Vibrating representation of a car
Although wheeled transport was
probably introduced around 3500 B.C., the vehicles were mostly load carriers
which did not require a high comfort. But when people wanted to reduce the
shaking, they thought in terms of smoothing the road surface rather than
developping suspensions.
About the middle of the 16th
century, people began to work on the insulation of the bodywork from the road
wheels. The first known system involved a chassis that looked like an upturned
table, with the carriage body slung by leather straps from the tops of the legs.
This was not a springing system as it only allowed the body and the wheels to
move seperately. This formed the basis for springing being called the
suspension.
The first
springing development was the replacement in 1665 of the table legs by big
C-shaped spring, a form of insulation that can still be seen on children's
prams. But this system had disadvantages in allowing forward and backward sway
an giving a high centre of gravity and a high roll axis. They also used to
spring only the body, leaving the heavy chassis unsprung.
By the time that
powered vehicles were developing, the springs had changed to an elliptical shape and were
bolted between the axle and the chassis. The primary function of suspension at
this time was to allow the four wheels to keep contact at all time with an
uneven road surface and any increase in the comfort of the passengers was a
bonus.
The need of a compromise between a suspension system keeping the wheels on the ground, and one that provides maximum insulation from the road shock for the passengers has always been a problem and is still unsolved today. The strength, or rate, of the springs between the wheels and the body cannot instantly altered according to the kind of bump the wheel is about to meet.
When a wheel hits a bump,
it moves upward from the surface and gain momentum in that
direction. If the wheel is unsprung, all the energy is transfered to the
body which moves in the same way. And the gravity which is the only force
acting this movement will eventually bring it back sharply to the ground.
By putting a spring
between the wheel and the chassis, much of the energy of the moving
wheel is transferred to spring instead of being transferred to the chassis
which therefore moves upwards through a smaller distance than the wheel. The
lighter the wheel and the other pieces connected to it, the smaller is the total
energy involved because this amount of energy depends linearly of the mass as
shown in its expression Ek=1/2mv2
A low rate for spring permit a
large wheel movement and so a low chassis movement but it must be strong enough
to bear the chassis properly. So although soft springs should give the greatest
comfort, they cannot do much good if the chassis squashes them totally. On the
other hand, a stiff spring that does not deflect much under the chassis does not
perform its duty and gives a hard ride.
The problem with springs is that they store the energy without dissipating it and so tends to release it by moving the wheel downwards hence a vibration of the spring of its natural frequency. It is possible for the vertical accelerations of the wheel to coincide with the natural frequency of the springs and in this condition, the movement can become large enough to lift the wheel off the road. To fight against this phenomenon, engineers designed a damping device known as shock absorber. Its function is to dissipate, generally by friction, the energy stored in the springs by providing a damping force proportional to the relative velocity between the two moving systems.
But the suspension is not only
made of springs and dampers, the tyres and wheels can also be considered as
springs as they absorb the small irregularities from the road surface.
Globally and from a vibrating,
physical point of view, the whole suspension system can be represented as follow
:
Vibrating representation of a car