Torsion bar question

[j911brick]

There is a lot of talk about torsion bar vs spring rates. But I have always had this question. Wouldn't torsion bar rates be effected by the exact metallurgy of the torsion bar? In other words: might the torsion bar from one manufacture have a different rate than the same size bar from a different manufacture? I know that both springs and torsion bars begin to sag as they get older; doesn't that mean the rate(s) are changing?

[IROC]

As I understand it, not so much. The characteristic that is important is called the Modulus of Rigidity, G, and it is basically the same for all steels. Kind of like the Modulus of Elasticity, E. Basically the same.

[j911brick]

Originally Posted by IROC

As I understand it, not so much. The characteristic that is important is called the Modulus of Rigidity, G, and it is basically the same for all steels. Kind of like the Modulus of Elasticity, E. Basically the same.

Ok, thanks.

[Magyar Kiwi]

Hi
Modulus of Rigidity, G is pretty much constant for all steels.
So torsion bar and coil spring rates are a function of diameter(s) and length.
When any steel spring sags or takes a "set" the rate does not change.
Cheers

[PeteKz]

Ditto what Magyar Kiwi and IROC said. For virtually all carbon steels and their alloys (e.g., 4130), the material constants are the same. The stiffness of a torsion bar depends on the diameter of the bar and the active length. The same size torsion bar from any quality manufacturer has the same spring rate.

[j911brick]

Originally Posted by PeteKz

Ditto what Magyar Kiwi and IROC said. For virtually all carbon steels and their alloys (e.g., 4130), the material constants are the same. The stiffness of a torsion bar depends on the diameter of the bar and the active length. The same size torsion bar from any quality manufacturer has the same spring rate.

OK, I think I get it. However, I know from experience that different grade bolts have different shear strengths. How does that play into this theory?

[Magyar Kiwi]

Hi
Yes different alloy and heat treatment steels have different shear and tensile strengths, but Modulus of Rigidity, G is pretty much constant for all steels.
Cheers

[PeteKz]

1. It's not theory, it's demonstrable fact.
2. Torsion bars are high carbon alloy steel that's heat treated to maximum strength. Similar to SAE Grade 8 or higher bolts.
3. The bolts you mentioned have different shear strengths, but up to the point where they begin to deform, they have the same "stretch" and "twist" spring rates.

If you seek more understanding, read this: https://civilenggblitz.com/understanding-stress-strain-curves-for-steel/