SSP-285 Running gear Audi A8 Part 2.pdf

7/29/2019 SSP-285 Running gear Audi A8 Part 2.pdf

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Brake System

Brake servo

Use is made of a tandem vacuum-type brakeservo (8+9 inch, basic design as for A4 andA6). In comparison to the A4 and A6, thetransmission ratio has been increased to 7:1.The inlet valve flow characteristics have beenoptimised and the valve closing distanceshortened. This results in far quicker andmore precise servo response accompaniedby a greatly improved operating feel.With V8 petrol engines, vacuum is suppliedby a suction jet pump driven by the intakemanifold vacuum.

An electric vacuum pump is employed for theV6 petrol engine.

Brake master cylinder

Use is made of a tandem brake mastercylinder.As compared to the A4 and A6, the pistondiameter was increased to 26.99 mm with atotal stroke of 36 mm (18/18).The central valves of both brake circuits(diagonal configuration) have been designedfor optimum flow. This permits the use of aself-priming ESP unit without a separatecharging pump.These changes and the above-mentionedmodifications to the brake servo result in asignificant reduction in pedal travel prior tobrake response in conjunction with lesspedal force. Active safety is thus enhancedby shortening the stopping distance.

For design reasons, the brake fluidreservoir is never to be completelydrained, as this would permit theingress of air into the pipes on accountof the position of the connections. Referto the current Workshop Manual forprocedure for changing brake fluid.

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Brake mastercylinder

Brake fluidreservoir

Brake servo

Brake fluid reservoir

The brake fluid reservoir is a separatecomponent fitted into the brake mastercylinder.


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Notes


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Electromech. Parking Brake

Summary

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Parking brake pressure switchF234

Right parking brakemotor V283

Electromechanicalparking brake controlunit J540

Left parking brake motorV282

Control unit with display in dash panel

insert J285


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Operation and display

The parking brake is actuated by the pressureswitch F234 in the centre console.The brake is applied by pulling the switch.It is released by pressing the switch and atthe same time pressing the brake oraccelerator pedal.

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Parking brake display

The electromechanical parking brakecan still be applied by pulling the switcheven when the ignition is off. The

ignition must however be switched onfor the brake to be released.

Application of the parking brake is indicated by the parking brake display in the dash panelinsert and a lamp in the switch.

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System components

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Parking brake motors V282/283

Design:

The brake pads are mechanically tensionedby way of a spindle mechanism. The thread

on the shaft is self-locking.The spindle is driven by a swash platemechanism.

The mechanism is driven by a DC motor.The mechanism and motor are flanged to thebrake caliper.

Control unit J540

The control unit is fitted beneath the batteryon the right side of the luggage compart-ment.From the battery, the left and right parkingbrake motors V282/283 are actuated sepa-rately.There are two processors in the control unit.Release decisions are taken by both proces-sors.Data transfer is by way of the drive system

CAN (refer to "Data transfer").The control unit contains an integratedmicromechanical tilt angle sensor.


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Operation:

Implementation of the parking brake functioninvolves translating the rotation of the drivemotor into a very short brake piston stroke.This is achieved through the use of a swashplate mechanism in conjunction with thespindle drive.

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Toothed belt

Toothed belt

Electric motorGear mechanism

There are three transmission stages. The firstreduction stage (1:3) is achieved by themotor/gear mechanism input toothed beltdrive. The swash plate mechanism isresponsible for the second stage.A speed reduced by a factor of 147 withrespect to the electric motor drive speed isavailable at the gear mechanism output.


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Brake pistonSpindle

Cylinder

Brake disc

Parking brake application:The nut moves forwards on the spindle. Thecylinder makes contact with the piston.Cylinder and piston are pressed against thebrake disc.

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Parking brake release:The nut is screwed back on the spindle, thusrelieving the load on the cylinder.The recovery of the sealing ring moves thepiston back and releases the brake disc.

A spindle which drives the brake piston isresponsible for converting the rotation into astroke.The spindle is driven directly by the swashplate mechanism. A cylinder is mounted suchthat it can slide axially in the brake piston.Two plane surfaces stop the cylinder turning.The flared section at the end of the cylinder isprovided with a forcing nut. Rotation of thespindle moves the forcing nut on the spindlethread.

The number of motor revolutions ismeasured by a Hall sensor.This enables the piston stroke to becalculated by the control unit.


