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Student Workbook

LV40 Suspension Systems (3)

kap all phase 2 & 3 6/11/03 11:37 am Page 35

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV40

Contents

Page … Page

Oil flow exhaust position to neutral

The benefits 3

Electronically Controlled Hydro-

Progress check 1 6 Suspension in firm mode 23

MacPherson strut hydro-pneumatic

Manual Control 7 Hydro-pneumatic trailing arm (rear) 24

Automatic control 7 Progress check 6 25

Hydro-pneumatic suspension

systems 8 Active Hydro-Pneumatic

Progress check 2 10 Suspension 26

Hydro-pneumatic suspension

damper action 11 Routine Maintenance: 27

Progress check 3 13 Self levelling and ride control

Hydro-pneumatic suspension suspension systems (hydro-

Swash plate hydraulic pump 14 Test action of height correctors 28

Hydro-pneumatic height corrector 15 Check the condition and security of

Progress check 4 16 the height corrector linkages 28

Height corrector construction 17 Other maintenance checks 28

Oil flow in neutral (negligible) 17 Common faults associated with self-

Oil flow neutral position to inlet levelling and ride control suspension

position 18 systems (hydro-pneumatic) 28 Oil flow inlet position to neutral

position 19

Page … Page

Self energising suspension systems 29 Electronic control component

Self energising suspension system functions 42

Underside view of unit and A-frame 31 Electronic circuit for air suspension 44 Ball joint and lower suspension unit Sensor and actuator relationship 45

mounting 31 ECU functions 46 Self-levelling and ride control Air suspension ECU 46 suspension systems (self- Skid control ECU 46 energising suspension systems) 32 Engine ECU 46 Self-energising suspension unit faults 32 Electronic control of vehicle roll centre 46 Progress check 7 33 Progress check 9 48

Electronic controlled air suspension 49

Electronic Controlled Air Common faults associated with

Suspension: 34 electronic controlled air suspension 51 Electronic controlled air suspension

(basic) 35

Electronic controlled air suspension

(detailed) 35

Motor driven air compressor dryer 36

Pneumatic suspension unit with

damper 37

Suspension control actuator details 38

Suspension control actuator position 38

Height control valves 39

Height control sensors 39

Vertical accelerator sensor 40

Location of height control sensor 41

Disc with vertical accelerator sensor 41

Introduction

On completion of this module you will be expected to effectively:

• describe the operating principles of self levelling suspension

• describe the operating principles of ride controlled systems

• explain the routine maintenance requirements for self levelling and ride

controlled suspension systems

• describe the common faults associated with self levelling and ride

controlled suspension systems

Self-Levelling Suspension

Self – levelling suspension systems have been used on cars for many years Some systems use only gas (pneumatic systems) and others are

arrangements involving the use of both hydraulic fluid and gas

(hydro-pneumatic) In addition self-energising systems use a large gas filled damper, together with a built in pump supporting normal springs and dampers

Hydraulics – the study of pressures in liquids

Pneumatics – the study of pressures in gases

The benefits

These suspensions systems are complex and relatively expensive, but they have the following advantages:

• constant ride height

• variable spring rate, which is dependent on load

• reduced body roll

• reduced pitching

Electronic control has been added to these systems to make them very

sophisticated and gives additional features over the standard layout These additional features are listed below:

• anti-dive and anti-squat suspension characteristics can easily be

incorporated

• improved damping features related to speed

• driver can alter spring stiffness and damper settings

• roll stiffness can be predicted by use of a steering wheel sensor

• ride height can be automatically lowered as speed increases giving better road holding and improved aerodynamics

Principle of Air Suspension (Pneumatic)

An air suspension system consists of three main components, namely

• an air spring (with a concentric damper) formed by a rolling diaphragm

enclosing the bottom of a chamber

• an engine driven air compressor/drier

• height control valves that are connected to the suspension and allow air in and out of the spring/damper unit

Small-bore pipes and hoses connect all components

The diagram below shows a typical air suspension layout as used on a luxury car Air suspension systems have been used on large goods vehicles for a number of years The variable rate spring characteristic proving particularly useful where the laden to un-laden ratio may be as high as 3:1

In the above diagram, you will also notice front suspension control actuators, which give the driver some control over the damper settings Air pressure

level is in under the command of the control valve on the compressor

The above diagram shows the motor driven piston type air compressor, which has an air dryer attached to remove the water from the compressed air The air dryer is necessary particularly when the humidity is high When air is

compressed, its volume is reduced dramatically, but since water is a liquid, it cannot be compressed Therefore, the water takes up a disproportionate

amount of the space as the compressed air cools and needs to be removed to prevent any corrosion or damage to the system

