The 10 speed XT rear derailleur seems to swing loosely around the second ‘pivot’ bolt (i.e. the one next to the one that bolts into the drop out) when there is no cable or chain attached? As though there is no spring in this bolt?

It swings between two stops and it seems that it is cable tension that will keep it taught when set up correctly?

Should it be so loose? Or have I broken something – I did hear an almighty ‘crack’ as I unbolted it off the frame but I assumed that was the threadlock breaking rather than anything snapping?

Shagged.

Should drop to smallest ring without cable.

It does. It stays ‘closed up’ as though in the smallest cog.

Its just that it pivots loosely round its pivot, without a chain or cable. There’s still a spring force in the parallelogram mechanism.

It’s fine, not sure of the purpose of it(other than a dropout saver, maybe), but the mech is free to rotate on it within it’s limits, chain tension holds it in place.

Ermmm I’d disagree with coatesy.

My SLX rear mech’ does swing around the pivot bolt but it has a spring that forces it backwards. With no chain or cable attached the mech’ should close up (parallelogram wise) and also swing backwards.

This applies tension to the chain and takes up the slack in the chain as you move up and down the cassette. In the smaller gear combinations it’s this spring tension that stops the chain hanging ‘slack’ between the bottom of the derailleur and the chain rings.

I think you’ve broken that spring 🙁

Should swing completely freely on the pivot below the frame hanger bolt, if that’s what you mean.

For further confirmation, check the exploded diagram on Shimano’s Techdocs, where you’ll see it’s just a pivot bolt, no springs or anything.

Of course it just swings on that bolt. It’s just a pivot as Coatesy says.

It’s a shadow so it will have in order running from drop out to lower jockey wheel:

1. Bolt to mech hanger which bolts the link part horizontal pointing backwards.
2. Free to swing pivot bolt (this is the bolt which attaches it to the frame on the ‘new standard’ shimano mechs without the little link part)
3. 1st spring pivot point at one side of the X part.
4. 2nd pivot point at the other end of the spring.
5. Pivot at the other end of the X.
6. Pivot point inline with the top jockey axle which has a rotary spring.
7. Bttom jockey axle.

It is the chain which acts as the spring acting on the pivot in question to stop it dangling.

Ermmm I’d disagree with coatesy.

Then you’re wrong.

There is no spring between the mounting bolt and the pivot bolt.

The chain and cable will hold the mech in the correct position.

coatesy always speaks the truth. 😉
Listen to the words of the wise.

Well, it seems to have worked ok in a 1×10 combination today, so I’d agree with those who say it should swing freely 🙂