Torsional Rigidity

[silver69camaro]

Deepblue68, your calculations are incorrect. You need to subtract the PMOI for the hollow sections, since we are talking about hollow tubes. You can compare round tubes in the manner which you did, but not round to non-circular.

The 4" tube will be more rigid for a given length. But for the manner in which you are talking about, it's all a wash. For a flat plane (chassis), I'd use rectangular tube for it's good torsional rigidity (4" tube isn't very practical) and great bending strength, something which it tough to get with round tube. The torsional stiffness of a flat chassis is very limited.

Now, if you triangulate the chassis (with a cage or similar), then it's a whole different ballgame.

[killer_datsun]

yeah I am going to put a cage on it as well

[DeepBlue68]

Deepblue68, your calculations are incorrect. You need to subtract the PMOI for the hollow sections, since we are talking about hollow tubes. You can compare round tubes in the manner which you did, but not round to non-circular.

Oh, wow, I guess that's what I get for trying to do those calculations during a 6 hour study binge for a Controls exam I had this morning. Geez, that's pretty said I forgot to subtract the hollow sections out. Anyway, thanks for the correction. Am I anywhere close in thinking that most of the stress seen by members of a rollcage is either axial or bending (vs. torsion)?

Also, you're saying you can't directly compare the polar moment of inertia of a rectangular cross-section to that of a circular cross-section?

[silver69camaro]

Wait, you're saying you can't compare the polar moment of inertia of a rectangular cross-section to that of a circular cross-section?

If you don't subtract the PMOI for the hollow sections, then no you can't. Once the hollow areas are accounted for, then the two numbers can be compared.

[silver69camaro]

Am I anywhere close in thinking that most of the stress seen by members of a rollcage is either axial or bending (vs. torsion)?

To update my post to your updated post...

When I design cages, I deal mostly with direct forces, and columnar bending with longer bars. There is some torsional issues that you can think about, but honestly, I let FEA do the work on that.

When you twist a roll cage, most of the forces are primarily axial like you mentioned. Very well designed cages exhibit the most direct tension and compression forces (obviously it's stiffer that way), instead of weird moments and bending forces at the joints.

[DeepBlue68]

Cool...what FEA program do you use? We've learned on NE Nastran here at school, and to be honest I'm not a huge fan of it. I've heard from more than one person that I've interviewed with that it's not the best or the easiest to master, and there's better FEA programs around. Your thoughts?

[killer_datsun]

well my question is pertaining using either 2x4 rectangular tubing or 4 inch diameter circular tubing for the chassis.