% Contribution of Main Spring to Roll Stiffness

[nm9stheham]

FWIW..... This post is mainly to try to help folks gain a better conception on the relative contributions of the main springs in the car versus that of the anti-roll bars in fighting body roll. This was prompted by a post over in another subforum that said that the anti-roll bars were 90% of the roll rate. It is rarely if ever anywhere close to that %. Rather than continue to discuss this matter in an engine thread, I thought the info would be better placed here.

(And seriously, I have no desire to make anyone feel bad or to be disagreeable, but only want folks to get a better grasp on this matter.)

1. T-bars, coil springs, leafs (or whatever main spring is used) have a spring rate at each wheel (wheel rate) that works EXACTLY the same in straight up and down motion and in roll. Roll motion is just up an down motion in opposite directions on each side. Roll control is indeed a big part of the purpose of the main springs; that is the misconception that I am wanting to correct. The sway/anti-sway bar is use to modify the roll rate of the main springs alone, to achieve the desired wheel rate in when in roll motion.
2. Here is a good illustration of this. It is foolish to dismiss this and claim that it does not apply to a Mopar A body suspension design since this discusses a Subaru STi; the design and operating principles are the same, and working out the wheel rate of main springs and anti-sway bars for a Mopar A-body will give similar results, and even moreso if you have installed heavier T-bars and/or rear leafs. The thread below shows that the sway bars contribute 56% and 45% to total roll rate (for front and rear respectively) for a car designed from the factory for very good handling. Those numbers are quite different than the 90% posted for the % of roll rate contributed by the anti-sway bars. Sway bars and total spring rates (why sway bars sometimes do too much) - NASIOC
3. As a side point, I had mentioned an example of a certain Porsche where a Z-bar was used instead of a straight anti-roll bar to reduce the roll rate versus that of the main springs alone. For the Porsche, like a Mopar A body suspension design, the roll rate (the total roll rate at the wheels) is a combination of the main spring (torsion bars in both cases) with the roll/anti-roll bar. In the case of the Porsche, the z-bar's roll rate is a subtraction from the main t-bar's rate. It is only different in that is subtracts rather than adds to the main springs wheel rate in roll, as is the case in most cars. But the same principle is in action: a sway, or anti-sway, bar's modification to the roll rate of the main springs alone.