If you would substitute the word 'deflection' into your arguments
in place of elasticity, it would make more sense. If you do, I
would agree.

As for the fatigue resistance, ductility arguments, take a look
at these frames:
http://www.jeraldsulky.com/horsecart...GoldJCart.html
4130 tube, heat treated to the 200kpsi range on tensile, 12 %
elongation in 2" (BETTER ductility than 6061-T6 !!!) These have
no springs, the driver just bounces along on a rough dirt track,
letting the frame flex. They have 'mixups' occasionally, not good
for horses, drivers, or frames. Typical sulky might get used for
1000 races (1250 miles) of race conditions. Never see any fatigue
issues. One got bent 90 degrees when a semi backed over it, still
didn't break. But the heat treating on these is precise. Pull
coupons attached to each batch.

I would agree that the need for a full high strength sway bar is
debatable in standard practice. But a couple things to consider:
heat treated parts tend to be much more uniform in
characteristics. I would much rather have a 4130 bar treated to a
low value than an equivilent CR bar if I wanted an accurate point
of stress beyond yield. The other thing is if the bar might need
to take a huge overload, say from a baja buggy landing on one
corner in a movie style launch. I have a picture sequence on a
buggy launching upward, 5' of air, slowly going nose down and
tilting off to one side, and coming down on one front tire. Next
pic in the sequence is the tire going a rather unusual direction.

Ed Huntress wrote:

"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