Suspension System: Principles of suspension

 Suspension System:

 Principles of suspension

The suspension system isolates the body from road shocks and vibrations which would otherwise be transferred to the passengers and load.
It also must keep the tires in contact with the road. When a tire hits an obstruction, there is a reaction force. The size of this reaction force depends on the unsprung mass at each wheel assembly.
The sprung mass is that part of the vehicle supported by the springs - such as the body, the frame, the engine, and associated parts.

Unsprung mass includes the components that follow the road contours, such as wheels, tires, brake assemblies, and any part of the steering and suspension not supported by the springs.
Vehicle ride and handling can be improved by keeping unsprung mass as low as possible. When large and heavy wheel assemblies encounter a bump or pothole, they experience a larger reaction force, sometimes large enough to make the tire lose contact with the road surface.
Wheel and brake units that are small, and light, follow road contours without a large effect on the rest of the vehicle.
At the same time, a suspension system must be strong enough to withstand loads imposed by vehicle mass during cornering, accelerating, braking, and uneven road surfaces.

Suspension force

Applying a force to this object deforms it. Removing the force lets it return to its original shape. This characteristic is called elasticity.
Automotive suspension systems generally use the elastic properties of metals, to provide the springing medium.
The springs are located between the frame and the axle assemblies, and are shaped to suit the application.
Leaf springs are normally semi-elliptical, and they absorb the applied force by flattening out under load.
Coil springs are formed in a spiral from a single steel rod, and absorb the force of impact by twisting.
Torsion bars are held rigid at one end, and twist around their centre as the suspension arm is deflected.
They all return to their original shape when the deflecting force is removed.
Non-metallic materials, like rubber, can provide the main springing action, but are more commonly used as stops, to limit extreme suspension movement.
The stops can also be shaped to provide an auxiliary springing function.
In light vehicle applications, air is normally used only for ride-height control.
When a vehicle strikes an uneven surface, the springs are deformed from their original shape. They return to their original position, but tend to overshoot, and set up oscillations.
This makes the vehicle bounce up and down, which makes the ride uncomfortable.
It can produce forces that make the tires bounce. And a bouncing tire won’t grip the road surface well.
Shock absorbers have a marked effect on how well tires follow a road surface. They damp the natural bounce over the road, and reduce spring oscillations.
There are different kinds of shock absorbers, but they all use a piston sliding in a cylinder filled with oil.
The dampening action occurs as a result of the piston movement, forcing the oil through valves in the piston, and at the foot of the shock absorber, which restrict oil flow.
The oil heats up from this continuous movement as the energy of motion of the suspension is transformed into heat. This heat is transferred through the body of the shock absorber to the outside air.

Unsprung weight

Most of a vehicle’s weight is supported by its suspension system. It suspends the body and associated parts so that they are insulated from road shocks and vibrations that would otherwise be transmitted to the passengers and the vehicle itself.
However, other parts of a vehicle are not supported by the suspension system, such as the wheels, tires, brakes and steering and suspension parts not supported by springs. These parts are all called unsprung weight. Generally, unsprung weight should be kept as low as possible.

Wheel unit location

When a vehicle is in motion, several forces operate to displace the wheel units - driving thrust, braking torque, and cornering force. These forces must be transferred to the frame of the vehicle, but while they act, the wheel units must stay aligned with each other, and with the frame.
They must be located longitudinally, and laterally, while still having the freedom to move vertically, to allow for suspension travel.
On vehicles with non-independent suspension, leaf springs provide a simple means of performing these functions.
On a rear-wheel-drive vehicle, the axle housing is located on the spring by the spring centre bolt, and clamped to the spring by U-bolts. Since the front eye of the main leaf is located on the frame at the fixed shackle point, the spring then prevents the axle moving longitudinally. A swinging shackle at the rear allows the semi-elliptical spring to flatten out under load, which provides for vertical movement. As well, the driving forces can be transferred through the spring to the frame.
Driving thrust is transferred from the tire contact patch, through the axle housing and front half of the spring, to the fixed shackle point. This pushes the vehicle along the road.
During braking, the torque tries to twist the axle housing around its centre, but it’s held firmly, clamped to the spring, and any twisting effect is resisted by the relatively stiff spring.
During cornering, lateral movement of the axle housing is prevented by its location on the spring centre bolts, and by the spring shackles.
Leaf springs perform these functions well, but the transfer of forces interferes with the suspension function, and ride quality is compromised.
On an axle of this type, the use of coil springs allows the spring to carry out the function of suspension only.
The axle housing is usually located longitudinally, by upper and lower control arms, and laterally, either by the angle of the control arms, or by a separate Panhard rod, or Watts linkage.
The control arms have flexible bushes at each connecting point, to a

Dampening

Different materials have different levels of elasticity. Up to a certain point, they can be deformed and released, and they will try to return to their original condition.
Beyond that point they stay deformed.
With some materials, if it returns to its original state too quickly, it can produce a bouncing effect called an oscillation.
Preventing or reducing this oscillation is called dampening. It can occur in many different ways. The dampening material absorbs the energy from the oscillation.
In vehicle suspension, a shock absorber reduces oscillation in the spring.