Dampers: Shock Absorbers
Unless a dampening structure is present, a car spring will extend and
release the energy it absorbs from a bump at an uncontrolled rate. The
spring will continue to bounce at its natural frequency until all of
the energy originally put into it is used up. A suspension built on
springs alone would make for an extremely bouncy ride and, depending on
the terrain, an uncontrollable car.
Enter the shock absorber, or snubber, a device that controls unwanted
spring motion through a process known as dampening. Shock absorbers slow
down and reduce the magnitude of vibratory motions by turning the
kinetic energy of suspension movement into heat energy that can be
dissipated through hydraulic fluid. To understand how this works, it's
best to look inside a shock absorber to see its structure and function.
A shock absorber is basically an oil pump placed between the frame of
the car and the wheels. The upper mount of the shock connects to the
frame (i.e., the sprung weight), while the lower mount connects to the
axle, near the wheel (i.e., the unsprung weight). In a twin-tube design,
one of the most common types of shock absorbers, the upper mount is
connected to a piston rod, which in turn is connected to a piston, which
in turn sits in a tube filled with hydraulic fluid. The inner tube is
known as the pressure tube, and the outer tube is known as the reserve
tube. The reserve tube stores excess hydraulic fluid.
When the car wheel encounters a bump in the road and causes the spring
to coil and uncoil, the energy of the spring is transferred to the shock
absorber through the upper mount, down through the piston rod and into
the piston. Orifices perforate the piston and allow fluid to leak
through as the piston moves up and down in the pressure tube. Because
the orifices are relatively tiny, only a small amount of fluid, under
great pressure, passes through. This slows down the piston, which in
turn slows down the spring.
Shock absorbers work in two cycles -- the compression cycle and the
extension cycle. The compression cycle occurs as the piston moves
downward, compressing the hydraulic fluid in the chamber below the
piston. The extension cycle occurs as the piston moves toward the top of
the pressure tube, compressing the fluid in the chamber above the
piston. A typical car or light truck will have more resistance during
its extension cycle than its compression cycle. With that in mind, the
compression cycle controls the motion of the
vehicle's unsprung weight,
while extension controls the heavier, sprung weight.
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