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Originally Posted by fe1rx
.... Testing is definitive. I did the math to be able to estimate the stiffness for a hypothetical different bar. That is certainly suspect now. A factor of 2 correction for arm bending and bushing deflection isn't appropriate for a wide range of bar diameters, because bushing deflection will become more significant for stiffer bars.
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The Puhn formula includes the arm bending, and you could add that to your calculated deflection, simple (PL^3)/3EI , with L = C from Puhn.
The factor of 2 correction has nothing to do with arm bending. I mentioned a factor of 2 necessary to go from the puhn bench test equation, to a cornering condition, assuming any jacking effect is negligible. I also mentioned that in our case of lever hole and bushing positions on the bar, we were off in our bench test stiffnessby a factor of ~2 on the soft side. Sorry for the confusion.
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The Puhn formula is only for a specific bar shape, never found in street cars.
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I have owned several cars where the shapes covered by puhn were identical to the actual bar ( mazda6 F&R bar, Rx7 FD F&R bars). But you have a point in that newer cars tend to have more curves in them esp rear bars in AWD, and they become more 3 dimensional vs the Puhn 2-D types.
[quote]That assumes that there is no jacking at the corner in question and the inside wheel's "up" matches the outside wheel's "down". Better, I think, to convert the bar stiffness to be a function of roll angle rather than one-wheel bump travel.[quote]
I think the Puhn formula is fine as is. Jacking is a function of suspension type and roll center positions, and should be delt with separately, or in a reliable software package (OptimumG). No one has likely experience jacking as I have, with rear swing arms on my old Triumph. In an autox, the scenery in the rear view mirror changed to blue sky as the rear jacked on a tight corner. That was fixed by having trivial rear roll stiffness on a swing-spring upgrade. The bar's stiffness does not change due to jacking.
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I am using OptimumG's "magic number" spreadsheet which takes care of that math.
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The bar will always have equal and opposite forces at the endlinks. I'd be interested in how significant jacking is in the case of your car, in terms of weight transfer bias.