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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. A sway bar increases the suspension's roll stiffness -- its resistance to roll in turns, independent of its spring rate in the vertical direction.
Principles
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. The effect of roll couple is positive camber of the wheels on the outside of the turn, which reduces their cornering grip.
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 use of anti-roll bars allows designers to reduce 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.
The spring rate of an anti-roll bar is based on the fourth power of the torsion bar's diameter, the inverse of the length of the lever arms (i.e., the shorter the lever arm, the stiffer the bar), the geometry of the mounting points, and the rigidity of the bar's mounting points. Some anti-roll bars, particularly those intended for use in auto racing, are adjustable, allowing their stiffness to be altered by increasing or reducing the length of the lever arms. This permits the roll stiffness to be tuned for different situations without replacing the entire bar.
Increasing the roll stiffness of the suspension increases the rate of weight transfer to the wheels on the outside of the turn. As more weight is applied to the outer wheels, the adhesion of the tires is increased until their limits are reached, increasing their slip angles. If the front and rear weight transfer is unequal, the slip angles of the end with the greater weight transfer will be larger, resulting in understeer or oversteer. The use of anti-roll bars allows the weight transfer of the front and rear wheels to be adjusted separately, compensating for unequal front/rear weight balance and "tuning" the vehicle's handling characteristics.
Drawbacks
Because an anti-roll bar connects wheels on the opposite sides of the vehicle together, 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.
See also
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