"Roy J" wrote in message
...
snip

However, it would surprise me if a sway bar designed for a
high-performance
car would even approach the elastic limit of untreated, cold-rolled
steel.
It shouldn't deflect a great deal. Much more likely to need fancy steel
and
heat treatment is an ordinary street sedan, in which the bar would be
less
stiff, and therefore would need more elasticity.

In any case, the difference in elastic limit between the normalized
condition and the hardened condition is not that great for 4130. It's
fairly
strong as it is, and it isn't tremendously strong even when heat
treated.

???????????

The modulus of elasticity is the same whether it is heat treated
or not. ie for a force of x, it will deflect y.

The modulus is, but the elastic *limit* depends on its yield strength. In
fact, the elastic limit IS the yield strength.

This is a field in which terms always get misconscrewed. By more
"elasticity" I mean the ability to deflect farther without exceeding the
yield strength.

The question is:
does the deflection make it exceed the yield strength?

That's what I said. g

Annealed
4130 is about half again stronger (higher yield strength) than
equivilent cold rolled mild steel. (80-90kpsi versus 50-60kpsi)
The heat treated version of 4130 can easily push past 200kpsi or
more than double its unheateated version.

I don't think you want to make a spring -- or an anti-roll bar -- out of 200
kpsi 4130. If it's going to fail, you want ductile failure, and the
elongation at 200 kpsi strength is close to zilch.

The bigger point, though, is this: If you're loading 4130 to, say, 150 kpsi,
you're getting a lot of spring deflection at that load. In a
high-performance car you don't want a lot of spring in your anti-roll bar,
so you make it of a larger section or you use shorter arms. Of course, by
using shorter arms you also demand more twisting displacement from the bar
for a given amount of travel, so you could wind up loading the bar quite
heavily toward its ultimate strength if you compensate for a
smaller-diameter bar by using shorter torque arms.

The ideal is the longest practical arms. For the kind of deflection you want
in a race car or high-performance car, that means you want a bar of larger
diameter, which does less twisting in use. That keeps the specific torsional
load (the load per unit of metal) relatively low; low enough, IIRC, that
there should be no benefit whatsoever from using a high-strength bar.

Ed Huntress