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Electricmotor

Toothed belt Input gear

Swash plate

Output gear

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Swashplate

Mode of operation of swash plate

mechanism

A wheel (swash plate) with bevelled splines ismounted on the input gear. It is mountedobliquely with respect to the input gear shaft.

This causes the plate to wobble as the inputgear rotates.The plate is fixed in position by keyways inthe gear housing. It cannot turn freely.

The swash plate has 51 teeth, the output gear50 teeth.

As a result of this so-called "pitch error", theswash plate teeth always make contact withthe flanks of the output gear and nevercoincide with the tooth spaces.Consequently, the output gear is moved onby a small angle of rotation.


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Two output gear teeth are meshed with twoswash plate teeth in the course of onerevolution of the input gear. The wobblecauses the second pair of teeth (position 2) tobe meshed after half a revolution of theswash plate. In position 1, the output gear ismoved on such that the tooth of the swashplate again makes contact with an outputgear flank in position 2. As a result of this

sequence, each half revolution moves on theoutput gear and the spindle connected to itby half a tooth face width.

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Swash platePosition 2

Position 1

Functions

The following functions are provided by the electromechanical parking brake:

Parking brake function Dynamic emergency braking function Adaptive starting-off assistant Brake pad wear recognition and play correction

Parking brake function

The tensioning force set by the system issufficient for all driving situations. A textmessage in the dash panel insert centredisplay warns the driver in the event ofgradients exceeding 30 %.Activated status is indicated by lamps in theswitch and dash panel insert.

The brake is automatically re-tensioned if thedisc cools down after parking the vehicle. Forthis purpose, the current disc temperature isconstantly established by way of a simulationmodel in the control unit.


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Dynamic emergency braking function

Pulling the parking brake pressure switchF234 slows the vehicle at a maximumdeceleration rate of 8 m/s2.Operation corresponds to that of thehandbrake lever. The vehicle is braked as longas the switch is pulled. Braking action isterminated on releasing the switch.

If the vehicle is travelling at a speed of morethan 8 km/h, braking is implemented by theESP. With the accelerator pedal still pressed,engine torque is reduced to idling level andbrake pressure is built up by the ESPassembly at all four wheel brakes. The cruisecontrol system is deactivated if in operation.

Actuation of the switch at vehicle speedsbelow 8 km/h causes the parking brake to beapplied.To prevent possible incorrect operation(triggered for example by the frontpassenger), active emergency brakingfunction is deactivated as soon as theaccelerator is pressed again.

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ESP (ECD)

8 km/h

Electromechanicalparking brake

Vehicle halt

Activation of emergency braking function


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Adaptive starting-off assistant

This function permits smooth hill startingand stops the vehicle rolling back. Thefunction is only activated if seat belt isfastened.The tilt angle is measured by a sensor in thecontrol unit. In addition, the control actionmakes allowance for engine torque,accelerator pedal position and gear selected.The parameters listed above govern the pointat which the parking brake is released whendriving off.

Automatic calibration of the tilt angle sensorand starting-off parameters takes placeconstantly.Whenever the vehicle is started on the flat, itsacceleration behaviour is evaluated andadjusted for control purposes to theparameter set stored in the control unit.The function can be deactivated at theworkshop but not by the driver.

V282Left parking brake motor

V283Right parking brake motor

Tilt angle

Tilt angle sensor(integrated into control

unit)

J220Motronic control unit

Selector lever position

Accelerator pedal

valueEnginetorqueEnginespeed

J217Automatic gearboxcontrol unit

Drive systemCAN

J540Electromechanical parking brakecontrol unit

F125Multi-function switch

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Acceleratorpedal value

J234Airbag control unit(belt interrogation)


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Brake pad wear recognition and play

correction

The pad thickness is automaticallydetermined cyclically (approx. every 500 km)with the vehicle stationary and the parkingbrake not applied. For this purpose the brakepad is moved out of neutral position (= endposition) towards the brake disc. The controlunit uses the value measured by the Hallsender to calculate the brake pad travel andthus the pad thickness.

Measurement is performed with the vehicleparked, the ignition lock applied and theparking brake released.

If drivers regularly use the parking brake, thewear measurement may be less precise thanif the parking brake is seldom applied.

Roadworthiness test mode

Metered braking on a dynamometer isnecessary for checking parking brakeoperation.Roadworthiness test mode is recognisedafter 3 seconds if the rear wheels are turningat a constant speed of between 3 and 9 km/hon the dynamometer roller.Terminal 15 must be on for this purpose.

