anti-sway bar

Sway bar (in black) in the rear axle of a Porsche

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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. The first stabilizer bar patent was awarded to the Canadian S. L. C. Coleman of Fredericton, New Brunswick on April 22, 1919.^[1]

Principles

A sway bar is usually a torsion spring that resists body roll motions. It is usually constructed out of a U-shaped piece of steel that connects to the body at two points, and at the left and right sides of the suspension. If the left and right wheels move together, the bar just rotates about its mounting points and does not bend. If the wheels move relative to each other, the bar is subjected to torsion and forced to twist.

The bar resists the torsion through its stiffness. The stiffness of an anti-roll bar is based on the fourth power of its radius, the stiffness of the material, 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. The stiffer the bar, the more force required to move the left and right wheels relative to each other. This increases the amount of force required to make the body roll.

In a turn, the sprung mass of the vehicle's body produces a lateral force at the center of mass (CG), proportional to lateral acceleration. Because the CG is usually not on the roll axis, the lateral force creates a moment about the roll axis that tends to roll the body. (The roll axis is a line that joins the front and rear roll centers (SAEJ670e)). The moment is called the roll couple.

Roll couple is resisted by the suspension 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 without making the suspension's springs stiffer in the vertical plane, which allows improved body control with less compromise of ride quality.

One effect of body (frame) lean, for typical suspension geometry, is positive camber of the wheels on the outside of the turn and negative on the inside, which reduces their cornering grip (especially with cross ply tires).

Main functions

Anti-roll bars provide two main functions. The first function is the reduction of body lean. The reduction of body lean is dependent on the total roll stiffness of the vehicle. Although Anti-roll bars reduce body lean, this comes at the expense of increased load transfer at the end it is applied to. To keep the body level in cornering, the bar pulls up on the inside tire (therefore under loading it) and pushes on the outside wheel (over loading it). Anti-roll bars therefore reduce overall grip at the end they are installed to. This is because load transfer always decreases the overall grip of a pair of tires, due to the fact that coefficient of friction decreasing with load. This decrease in grip from load transfer overwhelms the increase in grip from the more optimal camber caused by less roll. The net effect, then is less grip at that end of the car.

The other function of anti-roll bars is to tune the handling balance of a car. Understeer or oversteer behavior can be tuned out by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front will increase the proportion of the total load transfer that the front axle reacts and decrease the proportion that the rear axle reacts. This will cause the outer front wheel to run at a higher slip angle, and the outer rear wheel to run at a lower slip angle, which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle will have the opposite effect and decrease understeer.

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. Excessive roll stiffness, typically achieved by configuring an anti-roll bar too aggressively, will cause the inside wheels to lift off the ground during very hard cornering. This, of course, is only possible if the regular spring rate actually allows the outside wheels to handle the much increased load. This can be used to advantage, in fact many front wheel drive production cars will lift a wheel when cornering hard, in order to overload the other wheel on the axle, so limiting understeer.

Active systems

Some high-priced cars, such as the Range Rover Sport and BMW 7-series, have begun to use "active" anti-roll bars that can be proportionally controlled automatically by a suspension-control computer, reducing body lean in turns while improving rough-road ride quality. The first^{[citation needed]} to use this was the Citroen Xantia Activa, a medium sized sedan sold in Europe. The Activa system was an anti-roll bar that could be stiffened under the command of the suspension ECU during hard cornering. The car rolled at any time at most 2 degrees. Mercedes S-class ABC system uses another approach, the computer uses sensors to detect lateral load, lateral force, height difference in the suspension strut and uses hydraulic pressure to raise or lower the spring to counter roll. This system removes the anti-roll bar. Most active roll control systems allow a small degree of roll to give a more natural feel.

References