variables all added up I'd say.

philclubman

Matthewfalle wrote:

Looks like it's failed at the inside lower pivot and been dragged outwards by the spring no longer being held in straight tension snapping it at the highest leverage/weakest point on the outside.

Age, weakness following bash, number of variables all added up I'd say.

I think you might be right. Looking closely, that pivot could easily be where it failed first.

Not sure what lesson there is to learn here. I think I was just unlucky for it to break, but lucky it wasn't in a race where it would have caused a crash.

Matthewfalle

No lessons to learn - it's just fatigue and happened.

I personally think it's karma telling you to take the whole lot off, smash it with a hammer and throw it into next door's garden and replace it all with eTap.