Audi A6 242 Bedienungsanleitung

Service.

For internal use only

the right to make technical

changes.

AUDI AG

Dept. I/VK-5

D-85045 Ingolstadt

Fax 0841/89-36367

940.2810.47.20

Technical status 11/00

Printed in Germany

Pneumatic suspension system

Part 1

Selﬂevelling suspension

in the Audi A6

Design and Function

Self-study programme 242

242

242_067

Pneumatic self-levelling suspension system

The 4-level air suspension of the Audi

allroad quattro is described in self-

study program 243.

You will ﬁnd further information on the

Audi allroad quattro in self-study

programme 241.

Principles of spring suspension, damping and

air suspension

Self-levelling suspension, A6

The rear axle air suspension system for the

Audi A6 Avant is described here.

242_046

242_048

This self-study programme is divided into two

parts:

Contents

Principles

Vehicle suspension.................................................................. 4

The suspension system .......................................................... 6

Vibration ................................................................................... 8

Characteristic values of springs .......................................... 12

Conventional running gear without self-levelling ............ 14

The self-study programme is not intended as a workshop manual.

The self-study programme will provide you with

information on design and functions.

New

Note

Important:

Note

Page

For maintenance and repairs please refer to the current

technical literature.

Principles of air suspension

Self-levelling air suspension ............................................... 16

Characteristic values of air spring ...................................... 21

Vibration damping................................................................. 23

Shock absorbers (vibration dampers) ................................ 25

PDC shock absorbers ........................................................... 33

System overview ................................................................... 38

Air springs.............................................................................. 40

Air supply unit........................................................................ 42

Diagram of pneumatic system............................................. 43

Compressor ........................................................................... 44

Air dryer ................................................................................. 47

Discharge valve N111 ........................................................... 48

Valve for suspension struts N150 and N151....................... 51

Self-levelling suspension sender G84 ................................ 52

Self-levelling suspension control unit J197 ....................... 54

Self-levelling suspension warning lamps K134 ................ 55

Function diagram ...................................................................56

Interfaces................................................................................ 57

The control concept .............................................................. 58

Other features of the control concept ................................ 60

Self-levelling suspension, A6

Vehicle suspension

When a vehicle travels over irregular road

surfaces, impact forces are transmitted to the

wheels. These forces pass to the bodywork

via the suspension system and the wheel

suspension.

The purpose of the vehicle suspension is to

absorb and reduce these forces.

Principles

Wheel contact with the road surface, which

is essential for braking and steering, is

maintained.

The vehicle components are protected

against excessive stresses.

Unpleasant and unhealthy stresses to vehicle

passengers are minimised, and damage to

fragile loads is avoided.

242_003

Driving safety

Operating safety

Driving comfort

When we talk about the vehicle suspension

we can basically distinguish between the

suspension system

and the

vibration

damping system

By means of the interaction of the two

systems, the following is achieved:

During driving operation, the vehicle body is

subject not only to the forces which cause the

upward and downward motion of the vehicle,

but also the movements and vibrations in the

direction of the three spatial axes.

Along with the axle kinematics, the vehicle

suspension has a signiﬁcant inﬂuence on

these movements and vibrations.

242_048

Longitudinal axis

Transverse axis

Vertical axis

Drift

Pitch

Swerving (yaw)

Rising and sinking

Tipping (roll)

Jerking

The correct matching of the springs and

vibration damping system is therefore of

great signiﬁcance.

Principles

The suspension system

As ”supporting” components of the

suspension system, the suspension elements

form the connection between the wheel

suspension and the bodywork. This system is

complemented by the spring action of the

tyres and vehicle seats.

The suspension elements include steel

springs, gas/air and rubber/elastomers or

combinations of the above.

Steel spring suspensions have become well

established in passenger vehicles. Steel

springs are available in a wide variety of

designs, of which the coil spring has become

the most widespread.

Air suspension, which has been used for

many years in heavy goods vehicles, is

ﬁnding increasing application in passenger

vehicles due to its system-related

advantages.

242_047

In the case of the passenger vehicle we can

differentiate between

sprung masses

(body

with drive train and parts of the running gear)

and

unsprung masses

(the wheels, brakes

and parts of the running gear and the axle

shafts).

As a result of the suspension system, the

vehicle forms an oscillatory unit with a

natural frequency of the bodywork

determined by the sprung masses and the

matching of the suspension system (see

”Vibration” chapter).

