Cage attachmment to unibody type car

SpareChangeRacng

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Ok guys, I'm not positive on the best plate thickness to weld inbetween the stock floor and the tube when attaching a roll cage tube to the floor of a unibody car. I know I have read this before somewhere a long time ago, but have forgotten. I don't deal w/ unibody cars much so I have no clue. Anyways, the tube will be 1 1/2", .120 wall mild steel. I know you want as big of an attachment area as possible (about 6" sq.) to distribute the load, but I'm not sure if the plate should be 1/8" like the tube, or if it should be a little thinner like .090. What do you bug guys typically do when attaching the tub to the cage? Thanks. Steve
 

TimHayosh

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The SCCA rulebook gives some specifics about roll cage attachment in a unibody. In fact, they don't even consider a car with a frame. Anyway, here's a link to their 2003 rulebook: http://www.scca.org/news/fastrack/02-10.pdf roll cage info begins on page 7.

In short, they say the termination plate should be 10x the OD of the tube. I.E.: 1.5" tube needs 15" sq-in. A 4"x4" plate covers that!
 

ntsqd

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There's a trade-off to be made there. If you go too thin the plate is nearly pointless and the tube can cookie-cutter it too. If you go too thick you're not only carrying more weight than you need to, but it will also be harder to weld in place.
Rule of thumb, I'd say, is no thinner than 2/3rds the wall thickness of the tube and no thicker than the wall thickness. Round off the corners of the plates to at least a 1/4" radius.

The solution I like the best, when possible, is to poke the tube all of the way thru a box section in the stock body, then weld the tube to the body in both places. Next add a thin plate (a little thicker than the sheetmetal) on the top and the std. thicker plate on the bottom. You get a lot more bead length this way and you add bending moment resistance into the joint.
Also, bend the cage tight to the body and fish-plate everywhere you can. Unit bodies want load distribution, not point loads.
 

SpareChangeRacng

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Thanks Thom and Tim. I seemed to remember the plate thickness being about the same as the tube, but it seemed heavy and harder to weld to me too, which is why I figured I'd ask for others' opinions. Your suggestions just backed up my thoughts so I feel much better.
I plan on fish-plating the cage as much as possible, and I do understand what you were saying about the box section attachment. Before I posted I was actually looking at the car and trying to figure out how to do what you ended up suggesting (box section attachment), but seems like it will be pretty tricky w/ this car (72 Alfa Romeo GTV). Steve
 

ntsqd

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It nearly always is tricky to do it that way. Sometimes it's a matter of perserverence, sometimes it's just not possible.

I think VARA pretty much follows the SCCA Rulebook, so a copy of that may give a specific answer to your Q.
 
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