Sway Bars

A sway bar (also stabilizer bar, anti-sway bar, roll bar, or anti-roll bar, ARB) is an automobile suspension device. It connects opposite (left/right) wheels together through short lever arms linked by a torsion spring. more...

A sway bar increases the suspension's roll stiffness -- its resistance to roll in turns, independent of its spring rate in the vertical direction.

An anti-roll bar has a spring rate based on the length and diameter of the torsion bar and the geometry (and thus leverage) of its mounting points.

In a turn, the sprung mass of the vehicle's body rotates around an axis known as the roll center. If the vertical distance between the roll center and the center of gravity is not zero, a torque (roll moment) equal to the centrifugal force times the distance between the center of gravity and the roll center will be exerted on the sprung mass, causing the body to lean towards the outside of the turn. This force is called roll couple.

Roll couple is resisted by the suspension's roll stiffness, which is a function of the spring rate of the vehicle's springs and of the anti-roll bars, if any.

The presence of anti-roll bars allows designers to increase roll couple without making the suspension's springs stiffer in the vertical plane, which allows improved body control without compromising ride quality, at least on smooth roads. Because an anti-roll bar connects wheels on the opposite sides of the vehicle together, however, the bar will transmit the force of one-wheel bumps to the opposite wheel. On rough or broken pavement, anti-roll bars can produce jarring, side-to-side body motions (a "waddling" sensation), which increase in severity with the diameter and stiffness of the sway bars.

Some high-priced cars, such as the Mercedes S-class and BMW 7-series, have begun to use "active" anti-roll bars that can be connected or disconnected automatically by a suspension-control computer, reducing body lean in turns while improving rough-road ride quality.

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