The parking brake application action ismodified by the control unit:Each time the switch is actuated, the piston ismoved by a defined small amount and thebrake applied slightly more.

Special system functions

Pad change mode

Pad change is performed using the diagnosistester VAS 5051 with the parking brake not

applied.In basic setting function 5, the cylinder isfully retracted by the spindle drive (refer toReleasing parking brake on Page 38). The padcan be replaced after resetting the brakepiston with the special tool VAS T10145.

In basic setting function 6, the cylinder ismoved back towards the piston (refer to

Applying parking brake on Page 38).The pad thickness is entered in adaptionfunction 6 (for detailed information, refer tocurrent Workshop Manual).


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Emergency release

An applied parking brake can be releasedmechanically if electrical actuation is nolonger possible or in the event of mechanicalproblems with parking brake components.An emergency key is provided for thispurpose in the vehicle tool kit.The vehicle is to be jacked up and theappropriate wheel removed.

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The Torx head at one end of the key is used toremove the actuator from the brake caliper.The spindle can then be turned with theopposite end of the emergency key until thebrake is released.

Fault displays

Flashes constantly if parking brake has notbeen properly applied. Flashing on actuatingparking brake pressure switch F234 indicates

a wiring fault.

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Fault detected by control unit restrictingoperation.

System fault; vehicle should no longer bedriven for safety reasons.


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J540 Electromechanical parkingbrake control unit

Parking brake status (2) Tensioning force attained (2) Deceleration request (2) Deceleration request release (2) Terminal 15 status (2) Parking brake fault message (2) Fault lamp (6) Acoustic warning (6) Wear display (6) EPB text messages (6)

J533 Gateway (1)

Mileage (old) Time (old)

J104 ESP control unit (2) Vehicle speed Wheel speed TCS/MSR request ABS braking ESP/EBPD intervention Brake pressure Front brake temperature/EPB Deceleration available EPB message plausible Quattro drive

J220 Motronic control unit (3)

Engine speed Closed throttle position

information Accelerator pedal value Engine torque loss Driver input torque Clutch switch status

J217 Automatic gearbox control

unit (4)

Gearbox status Target gear/gear engaged Selector lever position Torque converter lock-up

clutch status

J255 Front climate control unit(7)

Ambient temperature

J285 Control unit with display in

dash panel insert (6)

Displays Acoustic signals (gongs) Mileage Date Time Standing time

J518 Entry and start

authorisation control unit (5)

Status of steering wheel lock Terminal 15 on S-contact

J234 Airbag control unit

Driver's belt buckleinterrogation

Convenience CAN

DashpanelinsertCAN

DrivesystemCAN

Diagnosis CAN

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Electromechanical parking brake CAN data exchange


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ESP

Summary

The Audi A8 `03 is fitted with the ESP 5.7 already used in the Audi A4. In addition to thenecessary software adaptation to the new vehicle, the main new features are as follows:

Communication interface ECD

(electronically controlled deceleration)

The interface enables other vehicle systemsto actuate the ESP. The ESP control unit J104

can be informed directly of decelerationrequests. The ECD request involvesdeceleration of the vehicle at a maximum rateof 8 m/s2.

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J428 Distanceregulation control unit

J104 ESP control unit

Wheel brake Wheel brake Wheel brake Wheel brake

Brake pressure is built up evenly at all fourwheels.

The interface in the A8 is used by theelectromechanical parking brake andadaptive cruise control functions.


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Active speed sensors

The new sensors detect the correspondingwheel speed directly at the wheel bearing byway of magnetic multipoles.Direction of rotation and size of air gap arealso determined (refer to Systemcomponents for design and operation).

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Active speed sensors

The new sensors detect the correspondingwheel speed directly at the wheel bearing byway of magnetic multipoles.Direction of rotation and size of air gap arealso determined (refer to Systemcomponents for design and operation).

New software modules

The TCS function has been upgraded toinclude improved traction on non-compactedsurfaces (e.g. deep snow).Greater wheel slip values are accepted foracceleration when travelling straight aheador with small steering angle.Directional stability has priority whencornering. The permissible slip values arereduced.

ESP control action is reduced if a sportydriving style is required. Directional stabilityis maintained but larger float angles arepermitted, leading to higher wheel slip valuesin transverse vehicle direction.A sporty driving style is recognised fromevaluation of accelerator pedal actuation.


