If we add a sway bar to our car, it has the same effect on chassis roll as adding wheel rate to the car. The car senses the spring and sway bar rates at the wheel, and those rates are reduced by the motion ratio or installed ratio. So we need to translate the sway bar rate to a rate at the wheel to understand the true effect. This is an important point, because if we mount the sway bar arm closer to the ball joint, we end up with a stiffer sway bar rate at the wheel. A sway bar arm mounted inboard toward the inner chassis mounts of the lower control arm has a much softer rate.

Going Big on Bars and Soft on Springs
When we install a large-diameter bar, we are effectively adding spring rate to our cars in a big way. Let's say we have a touring Late Model car that has 300-pound springs in the frontend. If we install a 111/42-inch sway bar, the effect on reducing body roll is just the same as if we added 1,237 pounds of spring rate to the car. We could install 1,550-pound springs in the front of the car and have the same diminishing effect on body roll.

The effect on body dive is not increased when using a large-diameter sway bar. So we may be able to increase the aero downforce on our cars by installing softer springs with the BBSS setups. The car will compress in the turns instead of rolling, and the overall attitude of the car will be lower. With a lower-sprung mass, the center of gravity will be lower and less weight will transfer from the left-side tires onto the right-side tires. More retained left-side weight means more overall traction. A lower body profile means less drag, more overall downforce, and more load on the tires.

This all seems like a win-win situation. It can be, except for one fact: We still need a balanced setup in order for our car to be consistently fast. When we reduce the front roll angle to a fraction of the old roll angle, we need to also reduce the rear roll angle by the same amount. So, the teams have found that they need to increase the right-rear spring rate a great deal and create a large spring split in the rear of the car to reduce the rear roll angle.

In past articles, we have explained the dynamics of a straight-axle suspension system. The lateral force vector is in a direction that is down and to the right, usually pointed between the rear tire contact patches. If we have a spring split in the rear, the effect of the spring split will cause either an increase or decrease in the rear roll angle, depending on which side is softer.

If the left-rear spring is softer than the right-rear spring, the car will roll less. If the right-rear spring is softer than the left-rear spring, the car will roll more. So, in order to reduce the rear roll to match the front stiffness, we see rear spring rates that might be as follows: left-rear = 175 and right-rear = 300. Some teams will run the same spring on the left-front, right-front, and right-rear corners and a much softer spring on the left-rear corner.

Note: The spring rates shown will not necessarily work for your particular car. Develop your own setups based on the information from your car. We do not recommend you use these spring rates. They are being used as an example of the general trends in setups.