Torsion Bar Questions for Racers

[ken180]

I am looking for opinions on Torsion Bars for my RACE car. (911 A arms)

Weltmeister ?
Sway-A-Way?
Sander?

Solid?
Tubular(hollow)?

Are Sander bars worth the extra $ ?

Are Hollow bars worth the extra $ ?

Has anyboby used the Sander 24mm torsion bar.

Ken

[Dave at Pelican Parts]

I don't know if the expensive ones are worth it or not.

The tubular ones are not. You get a reduction in weight, true, but it has to be less than a pound. And it's regular sprung mass, up on the body. You could have more of an effect by cleaning out your glovebox...

--DD

[philinjax]

Hollow torsion bars have greater torsion force for a given degree rotation than solid bars of the same diameter and obviously are lighter than solid bars. One can get away with a smaller hollow bar than solid bar and have equal performance. the superior performance of the hollow bar has to do with its greater surface area per diameter than the solid bars. I don't remember the math/physics behind this, in fact, I don't remember a lot of things any more, but I am quite certain of the theory.



Phil

[Dave at Pelican Parts]

Hollow bars have a slightly smaller amount of spring force than solid bars. Very slight, in general...

A solid bar's effective spring rate goes up with the fourth power of the diameter, if all other things are equal. To get the effective spring rate of a hollow bar, figure it out as if it were solid. Then subtract the effective spring rate of a solid bar the size of the inner diameter of the hollow bar.

Example--a solid bar of 24mm in diameter versus a hollow bar 24mm in diameter with a 10mm hole in the middle.

Solid 24: 24^4 = 331776 (times whatever)
Hollow 24: 24^4 - 10^4 = 331776 - 10000 = 321776 (times the same "whatever" as the solid above) ; about 97% of the effective rate of the solid 24mm bar.

So they have a very slightly less spring rate, and the weight savings is very miniscule. Not worth it.

--DD

[philinjax]

DD,

I stand corrected. my example apparently applies to the higher force required to "bend" multiple hollow concentric bars compared to a solid bar of identical outside diameter. Heck, I can't be right all the time.

Phil