Originally Posted by Epstein

A full cage is for 1) requirements of sanctioning bodies. 2) actual roll-over / crash protection. 3) a place to hang your multi-point belts. 4) impress high school girls with your "race car".

A foamed unibody provides none of these.

If you were to compare a 4-pt bolt-in cage to a foamed (rockers, cross bars, a-b-c pillars) car, the foamed car would be stiffer. A cage really isn't designed for chassis stiffness. If you weld it in and gusset it to other members, then yes you get some stiffening. Most of the cages around here aren't though. Ever get you hand pinched between the a-pillar and a front cage tube? That's the unibody still transferring all the suspension loads while the cage carries none. You're not making anything stiffer without transferring the load through the reinforcement.

So as far as a gussetted cage versus chassis foam, the cage in this comparison would be way stiffer. It would also be way more invasive, way more expensive, and way heavier.

As far as weight, simple 4/6pt cages are (very) generally a couple dozen pounds. This foam I used is 8 pounds / cubic foot. 1 cuft is 957 fl ounces. I put about 180oz of liquid component in to the car, which expanded to about ~400 fl oz worth of space. So I added about 3.5 pounds to the car. I probably need to add 2 more pounds before I finish.