why make things so difficult?

take the weight of the vehicle and the percentage front to rear and find a similar vehicle, use coils for that similar vehicle or have some custom made, build links according to link articles printed in Peterson's 4wheel and Offroad and Pirate4x4 Gen4x4 Bulletin Board section and Tech articles located in the Tech section.

there is no magic involved, and no complicated physics, just simple, "been done before" info floating around. just gotta know where to look.

now, are you looking for torsional stiffness of the body, the frame, the body and frame, the entire vehicle? and next, why, exactly? and last, what is this 3D you're looking for?