Nissan 240SX - Performance
By providing a link between the left and right side suspension, an anti-roll bar (AKA sway bar) keeps the left and right side of the suspension at nearly the same level vertically, decreasing body roll. Larger diameter sway bars make the suspension stiffer and transfer more weight to the end of the car where they are installed. If the car is understeering, you can increase rear sway bar diameter or decrease front sway bar diameter to restore balance. To correct oversteer with sway bars, it is necessary to install either a smaller rear bar or a larger front bar. Most sway bars have adjustable links that can be used to effectively increase or decrease the stiffness of the sway bar without buying a new one.
Anti-roll bars should be chosen to match your springs. If you are planning on installing stiff springs, there is no need for a large diameter sway bar. The combination of stiff springs and small sway bars is enough to control body roll. Large diameter sway bars are necessary if you will be using relatively soft springs. This is a popular configuration since the ride is not overly harsh, but the suspension is still stiff and body roll is reduced due to the sway bars.
Soft springs with large sway bars and stiff springs with small sway bars accomplish virtually the same goal of providing a stable suspension and reducing excessive weight transfer. A stiff springs/small sway bars setup is generally better than soft springs/large sway bars because stiff springs reduce front-to-back weight transfer. With soft springs, side-to-side weight transfer is controlled by the sway bars, but there is a fair amount of front-to-back weight transfer due to the soft springs. In other words, using soft springs can result in brake dive and acceleration squat, which are detrimental to overall handling.