AUDI A6 ALLROAD 1999 C5 / 2.G Pneumatic Suspension System

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0,52 0,260
-0,26-0,52
0,52 0,260
-0,26-0,52
0,52 0,260
-0,26-0,52
The force/stroke diagrams thus obtained can
be converted into force/velocity diagrams (f-v
diagrams).
These characteristic curves show the
relationship between the damping force and
the piston speed, thereby indicating the
shock absorber characteristics.
We differentiate between linear, progressive
and decreasing characteristic curves. Damping force
The damping force depends upon the oil
volume to be displaced (surface of the
damping valve), the ßow resistance of the
damper valves, the speed of the damper
piston and the viscosity of the damping oil.
The damping force is determined with the aid
of a test machine. At a constant speed, this
machine produces various rebound and
compression strokes thereby producing
differing rebound and compression speeds in
the damper.
242_066
F-v diagram characteristic curve progressions (speed constant for all strokes)
decreasing
progressive
linear
Traction
force
Compression
force
Traction
force
Compression
force
Traction
force
Compression
force
v in m/s
v in m/s
v in m/s 25 mm
50 mm
75 mm
100 mm
Stroke

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Principles of air suspension
Measures are taken during the design stage
to adapt the characteristic curves to the
requirements of suspension matching.
Shock absorbers with decreasing
characteristic curves are normally used.
Normal shock absorbers have predetermined
characteristic curves. They are adapted to
normal bodywork weights and can cope with
a wide range of driving situations in a well-
matched running gear.
Running gear matching is always a
compromise between driving safety (driving
dynamics) and driving comfort.
The degree of damping (damping effect of
sprung masses) is lessened as the load
increases, which affects the driving dynamics.
In contrast, the degree of damping is greater
when the vehicle is un-laden, which lessens
driving comfort.Note:
A distinctive feature of damper
matching is described in SSP 213,
page 28, ÒShock absorbers with load
and travel-dependent damping
characteristicsÓ.

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The PDC damper
In order to maintain the degree of damping
and thereby the handling characteristics at a
constant level between partially and fully
laden, the Audi A6 self-levelling air suspen-
sion and the Audi allroad quattro 4-level air
suspension both have a continuously variable
load recognition system Þtted to the rear axle.
Along with the constant natural frequency of
the bodywork, the vehicle bodywork
maintains virtually constant vibration
characteristics irrespective of the load thanks
to the air springs.
When the vehicle is partially-laden, good
driving comfort is achieved and body
movements are damped sufÞciently Þrmly at
full load.
The PDC damper (Pneumatic Damping
Control) is responsible for this. The damping
force can be varied according to the air spring
pressure.
242_043
PDC valve
Hoses
Air springs
1 1,2 1,4 1,6 1,82,0Body weight ratio
Degree of damping D
PDC damper
Conventional dampers242_057
Coaxial arrangement of air springs/PDC damper

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Principles of air suspension
The damping force is altered by means of a
separate PDC valve integrated into the
damper. It is connected to the air springs via a
hose.
A variable throttle in the PDC valve is
controlled by the air spring pressure acting as
a control variable proportional to the load.
This inßuences the ßow resistance and
thereby the damping force during rebound
and compression.
The air connector in the PDC valve is Þtted
with a throttle to counteract the undesirable
inßuence of the dynamic pressure changes
(compression and rebound) in the air springs.
0,13 0 0 200 400 600 800 1000
1200 1400 1600
0,26
0,390,520,650,78 0,91 1,04Piston speed in m/s
Damping force in N
Rebound
Compression
242_0426.5 bar 8 bar 9.5 bar242_087
PDC valve
Air springs
Separate arrangement of air springs/PDC damper

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Design and function
The PDC valve inßuences the ßow
resistance of the working
chamber on the piston rod side
(working chamber 1).
Working chamber 1 is connected
to the PDC valve via bore holes.
The PDC valve has a low ßow
resistance when the air spring
pressure is low (no load or small
partial load). Part of the damping
oil bypasses the damping valve,
thereby reducing the damping
force.
The ßow resistance of the PDC
valve has a Þxed relation to the
control pressure (air spring
pressure).
The damping force is dependent
on the ßow resistance of the
relevant damping valve
(compression/rebound) plus that
of the PDC valve.
242_033 PDC valve Working chamber 1
Bottom valvePiston valve with
sealing collar
Working
chamber 2BoresGas Þlling
Throttle in air connectorRebound stop

