Chassis Building 101. Ride Height

[Dr G]

Not aiming to be the smart-ass here but i think the important difference between the Big-10 chassis and a normal truck frame is the X-shaped structure in the center, that links all of the points that have some kind of force transmitted into them to eachother.
Without that, even an 11" tall lader type frame will twist like a fish...

I want to build a bolt-in X-structure like that for my projects frame.

For cars there's a lot more you can do than a roll cage (increasing the number of pinch welds, stitch-welding frame structures, adding reinforcement panels to link structures together, etc.)

An X-frame inside a wider rectangular section (Big-10) is different than an X-frame on it's own (like the Impala I showed). Generally you want to load tubes (or frame sections) in tension or compression, not in bending, in order to get the biggest increase in torsional stiffness. Also, I hope that I did not convey the impression that a truck can be stiffer than a car. With a truck the body panels do nothing for you in terms of stiffness. In a car, the existence of a roof make it stiffer than a truck, with or without a roll cage. But if you use the space available to you underneath a truck (which a car cannot do) then you can definitely improve stiffness a lot. Rob's Big-10 frame is the only one on the market that takes advantage of this possiblity.

In a previous life a was pretty into BMW M3's (E30 and E46) and I am aware that you can seam weld an entire unibody. But that is going to be an incremental thing (it's easy, cheap, does not add weight, so why not seam weld the body?). However, the biggest improvement in bending and torsional rigidity for a car will come from adding a roll cage.



As Rob mentioned, the ideal for torsional rigidity is a cylindrical tube (we're both engineers so this was taught us in college). Next up would be a square (or rectangular) tube. A truck frame can be thought of as a rectangular tube. At the risk of getting "wonky" here's a formula for the polar moment of inertia for a rectangular hollow section rectangular tube:



The polar moment of inertia dictates how stiff a tube will be in torsion about it's axis. If you look at the formula, you can see that the two relevant parameters are the height and width of the rectangular tube. Thus by analogy, for a truck frame, if you make it wider, taller, or both, you will make it stiffer. That is the point I was trying to make.

And for bending, making a truck frame taller is also a big improvement. Conclusion: if one is designing and building a truck frame with performance intentions, then if you do not make it as tall as possible, you are leaving meat on the table (so to speak).

Gustave