September 7, 2004

The Audi A4: The Dynamic Chassis
Source: Audi AG

Dynamics and comfort, easy handling and active safety at the very highest level: on the basis of these strengths, Audi is also demonstrating its proverbial "Vorsprung durch Technik" with the new A4's dynamic chassis. Agility and ride comfort are equally impressive.

This road behaviour is the result of extensive modifications in the area of elastokinematics as well as to the spring and shock absorber settings.

The predecessor's tried-and-test basic geometry has been adopted at both front and rear: four-link front suspension and the highly complex self-tracking trapezoidal-link rear suspension. Both consist primarily of light-alloy components resulting in a low unsprung mass.

All versions of the Audi A4 have disc brakes at the front and rear; the front discs are ventilated. Their dimensions have been adapted to the increased performance potential of the new generation of engines. Excellent controllability, spontaneous response and above all considerably reduced braking distances are the result.

The Front Suspension

On the four-link front suspension layout, each wheel is located by four aluminium control arms. Large-volume rubber components are used as the damper mounts on the new suspension design. This helps to effectively isolate the shock absorbers acoustically.

Several mounts as well as the considerably stiffer track rods on the front suspension have been adopted from the high-performance S4. The mounts for the control arm originate from the new Audi A6. 

Together with the new shock absorber settings, this results in a particularly sensitive response to steering movements and even more precise feedback from the road. In other words, extremely dynamic behaviour which is isolated almost completely from front-wheel-drive influences.

High-tech Rear Suspension

The new Audi A4 comes with one of the most complex, but also most efficient, rear suspension layouts currently available.

The trapezoidal-link rear suspension makes for top-quality ride comfort on all models, coupled with dynamic behaviour that will amaze the driver of many a competing model.

The trapezoidal link is made from hollow-section aluminium. This torsionally and flexurally rigid control arm absorbs a large portion of the forces acting on the wheel. Together with the track rod located behind it, it thus determines the elastokinematic behaviour of the axle.

This design is notable for its low weight as well as its outstanding space efficiency. The compact design permits a low load-area floor in all versions, coupled with a generous load-area width. 

All wheel control elements pivot on the resiliently mounted subframe via rubber-metal mounts which isolate axle vibration very effectively from the body. Moulded rubber mountings are also used for the suspension springs between the trapezoidal link and the body's longitudinal member. Here again, the material chosen provides excellent acoustic insulation.

The twin-tube gas-filled shock absorber is connected to the wheel carrier, so that a direct-ratio effect is obtained. Combined with the separately mounted spring, this ensures a particularly sensitive response from the shock absorber.

The trapezoidal links and wheel carriers on the new A4 generation now also originate from the S4, a car whose suspension has demonstrated its all-round superiority in numerous individual and comparative tests conducted by the trade press. However, several mounts and the shock absorbers, which are considerably larger than on the previous model, have been adopted again from the A6. 

Thanks to the precision work that has gone into modifying the rear suspension set-up, the dynamic qualities of this basic structure are demonstrated even more effectively. This all adds up to agility and excellent directional stability, outstanding straight-line running and a high level of comfort without any disturbing movements of the body.

The Steering: Precision and Lightness

The new Audi A4 has hydraulically assisted rack-and-pinion steering bolted rigidly to the suspension-strut cross-member.

The progress that has been made by detailed modifications can be felt above all at the steering wheel: the driver registers this as greater precision, an even more spontaneous response to steering movements and more precise feedback from the road.

Audi's developers have also achieved an optimum steering layout in terms of safety for the A4. Because the steering column is mounted in two universal joints, it can be telescopically disengaged in the event of a crash, to isolate it from the steering gear and protect it against bulkhead intrusion. 

The dictates of lightweight design even extend as far as the design of the steering wheels for the Audi A4: for reasons of weight, the single-section skeletons are die-cast in magnesium.

All new Audi A4 2.0 T FSI, 3.2 FSI and 3.0 TDI come as standard with hydraulic rack-and-pinion steering featuring speed-dependent power assistance, servotronic. The development goal here was to provide the highest conceivable standard of sporty steering precision and improved steering comfort. 

The refined servotronic system enables the driver to manoeuvre into parking spaces with a minimum of steering effort and maximum accuracy, and also provides a clear response from the road at higher speeds. The system also offers a high level of sensitivity around the straight-ahead position and even more comfortable initial response behaviour.

Road Behaviour: A Policy of Neutrality

An excellent standard of comfort and superb traction when coming out of even the tightest of curves, neutral, clearly defined self-steering characteristics and, finally, maximum agility. These were the requirements to be met by those whose task it was to develop the Audi A4's suspension. 

