Suspension question. What does sway bar preload do?
 

I was checking and un-winding some rear sway bar pre-load (i.e. the stress on the bar while the car is sitting flat/even on the ground) and I tied myself in mental knots trying to noodle out what sway bar pre-load does. My best guess is that it effectively increases spring rate in one corner, and reduces it in another, right?

And is that what the NASCAR teams adjust with a crank near the back window?

A sway bar is only pre-loaded in one direction (one drop link longer than the other). Unless you are only turn in one direction (NASCAR) you don't want any preload. You cannot preload both ends of the rear bar to effect spring rate at the rear (together, the same, equally) as all you will have done is lower the two ends (assuming your bushings are actually working and letting the bar rotate).

The NASCAR guy in the pit is actually removing/adding wedge to the car by changing the suspension perch height on that corner (effectively re-corner balancing the car). It may effectively impact preloading of the rear bar but I'm not sure of the linkages/geometry of NASCAR cars.

And I thought they were opening the window.

Some might disagree! ! Don't forget sway bars can effectively increase spring rates

Formula for sway bar stiffness of a solid steel bar

500,000 D^4
K (lbs/in) = --------------------------------------------------
(0.4244 x A^2 x B) + (0.2264 x C^3)


B
---------------------
A | / \ C
| / \

A - Length of end perpendicular to B (torque arm - inches)
B - Length of center section (inches)
C - Length of end (inches)
D - Diameter bar (inches)


Don't overlook things like endlink bushing compliance and motion ratio once you've got a bar-only stiffness but then you kind-a-sort-of sort or hinted at that..

A little heavy on one side? Might shift some weight around with sway bar adjustment. Lots,and lots of info on this on interwebs... some might even be true!

Martin is right. If you are looking for symmetrical turning performance you want zero preload on the swaybar. If you are setting up for circle racing you may want some preload, but you are on the wrong forum. We go both ways.

I guess I wasn't clear. I know that there should be no pre-load. My question is what happens when there is pre-load? What is happening to effective spring rates and what will it do to the car?

On the axel (front or rear wheel pair) in question one wheel will push down on the ground harder than the other. This will also change things with regards to the other axel but how much depends on a lot of other things. Static spring rate will go up on one wheel and down on the other. Net 0 On a car that has progressive springs this might result in an added V to the rate curve to calulate.

Ah, I think I need my TJ high school kid to help me with the math......:oops:

As others have said, it asymmetrically biases a bit more spring rate to one side of the car.

I'm typically not a fan of preload, as there are often more effective means of achieving the same results, and too much preload can really mess with the car's transitional capabilities.

The only exceptions I can think of, in recent history, are at MidO and Lime Rock. The types of corners in one direction are distinctly different than those in the other direction, so we played with preload as a means of fine tuning the spring rates. At Lime Roc, however, I also ran different springs L-R.

Re-reading your post you were quite clear - I misread it. :oops: