From HB Racing:
These B-Mounts allow racers to fine tune the front kick-up and roll center of recent HB Racing 1/8 buggies and truggies. The B-Mount controls the location of the rearmost end of the front suspension hinge pins. A higher B-Mount location makes the car smoother and easier to drive when the grip is high. They’re machined from aluminum, anodized black, and available in +0.0mm, +1.4mm, and +2.8mm. Compatible with: D817, D817T, E817, D815, D812.