what is a roll bar
The anti-roll bar is essentially a transverse-mounted torsion bar designed to reduce body-roll during turns.  It exerts no influence on the suspension when wheels bounce in unison.  If vertical movement on one side exceeds the vertical movement on the other, the anti-roll bar exerts an opposing force.  Along with its primary function of reducing body-roll, the anti-roll bar will also reduce the combined cornering force and the adhesion limits of the side-by-side tires that are being acted upon.   Consequently, the location and stiffness of the bar can be modified to influence the oversteering or understeering characteristics of the vehicle.

Body Roll is reduced by torsional effect of sway bars as they transfer the pressure exerted on the inboard wheels. The weight is more evenly distributed to all four wheels helping the vehicle stay more level with the road and improving control in turns.

What is the purpose of sway bars? They tie the left side wheels to the right side wheels on both the front and rear of your car.

What do they do: In a corner your vehicle wants to roll over in the corner due to centrifugal force.

With out sway bars on an off-road vehicle this causes excess chassis roll that compresses the outside suspension and tire and causes the inside tire to go into droop.

While on an on-road car this excess chassis roll overloads the outside tire and unloads the inside tire lowering overall traction.

on a car or truck a sway bar accomplishes the same thing, but the big difference is that on a car on road racing, it keeps both the inside and outside tires planted squarely on the ground increasing traction, by allowing the suspension (leafs or shock) to apply there forward traction in a corner and here is were a posi rear end also comes into play

The weight of your car or truck also determines the size of sway bar you need to run. A heavy monster truck with heavy large tires will need a large heavy sway bar. While a small light weight on-road car will need a much smaller lighter one.Also to keep in mind with learning how to adjust and tune your sway bars is your setup and driving style.

You may want to run different size sway bars on the front and rear. There is no carved in stone rules when it comes to sway bar adjustment. It all depends on your conditions and driving style.

Keep in mind that your sway bars do not work by themselves. The effectiveness of the sway bar you run depends also on the weight of spring you run.

With the sway bar tying both the left and right side together, the sway bar must overcome the weight of the spring on the opposite side as one side compresses.

If a light weight thin sway bar is used this allows a lot of movement on the outside suspension arms and very little on the inside suspension arms in a corner.

While a thick heavy weight sway bar so stiffly connects both sides of your suspension resulting in what feels like a straight axle suspension.

The trick is finding the sweet spot between these two extremes. Sway bars do not change the overall traction of your car or truck, they just affect your side grip in a corner.

Besides springs chassis flex plays an important role in the effectiveness of your sway bars. The stiffer your chassis is the more responsive your car or truck will be to nay changes to sway bar settings.

Adjusting sway bars is a balancing act, increase stiffness to a sway bar on one end, reduces the side grip of that axle, while increasing the side grip on the other end.

The net effect of sway bars on both on-road and off-road vehicles is almost the same.

A softer front bar:
1. Increases front chassis roll.
2. Increases front grip or traction, while decreasing rear grip or traction.
3. Slower steering response.
4. Increases off-power steering at corner entry.

A stiffer front bar:
1. Decreases front chassis roll.
2. Decreases front grip or traction, while increasing rear grip or traction.
3. Faster steering response.
4. Decreases off-power steering at corner entry.

A softer rear bar:
1. Increases rear chassis roll
2. Increases rear grip or traction, while decreasing front grip or traction.
3. Less on-power steering.

A stiffer rear bar:
1. Decreases rear chassis roll.
2. Decreases rear traction, while increasing front grip or traction.
3. Faster steering response in high speed corners and chicanes.
4. Increases on-power steering.