Shock absorbers have been in use since the early days of the automobile. It
would stand to reason that something that has been around for so long is
pretty well understood by the general driving public, right? Actually, this is
far from the truth, as even automotive enthusiasts will neglect and often
ignore the shock absorbers on their car, while degrading its handling
performance and ride comfort.
Even worse, worn shocks can be dangerous, especially during the severe
maneuvers that might be necessary to avoid an accident. As we will see, for
such a simple yet effective device, shock absorbers seem to get little respect.
The Gravity of the Situation
Strangely enough, despite their name, shock absorbers don't absorb shocks. In
reality, that is the job of the springs in a vehicle's suspension system. As a
wheel encounters a bump, the wheel moves upward, compressing and storing the
energy of the bump into the spring. This compression is actually what absorbs
the shock of the bump.
But now that the spring is compressed, it contains potential energy that must
be released. The spring does this by bouncing back to its original
uncompressed length, at the same time pushing the vehicle's body upward. In an
example of the old adage "what goes up, must come down," gravity pulls the
weight of the body back down, recompressing the spring. If the shock absorbers
are worn, the vehicle ends up bouncing its way down the road after every bump
until all of the energy is used up. In the worst cases, this bouncing can
actually pull a vehicle's tires off the ground, making the vehicle
uncontrollable.
Bring on the Dampers
Shock absorbers, more properly called dampers, are mounted alongside (or
inside) the springs at each corner of the vehicle. A shock absorber's job is
to provide resistance to the movement of the spring. Technically speaking, it
does this by taking some of the energy that is being used to compress the
spring and turning it into heat. So whether the vehicle is bouncing up or down
from a bump in the road, that motion is held in check by the shock absorber,
and once again some of the kinetic energy released by the spring is changed
into heat by the shock absorber. This conversion of energy keeps the vehicle's
body from bouncing more than once or twice, providing a controlled ride and
helping to keep the vehicle's tires safely in contact with the ground.
But How?
If you have ever waved your hand back and forth through water, then, in
principle, you know how a shock absorber works. The resistance to motion you
feel with your hand changes with speed – the faster you move your hand, the
more energy it takes to push against the resistance of the water.
A shock absorber works much the same way. Inside the shock absorber there's a
piston that moves inside a tube which is filled with oil. As the piston moves,
the oil is forced through tiny holes and valves within the piston, precisely
controlling the amount of resistance to movement. This resistance to the
motion converts the energy into heat. (Yes, a shock absorber that has been
doing its job over a rough road does get warm!)
What Types?
Although there have been many different designs for shock absorbers over the
course of automotive history, today there are four basic types available:
Twin-tube
Mono-tube
Gas-filled mono-tube
External reservoir
Sometimes these types of shock absorbers are incorporated into a strut-type
suspension that uses the shock absorber as part of the spring support, but the
basic principles still apply.
The twin-tube design is the most common and often times the least expensive.
These are the ordinary shock absorbers that typical passenger cars and trucks
use, and they are widely available at car dealerships, auto parts stores and
repair facilities. As the name implies, in a twin-tube shock, there are two
actual tubes – one the outer shock body and the other a cylinder inside in
which the piston moves. Tiny holes or orifices in the piston as well as
special valves between the inner and outer tubes restrict the flow of oil to
control wheel motion.
The limitations of twin-tube shocks become apparent when they are used over
very bumpy roads. In this case, the rapid motion of the piston can cause the
oil to overheat and to foam, reducing the shock absorber's ability to control
wheel motion. The result is a ride that becomes increasingly sloppy,
especially when traveling over a washboard surface. Heavy-duty twin-tube shock
absorbers are usually stronger, with more robust piston shafts and mounting
points, and they may use oils that are more resistant to foaming, but
ultimately they still have the same limitations.
Mono-Tube and Gas-Filled Shocks
The heat generated in the twin-tube shock tends to get trapped within the
walls of the shock absorber body, reducing its effectiveness in controlling
wheel motions. The mono-tube shock absorber uses a piston traveling within a
single tube that is exposed more directly to the air. The general advantages
and disadvantages of this design are:
By getting rid of heat more easily, mono-tube shocks are less susceptible to
overheating on rough roads.
They are more expensive to manufacture however, and the tolerances must be
higher and the seals better to keep the oil inside the shock body.
To prevent foaming and bubbles in the oil, which degrades shock-absorber
performance, a gas-filled mono-tube shock has a chamber of high-pressure
nitrogen above the oil chamber. This high-pressure gas makes it difficult for
bubbles to form in the oil, even when the shock absorber moves in and out very
quickly, as it might while traveling rapidly on a very rough or washboard road.
Gas-filled shocks are expensive, since they require strict manufacturing
tolerances, but they are very resistant to fade and consequently are popular
in off-road racing and rallying. Gas-filled shocks, by the way, are not the
same as "air shocks" which use an air chamber separate from the shock oil. An
air shock is actually an air spring that raises or lowers the vehicle when air
is added or removed through a valve.
