[GaryVoshol]

If the wheels are the farthest point outwards on your ROBOT, they define the FRAME PERIMETER (FP). Would a string stretched around your ROBOT hit the wheels? Then they're part of the FP. And the BUMPERS have to be mounted on the FP.

You may be able to use some standoff pieces between the wheels as places to mount the BUMPERS. But remember the BUMPERS must have backing at least every 8 inches. If you are using 8 inch wheels, you might have problems, because the 8 inch wheel would rub on mounts that are 8 inches apart. You might be able to put your mounts a bit above or below the midpoint of the wheels to make it work; you'd be spanning a shorter chord of the wheel rather than the full diameter.