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Once the loads on a chassis exceed a certain point it starts to take a permanent "set". That means bent in layman's terms. Most people are surprised to learn a mild steel chassis [1020] will be just as stiff as a chrome moly chassis [4130] since the modulus of elasticity is the same [30,000,000 psi]. What is different is the load at which they take a permanent "set."
The big tires and high horsepower of today's national midgets and all 360/410 sprint cars are capable of twisting even the best chassis. Our sportsman midgets and most other "lower classes" aren't able to produce the loads needed to create a permanent "set".
Think of all those Kurtis midgets that won races for 20 or 30 years; unless crashed they didn't get loaded enough to bent.
I was once told a midget really starts to handle after it has been upside down. Maybe it serves as a vibratory stress relief. Variation from chassis to chassis could be welding sequence or stress relief related. Theoretically 4130 requires a stress relief, 1020 doesn't. It has to do with carbon content.
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