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An anti-roll bar ( roll bar, anti-sway bar, sway bar, stabilizer bar) is an automobile suspension part that helps reduce the body roll of a vehicle during fast cornering or over road irregularities. It links opposite front or rear wheels to a torsion spring using short lever arms for anchors. This increases the suspension's body roll stiffness—its resistance to roll in turns.
The first stabilizer bar patent was awarded to Canadian inventor Stephen Coleman of Fredericton, New Brunswick on April 22, 1919.
Anti-roll bars were unusual on pre-WW2 cars due to the generally much stiffer suspension and acceptance of body roll. From the 1950s on, however, production cars were more commonly fitted with anti-roll bars, especially those vehicles with softer coil spring suspension.
In a turn, a vehicle compresses its outer wheel's suspension. The anti-roll bar forces the opposite (inner) wheel's suspension to compress as well, thereby keeping the body in a more level lateral attitude. This has the additional benefit of lowering its center of gravity during a turn, increasing its stability.
[[File:Antiroll Bar.svg|thumb|One way of estimating antiroll bar stiffness:
T=Vehicle track width (inches)
K=Fractional lever arm ratio (movement at roll bar / movement at wheel)
d=Bar diameter (inches)
R=Effective arm length (inches)
L=Half length of bar (inches)
S=Length of lever arm (inches)
Q=Stiffness (lb*in per degree)
(2025). 9781859608463, G.T. Foulis & Co. 1983. ISBN 9781859608463]]
When both front and rear anti-roll bars are fitted, their combined effect can help maintain a vehicle's tendency to roll towards the general slope of the terrain.
Each end of the bar is connected to an end link through a flexible joint. The link is connected in turn to a spot near a wheel or axle, transferring forces from the heavily loaded side of a suspension to the opposite.
Forces are therefore transferred:
The bar resists the torsion through its stiffness. The stiffness of an anti-roll bar is proportional to the stiffness of the material, the fourth power of its radius, and the inverse of the length of the lever arms (i.e., the shorter the lever arm, the stiffer the bar). Stiffness is also related to the geometry of the mounting points and the rigidity of the bar's mounting points. 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 centre of gravity (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). 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 lean, for typical unibody suspension geometry, is positive Camber angle of the wheels on the outside of the turn and negative on the inside, which reduces their cornering grip (especially with cross ply tires).
The other function of anti-roll bars is to tune the handling balance of a car. Understeer or oversteer can be reduced by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing it at the front increases the proportion of the total load transfer that the front axle reacts to—and decreases it in the rear. In general, this makes the outer front wheel run at a comparatively higher slip angle, and the outer rear wheel to run at a comparatively lower slip angle, increasing understeer. Increasing the proportion of roll stiffness at the rear axle has the opposite effect, decreasing understeer.
Excessive roll stiffness, typically achieved by configuring an anti-roll bar too aggressively, can make the inside wheels lift off the ground during hard cornering. This can be used to advantage: many front wheel drive production cars lift a rear wheel when cornering hard in order to overload the opposite wheel, limiting understeer.
As the anti-roll bar is required to control wheel position, the bars of a MacPherson strut suspension may be connected through ball joints. However many later "MacPherson strut" suspensions have reverted to using wishbones rather than the simplified track control arm of the original design.
The Mercedes-Benz S-Class Active Body Control system eliminates the anti-roll bar, instead using sensors to detect lateral load, lateral force, and height difference in the suspension strut, which then hydraulically raise or lower the spring to counter roll.
Toyota also uses a mechanical system called Kinetic Dynamic Suspension System (KDSS) that essentially disengages the stabilizer bars when off-road, allowing for greater vehicle articulation and ride quality.
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