Such targets can only be attained by coordinating all new and modified elements to a high standard of precision. The role of the new self-tracking trapezoidal-link rear suspension in the overall package is of course particularly significant. Its elastokinematic properties ensure above all the desired precision of response, while significantly pushing back the handling limits.

In steady-state circular motion, the most revealing gauge of behaviour at the limits of handling, it transpires that at moderate to high lateral acceleration, the Audi A4 requires only a slight increase in the steering wheel angle to remain on course.

This effect from the driver's viewpoint is clear enough: the Audi A4 remains unruffled even when close to the handling limits, and its behaviour is controllable and strictly neutral along its chosen line.

At the same time, body movements even in S-bends remain moderate, without any of the loss of comfort that might be expected from this dynamically firm layout. A positive effect that is preserved even in a wide variety of load conditions.

Safety Featured as Standard: New Generation of ESP

All new Audi A4 models come with the latest electronic stabilisation program, ESP 8. This system comprises ABS brakes with electronic brake-force distribution EBD and hydraulic brake assist which automatically increases brake power when the driver applies the brakes in an emergency. The traction control system ASR interacts with the electronic differential lock EDL to provide efficient yaw control as the foundation for enhanced ESP safety.

The system has been developed to an even higher standard in terms of both electronics and hydraulics. New pressure control ensures even more precise intervention of the ESP stability functions in accordance with the given situation, particularly when the car is understeering. In the event of excessive understeer, all four wheels are braked if necessary so that the vehicle can be returned to its original course.

Yet another new feature is the brake disc wiping function integrated into ESP. This new function helps to keep the brake discs largely dry on wet roads even when the driver does not apply the brakes for a relatively long time. To provide this effect, the brake linings are pressed against the discs under low hydraulic pressure at regular intervals, ensuring even more spontaneous brake response under wet conditions, too. This process goes unnoticed by the driver.

The new Audi A4 comes for the first time with a two-stage ESP deactivation button: pressing the button briefly, the driver is able to deactivate the traction control system ASR. This is sometimes necessary in deep snow or on gravel roads in order to pull away smoothly. Pressing the button a bit longer, on the other hand, the driver is able to deactivate ESP in all operating conditions. This is necessary on the race track, for example, to enable an intentional sideways slide through the use of the brakes. Even then, however, the EDL and ABS functions remain operative.

Wheels and Tyres

Even the four-cylinder and six-cylinder versions (above 96 kW) of the new Audi A4 come as standard with 7x16-inch alloy wheels shod with 205/60 R16 tyres. The particularly powerful six-cylinder versions and the 2.0 T FSI run on 7x16-inch wheels with 215/55 tyres.

As an option, the new Audi A4 is available with a choice of ten different wheels measuring up to 7.5 x 17 inches and running on tyres up to 235/45 R17. 
In winter, too, the A4 driver has the choice of three wheels up to 7x17 inches in size and running on 225/50 tyres.

All new Audi A4 models come as standard with Audi's tyre mobility system, but may also be provided with a spare wheel as an option.

A permanent tyre pressure monitoring system is also available as an option for the new Audi A4. The system monitors the tyre pressure and temperature of each individual wheel. In the event of a loss of pressure, it informs the driver optically and acoustically via the display in the instrument cluster. 

To provide this function, the signals from the wheel's electronic control unit are transmitted wirelessly to the central vehicle electronics. There are two warning stages: a pressure loss of more than 0.3 bar at a wheel activates a yellow warning lamp. This tells drivers to check their tyre pressure and top up the air to the correct level at the next opportunity. If tyre pressure is more than 0.5 bar below the limit required or if the tyre is losing pressure quickly, a red lamp lights up urging drivers to stop immediately and remedy the problem.

The system is also able to take minor changes in temperature and pressure caused, for instance, by intense sunshine, hot air coming from the engine or unusual weight distribution of the car into account, thus avoiding a false alarm.

As an Option: PAX wheels

The new Audi A4 will also be available with cast aluminium wheels with run-flat tyres - the so-called PAX system. These wheels and tyres allow drivers to continue for up to 200 kilometres at a top speed of 80 km/h, even if a tyre has suffered a complete loss of pressure.

Thanks to the run-flat properties of the PAX wheel, drivers can, in the event of a puncture, reach an Audi workshop of their choice instead of having to change the wheel themselves or call a breakdown recovery service - a new dimension to mobility that befits the luxury segment.

The flat tyre still offers a high level of residual ride comfort - driver do not perceive a loss of pressure as a permanent deterioration of running characteristics. However, to make drivers aware of the threat of a flat tyre or a gradual loss of pressure, the PAX system is only offered in combination with the automatic tyre pressure monitoring system. This means that driver are kept informed of the condition of all wheels and are in addition reminded of the maximum permitted speed of 80 km/h if one of the tyres suffers a puncture.