Shock Absorber Design


Shock Absorber Design

The lower half of a shock absorber is a twin tube steel unit filled with hydraulic oil and nitrogen gas. In some shock absorbers, the nitrogen gas is omitted. A relief valve is located in the bottom of the unit, and a circular lower mounting is attached to the lower tube. This mounting contains a rubber isolating bushing, or grommets. A piston and rod assembly is connected to the upper half of the shock absorber. The upper portion has a dust shield that surrounds the lower tube unit. The piston is precision fit in the inner cylinder of the lower unit. A piston rod guide and seal are located in the top of the lower unit. A circular upper mounting with a rubber bushing is attached to the top of the shock absorber. A nitrogen gas charge is located in the oil reservoir to prevent oil cavitation and foaming, which provides more positive action but does not provide for spring support.

Heavy-Duty Shock Absorbers

Some heavy-duty shock absorbers have a dividing piston in the lower oil chamber. The area below this piston is pressurized with nitrogen gas to 360 psi or 2482 kPa. Hydraulic oil is contained in the oil chamber above the dividing piston. The operation of the heavy-duty shock absorber is similar to that of the conventional type. The other main features of a heavy-duty shock absorber include a higher quality seal for longer life, single tube design to prevent excessive heat buildup, and a rising rate valve to provide precise spring control under all conditions.

Aftermarket spring-assisted shock absorbers are also available. These are conventional shock absorbers with a small coil spring mounted over them. The coil springs on the shock absorbers help the springs support the vehicle's weight.

Load-Leveling Shock Absorbers

Load-leveling rear struts or shock absorbers, often called air shocks, are used with an electronic height control system. An onboard air compressor pumps air into the rear shocks to raise the rear of the vehicle, and an electric solenoid releases air from the shocks to lower the rear chassis. An electromagnetic height sensor may be contained in the shock absorber, or an external sensor may be used. This sensor sends a signal to an electronic control module in relation to the rear suspension height. The module controls the air compressor and the exhaust solenoid to control air pressure in the shock absorbers. This action maintains a specific rear suspension trim height regardless of the load on the rear suspension. If a heavy package is placed in the trunk, the vehicle chassis is forced downward. However, the load-leveling shock absorbers extend to restore the original rear suspension height.

Aftermarket air shock absorbers are available. These contain an air valve connection ,and a shop air hose is used to supply the desired pressure in these air shock absorbers.

Shock Absorber Ratios

Most automotive shock absorbers are a double acting and control spring action during jounce and rebound. The piston and valves in many shock absorbers are designed to provide more extension control than compression control. An average shock absorber may have 70 percent of the total control on the extension cycle, and thus 30 percent of the total control is on the compression cycle. Shock absorbers are so designed because they must control the heavier sprung body weight on the extension cycle. The lighter unsprung axle, wheel, and tire weight are controlled by the shock absorber on the compression cycle. A shock absorber with this type of design is referred to as a 70/30 type. Shock absorber ratios vary from 50/50 to 80/20.

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