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Principles of air suspension
Function during rebound at low air
spring pressure
The piston is drawn upwards, part of
the oil ßows through the piston valve,
the remainder ßows through the bore
holes in working chamber 1 to the
PDC valve. As the control pressure (air
spring pressure) and consequently
the ßow resistance of the PDC valve is
low, the damping force is reduced.
242_052
242_051
Function during rebound at high air
spring pressure
The control pressure and
consequently the ßow resistance of
the PDC valve is high. Most of the oil
(depending on the control pressure)
is forced to ßow through the piston
valve, thereby increasing the
damping force.
Low air spring
pressure
High air spring
pressurePDC valve open
PDC valve closed

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Function during compression at low
air spring pressure
The piston is pushed downwards and
damping is determined by the
bottom valve and to a certain extent
by the ßow resistance of the piston.
The oil displaced by the piston rod
ßows partly via the bottom valve into
the reservoir. The remainder ßows
through the bore holes in working
chamber 1 to the PDC valve. As the
control pressure (air spring pressure)
and consequently the low ßow
resistance of the PDC valve is low,
the damping force is reduced.
242_070
Function during compression at high
air spring pressure
The control pressure and
consequently the ßow resistance of
the PDC valve are high. Most of the oil
(in relation to the control pressure)
must ßow through the piston valve,
thereby increasing the damping
force.
Low air spring
pressure
242_069 High air spring
pressure
PDC valve closedPDC valve open

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The Audi A6 air suspension system comprises
the following main components:
Air springs with U-bellows are used as
suspension elements.
PDC dampers as used as shock absorbers (see
page 33).
The air supply unit with integrated air dryer,
control valves and control unit are contained
in a metal box within the air supply unit.
A level sensor detects the actual vehicle level. The following chapter deals with the self-
levelling air suspension system in the Audi A6
Õ98. Basic information about air suspension/
self levelling has already been given in the
ÒPrinciplesÓ chapter. As this information and
knowledge forms the basis for the next
chapter we recommend making yourself
familiar with the principles before continuing.
Overview of system
In the case of the Audi A6, an air suspension-
based self-levelling system is offered as an
optional extra. The air suspension system is
designed speciÞcally for the rear axle
because only small loads are applied to the
front axle and consequently only small level
changes occur as a result of loading the
vehicle.
Self-levelling suspension, A6
Self-levelling suspension, A6 front
wheel drive
242_040
Air springs PDC damper
Level sensorAir supply unit

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¥ Environmentally friendly, uses air
¥ Good operating safety due to great
stability.
¥ Electronic control system with
comprehensive self-diagnosis functions
¥ Maintenance-free Along with the principle advantages of self-
levelling (see Principles), the system realised
in the A6 has the following advantages:
¥ Virtually load-independent suspension
and vibration behaviour.
¥ Little space requirement due to compact
design, especially in the axle area.
¥ Self-levelling even available when engine
is off.
¥ Rapid raising and lowering times
¥ Low energy requirement
Air springs with PDC damperSelf-levelling suspension, A6 quattro
drive
Air supply unit242_041

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The air springs
The installation of the air springs on the front-
wheel drive and the quattro drive is the same
as in the steel spring version. This allowed the
use of the axle design from the production
running gear with few modiÞcations.
In the front wheel drive version the piston is
conical in shape to allow sufÞcient clearance
for the spring movement between the
bellows and the piston.
In the quattro drive the air springs are
combined coaxially with the dampers to
act as a suspension strut.
Self-levelling suspension, A6
242_043
quattro drive
Coaxial arrangement of air springs/PDC damper
242_042
Front-wheel drive
Separate arrangement of air springs/PDC damper
Air springs may not be moved while
at atmospheric pressure since the
U-bellows cannot uncoil on the piston
and would be damaged.
In a vehicle with depressurised air
springs, the corresponding air springs
must be Þlled with the aid of the
diagnostic tester (see Workshop
Manual) before raising or lowering
the vehicle.