A sway bar or anti-roll bar or stabilizer bar is a part of an automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 suspension

Suspension (vehicle)

Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...

 that helps reduce the roll of a vehicle that is induced by cornering or road irregularities. It connects opposite (left/right) wheels together through short lever

Lever

In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...

 arms linked by a torsion spring

Torsion spring

A torsion spring is a spring that works by torsion or twisting; that is, a flexible elastic object that stores mechanical energy when it is twisted. The amount of force it exerts is proportional to the amount it is twisted. There are two types...

. A sway bar increases the suspension's roll

Body roll

On wheeled or tracked vehicles, body roll is a reference to the load transfer of a vehicle towards the outside of a turn. When a vehicle is fitted with a suspension package, it works to keep the wheels or tracks in contact with the road, providing grip for the driver of vehicle to control its...

 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

Fredericton, New Brunswick

Fredericton is the capital of the Canadian province of New Brunswick, by virtue of the provincial parliament which sits there. An important cultural, artistic, and educational centre for the province, Fredericton is home to two universities and cultural institutions such as the Beaverbrook Art...

 on April 22, 1919.

Principles

A sway bar is usually a torsion spring that resists body roll motions. It is usually constructed out of a U-shaped steel bar 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

Torsion (mechanics)

In solid mechanics, torsion is the twisting of an object due to an applied torque. In sections perpendicular to the torque axis, the resultant shear stress in this section is perpendicular to the radius....

 and forced to twist. Each end of the bar is connected to an end link through a flexible joint. The sway bar end link in turn connects to a spot near a wheel or axle, permitting forces to be transferred from a heavily-loaded axle to the opposite side.

Forces are therefore transferred:

1. from the heavily-loaded axle
2. to the connected end link via a bushing
3. to the anti-sway (torsion) bar via a flexible joint
4. to the connected end link on the opposite side of the vehicle
5. to the opposite axle.

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

Sprung mass

In a vehicle with a suspension, such as an automobile, motorcycle or a tank, sprung mass is the portion of the vehicle's total mass that is supported above the suspension, including in most applications approximately half of the weight of the suspension itself...

 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 (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

Ride quality

Ride quality refers to the degree of protection offered vehicle occupants from uneven elements in the road surface, or the terrain if driving off-road. A car with very good ride quality is also a comfortable car to ride in. Cars which disturb vehicle occupants with major or minor road...

.

One effect of body (frame) lean, for typical suspension geometry, is positive camber

Camber angle

thumb|100px|From the front of the car, a right wheel with a negative camber angleCamber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or...

 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. 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 CG height and track width.

The other function of anti-roll bars is to tune the handling balance of a car. Understeer

Understeer

Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Simply put, oversteer is what occurs when a car turns by more than the amount commanded by the driver...

 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. In general this will cause the outer front wheel to run at a comparatively higher slip angle, and the outer rear wheel to run at a comparatively 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 can be used to advantage: many front wheel drive production cars will lift a wheel when cornering hard, in order to overload the other wheel on the axle, limiting understeer

Understeer

Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Simply put, oversteer is what occurs when a car turns by more than the amount commanded by the driver...

.

Adjustable bars

Some anti-roll bars, particularly those intended for use in auto racing

Auto racing

Auto racing is a motorsport involving the racing of cars for competition. It is one of the world's most watched televised sports.-The beginning of 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 JGTC. This allows the 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.

Active systems

Some high-priced cars, such as the Range Rover Sport

Range Rover Sport

The Range Rover Sport is a luxury-type sport utility vehicle produced by the Jaguar Land Rover business unit of India's Tata Motors.-Chassis:...

 and BMW 7-series

BMW E65/E66

|name=BMW 7-Series E65/E66/E67/E68|manufacturer=BMW|assembly=Dingolfing, GermanyRayong, ThailandCairo, EgyptToluca, MexicoKaliningrad, Russia|production=2002–2008|image=|wheelbase= |length=...

, have begun to use "active" anti-roll bars that can be proportionally controlled automatically by a suspension-control computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

, reducing body lean in turns while improving rough-road ride quality. The first to use this was the Citroen

Citroën

Citroën is a major French automobile manufacturer, part of the PSA Peugeot Citroën group.Founded in 1919 by French industrialist André-Gustave Citroën , Citroën was the first mass-production car company outside the USA and pioneered the modern concept of creating a sales and services network that...

 Xantia Activa

Citroën Activa

The Citroën Activa and Activa 2 were two concept cars produced by the French manufacturer Citroën as a means to test and to showcase features intended for future use in their production cars...

, a medium sized sedan sold in Europe. The Activa system featured 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

Mercedes-Benz W220

The Mercedes-Benz W220 was a series of flagship sedans which constituted the Mercedes-Benz S-Class during the early-to-mid 2000's. Development for the W220 S-Class started in 1992, with the final design being approved in 1996 spanning a total of 38 months. The completed prototypes were presented in...

ABC 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. Most active roll control systems allow a small degree of roll to give a more natural feel.