Lesson 3/Learning Event 1
The cam moves to the right when the vehicle spring is compressed. The piston spring
forces the piston to follow the cam, opening the intake valve and allowing oil to flow into
the cylinder. This valve has a large port opening, so it offers little resistance to the flow
of oil. Therefore, it has little effect upon the action of the piston when the spring is
When the vehicle spring rebounds, the cam is moved in the reverse direction. This
forces the piston to the left against the oil in the cylinder, closing the intake valve. The
motion of the piston forces the oil from the cylinder through a small opening in the relief
valve. Liquids are not compressible under ordinary pressures, and it takes a certain
amount of time for oil to flow through the small relief valve opening. Therefore, the
rebound of the vehicle spring is slowed down. The flow of oil and the vehicle spring
rebound are regulated by the size of the valve opening. In addition, some regulation is
also obtained by the valve spring pressure holding the relief valve against its seat. The
instant the vehicle spring stops its rebound, the relief valve is closed by its valve spring.
The operating principle of the double-acting, cam-operated shock absorber is the same as
the single-acting shock absorber except that it checks spring action in both directions. A
cam-operated, double-acting shock absorber has two pistons. One regulates vehicle spring
rebound, and the other regulates spring compression.
The shock absorber most widely used is the direct-acting shock absorber. These shocks
are often referred to as airplane-type shocks. The direct-acting shock is mounted directly
to both the vehicle frame and the axle or suspension arm. As the frame rises and falls in
relation to the axle, the shock absorber must telescope out and in. The shock's resistance
to telescopic movement dampens (hinders or slows down) this up-and-down movement.
The shock may have eyes made on its ends for mounting, or it may have threaded studs.
Rubber bushings are used inside the eyes and rubber grommets on the studs to prevent
metal-to-metal contact and to provide flexible mounting.
The direct-acting shock absorber consists of an inner cylinder, an outer cylinder, a piston,
a piston rod, and, in most cases, an outer dust-and-rock shield. A series of valves in the
piston and at the bottom of the inner cylinder control the movement of oil within the
shock. A reservoir which contains a supply of oil surrounds the inner cylinder.