There are sound technical reasons for these changes. First, our material spec. is "magnetic steel", mostly because it is what we can reliably determine at the hill. We can reliably measure wall thickness, and Tech (Don) will be furnished with a guage to do so, with calibration traceable to NIST. Basically, with rare exception, commercially available tubing meeting the wall thickness & magnetisim spec. will be suitable for rollcage construction, if it is welded correctly.
Submitted for your consideration: Physical properties of steel tubing, as published by Thyssen-Krupp at their Online Metals website.
As you can see, the most common materials - 1026 DOM, 4130, and 1018 erw share common properties. they have yeild strengths over 50,000 psi, tensile strengths over 65000psi, and elongation of less than 25%. A-36 pipe is marginal, but it's most commonly available spec (Schedule 40) is substantially thicker than our minimum - so strength will be roughly = to 1018, but with a weight penalty. 1008 exhaust tubing is generally not available in wall thickness sufficent to meet spec. which is good - 'cuz the stuff is gooey soft, and will bend (%elongation) quite easily. Both of the stainless tubings listed fail the magnet test. And that's good, 'cuz although their tensile strength is quite good, they are very ductile (softer than a sneaker full of s#@*!), and nearly impossible to tell apart in the field.
It is also important to note that "Chrome moly/4130" is not substantially stronger than 1026 DOM, unless it is heat treated to a point where embrittlement is a real possibility, particularly if it is welded after heat treat. And the common practice of torch annealing the welds to prevent embrittlement renders it no stronger than 1026 DOM - thus our discontinuance of a thinner wall spec for 4130.
