Torsional Rigidity

[killer_datsun]

well this is for a long term project where I am going to try and build almost everything myself. so I am not really starting with any car as a basis

[Matt@RFR]

Why are you stuck on 4" round tube?? I've never heard of that being done, and see no reason for it either. Look at the attached picture. Every single one-off chassis I've ever been around has been built using the same basic materials (meaning 1.375" - 1.75" round tube for everything), and with a few tweaks, would make an excellent road race chassis.

I'm not about to spend the time to do the math, but I'm willing to bet you'd end up lighter using round tube versus RECTANGULAR tube.

(2x2 would be square, 2x4 would be rectangular)

Me thinks you've got a long way to go before you're ready to start designing your own chassis.

[killer_datsun]

I am not stuck on 4 inch round tube it is just something I threw out there to compare to rectangular tubing of the same heigth

[v7guy]

when building something like this you need to take in to account alot more than just the hieght of the material. The width is going to be just as important both from the standpoint of ergonomics and strength. Thought I would bring it up in case it wasn't just a typo in your last post.

If you haven't I would consider doing some reading. There are some really good books out there.