Sway Bar
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An anti-roll bar (roll bar, anti-sway bar, sway bar, stabilizer bar) is a part of many
automobile A car or automobile is a motor vehicle with Wheel, wheels. Most definitions of ''cars'' say that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, pe ...
suspensions In chemistry, a suspension is a heterogeneous mixture of a fluid that contains solid particles sufficiently large for sedimentation. The particles may be visible to the naked eye, usually must be larger than one micrometer, and will eventually ...
that helps reduce the
body roll Body roll is the axial rotation of a Vehicle, vehicle’s Vehicle frame, body towards the outside of a turn. Body roll occurs because the Compliant mechanism, compliance in Car suspension, vehicle suspension allows the vehicle body, which sits upo ...
of a vehicle during fast cornering or over road irregularities. It connects opposite (left/right) wheels together through short
lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or ''fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is div ...
arms linked by a
torsion spring A torsion spring is a spring that works by twisting its end along its axis; that is, a flexible elastic object that stores mechanical energy when it is twisted. When it is twisted, it exerts a torque in the opposite direction, proportional ...
. A sway bar increases the suspension's
roll Roll or Rolls may refer to: Movement about the longitudinal axis * Roll angle (or roll rotation), one of the 3 angular degrees of freedom of any stiff body (for example a vehicle), describing motion about the longitudinal axis ** Roll (aviation), ...
stiffness—its resistance to roll in turns—independent of its
spring rate A spring is an elastic object that stores mechanical energy. In everyday use the term often refers to coil springs, but there are many different spring designs. Modern springs are typically manufactured from spring steel, although some non- ...
in the vertical direction. The first stabilizer bar patent was awarded to Canadian inventor Stephen Coleman of
Fredericton, New Brunswick Fredericton (; ) is the capital city of the Canadian province of New Brunswick. The city is situated in the west-central portion of the province along the Saint John River, which flows west to east as it bisects the city. The river is the do ...
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.


Purpose and operation

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. With the bar removed, a vehicle's wheels can tilt away by much larger distances, as shown in the
SUV A sport utility vehicle (SUV) is a car classification that combines elements of road-going passenger cars with features from off-road vehicles, such as raised ground clearance and four-wheel drive. There is no commonly agreed-upon definiti ...
image at right. Although there are many variations in design, a common function is to force the opposite wheel's
shock absorber A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most sh ...
, spring, or suspension rod to lower or rise to a similar level as the other wheel. In a fast turn, a vehicle tends to drop closer onto the outer wheels, and the sway bar soon forces the opposite wheel to also get closer to the vehicle. 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 each pair of wheels is cross-connected by a bar, the combined operation causes all wheels to generally offset the separate tilting of the others and the vehicle tends to remain level against the general slope of the terrain.


Principles

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 Torsion may refer to: Science * Torsion (mechanics), the twisting of an object due to an applied torque * Torsion of spacetime, the field used in Einstein–Cartan theory and ** Alternatives to general relativity * Torsion angle, in chemistry Bi ...
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: # 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. 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 (or sprung weight), in a vehicle with a suspension, such as an automobile, motorcycle, or a tank, is the portion of the vehicle's total mass that is supported by the suspension, including in most applications approximately half of the ...
of the vehicle's body produces a lateral force at the
centre of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
(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 Ride quality refers to a vehicle's effectiveness in insulating the occupants from undulations in the road surface (e.g., bumps or corrugations). A vehicle with good ride quality provides a comfort for the driver and passengers. Importance Good r ...
. One effect of body (frame) lean, for typical suspension geometry, is positive
camber Camber may refer to a variety of curvatures and angles: * Camber angle, the angle made by the wheels of a vehicle * Camber beam, an upward curvature of a joist to compensate for load deflection due in buildings * Camber thrust in bike technology * ...
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 and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Oversteer is what occurs when a car turns (steers) by more than the amount commanded by the driver. Conversely, understeer is what occu ...
or
oversteer Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Oversteer is what occurs when a car turns (steers) by more than the amount commanded by the driver. Conversely, understeer is what occu ...
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 increases the proportion of the total load transfer that the front axle reacts to—and decreases the proportion that the rear axle reacts to. 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, which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle has the opposite effect and decreases understeer.


