Rick;
Something to keep in mind when designing a chassis is that crash energy
(kinetic energy due to speed) is absorbed by deformation of the structure--
when a piece of tubing is bent permanently, it absorbed energy-- if it bent
and then bounced back, it just stored energy temporarily and then returned
it.
Mild steel tubing actually absorbs more energy in a crash than 4130. The
higher strength of chromemoly lets it bend without being permanently
deformed; of course if the forces are high enough, it will be permanently
deformed, too, but the total absorbed energy is lower than it would have
been with mild steel. Stronger isn't necessarily better in all cases.
Of course, you want the chassis to deform to absorb crash energy but not
deform so much that driver protection is compromised. Ideally, you want a
very strong ridged protective cage around the driver and an outer
deformable, energy absorbing structure. Any time pieces are torn off in a
crash (wheels, body sections, etc), energy is being absorbed and the car is
being decelerated. Speed doesn't kill you-- it's the high deceleration
forces involved in a crash. By absorbing energy, the car is decelerated more
slowly over a longer time span.
The trick to surviving is to let the structure deform as it slows the car;
this transfers fewer "Gs" to the driver. A really poor design would be
something like a tank hitting a brick wall-- the vehicle might be brought to
an abrupt halt without much structural damage but the driver would undergo
severe G- forces.
4130N is good stuff-- strong, good welding properties, and it is made to
high quality standards. Lots of successful race cars have been fabricated
with it. Many times it would have been just as good to have used a good
quality mild steel in the chassis and save the chromemoly for highly-
stressed members such as suspension, etc.
I guess it might sound ridiculous to say this but DO NOT fabricate chassis
from EMT electrical conduit or exhaust- pipe tubing. Don't laugh, it's been
done and-- it has worked-- but it isn't something which inspires confidence
that a crash would be survivable. I don't mean to imply that anyone on the
LSR list would do this but it's something that we should discourage other
novices from doing.
Sorry for the long- winded rant.
Regards, Neil Tucson, AZ
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