First up, if you're going to build in a jacking point (common, and worthwhile) utilising the factory mounting points inside the chassis rails you want to use box section for the mounting
The reason for this is to distribute load evenly when jacking the car up. If you use tube instead, the mounting points in the chassis rails will bend to the shape of the pipe (rounded). Yeah, i learnt this from experience lol
Don't over engineer it - you want it to have crush zones in the most likely impact points. That way the bar takes the impact, and doesn't transfer the impact to the chassis rails. In a crash that heavy you'd need a new crash bar anyway, so why not design it to protect the chassis rails.
If i was building another one I'd:
- Make the section between the chassis rails the strongest, protecting a fuel cell from a square-on rear hit and providing enough stiffness for jacking the car up.
- Either side of the jacking point, add some strong loops for hooking tow straps to for easy access mounting points when towing your car
- Make the bars that wrap around from the mounting points weak enough to bend on their own if they take a hit - these are the ones that get fucked up the most due to wall taps / wheel-bumper impacts during tandem. You might want to consider running a smaller thickness tube here or smaller overall OD tube. Maybe even consider removable, sleeved corners that you can replace easily
- Wrap these corner bars around - don't leave them open ended and pointing somewhat outwards. If you have to spin and tap someones door with your rear quarter they turn lethal. I've seen these corner bars punch straight through doors and miss drivers legs by millimeters.
- Weld M6 nuts on top of the top bar every foot and thread in wide headed bolts. These act as adjustable mounting points to get your rear bumper sitting flush with the garnish
