Suspension System: Shock absorber types

 Suspension System:

Hydraulic shock absorbers

The most widely-used hydraulic shock absorber is the direct-acting telescopic type. It can be fitted as a self-contained unit, or combined with a suspension strut. The strut type uses the same principle of operation but it is considerably larger.
The hydraulic shock absorber provides its dampening action by transferring oil, under pressure, through valves which restrict the oil flow.
The twin-tube type is the most common. The outer tube is normally attached to the suspension member at its base, and the inner tube provides a working cylinder for a piston which is attached to a piston rod. The piston rod is connected to the frame at its outer end, and a bearing at the top of the outer tube keeps the rod in alignment as it moves in and out of the shock absorber, with suspension action.
A seal above the bearing prevents oil leakage, and keeps out dirt and moisture. A shroud protects the rod from damage.
During bumps, or compression, the rod and its piston move into the shock absorber. In rebound, or extension, the rod and piston move out of the shock absorber.
For dampening to be effective, resistance is needed in both directions. This is provided by the oil, and by disc valves attached to the piston and the base of the inner tube. Oil fills the inner tube and surrounds its outer surface to a level which allows a free space or reservoir to exist above it, between the inner and outer tubes.
On bump, or compression, the piston and rod move downwards in the cylinder, resulting in a small pressure drop in the chamber labeled A, above the piston. At the same time, the volume of the chamber labeled B, below the piston, is reduced, causing a high fluid pressure. This unseats the piston intake valve, and fluid flows up through the outer passages in the piston, and into chamber A. But the piston rod is also now entering A, and displacing a quantity of fluid equal to its volume, so, all of the oil in B cannot flow into A, The displaced fluid is forced down through a base valve, and out into the reservoir, labeled C.
In the rebound, or extension phase, the piston and rod move upwards and the volume of chamber A is reduced. Chamber A becomes a high-pressure area, and fluid flows through the extension valve in the piston, into chamber B. However the withdrawal of the piston from B greatly increases its volume, and fluid flow from a is insufficient to fill the space. Pressure in B falls below that of the reservoir, causing the base intake valve to be unseated. Fluid flows from the reservoir into chamber B, keeping the inner tube full.
The valves provide control over the amount of force required to pass fluid through them at any given piston velocity. They can be made to open in stages, according to fluid pressure. This allows light resistance to motion, when the piston moves slowly, and heavy resistance when piston velocity is high.
The rapid movement of the piston, continually forcing the oil backwards and forwards through the valves, causes it to heat up, as it absorbs the energy of motion of the spring, and converts it into heat. The heat is transferred through the outer tube to the outside air.
However, the hotter the oil becomes, the greater its tendency to aerate. Aeration occurs because of the high velocity of the oil as it passes through the small passages in the valves. If the velocity is high enough, air dissolved in the oil, comes out of solution as small bubbles, and forms a foam.
Aerated oil has a certain amount of compressibility, so it is unable to provide the dampening force previously achieved in the non-aerated condition. The performance of the shock absorber is thus considerably reduced. This effect is called shock absorber “Dissolve”.

Gas-pressurized shock absorbers

In a hydraulic shock absorber, the oil heats up as the energy of motion of the suspension is dampened. The rapid piston movement as the vehicle moves over the road causes the hydraulic fluid to aerate. This reduces the dampening effect, and the shock absorber’s performance very quickly deteriorates. This condition is called shock absorber Dissolve. It can be reduced substantially by pressurising the fluid with gas, usually nitrogen.
In this mono-tube design, fluid fills the chambers above and below the piston. As the piston moves in the cylinder, valves control the movement of oil from one chamber to the other.
Pressure on the oil is provided by nitrogen gas at the base of the cylinder, acting on a free-floating separation piston which separates the gas from the oil.
On bump, the piston moves downwards, and the penetration of the piston rod displaces a quantity of oil equal to its volume. The separation piston is displaced accordingly, and gas pressure increases.
On rebound, the piston and rod move upwards, and gas pressure reduces as the separation piston follows the movement.
Pressure on the oil is maintained, even when the piston and rod are at the top of their stroke.

Load-adjustable shock absorbers

When vehicles carry heavy loads, their suspension is compressed, causing the rear of the vehicle to be lower than normal.
As a result, steering becomes lighter, the alignment of the headlights becomes too high, and the compression length of travel of the suspension over bumps is reduced, causing discomfort to passengers.
To reduce these effects, a manually adjustable air spring can be incorporated into each rear shock absorber. The air spring consists of a flexible rubber cylinder which seals the outside of the upper and lower halves of the shock absorber.
The shock absorber is a standard hydraulic type, providing normal dampening action, but when a heavy load is placed on the rear of the vehicle, the rubber air cylinder can be pressurised to assist the suspension springs.
By changing the air pressure in the cylinder, the ride height can be adjusted, as well as the stiffness of the suspension.
Compressed air in the pneumatic cylinder can absorb smaller road shocks, and provide better ride characteristics than just stiff springs alone.
The rubber air cylinder is connected to a filling valve by a flexible plastic hose. Air from a tire pump or a hand unit forces more air into the rubber cylinder, allowing the suspension to support more weight.
The maximum air pressure setting must not be exceeded as this can damage the shock absorber and its mounting points on the vehicle frame.
When the load is removed, the extra air is released through a filling valve, which allows the suspension to return to its original settings.
A minimum air pressure must be maintained in the cylinder to prevent tearing of the rubber as it collapses internally with shock absorber action.

