An anti-roll bar (roll bar, anti-sway bar, sway bar, or stabilizer bar) is a part of many automobile suspensions that helps reduce body roll during fast cornering or over road irregularities. 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 Canadian inventor Stephen Coleman in 1919. Anti-roll bars were unusual on pre-war 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.
Think about what happens to a car in a sharp turn. If you are inside the car, you know that your body gets pulled toward the outside of the turn. The same thing is happening to all the parts of the car. So the part of the car on the outside of the turn gets pushed down toward the road and the part of the car on the inside of the turn rises up. In other words, the body of the car "rolls" 10 to 30 degrees toward the outside of the turn. If you take a turn fast enough, the tires on the inside of the turn actually rise off the road and the car flips over.
Roll is bad. It tends to put more weight on the outside tires and less weigh on the inside tires, reducing traction. It also messes up steering. What you would like is for the body of the car to remain flat through a turn so that the weight stays distributed evenly on all four tires.
A stabilizer bar tries to keep the car's body flat by moving force from one side of the body to another. To picture how a stabilizer bar works, imagine a metal rod that is an inch or two in diameter. If your front tires are 5 feet apart, make the rod about 4 feet long. Attach the rod to the frame of the car in front of the front tires with bushings in such a way that it can rotate. Now attach arms from the rod to the front suspension member on both sides.
When you go into a turn now, the front suspension member of the outside of the turn gets pushed upward. The arm of the sway bar gets pushed upward, and this applies torsion to the rod. The torsion them moves the arm at the other end of the rod, and this causes the suspension on the other side of the car to compress as well. The car's body tends to stay flat in the turn.
If you don't have a stabilizer bar, you tend to have a lot of trouble with body roll in a turn. If you have too much stabilizer bar, you tend to lose independence between the suspension members on both sides of the car. When one wheel hits a bump, the stabilizer bar transmits the bump to the other side of the car as well, which is not what you want. The ideal is to find a setting that reduces body roll but does not hurt the independence of the tires.
A sway bar is usually a torsion spring that resists body roll motions. It is usually constructed out of a cylindrical steel bar, formed into a "U" shape, 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 rotates about its mounting points. If the wheels move relative to each other, the bar is subjected to torsion and forced to twist. Each end of the bar is connected to an end link through a flexible joint. The sway bar end link connects in turn to a spot near a wheel or axle, transferring forces from a heavily loaded axle to the opposite side.
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. (See below, Estimating Anti-Roll Stiffness).
- from the heavily loaded axle
- to the connected end link via a bushing
- to the anti-sway (torsion) bar via a flexible joint
- to the connected end link on the opposite side of the vehicle
- to the opposite axle.
In a turn the sprung mass of the vehicle's body produces a lateral force at the center of gravity, proportional to lateral acceleration. Because the center of gravity 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). This is called the roll couple.
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. Increasing the total roll stiffness of a vehicle does not change the steady state total load (weight) transfer from the inside wheels to the outside wheels, it only reduces body lean. The total lateral load transfer is determined by the center of gravity height and track width.
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.
An anti-sway or anti-roll bar is intended to force each side of the vehicle to lower, or rise, to similar heights, to reduce the sideways tilting (roll) of the vehicle on curves, sharp corners, or large bumps. As a result, the vehicle tends to "hug" the road closer in a fast turn, where all wheels are closer to the body. After the fast turn, then the downward pressure is reduced, and the paired wheels can return to their normal height against the vehicle, kept at similar levels by the connecting sway bar.
Because an anti-roll bar connects wheels on opposite sides of the vehicle, the bar transmits the force of a bump on one wheel to the opposite wheel. On rough or broken pavement, anti-roll bars can produce jarring, side-to-side body motions.
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.
Some anti-roll bars, particularly those intended for use in auto racing, are externally adjustable while the car is in the pit whereas some systems can be adjusted in real time by the driver from inside the car, such as in Super GT.
The MacPherson strut is a common form of strut suspension. The anti-roll bar forms an integral and essential part of the suspension, in addition to its usual function in controlling body roll.
Semi-Active Anti-Roll Bars
Various methods of decoupling the anti-roll bar have been proposed. The first production car to use an active anti-roll bar was the 1988 Mitsubishi Mirage Cyborg. The 2018 Jeep Wrangler also has a switchable de-coupler on some models, to increase wheel articulation for off road work.
The first car to use an active anti-roll bar in 1994 was the Citroen Xantia Activa. That system featured an anti-roll bar that could be stiffened under the command of the suspension ECU during hard cornering.
In 2001 the BMW 7 Series introduced Active Roll Stabilization anti-roll bars that could be proportionally controlled automatically by a suspension-control computer.
The Mercedes-Benz s-Class Active Body Control system uses another approach- the computer uses sensors to detect lateral load, lateral force, and height difference in the suspension strut, then
uses hydraulic pressure to raise or lower the spring to counter roll. This system removes the anti-roll bar.