Active suspension
eSelf Study Program 960393

Audi of America, LLC
Service Training
Created in the U.S.A.
Created 2/2020
Course Number 960393
©2020 Audi of America, LLC
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The eSelf-Study Program (eSSP) teaches a basic understanding of the design and mode of operation of new models, new
automotive components or new technologies.
It is not a repair manual! Figures are given for explanatory purposes only and refer to the data valid at the time of
preparation of the SSP.
For further information about maintenance and repair work, always refer to the current technical literature.

Release: February 2020
2

Introduction
Running gear concepts compared

Basic principle
Design of the system

System components
Actuators for running gear stabilization
Harmonic drive gear
Electric motor
Suspension stabilization control modules
Drivetrain Control Module J775

System functions
Additional
Additional
Additional
Additional
Additional

preview function
crash lifting function
elevated entry function
corner tilting function
helicopter function

Operation and servicing
Operation and driver information
System behavior in the event of a fault
Service operations

Knowledge assessment

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3

Introduction
The Audi active suspension is an electromechanically operated suspension system. It can increase or reduce the load
on each wheel individually to adjust to the road as needed.
This means the system can actively control the position of
the vehicle body in every driving situation.
For each wheel there is one electric motor supplied by a
48-volt electrical system. The Driver Assistance Systems
Control Module sends control signals to the active suspension every five milliseconds. A single Suspension Stabilization Control Module per axle processes the signals for the
electric motors. A belt drive and compact gearing arrangement step up the torque of the electric motor and transfer it
to a steel rotary tube. The tube houses and is rigidly joined
to a titanium torsion bar. This bar is more than 15.7 in
(40 cm) long, approximately 0.9 in (22 mm) thick and,
despite its high strength, can be twisted more than 20
degrees. The force is transferred from the end of the torsion
bar to the suspension via a lever and a coupling rod. This
force is exerted at the front axle on the pneumatic strut of
the adaptive air suspension and at the rear axle, on the
transverse link.
The range of ride characteristics takes on a whole new
dimension thanks to the flexibility of the active suspension.
If the driver chooses “Dynamic” mode in the Audi drive
select system, the car becomes a sports car. It turns firmly
into corners and body roll angles are reduced by half compared with a conventional suspension. The body hardly dives
at all during braking. In “Comfort” mode, however, it glides
smoothly over any and all road surface irregularities. The
active suspension settles the superstructure by continuously
adding energy to or removing energy from the body depending on the respective driving situation.
The active suspension also enhances passive safety. The
system uses the sensors networked in the Driver Assistance
Systems Control Module (zFAS) to detect risks of a collision
around the vehicle. In the event of an imminent side impact
of more than 15.5 mph (25 km/h), the suspension actuators
raise the body on the exposed side by up to 3.1 in (80 mm)
within half a second. As a result, the collision is directed to
the even stronger areas of the vehicle, such as side sills and
floor structure. The load on occupants is reduced by up to 50
percent compared with a side impact when the body is not
raised.

677_008_03

677_001
4

Running gear concepts compared
Conventional running gear
Springs, dampers and anti-roll bars for conventionally
constructed axles are designed to the special conditions for
which the vehicle is used. These systems, which cannot
change when the vehicle is moving, are always a compromise. The usage possibilities of the vehicle are limited.
In addition, there is a conflict between designing the components with a focus on dynamic driving or on comfort.
Running gear designed for sporty driving cannot offer the
comfort which can be provided by running gear designed
for comfort.

Front axle of conventional, non-regulated running gear

Conversely, a vehicle with running gear designed for
comfort will not be able to offer the driving dynamics which
can be offered by a sporty design. Furthermore, the
self-steering behavior of the vehicle is defined by the relation between the fixed anti-roll bar rigidities on the front
and rear axles. Depending on the design of the anti-roll
bars, the vehicle will understeer, oversteer or handle
almost neutrally when driven on the limit.
In the past, springs with progressive rates were used along
with dampers on which the damping characteristics
changed depending on the spring travel. However, this also
meant no opportunity to react to external conditions or the
driver’s wishes via a targeted adjustment of the spring and
damping forces.

