I don't know how it happened but my XT rear dérailleur broke today. The chain was hanging down so the dérailleur was not tensioning it – I guess the spring in the "knuckle" has broken or come loose.

Given the cost of a new one, can this be repaired? Has anyone done something similar?

Yes!

I first removed and cleaned the derailler.

Then unscrewed the hex-head pin holding the upper jockey wheel, carefully removing the wheel. While I was puzzling over how to remove the cage from the knuckle, I noticed a tiny grub screw on the top of the knuckle. I unscrewed this and the cage popped off the knuckle with a quick tug. The "axle" of the cage has a recess at the end that allows the grub screw to retain it in-place when screwed up, unscrewing it allowed the cage to be removed. In fact I could probably have left the jockey wheel on.

Inside there is a stiff coil spring with both ends bent out. One end sticks into a hole on the cage, the other into a hole at the back (inside) the knuckle. By fitting the spring in these holes and rotating the cage, the spring is tensioned, allowing the derailler to tension the chain. The failure was caused the end inside the knuckle coming out of its hole. After carefully replacing the spring and cage I just rotated the cage 360 degrees, with a bit of thrutching, and replaced and tightened the screws.

And it works again, at least for now.

why did this have to posted ?

its like phew ,,,,,,im so glad your rear mech is working again….

maybe the forum should be for interesting things?

I had a creak on my bike recently , took me 3 weeks to figure it out.

I didnt once thing about posting my probs on some forum???

luv x

If only this was posted 2 weeks ago.

Mine failed in the same way and I ended up buying a new one as I thought it was fecked – doh!!!! 😕

"why did this have to posted"

Fairly obvious I'd of though… maybe you're a bit dim 😉

Buzz, you beat me to it 😉