The above shows the construction of a front and rear rolling bellows air spring

or pneumatic cylinder unit Note: The concentric damper position, which also forms the bottom suspension unit mounting, and the sub chamber mounted above the main chamber The sub chamber can be connected and

disconnected from the main air chamber to decrease and increase the spring rate respectively, i.e make the spring softer or return it to normal Open and closing the sub chamber is carried out by the suspension control actuators, which also control the damper settings

Progress check 1

Answer the following questions:

1 Name the three main types of self-levelling suspension systems

2 How can these systems be made more effective?

Ride Control Systems

These come under two headings:

Manual control

This is where the driver has some control over the ride height and damper

settings The first application of this system was the use of manually

adjustable dampers, which involved the driver reaching under the wheel arc and twisting the damper body to increase or decrease the stiffness This was later modified to cable operation to enable the driver to adjust damper settings from his or her seat whilst on the move

Electric control was later added to make operation easier As mentioned

earlier, the use of air suspension allows the spring rate to be altered If metal springs are used it is not possible to alter the spring rate or stiffness since this

is determined by the physical size and length of the spring

Automatic control

Usually controlled electronically by an ECU receiving signals from various

sensors around the vehicle In this case the ride height and damper settings (and spring rate with air suspension) are controlled automatically by an ECU, which has set suspension parameters programmed to control the suspension system to give the best ride and road holding characteristics

In this system the driver may have some control over the damper settings, i.e hard or soft ride, but the ECU will override these if vehicle handling or safety is compromised

follows the road surface, but for the travel to be resisted on the way down on rebound

Hydro-pneumatic suspension systems

The principle of this suspension is very simple Spherical gas springs are

used and suspension movement is transferred to the spring hydraulically by means of a piston running inside a cylinder The piston rod is attached to the upper suspension arm in a double wishbone suspension layout MacPherson struts are also used, as shown later

Each sphere contains a diaphragm behind which a quantity of nitrogen is

trapped under a pressure of approximately 50 bar

A height corrector is attached to the anti roll bar and when a load is placed in the car, the car body sinks This movement is registered by the height

corrector, which opens a valve to admit hydraulic fluid under pressure to

"lengthen" the hydraulic strut and thereby re-establish the correct ride height

Hydro-pneumatic suspension

Nitrogen gas

Height corrector

Mineral based hydraulic fluid Damper valve

This diagram illustrates the layout of a double wishbone IFS hydro-pneumatic suspension system Note: The position of the damper valve between the

cylinder and the oil space below the sphere and the height corrector, which is linked to the anti-roll bar Later diagrams show that only one height corrector

is used at the front and only one at the rear

Progress check 2

Answer the following questions:

1 Give two reasons why a liquid is used to transfer wheel movement to the gas spring

2 Name the bases for the two liquids and give an advantage and an

advantage and disadvantage for each type

Hydro-pneumatic front suspension Trailing-arm rear suspension

The above diagrams show a hydro-pneumatic suspension employing a

MacPherson strut IFS and a trailing arm rear suspension layout respectively Note: The position of the gas spheres on front suspension and the central

mounting of the height corrector on the rear suspension The front gas

spheres are mounted horizontally on the top of the MacPherson struts

because it will be appreciated that there is insufficient room to mount them

vertically Refer to the circuit layout diagram for further details

Hydro-pneumatic suspension damper action

As fluid is displaced from the lower part of the suspension unit it passes

through the damper valve in the same way as a normal damper

Only a relatively simple damper is needed because; (a) the natural frequency

of vibration of the gas spring is low and (b) because of natural damping effect due to friction between the large suspension piston and its cylinder

Oil flow

Bump

Rebound Disc valves

The above drawings show the two stagedamper action similar to that which takes place in a damper fitted to a conventional suspension layout

Stage 1 Slow action due to small vertical wheel movements cause oil to flow through to central passage only

Stage 2 Rapid and large suspension movements cause the disc valves to

open and allow oil through the large drillings

Height correctors Small pipes are for leak off

Progress check 3

Answer the following questions:

1 Why is nitrogen gas used as spring medium?

2 What is an inert gas?

3 Why is air used on some suspension systems instead of nitrogen?

Hydro-pneumatic suspension accumulator

The purpose of the accumulator is to store a reasonable quantity of hydraulic fluid under pressure (150-160 bar) This enables the suspension to react

quickly if necessary The pump is unable to supply a large volume of oil

quickly and is used simply to keep the accumulator charged up The swash plate pump spends most of its time idling (but running at engine speed or a percentage of engine speed) once the hydraulic pressure has been built up in the accumulator and there is little or no demand for a supply of high-pressure fluid e.g when the vehicle is being driven at a steady speed over a normal

road surface

Can you think of an analogy with another system on a motor vehicle?

