Building a pretty comprehensive cage in the TR8 coupe that extends well beyond the firewall. Want to build it strong, but don't want needless tubes adding weight. If the cage fits real tight, and has dozens of points of contact where its welded to the body, how important is a diagonal or X across the top of the cage inside the roof panel? On my ITS cars, I never installed one, but this is more of a EP or GT style cage. After the cage is completed, the roof panel will have several points of contact where it will be welded to the cage. Is the extra resistance to bending, worth the extra weight located at the highest point? If I was going to use this as a rally car where a tree could possibly poke thru the roof in a roll over, it would be a no brainer.
Keep in mind a few things: a single diagonal will increase strength a lot(turns the square roof section into two triangles. Triangles are strong) without all the extra weight of the "X."
And remember you don't need to use the same thickness or even diameter as the rest of the cage like the B pillar which is more likely to take a direct sideways hit.
So you can end up with maybe 80-85% of the strength benefit, with only a 2-3lb deficit, not 6-8 or whatever.
There IS a middle ground.
Entire cage is 1.5X.095 DOM. Thats as middle ground as I'm willing to go in a 2500 pound car. It will probably weigh less, but thats the target max weight with the LS engine. The last one I built similar to this one weighed in right at 2000 pounds with the Rover V8, less tubes, less gutting, smaller wheels, brakes etc.
My first question is what are you using the car for?
Some sanctioning bodies consider additional cage to bodymounts "performance" related and will change you class as a result. If this is a race car, what class/organization is targeted?
I know this is probably aside from your conversation question, but I believe it matters. If this is a street car (doubt it) I would consider the X un-necessary.
Check your sanctioning body rules. Most rally rules I've seen default to the FIA, and they require something in the roof- either an X, or a V iirc. In a low roof car like a TR8, its a huge pain in the ass to try to fit your head up there along with the jungle gym they want.
Its being built as an ultimate street car/weekend track beast. Built enough race cars following the rule book, but not this time. Plan is to build the fastest, safest, lightest TR8 possible on a budget that is just barely street legal. I have other cars designed for chasing plastic trophies. As far as the low roof thing, you've obviously never driven a TR8 coupe. Lots of headroom. Many TR8 owners are well over 6 feet tall and many are over 300 pounds. This isn't a tiny Miata cockpit. For a small car, there is an insane amount of interior space. My gut feeling is to leave it out and use that 4 feet of pipe for a third drivers door bar, or a pair of second front down tubes.
I'm 6'2" with a 32 inch inseam. I worry about head room in every vehicle ever built...