Drawbacks

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 (a "waddling" sensation), which increase in severity with the diameter and stiffness of the sway bars. Other suspension techniques can delay or dampen this effect of the connecting bar. 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 Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Oversteer is what occurs when a car turns (steers) by more than the amount commanded by the driver. Conversely, understeer is what occu ...
.


Adjustable bars

Some anti-roll bars, particularly those intended for use in
auto racing Auto racing (also known as car racing, motor racing, or automobile racing) is a motorsport involving the racing of automobiles for competition. Auto racing has existed since the invention of the automobile. Races of various sorts were organise ...
, 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 Super GT (stylized as SUPER GT) is a grand touring car racing series that began in 1993. Launched as the , generally referred to as either the JGTC or the All Japan Grand Touring Car Championship, the series was renamed to Super GT in 2005. It is ...
. This allows the stiffness to be altered, for example by increasing or reducing the length of the lever arms on some systems, or by rotating a flat lever arm from a stiff edge-on position to a more flexible flat-side-on position on other systems. This lets a mechanic tune the roll stiffness for different situations without replacing the entire bar.


MacPherson struts

The
MacPherson strut The MacPherson strut is a type of automotive suspension system that uses the top of a telescopic damper as the upper steering pivot. It is widely used in the front suspension of modern vehicles, and is named for American automotive engineer Ear ...
is a common form of strut suspension. This was not the first attempt at strut suspension, but in MacPherson's original patent, the anti-roll bar forms an integral and essential part of the suspension, in addition to its usual function in controlling body roll. A strut suspension like MacPherson's requires a hinged lower member between the chassis and wheel hub to control the wheel position both inwards and outwards (controlling the track), and also forwards and backwards. This may be provided by a
wishbone Wishbone commonly refers to: * Furcula, a fork-shaped bone in birds and some dinosaurs Wishbone may also refer to: * Wish-Bone, an American salad dressing and condiment company * Wishbone formation, a type of offense in American football * Wish ...
with a number of joints, or by using an additional
radius rod A radius rod (also called a radius arm, torque arm, torque spring, and torsion bar) is a suspension link 300px, 5-link live axle suspension In automotive suspensions, a suspension link, control link or link is a suspension member, that attach ...
. MacPherson's design replaced the wishbone with a simpler and cheaper
track control arm The MacPherson strut is a type of automotive suspension system that uses the top of a telescopic damper as the upper steering pivot. It is widely used in the front suspension of modern vehicles, and is named for American automotive engineer Ear ...
, with a single inboard joint, to control the track. Forward and backward position was controlled through the anti-roll bar. Overall this required a simpler and cheaper set of suspension members than with wishbones, also allowing a reduction in
unsprung weight The unsprung mass (colloquially unsprung weight) of a vehicle is the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them. This contrasts with the sprung mass (or weight) supported by the ...
. 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.


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 The Mitsubishi Mirage is a range of cars produced by the Japanese manufacturer Mitsubishi from 1978 until 2003 and again since 2012. The hatchback models produced between 1978 and 2003 were classified as subcompact cars, while the sedan and stat ...
Cyborg. The "Dual Mode Suspension" equipped with the 16-v turbo model has front active anti-roll bar that has a hydraulic actuator built in the anti-roll bar link. The actuator can be operated with a switch on the dash board, changing the effectiveness of the anti-roll bar between the sport mode and touring mode. The 2018 Jeep Wrangler also has a switchable decoupler on some models, to increase wheel articulation for off road driving.


