Conventional shock absorbers can be combined with air suspension springs – another way to achieve ride height control and self-leveling suspensions. In electrorheological fluid dampers, an electric field changes the viscosity of the oil. This principle makes semi-active dampers applicable in automotive and various industries.
Magnetic field variation: Magnetorheological dampers change the properties of their fluid by means of electromagnets. Usually a compressible gas is used as the working fluid or it is mounted with rubber bushings to limit the effect of the shock absorber at high (sound) frequencies.
Hysteresis of structural materials, such as the compression of rubber discs, the stretching of rubber bands and ropes, the bending of steel springs or the twisting of torsion bars. Hysteresis is when originally elastic materials rebound with a force less than that required to deform them. Simple vehicles without independent shock absorbers are damped to some extent by the hysteresis of the springs and frame.
Dry friction is used for wheel brakes by using a disc (usually made of leather) at the pivot of the lever, with friction applied by the spring. Used in early cars, such as the Ford Model T, up to some British cars of the 1940s and the French Citroën 2CV of the 1950s. Although now considered obsolete, the system has the advantage of being mechanically simple; the degree of damping can be easily adjusted by tightening or loosening the screws that clamp the discs, and it can be easily rebuilt with simple hand tools. The disadvantage is that the damping force tends not to increase with the speed of vertical motion.
Solid cone chain shock absorbers, using one or more conical, axially arranged pellet balls, usually made of a metal such as Nitinol, placed in a housing. Fluid friction, such as the flow of a fluid through a narrow orifice (hydraulic), makes up the vast majority of automotive shock absorbers. This design first appeared on the Mors racing car in 1902. An advantage of this type is that the shock absorber can be made relatively soft for compression (allowing a soft response to a bump) and relatively stiff for extension, by using special internal valves, thereby controlling “rebound”, the vehicle’s response to the energy stored in the springs; similarly, a series of valves controlled by the springs can vary the stiffness depending on the speed of the bump or rebound. Shock absorbers specifically designed for racing allow the front end of a car to rise with minimal resistance under acceleration, then strongly resist letting it settle, thus maintaining ideal rear weight distribution for enhanced traction.
Gas compression, such as in pneumatic shock absorbers, acts like a spring to resist forces acting on the shock absorber as air pressure builds up. Enclosed gas is compressible, so the device is less susceptible to damage from impacts. This concept was first used in production Citroën cars in 1954. Today, many shock absorbers are pressurized with compressed nitrogen to reduce the tendency of the oil to cavitate under frequent use. This causes blistering, which temporarily reduces the damping capacity of the unit. In very heavy-duty units for racing or off-road use, there may even be an auxiliary cylinder attached to the shock absorber that acts as a reservoir for both the oil and pressurized gas. In aircraft landing gear, air shock absorbers can be combined with hydraulic damping to reduce bouncing. Such struts are called oleo-struts (combined oil and air).
The shock absorbers of the Citroën 2CV resist inertial acceleration, dampening wheel bouncing without external moving parts. These shock absorbers consist of a spring-mounted 3.5 kg (7.75 lb) iron block inside a vertical cylinder[4], similar to the tuned mass dampers used on high-rise buildings, but much smaller.
Hybrid hydro-pneumatic suspension combines many suspension elements in one device: spring action, shock absorption, ride height control and self-leveling suspension. This combines the advantages of the compressibility of gas with the force multiplication capability of hydraulic mechanics.