Swash plate hydraulic pump

Swash Plate Type

A swash plate pump is shown above It is used for this particular application because it is reliable, capable of generating high pressures, produces a low noise level and has a high efficiency

Hydro-pneumatic height corrector

Height corrector

Anti-roll bar Spring linkage

The diagram illustrates the arrangement and location of a height corrector

showing the spring linkage between the anti-roll bar and unit itself

Progress check 4

Answer the following question:

Why do height correctors and levelling valves have a small time lag built into them?

Height corrector construction

Flow and return from suspension units

Feed from pump

& accumulator Return to reservoir

Diaphragm Valve

The height correctors and levelling valves are fitted to maintain a set ride

height and increase roll stiffness The construction of a height corrector is

there will always be slight leakage because of clearance between the spool valve and the height corrector body This will also allow oil to leak into the

outer left and right hand chambers and will fill the body of the unit in order that the action of the spool valve can be damped to introduce the delay effect

If a force is applied to the spool valve the movement will be delayed due to the damping effect of trying to force oil from one side to the other through the

restricted passage

Remember also that the movement of the anti-roll bar causes a spring linkage

to move the spool valve so that any resistance to movement by the valve will cause the spring linkage to bend and it will not necessarily move the valve There is only one height corrector at each end of the vehicle so during roll

conditions the anti-roll bar will twist but may not cause linkage to move

Unless, the outer gas spring is compressed more than the inner spring is

extended, in this case oil will be fed to the outer spring and the roll stiffness will be increased

Oil flow neutral position to inlet position

The diagram above illustrates the position of the height corrector as it moves from the neutral to inlet position to raise the suspension, in response to an

increase in its passenger load for example Oil from the right hand chamber is displaced to the left hand chamber via the restricted passage in the upper part

of the body as the spool valve is moved to the right The passage from the accumulator is opened and oil flows down through the height corrector to the suspension units raising the vehicle body

Oil flow inlet position to neutral position

The next phase of operation is shown above as the spool valve moves from inlet position back to neutral position

Once the suspension has returned to its correct ride height the spring link will pull the spool valve to the right This will cause a drop in pressure in the right hand chamber, as its volume increases, and the right hand valve will be lifted from its seat by the higher pressure in the left hand chamber and unrestricted passage Note: The left hand valve is held open by the end of the spool

valve This will allow the spool valve to quickly return to the neutral position to retain the ride height

Progress check 5

Answer the following question:

Look again at the position of the spool valve, its movement, and the oil flow through the horizontal passages in the last two slides What are the

differences, since the oil is still being fed from accumulator to suspension

unit?

Oil flow exhaust position to neutral position

The diagram shows the valve moving from exhaust position to neutral

position When a load is removed from the vehicle e.g three passengers get out, the suspension will rise on the springs and the spring link will cause the spool valve to be moved slowly to the right, as oil bleeds through the restricted passage Oil will be exhausted from the suspension units and the suspension will drop to its correct ride height The spool valve will then be quickly moved

to the left and a pressure drop will now occur in the left hand chamber This valve will open and oil will flow from left to right through unrestricted passage allowing the spool valve to return quickly to its neutral position

Electronically Controlled Hydro-Pneumatic Layout

The system switches from "soft" to "firm" modes according to a number of

parameters programmed into the ECU

Sensors from steering wheel, accelerator, brake circuit and transmission

(speed) provide the necessary input to the ECU to determine which mode is

appropriate

A central suspension sphere on each axle is switched in and out of the circuit

to alter the amount of suspension travel and damping The introduction of this extra sphere or gas spring has the same effect as making a pair of coil springs 50% longer and hence softer

The next three diagrams show the layout and circuit diagrams of this

electronically controlled hydro-pneumatic layout

1 ECU 2 Steering wheel sensor 3 Accelerator sensor 4 Brake sensor

5 Speed sensor 6 Body movement sensor 7 Electro-valve 8 Stiffness

regulator 9 Extra sphere 10 Front suspension sphere 11 Rear suspension

sphere

Suspension in soft mode

The above diagram shows the suspension in soft mode with the additional

sphere or gas spring in the circuit

Suspension in firm mode

MacPherson strut hydro-pneumatic (front)

Gas springs

Additional sphere or gas spring

This diagram shows the layout of a MacPherson strut IFS and the position of the additional gas spring

Hydro-pneumatic trailing arm (rear)

Gas springs

Additional sphere or gas spring

The above diagram illustrates the trailing arm rear suspension with the

additional gas spring

Progress check 6

Answer the following questions:

1 What type of pump is used in a hydro-pneumatic suspension?

2 How is it possible to ensure there is a sufficient quantity of high pressure oil available?