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ESP

System components

Hydraulic modulator

The basic version of the hydraulic modulatorcorresponds to that in the Audi A4.Compliance with noise level requirementswith adaptive cruise control actionnecessitates the use of integrated suctiondampers. These take the form of smallchambers which dampen brake fluidpulsation by way of rubber diaphragms.This modified modulator is used exclusivelyin vehicles fitted with adaptive cruise control.

Attainment of a high level of braking comfortrequires the use of the linear solenoid inletand switching valves developed for theESP 5.7.

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Solenoid switching valve

Solenoidintake valve

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Solenoidoutlet valve

Solenoidinlet valve

Returnpump

Solenoidoutlet valve

ECD request not active: Valves deenergisedDriver can regulate brake pressure by way ofopen solenoid switching and inlet valves.

Pressure build-up in response to ECD request:Solenoid switching and intake valvesenergised, return pump suction action viaopen solenoid intake valve and pump

regulates brake pressure.


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Spring

Coil

Seat

Sealing element

FF + FH

FM

Varying the current level makes it possible toset different opening pressures.In addition, the valve stroke (= valve openingcross-section) can be set in the rangebetween valve closed and valve fully open.This mode of operation enables the brakepressure to be varied as required.Such action is a prerequisite for comfortablevehicle deceleration.

Mode of operation of linear solenoid valves

Application of current to the coil of thesolenoid valve causes a magnetic force FM toact on the sealing element.The sealing element is pressed onto the seatin the valve housing. The forces exerted bythe spring (FF) and the hydraulic fluid (FH) actin opposition to the magnetic force.If FF+FH becomes greater than FM, the sealingelement is lifted off the seat and the valveopens. The higher the valve actuation current,

the greater the hydraulic fluid pressure mustbe to open the valve.


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ESP

Speed sensors G44-47

Design:

The measuring element is a Hall sensorconsisting of three Hall elements.The conventional sensor ring is replaced by amagnetised wheel bearing seal carrying48 pairs of north/south poles (multipole).

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Element A

Element B

Element C

Operation

The sensor detects changes in magneticflux density.

The three Hall elements are in offsetarrangement. The gap between theelements is selected such that element Asenses a magnetic maximum when Cdetects a magnetic minimum.


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A difference signal A-C is formed in the sensor.

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Signal A

Signal C

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ESP

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Difference signal A-C

Signal B

Negativemaximum

Trailingsignal edge

Hall element B is located centrally between Aand C. Element B detects a magneticmaximum if signals A and C and thus also the

difference signal are at zero.The point at which signal B reaches itsmaximum value (positive or negative) isevaluated for detection of direction ofrotation.

If, for example, zero crossing of the differencesignal A-C is reached by a trailing signal edgeand the signal B maximum is then negative,

anti-clockwise rotation is recognised.

Electrical configuration

The speed sensor is connected by way of acurrent interface to the ESP control unit,which is fitted with a low-impedance shunt R.The speed sensor has two electrical

connections and forms a voltage dividertogether with the shunt.

Battery voltage UB is applied betweenconnections 1 and 2. The sensor signalproduces a drop in voltage US at the shunt.This signal voltage is evaluated by the control

unit.


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SensorIS

R

3

US

UB

1

2

The output signal of the speed sensor is aPWM signal (PWM = pulse width modulation).The number of pulses in a specific time unitprovides the

speed information.

The following information is encoded bymeans of the pulse width: Direction of rotation Size of air gap Installation position Detection of stop

The correct air gap size is important for

system operation and is detected andevaluated for system self-diagnosis.

North

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Width

South North

Speed information

Diagnosis

Within the scope of self-diagnosis the systemis monitored for mechanical faults, electricalfaults and implausible signals.

The most important system data are stored inthe measured value blocks and can be readout with the diagnosis tester VAS 5051.

Detailed information can be found in therelevant Workshop Manual.


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Tyre Pressure Monitoring

Summary

The Audi A8`03 is fitted with a new version of the tyre pressure monitoring system, theprincipal new features of which are described in the following.

Aerials

Use is made of active aerials.The radio signals transmitted by the tyrepressure sensors are converted by the aerialsinto digital signals.

There are two aerial versions which differ interms of the carrier frequency (433/315 MHz)to be processed.

The major advantage of this method is theminimal susceptibility to electromagneticinterference.

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Aerial

Sensor

HF cableHF cable

Control unit/receiver

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Aerial/receiver

Sensor

LIN bus

Control unit

Tyre pressure monitoring control unit

The control unit is located beneath the rearseat bench. There is no sensor signalconditioning in the control unit.

