Steve D wrote:
[quote]Ranger wrote:
OK - one last try then I have to get some work done. 
The sway bar is a spring. (See? Chuck and I do agree on some things. :laugh: )
A thinner diameter bar is ‘softer’ (in racer parlance). A bar with longer legs on the U is softer. A bar carrying more weight is softer. By softer, we mean it is less able to transfer weight from the inside wheel to the outside wheel because the bar is deflecting. Hence, that outside wheel is farther from reaching its limits of adhesion. Ergo, no pushy pushy.
Short of self-producing various sets of bars (I figure a 45-60 bar run with different ‘leg’ lengths would give you 3-4 bar choices and would meet the rules requirement), we are stuck with the commercially-available, non-adjustable, spec-diameter bar. The only way I know you can adjust the spring rate of that bar is with wheel spacers and weight (whether that weight be static or aero).
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It’s confusing to think of the sway bar as a spring.
Think of a sway bar as a means to resist roll. The reason you want to resist roll is because body roll results in positive camber on the outside wheel in a turn. The problem with the sway bar is that in order to resist roll it steals traction. It lifts the inside tire and pushes the outside tire down. The inside tire loses more grip then the outside tire gains.
Stiffer bars resist roll more then softer bars. It doesn’t matter too much how you got it softer…maybe it’s thinner or maybe it has longer legs, softer is softer.
Springs impact body roll too so there is an important interaction between the spring and sway bar. There is less interaction between shocks and the sway bar because shocks are velocity devices whereas springs and swaybars are position devices.
Center of roll resistance. As long as the body is fairly stiff, the car will roll as a unit and not twist longitudinally. That means that you can choose to resist body roll at the front or rear. A stiff rear sway bar moves the car’s “center of roll resistance” rearward. Since sway bars steal grip, moving the car’s center of roll resistance rearward (stiff rear bar) means you are stealing grip from the rear.
So stiffening the rear bar resists body roll. Reduced body roll means wheel camber is preserved and the tires are less likely to go to positive camber. But you stole traction from the rear to do it. By stiffening the front sway bar instead, you could have stolen front traction in order to reduce body role. By playing with each you can turn an understeering car into an over steering car. Even someone as clumsy as me can feel a 5mm change in rear sway bar length.
Consider the case where both front and rear swar bars are too thin for your gripper tires and high center mass. This means too much body roll and your grippy tires go to positive camber
If your sway bars are too thick then in turns you end up with -1deg camber and you steal a lot of traction to do it. Softer sway bars (meaning thinner or longer) would put you at neutral camber in hard turns and not steal so much traction. And neutral camber is the objective because that maximizes grip.
Interaction between sway bars, springs and shocks. This is where experience comes in. You know, the stuff I lack. All I have is book-reading. The suspension is a system and there are interactions between all elements. It takes a lot of experience to choose good combinations of sway bar, spring, and shock. Our situation is vastly simplified by our tight ruleset.
Re. the bar carrying more weight is softer. That’s sort of right, but for us I’m not sure that it matters much. If you remove 100lbs from a car then it’s desire to roll (this kind of force is called a “Moment” ) is reduced. So a softer sway bar would resist rolln as well on the lighter car as the stiffer sway bar resisted roll on the heavier car. If you leave the stiff bar on the lighter car, you have, in effect, stiffened the bar. And, of course, the stiffer bar stole more traction.
Re. “A bar carrying more weight is softer. By softer, we mean it is less able to transfer weight from the inside wheel to the outside wheel because the bar is deflecting.” This isn’t correct. The bar only knows about a difference in the load on each wheel. Additional weight on the bar doesn’t cause it to deflect more. Additional weight just causes the bar to rotate in the bushings that attach it to the frame.
Damn website hosed my post. It logged me out after I hit Submit. But I’ve learned it’s nasty ways and I’d copied the post just in case.
