The wheel mounting pad is the back suface of the rim that contacts the adapter, the round sleeeve nuts fit into the holes of the wheel at the mounting pad. No material should be removed from the adapter for structural and safety reasons, but some can be removed (within reason) from the wheel, I'm sure there is a safe minium dimension however. The wheel manufacturer can advise how much material can be removed. Material is removed from the wheel as part of the manufacturing process to arrive a correct dimension for varying setups.
Scrub radius:
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