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# Flat torsion bar equivalent to round torsion bar?

## Flat torsion bar equivalent to round torsion bar?

(OP)
This is for an autocross race car suspension - I have a 22mm solid round torsion bar (sway control). I have run the dimensions through a "beam section properties" online calculator that outputs the properties listed below.

I would like to fabricate a flat torsion bar to achieve 50% greater torsional stiffness than the round bar. The "beam section properties" calculator also finds the same properties for flat bars, but I'm not sure which property is proportional to the torsional stiffness. The options are (about centroid):

Moments of inertia, Ixx,Iyy
Product of inertia, Ixy
Section modulus
torsional constant
centroid
shear center

Are any of the above directly proportional to torsional stiffness? Make sure to specify xx or yy

Thanks!

### RE: Flat torsion bar equivalent to round torsion bar?

The torsional constant should be the property you want.  Check to make sure it is in units of (Length)^4.

The equation for rotation is
theta=(T*L)/(J*G)
where
theta is the rotation
T is the torque
L is the length of the member
J is the torsional constant (or polar moment of inertia)
G is the shear modulus.

So to acheive 50% more torsional stiffness, you will need a torsional constant 50% greater, assuming the shear modulus stays the same (same material).

### RE: Flat torsion bar equivalent to round torsion bar?

(OP)
Panars - thanks for the answer. The results indicate that the round bar is torsionally much stiffer (~10X for the dimensions I used) for a given cross-section area than a flat bar is. Is that what you would expect?

### RE: Flat torsion bar equivalent to round torsion bar?

Hi funcrew

Yes a round section for torsion is the most efficient section. If your trying to reduce mass be preserve stiffness try using circular tubing.

regards desertfox

### RE: Flat torsion bar equivalent to round torsion bar?

(OP)
OK - Thanks for the assist!

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