Suspension System: Type of suspension

 Suspension System:

Suspension systems

The purpose of the complete suspension system is to isolate the vehicle body from road shocks and vibrations which would otherwise be transferred to the passengers and load.
It must also keep the tires in contact with the road, regardless of road surface.
A basic suspension system consists of springs, axles, shock absorbers, arms, rods, and ball joints.
The spring is the flexible component of the suspension. Basic types are leaf springs, coil springs, and torsion bars.
Modern passenger vehicles usually use light coil springs.
Light commercial vehicles have heavier springs than passenger vehicles, and can have coil springs at the front and leaf springs at the rear.
Heavy commercial vehicles usually use leaf springs, or air suspension.
Solid, or beam, axles connect the wheels on each side of the vehicle. This means the movement of a wheel on one side of the vehicle is transferred to the wheel on the other side.
With independent suspension, the wheels can move independently of each other, which reduces body movement. This prevents the other wheel being affected by movement of the wheel on the opposite side, and this reduces body movement.
When a wheel strikes a bump, there is a reaction force, and energy is transferred to the spring which makes it oscillate. Oscillations left uncontrolled can cause loss of traction between the wheel and the road surface.
Shock absorbers dampen spring oscillations by forcing oil through small holes. The oil heats up, as it absorbs the energy of the motion. This heat is then transferred through the body of the shock absorber to the air.
When a vehicle hits an obstruction, the size of the reaction force depends on how much unsprung mass is at each wheel assembly.
Sprung mass refers to those parts of the vehicle supported on the springs. This includes the body, the frame, the engine, and associated parts.
Unsprung mass includes the wheels, tires, brake assemblies, and suspension parts not supported by the springs.
Vehicle ride and handling is improved by keeping unsprung mass as low as possible. Wheel and brake units that are small and light follow the road contours without a large effect on the rest of the vehicle.

Solid axle

The solid or beam axle provides a simple means of locating and mounting the hub and wheel units. Together with leaf springs, it forms an effective, non-independent suspension system.
Similarly, with coil springs.
It is still used in the rear suspension of many front-engined, rear-wheel-drive cars, and light commercial vehicles, and as the front suspension on many heavy commercial vehicles.
On rear-wheel-drive vehicles, with leaf springs, the axle housing is held in place by the springs, and no other form of location is needed.
The drive is transmitted through the final drive unit and axles to the wheels, and therefore the axle is referred to as a live axle.
When a vehicle accelerates from rest, the resistance of its mass causes a torque reaction, producing a tendency for the axle housing to rotate in the direction that is opposite to wheel rotation.
A similar effect occurs during braking, but with the twisting effect in the direction of wheel rotation.
In both cases, this tendency can cause leaf spring wind-up, and the twisting action can interfere with suspension motion.
It is usually controlled by mounting the axle housing closer to the fixed shackle point, so that the spring’s front section is shorter than the rear section. The short, stiff

Dead axle

On front-wheel-drive vehicles, a simple beam axle can be used on the rear, with coil spring suspension and control arms for location. This is called a dead axle, since it only supports the vehicle and doesn’t transmit any drive. It is also non-independent, as deflection of a wheel on one side of the vehicle will be transferred to the other wheel.
On some vehicles, this is reduced by using a U-shaped axle beam, with a torsion bar mounted inside it.
Trailing arms are welded to the beam, to locate the axle longitudinally.
A lateral rod prevents lateral movement when cornering, and coil springs provide for suspension. The torsion bar is connected between the left and right wheel units, and deflection of the wheel on one side causes the axle and its torsion bar to twist together.
Passenger cars no longer use beam axle front suspension, but it is still common on heavy commercial vehicles, and some 4-wheel-drives.
Trucks use an I-beam, in most cases located by leaf springs.
4-wheel-drives, with rigid axles, may use leaf springs for front and rear suspension.
Coil springs may also be used for front and rear, and as with other beam axle designs, control arms and a lateral rod must be used for location.

Independent suspension

One of the main benefits claimed for independent suspension is that unsprung mass can be kept low.
Also, if a wheel on one side hits a road irregularity, it won’t upset the wheel on the other side on the same axle.
And it allows wheel camber to be adjusted individually, when provided for by the manufacturer.
One of the simplest, and most common, independent suspension systems is the McPherson strut type. It can be used on the front and rear of the vehicle.
It consists of a spring and shock absorber unit called a strut.
The lower end of the strut is located by a ball joint, fitted to the end of the suspension control arm. Its upper end is located in a molded rubber mounting.
If the unit is on the front, the upper mounting includes a bearing to allow the complete strut to rotate with the steering.
A tension rod, or stay bar, extends from the body sub-frame, to the outer end of the control arm.
This maintains the location of the control arm during braking, and accelerating.
In this front-wheel-drive suspension, the control arm is a wishbone shape with 2 widely-spaced mounting points. This prevents backward and forward movement, so a tension rod is not needed.
Wishbones can also be used in a parallel link system. They can be used in pairs with the coil spring between the lower wishbone, and the suspension cross-member.
Alternatively, the upper link may be a wishbone, with the coil spring mounted above, combined with a single-pivot lower link, located by a tension rod.
On some vehicles, a torsion bar provides the springing medium. The torsion bar is attached at the inner fulcrum point of the wishbone, or control arm. As the suspension is deflected, it twists around its centre.
It can be fitted to the upper, or the lower link, depending on the type of vehicle. The upper link is shorter than the lower one - irrespective of the springing method used. When the suspension is deflected, the unequal lengths allow the track of the vehicle to be maintained near constant, but with some changes to camber angle.
Generally, when the car leans during cornering, the inner wheel leans outwards at the top, and the outer wheel leans inwards. This helps to maintain maximum tire contact with the road surface.

