Body stiffness

[astroracer]

A perimeter frame will add a lot of longitudinal strength but you also need to add some torsional stiffness in there as well. "A", "B", & "C" pillar roll cage hoops should be tied into the floor as well to keep the lower frame from folding up on itself.
I did this pic very quickly in paint so you can get an idea of what I am talking about. It's not pretty but you should get the idea.

Mark