Torsional Constant For A Ladder Chassis

[thezhan93]

Hey guys! I'm currently designing a ladder chassis for a vehicle design assignment. I'm having troubles finding the torsional constant, J of a ladder chassis which is needed to find the torsional stiffness.

The ladder chassis will consist of 2 side members and 3 cross-members spaced equally along its length.
All the members are C-sections.

Am I suppose to sum up all the area moment of inertias, e.g. J = 2*I(side member) + 3*I(crossmember)
OR
Is this analysis only possible using a software?

 

[BrianPetersen]

It's not going to be that simple, and it's going to depend on how the crossmembers are connected to the main beams. If they're tacked on with a couple of rivets, they're not going to do much. A proper solid (welded-all-around) connection will do more. It is certainly not going to be as simple as the total of all three.

 

[thezhan93]

Wow that was a quick reply, thanks alot! Okay, lets assume that the connections are properly welded.
How about if I simplify the model by using a frame with only 2 cross-members located at both ends (essentially making it a square when viewed from top). Is it possible to find the torsional stiffness by hand now?

 

[Tmoose]

C sections/channels like andy "open' section will have mighty low torsional stiffness.
C and I sections are used when Low torsional stiffness is sometimes desirable.

http://www.vanpeltsales.com/FH_web/FH_images/FH_chassis-pics/Flathead_Suspension_1932-37passngr.jpg

http://ww2.justanswer.com/uploads/molurch/2010-11-15_014205_coil.png

Omer Blodgett says open section lateral Crossmembers don't add much.

http://weldingdesign.com/archive/no-twisters-or-swingers-allowed

More detailed info in Lincoln Electric's bargain Design of Weldments.

http://www.jflfoundation.com/ProductDetails.asp?ProductCode=DW

As I recall simply "boxing" the channels increased torsional stiffness