Active systems

The first car to use an active anti-roll bar in 1994 was the
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Xantia Activa, a medium-sized sedan sold in Europe. The SC.CAR (Systeme Citroën de Contrôle Actif du Roulis) system featured an anti-roll bar that could be stiffened under the command of the suspension ECU during hard cornering. The car rolled a maximum of 2 degrees. In 2001 the
BMW 7 Series (E65) The fourth generation of the BMW 7 Series consists of the BMW E65 and BMW E66 luxury cars. The E65/E66 was produced from 2001 to 2008 and is often collectively referred to as the E65. The E65 replaced the E38 7 Series and was produced with petro ...
introduced Active Roll Stabilization (ARS) "active" anti-roll bars that can be proportionally controlled automatically by a suspension-control
computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as C ...
, reducing body lean in turns while improving rough-road ride quality. In 2006
Toyota is a Japanese multinational automotive manufacturer headquartered in Toyota City, Aichi, Japan. It was founded by Kiichiro Toyoda and incorporated on . Toyota is one of the largest automobile manufacturers in the world, producing about 10 ...
introduced its
Active Stabilizer Suspension System Active Power Stabilizer Suspension System (APSSS), is an electric active suspension system with active anti-roll bars developed by Toyota Motor Corporation for its high-end vehicles including Lexus models. By altering stabilizer bar stiffness, ...
. By altering anti-roll bar stiffness, this system acts to reduce body tilt during cornering, keeping the vehicle more level during turns and improving handling, as opposed to the natural tendency of a vehicle to roll due to the lateral forces experienced during high-speed maneuvering. The active stabilizer system relies on vehicle body sensors and electric motors. The first production usage of this system was introduced in August 2005 with the Lexus GS430 sport sedan.
Porsche Cayenne The Porsche Cayenne is a series of mid-size luxury crossover sport utility vehicles manufactured by the German automaker Porsche since 2002 (Type 9PA or E1), with North American sales beginning in 2003. It is the first V8-engined vehicle built b ...
introduced Porsche Dynamic Chassis Control (PDCC), a system with active roll-bars in 2007 (on European market). In 2011, the third generation Mercedes-Benz M-Class introduced a similar system: ACTIVE CURVE SYSTEM.
Range Rover Sport The Land Rover Range Rover Sport, generally known simply as the Range Rover Sport, is a Executive car, mid-size luxury SUV produced under their Land Rover, Land Rover marque, from the British manufacturer Land Rover, and later Jaguar Land Rover. ...
introduced Dynamic Response active anti-roll bars.
Mercedes-Benz S-Class The Mercedes-Benz S-Class, formerly known as ''Sonderklasse'' (German for "special class", abbreviated as "S-Klasse"), is a series of full-sized luxury vehicle, luxury sedans, limousines and Armored car (VIP), armored sedans produced by the Germa ...
Active Body Control Active Body Control, or ABC, is the Mercedes-Benz brand name used to describe electronically controlled hydropneumatic suspension. This suspension combines a high level of ride quality with control of the vehicle body motions, and therefore vi ...
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. Toyota also uses a mechanical system called
Kinetic Dynamic Suspension System The Kinetic Dynamic Suspension System (KDSS) technology was employed initially in the Lexus GX 470, and subsequently the 200 Series Toyota Land Cruiser. The system was invented and developed by Kinetic Pty Ltd, a small R&D company based in Duns ...
(KDSS) that essentially disengages the stabilizer bars when off-road, allowing for greater vehicle articulation and ride quality.


See also

*
Panhard rod A Panhard rod (also called Panhard bar, track bar, or track rod) is a suspension link that provides lateral location of the axle. Originally invented by the Panhard automobile company of France in the early twentieth century, this device has been ...
*
Torsion bar suspension A torsion bar suspension, also known as a torsion spring suspension, is any vehicle suspension that uses a torsion bar as its main weight-bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end termi ...


References

{{Chassis control systems Canadian inventions Automotive suspension technologies Automotive safety technologies Auto parts Mechanical power control