A stabilizer bar — also known as a sway bar, anti-sway bar, or roll bar — is a series of tubular rods that connect opposite wheels and suspension units on a car to each other. These rods are connected by torsion springs. Their purpose is to reduce body roll, or lateral motion, during a vehicle's turning. The bar is part of a vehicle's suspension system and is designed to increase the vehicle's lateral roll stiffness.
Typically constructed of tubular steel, the U-shaped stabilizer bar is often connected to the frame of the vehicle at two points, as well as to the suspension on both sides (left and right) of the vehicle. When the wheels move in the same direction (i.e. forward or backward), the bar does not bend; but if the wheels move in different directions, even slightly, it twists, causing extra stiffness. This design is intended to reduce body lean during cornering, which can affect the tire's grip on the road and therefore the stability of the vehicle's cornering ability.
While the stabilizer bar performs an important task by connecting both sides of the vehicle to each other, it also has drawbacks. Both wheels are essentially connected to each other, so bumps from one wheel can be transferred to the other, causing side-to-side motion that can be uncomfortable; this motion also affects the handling of the vehicle. The stiffer the bar is, the more jarring the motion will become. In more extreme situations, the stabilizer can cause the wheels on the outside of a turn to lose contact with the road, causing both a handling problem and a safety issue.
Higher-end vehicles today sometimes come with active stabilizer systems that are controlled by a computer and can change the amount of stiffness of the system. They generally allow for a small amount of roll so that cornering feels more natural, but the computer can react to different amounts of torsion being placed on the bar and adjust accordingly. This is a fairly high-end feature and does not come standard on most vehicles, but it greatly enhances the feel and performance of the car since the vehicle will react to steering situations as they arise, rather than acting as a static system that only reacts to forces placed on it.