I have a Transition Smuggler 29er set up with XT M8000 rear mech, shifter, chain and cassette (11-42) and a Race Face Turbine crank with Wolf Tooth 32t NW chainring. In lower gears (1-3) if I’m pedalling hard the drivetrain really rumbles and grumbles. It doesn’t do it in other gears under the same pressure and the B tension screw is correctly adjusted.

Is there any reason for this? I thought it might be the chainring, and when I look down when it’s doing it the bottom part of the chain, between the ring and the mech, is really bouncing around. I swapped the cranks to a set of FSA K-forces with a Works Components 30t NW ring from another bike and it’s still doing it.

Is it a chainline thing?

Even though you’ve adjusted the b-screw, have you checked the top jockey wheel isn’t hitting the cassette ?

Yes, it can be caused by chain-line. Put the bike in gear number 5 or 6 and look along the line of the chain, it should be reasonably parallel to the chainring. There’s a risk of offset chainring when you mix-and match parts, so you might even need to do something like swap the 2.5mm BB cup spacer from RHS to LHS (perfectly do-able on a 1x setup).

Quite likely to be chainline. Wolf Tooth do different chainrings for boost and non-boost. If you’re using a boost (3mm offset) on a non-Boost bike, it’ll definitely be too wide.

Even if you’re using a non-boost (6mm offset) it’s probably still wider than “ideal”. There’s quite a good explanation here:

https://www.wolftoothcomponents.com/pages/boost-chainline-and-chainrings

The reason we don’t make 46-47mm chainline rings for non-boost bikes is that most bikes have inadequate chain clearance for “average” sized chainrings (30t-32t) at that narrow chainline.

It seems that 1x chainline is an unhappy compromise between good chainline and chainstay clearance. If you’ve got the clearance, space inwards if you can, although that may be tricky if you’re already on 6mm offset direct mount rings. I spaced my stock M8000 setup by 4mm (see this thread).

Mine does it when its filthy in the easiest two gears, doesn’t really bother me that much if I’m honest

Right, it’s going in the workstand tonight for a new gear cable anyway so I’ll double check the B screw and if that’s not causing a problem I’ll swap the BB shell spacers around (clearance allowing).

It seems that 1x chainline is an unhappy compromise between good chainline and chainstay clearance

There’s some interesting thoughts on 1xchainlines at OneUp and on SRAM’s tech pages Here

The design chainlines in SRAM land are 49mm for non boost and 52mm Boost.

Interestingly, the chainline for SRAM 1x road stuff is somewhat lower, despite having a slightly higher rear chainline. This doesn’t give a straight figure for 1x chainline, but comparing W3 (road) with W2 (MTB) suggests road chainline is 46.2mm. I measured mine as closer to 44mm. Either way it does suggest that MTB chainline is higher than “ideal”, presumably for clearance, and Shimano default to even wider.

I’ll swap the BB shell spacers around

Don’t expect miracles: sprockets are nearly 4mm apart, so 2mm won’t make a big difference if you’ve got problems over 3 gears.

Is it all new? Could just need the teeth to wear in a bit as you are obviously forcing an extreme chainline so any freshly cut edges on the teeth may need to round off a bit.

But double check everything with the RD set up make sure your indexing is spot on as if your chain is jumping around it could be that it’s not quite set up right for the gear.

This might be completely irrelevant to the issue but I now run a sram chain with my xt m8000 set up now. A much nicer experience all around. Thats xt m8000 cassette, derailleur and xtr m9000 crank and xtr chain ring, with a sram pc1170 chain. It’s taken away all those little niggly noises and shifts better, even when the chain is minging.