Rear independent suspension

The kind of independent suspension used on the rear of a vehicle depends in part on whether it is front-wheel-drive, or rear-wheel-drive.
If it is front-wheel-drive, it may use a McPherson strut system at the rear, similar to the front suspension system. There is normally no steering on the rear wheels, so there is no need for the bearing in the upper mounting.
On rear-wheel-drive vehicles, the suspension arrangement has to allow for the external drive shafts to transfer the drive to the wheels.
The final drive assembly is normally fixed to a cross-member, and since it must absorb the torque reaction, it must be secure. Drive shafts, either with conventional or constant velocity joints, transmit the drive to the wheels.
When conventional universal joints are used, each drive shaft may have a splined-section to accommodate changes in shaft length, due to changes in wheel camber, with suspension action. However, the drive shaft itself can be used as the upper link of the suspension, providing the pivot point. The splined-section is unnecessary, and the shaft can be made as a one-piece.
As with the front suspension, the lower link has widely-spaced pivots to provide stability, and the unequal-length links maintain the track nearly constant, although, with deflection, some camber change does occur.
In some designs, the wheel units are located at the outer ends of semi-trailing arms. The arm are attached to their cross-member pivot-points by rubber bushes, and constant-velocity joints are used at each end of the external drive shafts.

ear wheel drive independent suspension

On rear-wheel-drive vehicles with independent suspension, the final drive unit is fixed to the vehicle frame. Drive is transmitted to each wheel by external drive shafts.
Suspension is normally by coil springs, and each wheel unit is located by a combination of lateral, and longitudinal control arms, or by semi-trailing arms to the frame.
Front suspension systems are normally independent, but in addition to their suspension function, they have to allow for swiveling of the front wheels during steering. This is catered for by ball joints, or bearings, that also allow for suspension movement. The ball joints locate the wheel assemblies on lateral wishbones, or control arms.
Depending on the design, and the location of the springs, the lower arm may be a wishbone, with 2 pivot points, or a control arm with 1 pivot point.
Since the front wheels perform most of the braking, the wishbone pivots are widely spaced, to sustain the braking torque.
With a single arm, an angled bracing strut is needed. It locates the arm at its outer end and during braking; it prevents forward movement, and rearward movement. Similarly, during acceleration.
In front-wheel-drive vehicles, the driving thrust from the wheels pulls the vehicle along the road.
This additional force is generally accommodated by more substantial bracing, or by setting the pivot points on wishbones, further apart.
The rear suspension on front-wheel-drive vehicles must maintain alignment of the rear wheels with the front, and also with the frame.
On vehicles with 4-wheel steering, the rear suspension must also allow for swiveling of the rear wheels.
External forces, such as kerb impact or a collision, can damage control arms or linkages and move the wheel units from their correct position.
This can make a vehicle pull to one side, cause abnormal wear in the tires, and make the vehicle difficult to drive.

Adaptive air suspension

Adaptive air suspension is an electronically controlled air suspension system at all four wheels with a continuously adaptive damping system.
It combines sporty handling with a high level of ride comfort.
Additionally, the air suspension allows the speed-dependent lowering of the body – this change in ride height means a low center of gravity and significantly increased directional stability as a result. The vehicle’s handling characteristics are improved at the same time. Some European vehicles have air suspension struts at all four wheels.
The information obtained from sensors on the axles and acceleration sensors on the body is evaluated in the adaptive air suspension's central control unit. This computer can control the adjustment of the individual shock absorbers in milliseconds, depending on driving situation.
Provided no higher damping forces are required – for instance when driving straight ahead on good roads – the damper settings remain comfortably soft.
Specific adjustments to the damping force at individual wheels eliminates body movement, which could affect occupant comfort.
When cornering, braking or moving away, adaptive damping can, in some instances, automatically reduce rolling or pitching movements.
Adaptive air suspension also offers the advantages of:

• The vehicle's suspension height remaining constant irrespective of the load it is carrying;
• Adjustable dampening characteristics and ride height using a single process, via the manufacturers installed ‘Car Performance’ menu;
• and, generally, an ability for the driver to influence the suspension characteristics, and thus the operating dynamics, as individually preferred.

Adaptive air suspension operation

When the ignition is switched on, or when the vehicle’s door is opened before ignition, the control system is activated. The height sensor uses the induction principle to constantly monitor the distance between the vehicle’s axle and its chassis.
When the vehicle is being loaded, unloaded, or lowered due to driver command or vehicle speed, the electronic readings from the height sensor monitor the change. This is picked up by the electronic control unit and compared to the stored reference values.
The ECU either activates the electric motor of the compressor, or the exhaust solenoid valve. This also requires the solenoid valve block to be actuated, in order to maintain the required level. The corner solenoid valves are subject to stringent leakage requirements to maintain the vehicle’s height even without system operation.
When the vehicle is being loaded, the compressor delivers air into the four air suspension bellows, until the normal level has once again been reached. For additional air delivery or rapid response, the reservoir solenoid valve is opened and air flows directly from the reservoir.
When the vehicle is being unloaded, the solenoid valve block is activated. This results in airflow from the air suspension bellows being removed via the air dryer solenoid valve in the compressor, then via the relay valve. The air is then exhausted into the atmosphere.
Any dynamic air spring movement while the vehicle is in motion is ignored and does not cause the control system to respond