Bill Babcock wrote:
> My only suggestion is that unless you are a really great welder, or know
> one--don't use chrome moly. It's very difficult to do right and if you do
> it wrong you'll have a weaker cage than if you used mild steel.
As Bill suggests, 4130 requires annealing after welding (with some fairly
accurate temperature control), and that's
difficult to do with something as large as a roll cage. Welded joints in 4130
must be annealed uniformly for at least a
foot on either side of the joint.
And, whether or not the rules allow it, 4130 is not good for a roll cage. It
has high tensile strength, and has pretty
good toughness, but that's relative. The operative function here is Young's
modulus. For all steels (except some
stainlesses), that modulus (stress times strain) is the same. That means the
higher the tensile strength, the lower the
elongation before failure.
In layman's terms, that means that the differences in load necessary to make a
part yield and then break get smaller and
smaller as tensile strength goes up. If that roll cage is loaded beyond the
point of failure (imagine a 100 mph
end-for-end two or three times and a couple of nose pirouettes followed by a
body slam into the pavement), the cage
_breaks_, rather than bends. If that happens, then a) your cage isn't around
you any longer, and b) one of those broken
pieces can go through an essential part of you.
Use the specs for mild steel. The principle here is the same as controlled
crush in a unibody car. As the material
bends, it's absorbing energy. And it's much more likely to be there around you,
because it has bent, rather than broken.
4130 and similar materials are great for some purposes. But not for roll bars
or cages.
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto:mporter@zianet.com]
Never let anyone drive you crazy when you know it's within walking distance.
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