There are two primary and conflicting references for this cage. The Hellucination Charger (minimalist road car), and the Rampage Camaro (maximalist time attack), with a dash of 4400 class in the form of a structural center spine.
roadstershop.com
My design violates both the SCCA and NASA rulebooks since it has no diagonal brace in the same plane as the main hoop, and the main hoop itself will have more than 4 bends (top is radiused, though they might let that slide and treat it as the allowable minor roof bend). It also doesn't meet either of their material minimums. Both of those rulebooks explicitly prioritize roll safety and say something like, "chassis rigidity is a side benefit." I approached this with chassis rigidity and passenger comfort as the priorities, and in that order.
The structural compromise that comes from lower door bars is offset by the structural spine. The torsional strength that comes from a diagonal brace is (attempted to be) made up in the shorter diagonal braces that still allow passengers (my dogs) to get in the back. Also, the spine is tied into the firewall and the rear suspension structure, so that's something similar to a shear panel at the front and maybe 1/2 of a shear panel at the rear.
At least, that's how it all works together in my imagination.
If I end up with a minimalist design like Speedkore typically goes for, then this is a rowdy road car that can run in the Optima events. Optima's cage guidelines are essentially one sentence that says, "you should think about putting one in your car." That might be enough, since this is the wrong car for competitive autocross (fun, but not competitive). My local track has its own tech inspection, and I can participate in their open track days. Plus, there's always the fun public roads like the 3 Sisters in Central Texas.
I drew all this in bendtech to get my different thoughts "down on paper" and roughly estimate how much tubing I would need. 4130 is a common domestic alloy, but I want to get ahead of any price increases that come from the general pricing lift provided by import taxes. Right now, it's still reasonable, but hope is not a plan. Anything leftover will find a home on the suburban or waggy or whatever.
This is a very good reason to at least reference their design rules, even if I don't follow the wall thickness minimums.
David