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Anti-sway bar spring constant

Anti-sway bar spring constant

Anti-sway bar spring constant

(OP)
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

RE: Anti-sway bar spring constant

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

RE: Anti-sway bar spring constant

(OP)
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

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