677_002

5

Adaptive air suspension
To deal with the compromises of conventional running
gear, dampers with the capability of adjustment while the
vehicle is moving can be used.
Using air springs instead of steel springs also allows for
regulation of the vehicle ride height. Corresponding sensors
record dynamic driving parameters such as vehicle speed,
accelerations and torques. The damping forces are adjusted
to the situation depending on the dynamic driving conditions. The driver can also choose between sporty, balanced
and comfortable handling by selecting specific driving
programs which activate specific characteristic maps.

Thanks to the option of changing the volume of air in the
air springs, different vehicle ride heights can be achieved. If
the vehicle ride height remains the same, changes to the
vehicle’s load cause corresponding changes to the air pressure in the air springs. The natural frequency of the vehicle
body remains almost exactly the same as a result, irrespective of the load. As a result, the driver does not notice any
significant changes to vehicle comfort and driving dynamics, even if the load is changed.
Because air is compressible, a short amount of time is
needed between when the pump starts delivering and an
increase in volume in the air springs can be measured.
Therefore, changes to the vehicle level/vertical forces
cannot occur in real time. Furthermore, it is not possible to
initiate forces in the opposite direction; that is to have the
wheel “pull” on the vehicle body.

677_003

Front axle of regulated running gear with air suspension and damping control (adaptive air suspension)

6

Adaptive air suspension with Electromechanical Roll Stabilization (ERS)
This system was offered for the first time as an option in
the Audi SQ7 in other markets. With the use of
electromechanical control elements, the torsional rigidity
of the anti-roll bars on the front and rear axles can be
varied separately. The control elements connect the antiroll bar “halves” on one axle and tension them against
each other with correspondingly regulated forces. As a
result, it is possible to increase the support provided by
the outer wheels in corners, which effectively restricts the
roll inclination of the vehicle body.

In addition, the self-steering behavior of the vehicle is
affected by achieving a defined relation between the antiroll bar rigidities on the front and rear axles.
The force-transmitting connection between the wheels on
an axle via the anti-roll bar can be reduced to a minimum,
but the application of forces via a corresponding rigidity
change always occurs alternately on both wheels of an axle
at the same time.

Sway Stabilization
Control Module
J924

“Anti-roll bar half” permanently
connected to the actuator

Left Front Suspension
Stabilization Actuator
V634

“Anti-roll bar half” bolted to the
actuator - non-detachable connection

Front axle of the Audi SQ7 with Electromechanical Roll Stabilization (ERS)
677_004

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Vehicles with active suspension
The active suspension is an electromechanical suspension
system. With this system, four separate vertical forces per
wheel plus their direction can be set between the vehicle
body and the wheels.
As a result, the wheels on a single axle are no longer
coupled (as on the ERS system) and the corresponding
application of identical (opposing) forces on both wheels
no longer occurs. Force can be applied to a wheel without
affecting the other wheel on the corresponding axle.
In addition, the vertical forces can be applied in both
directions, therefore making it possible for the vehicle body
to be “pulled” in the direction of the road by the wheels.
This method of applying force to all four wheels separately
provides additional new options for influencing driving
dynamics and comfort.

Audi A8 with active suspension

8

The active suspension combines the following functions:
›
›
›
›

Active roll stabilization.
Active pitch stabilization.
Active vertical body control.
Dynamic height adjustment
(elevated entry function, crash lifting function).

The “classic” conflict between driving comfort and dynamic
driving is resolved with the active suspension. The driver
experiences a vehicle with the dynamics of a sports car and
the comfort of the luxury class.

677_005

The illustration below compares the driving dynamics and
comfort characteristics of the adaptive air suspension
(AAS, red) with those of the electromechanical active roll
stabilization (ERS, yellow), the Active suspension basic
system (green) and the Active suspension basic system
with additional functions (blue).

By selecting the desired driving program in Audi drive
select, the driver can decide whether to prioritize driving
dynamics (sportiness) or to drive more comfortably. This is
made possible by the system-specific spread which is also
illustrated in the diagram.

Driving dynamics

The adaptive air suspension was used as the reference.

