Shock Absorber Operation


Shock Absorber Operation

A shock absorber is bolted between the axle and chassis. The mounting bolts pass through isolating bushings on each end of the shock absorber. The isolating bushings prevent vibration and noise.

Front shock absorbers are usually mounted between the lower control arms and the chassis. When a vehicle wheel strikes a bump, the wheel and suspension move upward in relation to the chassis. Upward wheel movement is referred to as jounce travel. This jounce action causes the spring to deflect or compress. Under this condition, the spring stores energy and springs back downward with all the energy absorbed when it deflected upward. This downward spring and wheel action is called rebound travel. If this spring action were not controlled, the wheel would strike the road with a strong downward force, and the wheel jounce would occur again. Therefore, some device must be installed to control the spring action, or the wheel would bounce up and down many times after it hit a bump, causing passenger discomfort, directional instability, and suspension component wear.

Shock absorbers are installed on suspension systems to control spring action. When a wheel strikes a bump and jounce travel occurs, the shock absorber lower tube unit is forced upward. This action forces the piston downward in the lower tube unit. Since oil cannot leak past the piston, the oil in the lower unit is forced through the piston valves to the upper oil chamber. These valves provide precise oil flow control and control the upward action of the wheel and suspension, which is referred to as a shock absorber compression stroke.

When the spring expands downward in rebound travel, the lower shock absorber unit is also face3d downward. When this occurs, the piston moves upward in the lower tube unit, and hydraulic oil is forced through the piston valves from the upper oil chamber to the lower oil chamber. Since the valves restrict oil flow with precise control, the downward suspension and wheel movement is controlled.

When the shock absorber's piston moves, oil is forced through it. Since the piston's valves and orifices resist the flow of oil, friction and heat are created. The resistance of the oil moving through the piston must be calibrated as close as possible to the spring's deflection rate. Suspension systems deflect as different speeds depending on the type and size of bump and the vehicle's speed. The resistance of a shock absorber increases with the square of its speed. If a wheel strikes a large bump at high speed, the wheel deflection and rebound can be effectively locked by the shock absorber. Shock absorber engineers prevent this action by designing shock absorber valves and orifices to provide enough friction to prevent the spring from overextending on the rebound stroke. The valves and orifices must not create excessive friction, which would slow the wheel from returning to its original position.

Shock absorber pistons have many different valves and orifices. In some, small orifices control the oil flow during slow wheel and suspension movements. Stacked steel valves control the oil flow during medium speed wheel and suspension movements. During maximum wheel and suspension movements, larger orifices provide oil flow control. In other pistons, the stacked steel valves provide oil flow control. Regardless of the design of the orifice and valve design, a shock absorber must be precisely matched to absorb the spring's energy.

During fast upward wheel movement on the compression stroke, excessive pressure in the lower oil chamber forces the base valve open and allows oil to flow through the valve to the reservoir. Nitrogen gas provides a compensating space for the oil. Since the gas exerts pressure on the oil, cavitation - of foaming - of the oil is eliminated. When oil bubbles are eliminated, the shock absorber can provide continuous damping for wheel deflections as small as 0.078 in. (2.0 mm). A rebound rubber is located on top of the piston. If the wheel drops far, the shock absorber can become fully extended, and the rebound rubber cushions the action.

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