Frame Design

[vinniez]

Big question is what are the pros or consto a parameter frame vs one that the frame rails are running inboard (say 40"apart)?

[Twentyover]

Torsional stiffness

[vinniez]

Which one is stiffer and how?

[Bryce]

The moment of inertia equation is 1/12*b*h^3. This is for a box structure. If you cannot make the frame taller make it wider.

Also give a place to mount the cage. More side impact protection. Not much heavier than a parallel frame.

[Norm Peterson]

Big question is what are the pros or consto a parameter frame vs one that the frame rails are running inboard (say 40"apart)?

What kind of car are you talking about?

At 40" apart you aren't going to get any real help from the sill structures, so even though the load paths are straighter with an inboard design than with a perimeter frame, the frame members themselves won't be as rigid. Two cross sections that are at least reasonable solidly connected to each other along their length or at several/many individual connection points is going to be much stiffer than the sum of the stiffnesses of the two sections taken individually.

Designing for torsional stiffness ends up being a "3-D" problem, usually either by very large tube or box sections or by a carefully designed cage where the tubes are mostly loaded in tension or compression rather than bending. The poster-boy example of a "large box section" would be the 1960's Jaguar XKE. You'd only have to crawl over those sills (that came well up into where you'd normally expect part of the door to be) just once to know what I'm getting at, although the sills of more recent cars have been made significantly larger than they used to be in order to satisfy crash performance standards.

A really good reference for frame design is Omer Blodgett's "Design of Welded Structures", which gets into bending, shear, and torsion. You won't need to look at all of the topics and you won't be working with metal thicknesses that are anywhere near as heavy as you might see mentioned, but the concepts in the torsion section and the tips for making a structure torsionally strong/rigid are worth more than the cost of the book all by themselves. Another of his books, "Design of Weldments", contains a lot of the same information.


Norm

[vinniez]

I am building a 1959 Rambler American 2 door wagon. 351 windsor, 5 speed trans and a early vette rear,