Shock Absorbers

Shock absorbers are devices which are installed between the body of a motor vehicle and the wheel suspension and which perform the function of damping the objectionable spring oscillations that may be caused by irregularities of the road surface.

In modern cars the shock absorbers are almost invariably of the hydraulic type: a piston moves inside a cylinder and forces oil through narrow holes or valves. In this way a high resistance to the movement of the piston is developed, so that it is greatly retarded.

In the older types of shock absorber the piston travels to and fro in an oil-filled casing. The movements caused by the road irregularities are transmitted to the piston through a lever system connected to the axle of the vehicle.

When the lever moves upward, the piston goes to the left and forces oil through a passage and a valve to the other side of the piston. This upward movement must be only slightly damped, in order not to impair the spring action.

A valve with a large orifice is accordingly used. This allows the oil to escape quickly, so that only a small damping effect is obtained. On the other hand, the return motion of the springing must be powerfully damped; for this reason a much narrower orifice is used for the other valve, which opens when the piston travels to the right and the lever moves downward. This shock absorber is therefore double-acting, but with different intensities of damping in the two directions.

The type of shock absorber most frequently used nowadays is the telescopic shock absorber. Its operating principle is the same as that of the lever type. It comprises two tubes, one fitting inside the other, the piston rod being connected to the outer tube; the piston moves in the oil-filled inner chamber of the inner tube. The piston contains so-called flap valves which alternately allow oil to pass in one direction only. They constitute the throttle valves that produce the damping action.

When the piston travels to the right (compression of the springs), the oil is forced through a flap valve or through holes into the left-hand chamber; when the piston moves back to the left (decompression of the springs), the oil flows through another valve back into the right-hand chamber. Since the left-hand chamber is smaller than the right-hand chamber (due to the presence of the piston rod in the former), some of the oil must, when compression of the springs occurs, flow through the bottom valve into the storage space.

The bottom valve is so designed that it allows oil to pass in both directions, except that the flow resistance it presents when the springs are compressed is much higher because in that case the oil is not discharged through the orifice of the valve but is forced through the narrow gaps of the plates.

On decompression of the springs, the oil which was forced into the storage space flows back into the inner chamber through the relatively large valve orifice, i.e., without encountering much resistance.