Q: How can I prevent/fix the Nerf bars from rubbing on my plastic fairing?
This is typically a case where the center "U" shaped bracket needs to be adjusted to balance the clearance on both sides.
Basic Steps:
This should help balance the left and right side to have enough clearance at all points
The key, from a person who has installed many of these over the years, is that the aluminum bracket at the frame tube (and thus the front "U" bar) must be properly placed and tightened first and then the rest of the Nerf will fall into proper alignment.