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. Karl Benzs Velo model (1894) - entered into the first automobile race An automobile (or motor car) is a wheeled passenger vehicle that carries its own motor. ... The front suspension components of a Ford Model T. Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. ... The principle of the lever tells us that the above is in static equilibrium, with all forces balancing, if F1D1 = F2D2. ... A torsion spring or torsion pendulum is an elastic material that reacts against torsion (twisting motion). ... Hookes law accurately models the physical properties of common mechanical springs at small extensions. ...

Principles

In a turn, the sprung mass of the vehicle's body rotates around its roll axis. The roll axis is a line that joins the front and rear roll centers (SAEJ670e). If the vertical distance between the roll axis and the center of gravity is not zero, a torque (roll moment) equal to the centrifugal force times the distance between the center of gravity and the roll axis will be exerted on the sprung mass, causing the body to lean towards the outside of the turn. This force is called the roll couple. 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 radial tires). In a vehicle with a suspension, such as an automobile or a tank, sprung mass (or sprung weight) is the portion of the vehicles total mass that is supported by the suspension. ... This article or section may contain original research or unverified claims. ... Torque applied via an adjustable end wrench Relationship between force, torque, and momentum vectors in a rotating system In physics, torque can informally be thought of as rotational force or angular force which causes a change in rotational motion. ... In a vehicle suspension, roll moment is the moment of inertia of the vehicles sprung mass (the portion of its weight supported by the suspension). ... Centrifugal force (from Latin centrum center and fugere to flee) is a term which may refer to two different forces which are related to rotation. ... Wheel alignment Wheel alignment is a (mostly) computerized procedure routinely done in most cross-specializing mechanics shop, and in some tire shops. ...

Roll couple is resisted by the suspension's 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 body lean 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 the degree of protection offered vehicle occupants from uneven elements in the road surface, or the terrain if driving off-road. ...

The spring rate of an anti-roll bar is based on the fourth power of the torsion bar's diameter, 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. Auto racing (also known as automobile racing, autosport or motorsport) is a sport involving racing automobiles. ...

Increasing the roll stiffness of the suspension increases the rate of weight (load) transfer to the wheels on the outside of the turn. The load transfer at the other end of the vehicle is equally decreased. As more load is applied to the outer wheels, their slip angles tend to increase, and become unlimited as the limit of adhesion is reached. If the front and rear weight transfer is unequal, the slip angles of the end with the greater weight transfer will tend to be larger, resulting in understeer or oversteer. The use of anti-roll bars allows the weight transfer of the front and rear wheels to be adjusted separately, compensating for other factors such as unequal front/rear weight balance and "tuning" the vehicle's handling characteristics. In car handling, slip angle is the angle between a wheels actual direction of travel and the direction towards which it is pointing. ... Understeer is a term for a car handling condition during cornering in which the circular path of the vehicles motion is of a markedly greater diameter than the circle indicated by the direction its wheels are pointed. ... Oversteer is the phenomenon ocurred in an automobile when the rear-end of it doesn´t follow the trajectory of the front-end tending to overtake this one causing the car to spin. ... Car handling and vehicle handling is a description of the way wheeled vehicles perform transverse to their direction of motion, particularly during cornering and swerving. ...

Reducing body lean has other benefits such as reducing the sideways of force on the driver, passengers and cargo, allowing the car to respond more quickly to changes in steering and reducing the chance of exceeding the limits of suspension travel.

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 providing limit understeer. Understeer is a term for a car handling condition during cornering in which the circular path of the vehicles motion is of a markedly greater diameter than the circle indicated by the direction its wheels are pointed. ...

Some high-priced cars, such as the Mercedes S-class and BMW 7-series, have begun to use "active" anti-roll bars that can be connected or disconnected automatically by a suspension-control computer, reducing body lean in turns while improving rough-road ride quality. The Mercedes-Benz W220 sedans (and similar C215 coupes) have been produced since 1999. ... E66 BMW 745iL The BMW E65/E66 automobile platform is the basis for the 2002 through present 7-series of automobiles. ... A Lego RCX Computer is an example of an embedded computer used to control mechanical devices. ...

See also

• Torsion beam suspension

Torsion beam suspension, also known as a torsion bar or torsion spring suspension, is a vehicle suspension system. ...

External links

• How Stuff Works: How do stabilizer bars work?
• Sway bar removal
• Suspension tuning via sway bars