I'm trying to mock up the rear down bars and have an issue using the 6"X6" plate. The way the floor is contoured I only have a 4 1/2" X 12" area that is flat. I can't beat the floor into submission because of the rear shock towers. It's a prefab kit from Rhodes and they made the rear down bars terminate at the shock towers. Is it legit to use a 4 1/2" X 8" plate instead of the 6"X6" plate?

The rear shock crossmember is considered part of the unibody frame. The crossmember is a U shaped channel that is welded to the frame rail and the floor is spotwelded to the lip on top. I have seen where the sheetmetal was cut out over the opening and plate welded in place. Then weld the tube to the plate. Also seen this done on rear frame rails. Actually this is how my first cage was done in my car. It was legal for cert every time it was checked. Was always told the 6X6 rule is for floorboards with no frame underneath.

Leon

I remove the sheet metal over the framerail and weld a 3/16" plate. Weld the bar to it.

When Marty put the roll cage in my car, a few of the places I wanted tubes was not flat in a large enough area. He took the 6 x6 plates and bent them to match the floor and up the side of the inner rocker panel and welded them in place, then welded the tube where I wanted them. On the x-brace going to the rear shock cross member, he removed some of the sheet metal from over top of the crossmember, welded the tubes in place, then welded the sheet metal back in place. (The sheet metal was cut into two pieces to go around the tube)

Brian

That looks like a nice weld job BTW

And to the OP, your hidden question is, is 36 sq in the requirement or 6x6?

I don't care about cert in my case, I have perimeter welded the plates to the floor, and then use a bunch of grade 8 3/16" allens to additionally sandwich the three pieces together.

You can box it as crizila has or contour the plate to the floor. Either is acceptable.