Sprung mass

Unsprung mass

Suspension element

The unsprung masses

The aim in principle is to minimise the volume

of unsprung masses and their inﬂuence on

the vibration characteristics (natural

frequency of the bodywork). Furthermore, a

low inertia of masses reduces the impact load

on the unsprung components and

signiﬁcantly improves the response

characteristics of the suspension. These

effects result in a marked increase in driver

comfort.

Examples for the reduction of unsprung

masses:

• Aluminium hollow spoke wheel

• Running gear parts (swivel bearing, wheel

carrier, links etc.) made of aluminium

• Aluminium brake callipers

• Weight-optimised tyres

• Weight optimisation of running gear parts

(e.g. wheel hubs)

213_091

213_068

See also SSP 213, chapter “Running

gear”.

213_041

Principles

The natural frequency of the bodywork

The vibrations are deﬁned by the degree of

amplitude and its frequency. The natural

frequency of the bodywork is particularly

important during matching of the

suspension.

The natural frequency of unsprung parts is

between 10 Hz and 16 Hz for a medium-size

vehicle. Appropriate matching of the

suspension system reduces the natural

frequency of the bodywork (sprung mass) to

between 1 Hz and 1.5 Hz.

Vibration

If a mass on a spring is deﬂected from its rest

position by a force, a restoring force develops

in the spring which allows the mass to

rebound. The mass

oscillates

beyond its rest

position which results in a further restoring

force being exerted. This process is repeated

until air resistance and the internal friction of

the spring causes the vibration to cease.

242_021

Rest position

Mass

Spring

Vibration

Rebound

Compression

1 cycle

Amplitude

The natural frequency of the bodywork is

essentially determined by the characteristics

of the springs (spring rate) and by the sprung

mass.

Greater mass or softer springs produce a

lower natural frequency of the bodywork and

a greater spring travel (amplitude).

Smaller mass or harder springs produce a

higher natural frequency of the bodywork and

a lesser spring travel.

Depending on personal sensitivity, a natural

frequency of the bodywork below 1 Hz can

cause nausea. Frequencies above 1.5 Hz

impair driving comfort and are experienced

as shudders above around 5Hz.

242_072

Deﬁnitions

Vibration Upward and downward

motion of the mass

(body)

Amplitude The greatest distance of

the vibrating mass from

the rest position

(vibration extent, spring

travel)

Cycle Duration of a single

vibration

Frequency Number of vibrations

(cycles) per second

Natural

frequency of

the bodywork

Number of vibrations of

the sprung mass (body)

per second

Resonance The mass is disturbed in

its rhythm by a force

which increases the

amplitude (build-up).

Greater mass or softer springs

Smaller mass or harder springs

Spring travelSpring travel

Low natural frequency of the

bodywork

High natural frequency of the

bodywork

1 cycle

Time

The degree of damping of the vibration

damper has no signiﬁcant inﬂuence on the

value of the natural frequency of the

bodywork. It inﬂuences only how quickly the

vibrations cease (damping coefﬁcient). For

further information, see chapter “Vibration

damping”.

Matching of the natural frequency of the

bodywork

The axle loads (sprung masses) of a vehicle

vary, at times considerably, depending on the

engine and equipment installed.

To ensure that the bodywork height

(appearance) and the natural frequency of the

bodywork (which determines the driving

dynamics) remains practically identical for all

vehicle versions, different spring and shock

absorber combinations are ﬁtted to the front

and rear axles in accordance with the axle

load.

For instance, the natural frequency of the

bodywork of the Audi A6 is matched to 1.13Hz

on the front axle and 1.33Hz on the rear axle

(design position).

The spring rate of the springs therefore

determines the value of the natural frequency

of the bodywork.

The springs are colour-coded to differentiate

between the different spring rates (see table).

Principles

For standard running gear without self-

levelling, the rear axle is always

matched to a higher natural frequency

of the bodywork because when the

vehicle is loaded, it is principally the

load to the rear axle which increases,

thus reducing the natural frequency of

the bodywork.

242_073

Vehicle heightNatural frequency of the bodywork

Component tolerance band

Natural frequency tolerance band

Usable load range

of a spring

Height tolerance

Axle load800 kg 850 kg 900 kg 950 kg

1.13 Hz

= 33.3 N/mm

= 35.2 N/mm

= 37.2 N/mm

= 39.3 N/mm

= 41.5 N/mm

= 43.7 N/mm

Spring rate levels of the front axle for the A6

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