Spread due to Audi drive
select setting chosen

Driving comfort

677_006

Key:
AAS (Adaptive air suspension)
AAS + ERS (Adaptive air suspension + ERS [active roll stabilization])
Active suspension (basic system)
Active suspension (basic system)+ additional functions

Audi drive select setting “auto”

Audi drive select setting “dynamic”

Audi drive select setting “comfort”

9

Basic principle
Design of the system
The principle of the active suspension system can be
depicted using the electromechanical roll stabilization as a
basis. On the roll stabilization system, the anti-roll bar is
divided into two components, approximately in the center.
The two components are connected by an electromechanical actuator. This actuator tensions the two components
against each other, which causes the torsional torque and
therefore also the vertical forces which act on the axles of
the vehicle body to increase as the tension increases.

Compared with a hydraulic system, the electric actuation
also significantly reduces the energy required, in addition
to many other benefits. The electric motors are activated by
a single Suspension Stabilization Control Module per axle.
Drivetrain Control Module J775 activates the two control
modules via a sub-bus.

To regulate individual wheels, the two “halves” of the
anti-roll bar on the active suspension are not connected
and tensioned against each other. Each “half” of the antiroll bar is tensioned by a separate actuator. The actuators
are driven by electric motors.

Adaptive suspension with ERS

ERS actuator to tension
the torsion bar halves

Active suspension

Separate actuators to
twist the torsion bars

10

677_008

Suspension Stabilization
Control Module 1
J1152

Drivetrain Control Module
J775

Suspension Stabilization
Control Module 2 J1153
677_009

J775 calculates the regulating inputs required at the front
and rear axles. The two chassis stabilization control
modules implement the specifications by activating the
electric motors on the front and rear axles for each wheel
individually.
The actuators are bolted to the vehicle underbody (bodymounted) together with the subframes for the front and
rear axles. When an actuator is operated, the lever which it
actuates turns. A coupling rod connected to the end of the
lever is connected to the damper stalk on the front axle and
the transverse link on the rear axle.
Because of the turning movement of the lever, a vertical
force component acts on the corresponding damper stalk
or the corresponding transverse link via the relevant coupling rod. As a result, the distance between the wheel and
the vehicle body is reduced or increased depending on the
direction in which the actuator is turning (springs moving
up or down).

Front Suspension
Stabilization Actuator

677_011

Front Suspension
Stabilization Actuator
677_012
11

The illustration below shows the activation of a front axle
actuator as an example. The actuator lever is connected to
the damper stalk via the coupling rod. The actuator supports
itself on the wheel contact point via the lever, coupling rod,
damper stalk and transverse link. The turning movement of
the lever illustrated (shown in red) causes the actuator and
the vehicle body directly connected to it to be raised.

The piston rod of the damper is put under tension (rebound
damping). If the lever is turned in the opposite direction,
the vehicle body is lowered. The suspension strut
compresses and the piston rod of the damper is put under
load (compression damping).

677_010

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System components
Actuators for running gear stabilization
Design
The coupling rod is connected to the upper bearing at the
damper stalk of the front axle damper and at the top transverse links of the rear axle. The lower bearing of the coupling
rod is connected to a lever. The end of the lever is form-fit to
the torsion bar. The other end of the torsion bar is form-fit
to the end of the torque tube.

The torque tube forms the extension of the output of the
harmonic drive gear (strain wave gearing). The input shaft
of this gearing is connected to the rotor of the electric
motor via a toothed belt. The power is transmitted from
the electric motor via the toothed belt to the harmonic
drive gear, where it is output to the coupling rod via the
torque tube, torsion bar and lever.

Upper bearing for
connection to
damper stalk

Coupling rod

Lever

Form-fit

Harmonic drive
gear

Form-fit

Electric
motor

Toothed
belt

Torsion bar

Torque tube
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Function
The actuator provides a torque of approximately 811.3 lb-ft
(1,100 Nm) at the drive gear output; the maximum force at
the coupling rods is approximately 1124.40 lb (5.0 kN) on
the front axle and approximately 1011.65 lb (4.5 kN) on
the rear axle.

Lever angles of +/- 42° can be set from the zero position.
The body can be raised by approximately 3.34 in (85 mm)
from the center position at all four corners in just five
tenths of a second.