Are after-market BBs compatible w/ an EKAR crankset? I have a Niner RLT RDO frame w/a PF30 BB shell. Can I use a Wheels Manufacturing PF30 w/ the EKAR set-up? Thank uou!
No. With Ekar, the bearings are part of the crankset, and the Ekar BB simply consists of 2 cups that form an adapter between the frame's BB shell and the crankset's bearings.
"Regular" BB's like the Wheels Manufacturing's one have bearings too, so that's one set of bearing too many!
The Ekar BB does come in a PF30 variant, so you should be able to use that on your bike. If your bike uses internal cable routing through the BB shell, you also need the EPS cable guide/shell to put between the BB cups. Route the cables line around the cable guide to protect them from the spindle. The Niner RLT RDO's specs claim internal cable routing, but through a "fully sleeved guide tube", which might make the cable guide unnecessary, better check on your actual frame.
This is how the disassembled Ekar crankset looks like, you can clearly see the bearings:
These are the Ekar BB cups, which as you see don't contain any bearings or other moving parts: