Rick,
There are some good books on chassis engineering. Buy one and read up. It'll give you a bloody good understanding of exactly what you are looking at. i.e a chassis.
I've read a number of books recently, they're all at home can't remember the exact titles. But try a search on Amazon etc.
In most basic terms - a rectangle is weak, that is until it has a diagnol through it. Hence, it has been triangualted.
Regards,
J.P
Happy reading. /ubbthreads/images/graemlins/grin.gif