Manual adjustable-rate shock absorbers

Adjustable rate shock absorbers provide a means of changing the rate of dampening of the spring oscillations, to suit varying road conditions, or driver preference. This shock absorber has a manual, external damper rate adjustment.
The position of the valves in the piston can then be changed, to vary the number of restrictions the oil has to pass through, and to vary the force needed to open the valves.
In this position, all of the orifices are open, and a small dampening effect is applied to the oil. The spring force applied to the valve is also reduced to allow the valve to open more easily. This means the oil can flow through the valves more easily, which gives a softer ride, but can also allow more rolling and pitching of the body of the vehicle.
Closing some of the orifices, and increasing the spring force applied to the valves, makes it harder for fluid to flow through the piston. This increases the dampening effect of the shock absorber.
The method of changing the position of the valve varies. On this model, it is adjusted by a spindle that reaches down inside the plunger.
On this shock absorber, when it is extended to its maximum length, a pin is depressed, locking in an adjusting slide on the piston assembly. Twisting the 2 halves of the shock absorber changes the number of orifices, and the spring force on the valves.

Electronic adjustable-rate shock absorbers

Adjustable rate shock absorbers provide a means of changing their rate of dampening of the spring oscillations, to suit road conditions. Electronic controls let the changes occur either automatically, or as the driver prefers.
Each shock absorber has a rotary solenoid that can alter the dampening rate by changing the number of restrictions the oil must pass through.
In this position, all orifices are open. Oil can flow more easily through the passageways in the piston. Only a small dampening effect is applied to the oil.
This provides a dampening force that emphasizes ride comfort when traveling at low speeds.
Closing some orifices makes it harder for fluid to flow through the piston. This increases the dampening effect of the shock absorber, providing a firmer ride, more suitable for higher speeds, and faster cornering.
The solenoid is operated by an electrical signal from the electronic control unit, or ECU.
The ECU allows different modes of operation, according to a selector switch on the dash-board. In the Auto position, the dampening effect at the front wheels is increased at road speeds above 80 kilometers per hour.
This improves vehicle stability at high speeds. The rear shock absorbers stay at their normal setting.
The Manual position has two settings - Normal or Sport. In Normal setting, all shock absorbers remain at a rate suited to ride comfort. There is no change to the settings at high speeds.
The Sport setting increases the dampening rate of all the shock absorbers. This is more suited to brisk driving, with heavy acceleration and cornering.

Automatic load-adjustable shock absorbers

This section examines automatic load-adjustable shock absorbers. They are also called self-leveling.
When vehicles carry heavy loads, their suspension is compressed, causing the rear of the vehicle to be lower than normal.
As a result, steering becomes lighter, the alignment of the headlights becomes too high, and the compression length of travel of the suspension over bumps is reduced, causing discomfort to passengers.
A lower vehicle handles better on smooth roads, but on a rough road, reduced suspension travel can let harsh road shocks be transmitted to the passenger compartment, and cause discomfort.
An automatic load adjustable suspension system controls the vehicle ride height automatically, according to the load placed over the rear axle.
It consists of air-adjustable shock absorbers fitted to the rear suspension, an electrically-driven compressor and air-dryer assembly, and an electronic control unit, and associated wiring and tubing.
The ECU is mounted to the cross-member over the rear axle and a moveable link connects it to a rear suspension member.
As the vehicle is loaded, the normal suspension springs are compressed, which lowers the height of the vehicle.
When the ignition is switched on, the ECU senses the lowered ride height and switches on the air compressor. Air is directed to the shock absorbers, causing the airbag around them to expand and raise the suspension to the normal trim height.
If the load is removed, the suspension springs expand, raising the height of the vehicle.
The ECU senses the raised ride height, and air is exhausted from the shock absorbers, causing the airbag to deflate, and lower the suspension to the normal trim height.
During normal suspension operation, continual adjustment of vehicle ride height is prevented by a time delay, in the ECU.
This allows the trim height to be adjusted only when the ECU reads an out-of-trim signal for 5 to 15 seconds.
The compressor run-time or exhaust-time is limited to 2 minutes.
This prevents it continuing to operate, if the system develops an air leak, or if an exhaust vent remains open.