My set up is RaceFace chainrings on Shimano XT hollowtech II cranks, SRAM PC991 chain, XT 11-32 cassette, SRAM X9 medium cage mech. If I'm in the middle ring and largest sprocket and back pedal, just half a revolution of the cranks, the chain comes off the bottom of the middle ring and gets wedged trying to get onto the small ring. I can eliminate frame, chain, wheels, cables as they've all been changed. Mech cage looks vertical. So what's causing this?!

It does it in the big ring as well but I try and avoid this gear. I do need to back pedal 1/2 a revolution for alining my pedals up when setting off, etc. 🙂

Worn chainrings? Dunno, with that mishmash of bits could be anything really

If its a must for you, do you have enough space between the front rings and frame to run the 2 BB spacers on the left side instead of drive side?

My new chain didn't skip when I put lots of force through it, so I think the rings aren't worn.

do you have enough space between the front rings and frame to run the 2 BB spacers on the left side instead of drive side?

So could been an chainset alignment problem then? There would be room to try this.

I had problems with a sticky rear freehub – it was catching up on backpedal and the chain was riding up the front cog as a consequence.

If I'm in the middle ring and largest sprocket and back pedal, just half a revolution of the cranks, the chain comes off the bottom of the middle ring and gets wedged trying to get onto the small ring

Don't backpedal = Problem solved.
🙂

Stiff jockey wheel will do that to you.

Backpedalling is prone to making the chain fall off, especially if you have the chain at an angle, e.g. low gear at back, middle at front. This is a little bit like how a car can pull a trailer round tight corners easily, but will jacknife if attempting to do it in reverse. If you have the chain orientated straight w.r.t to the chainrings it shouldn't happen*. The way to solve it is to pedal forwards, not backwards.

*All in IME, not necessarily true.

If it comes off at the bottom it's because the mech is pulling it over too far, this could be due to the ring tooth shape or the chainline (is the middle ring further out than the middle of the cassette?). But using the full span of the block in the middle ring isn't ideal IME on any bike.

check the free hub as wwaswas said ^^^ also check rear mech is correctly adjusted and isn't slightly in between gears.

Thanks all, I appreciate what's being said about not ideal chain alignment etc, it's particularly noticeable on this bike whereas if I do it on my Trek, same set up except Shimano rings and long cage mech or Giant, same except SLX double chainset, I can get away with a lot more back rotations (5 or 6) before any derailment happens, occurs on the cassette instead.

I'll give the other suggestions a go!

Consider if the chain is a bit long and the derailler is not able to pull it tight enough.

Back pedalling deffo causes chain offs.

Stiff link or look at the mech. Old worn mechs don't like being back pedalled. Or freehub could be sticky.

If it used to be fine and now it is not, check your rear mech hanger is not bent and/or also check the mech cage is not bent out of position. Either can put the lower jocky wheel in an odd position