A suspension system that is a combination of hydraulic fluid and air has been developed in which the elastic medium is a sealed-in, fixed mass of air, and no air compressor is required. The hydraulic portion of each spring is a cylinder mounted on the body sill and fitted with a plunger that is pivotally attached to the wheel linkage to form a hydraulic strut. Each spring cylinder has a spherical air chamber attached to its outer end. The sphere is divided into two chambers by a flexible diaphragm, the upper occupied by air and the lower by hydraulic fluid that is in communication with the hydraulic cylinder through a two-way restrictor valve. This valve limits the rate of movement of the plunger in the cylinder, since fluid must be pushed into the sphere when the body descends and returned when it rises. This damping action thus controls the motion of the wheel with respect to the sprung portion of the vehicle supported by the spring. They are also called as pneumatic suspensions.
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The typical electronically controlled suspension system consists of several components in addition to the normal suspension components. Perhaps the most important component is the computer, which interprets input from various sensors that monitor such information as the automobile's height, pitch, and roll; how fast the wheels are spinning; and how quickly the automobile is turning.
The simplest electronically controlled suspension systems merely maintain a level ride height, counteracting the tendency of the weight of passengers and luggage to lower the rear end. Systems with four-wheel height adjustment lower the automobile's ride height to reduce aerodynamic drag and improve fuel economy at highway speeds. In off-road vehicles, these systems can raise the vehicle to increase ground clearance over rough terrain.
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Other systems are adjustable and allow the driver to switch manually between a soft-ride mode and a firm-handling mode. Some systems also offer intermediate choices. The most advanced systems automatically switch back and forth between soft and firm modes in milliseconds, depending on the condition of the road. These systems also work to keep ride height constant and to minimize roll. Consequently, the automobile has a better combination of ride and handling characteristics under various conditions than do cars with conventional suspension systems.
Electronically controlled suspension systems cost considerably more than conventional suspension systems. They are typically found only on relatively expensive luxury-class automobiles, high-performance sports cars, racing cars, and more expensive sport utility vehicles.
The latest innovation in the world of suspension systems is the active suspension. The system incorporates a microprocessor to vary the orifice size of the restrictor valve in a hydraulic suspension or shock absorber. This changes the effective spring rate. Control inputs may be vehicle speed, load, acceleration, lateral force, or a driver preference.
