Anti-sway bar spring constant

[CSBM5]

I'm in a back-and-forth with a guy on a car board about the stiffness of an anti-sway bar. From what I recall (and from what I've found in my old texts), a solid bar in torsion has a spring constant that varies with the 4th power of the bar diameter. The spring constant is given for a soild bar in torsion in my old (8th edition) Mark's Handbook as: k=G*Ip/L where Ip is the polar moment of inertia which is defined as pi*d^4/32 where d = diamter of the solid bar.

Hence my assumption that the stiffness of a sway bar was proportional to the 4th power of the diameter. Is this correct? Here's the link to the thread in question:

http://www.bimmerfest.com/forums/showthread.php?t=145483&page=2

(post #32 and lower).


Thanks,
Chuck

 

[NormPeterson]

The units for moments of inertia are [length]^4, so appropriate functions of Do^4, (Do^4 - Di^4), and, as an approximation for a thin-wall tube, Dmean^3*twall, are dimensionally correct.

[length]^3 terms such as Do^3 or (Do^4 - Di^4)/Do are involved in section modulus quantities, which are related to component stress under load, not stiffness.


Norm

 

[CSBM5]

Thanks Norm. It's been 25+ years since I studied this in undergrad school, and I specilized in heat transfer and fluids in grad school, but I've been out of the engineering world now for more than 15 years.

Your response has pointed me down the right path to reviewing/understanding this subject once again.

Cheers,
Chuck