Mounting bumpers with L angle

We have long rails running the across the perimeter of our frame at a height of 9 inches. We were considering using L angle attached to the back of the bumpers to sit on (and mount to) these rails to support and mount the bumpers. The L angle is 1/8" wall thickness and 1" square otherwise.

This sounds legal to us per R-21-F, but there’s a small drawing (FIGURE 4-7) that’s confusing us. It shows “hard parts” may not extend back more than 1". This is shown across the top of the bumper though, not where it may attach.

Am I misinterpreting the diagram in 4-7?

Look at this thread. The same question was discussed there. It has been addressed in an official Q&A response (Q744).

Thanks Mr. Wallace. That team had literally the same idea as us. Will close thread.

I reopened this thread for two reasons.

First, it could serve to provide guidance to other teams that are contemplating a similar solution.

Second, I’m not sure if this is a valid solution. You said the angle would sit on top of your structure that defines the FRAME PERIMETER, and that’s all well and good. So the 1/8 inch of material that hangs vertically outside your FRAME PERIMETER would be counted in the “hard parts”. But I see lots of layers of fabric between that aluminum angle and presumably the 3/4" plywood that it surrounds. If the thickness of those layers of fabric is greater than 1/8 inch, it will push the outer edge of the plywood to more than 1 inch outside the FRAME PERIMETER. Something that you need to examine before you conclude this is entirely legal.

Thank you for the tip. We will double-check the fabric thickness. The real bumpers should only have a single layer of Cordura across the back.