1st generation components and networking(Audi A8 up to `03)

Data transfer

The digital signals are transferred from theaerials to the tyre pressure monitoringcontrol unit via the LIN bus.

2nd generation components and networking(Audi A8 as of `03)

Only one control unit frequency version isrequired as the signals are alreadyconditioned in the aerials.


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Control concept

System operation forms part of the MMI

control concept.Pressing the CAR button and selecting"Systems"- "Tyre pressure monitoring system"provides a display of the current tyrepressures and temperatures or enables thefollowing settings to be made:

System activation/deactivation

Storage of tyre pressures

(Refer to operating manual for detailedinformation)

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Warning displays

Warnings continue to be displayed in the dash panel insert.The two-stage warning priority system has been retained(refer to operating manual for detailed information).


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J502 Tyre pressure monitor controlunit

Allocation of identifiers to aerials(1)

Request for aerial messages (1) System status (all) Output of information (warnings,

system faults, pressure,temperature) (2, 3)

J285 Control unit with display in

dash panel insert (2)

Status of warning and systemfault displays

J104 ESP control unit

Vehicle-speed signal

J523 Front information display and

operating unit control unit (3)

User request

J220 Motronic control unit

Engine speed

J533 Gateway

J644 Energy management control

unit

Load deactivation (LIN driver)

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Convenience CAN

LIN

Dash panelinsert CAN

Drive system CAN

MOST

Tyre Pressure Monitoring

Tyre pressure monitoring system CAN data exchange

R59...R62 Tyre pressure monitoringaerials (1)

Tyre pressure, temperature,battery status of sensors

Level of signals received

J518 Entry and start authorisation

control unit

Terminal 15 status


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Service

The range of service diagnosis options has

been extended.Aerial diagnosis is performed constantly anddoes not have to be started separately.

(For details, refer to current Workshop Manual

and assisted fault-finding)


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Run Flat System - PAX

Summary

For the first time in the luxury class, a "run flat" wheel system is available as an option for theAudi A8 `03. As compared to other systems, the PAX run flat system represents an optimumcompromise between handling, comfort and durability requirements. Both a summer and awinter version are available.

Support ring

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Tyre

Support ring

Wheel

Glycerine gel

Design:

The system consists of rim, support ring, tyreand tyre pressure sensor. All components

have been newly developed.The rim is of a completely new geometricaldesign.The support ring is fitted onto the centre ofthe rim and is made of a heavy-duty plasticwith honeycomb structure.The tyre is no longer tensioned behind therim flange by means of its bead, but ratherinserted in the rim seat.

The geometrical and structural design of thePAX tyre differs considerably from

conventional tyres, above all in the area ofthe side wall and bead.A glycerine gel is applied to the inner surfaceof the tyre to reduce the friction betweensupport ring and tyre in run flat mode.


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Operation:

In the event of partial or total loss ofpressure, the tyre rests on the support ring.The special design of the bead seating on therim stops a flat tyre coming off. This isparticularly critical in situations involvingcornering with the tyre side wall subject totensile load.The tensile force Fz causes the tyre bead torotate about the bead core, thus producing aforce Fw in the outer bead area which presses

the bead more firmly onto its seat.

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Fz

Fw

PAX enables a fully laden vehicle to be drivenfor a maximum of 200 km at a speed of max.80 km/h even with a completely flat tyre.Despite the use of the gel, componenttemperature and hence wear increase, inparticular on account of the friction betweentyre and support ring. A high degree of ridecomfort is maintained even in run flatsituations. A loss of pressure is thus not

always immediately apparent. For this reason,PAX always includes the tyre pressuremonitoring function.Run flat mode is indicated on the dash panelinsert centre display.

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Run Flat System - PAX

New tyre designation

PAX tyres have a new designation.

Arithmetically, the wheel used for the A8 corresponds to a wheel size of 18.3".

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Tyre width [mm]

Tyre diameter [mm]

Wheel diameter [mm]

Speed symbol

Load index

Service

Tyre removal/fitting involves completely new procedures.New tyre fitting machines and PAX attachments for conventional fitting machines areavailable.


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All rights reserved. Subject totechnical modification.Copyright* 2002 AUDI AG, IngolstadtDepartment I/VK-35D-85045 IngolstadtFax 0841/89-36367000.2811.05.20Technical status as at 07/02

Printed in Germany

SSP-285 Running gear Audi A8 Part 2.pdf

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