2008 Audi A4

Safety and driving pleasure

ESP and dynamic steering work together closely to improve handling characteristics and vehicle safety. Dynamic steering supports the stability program, because it can perform a countersteering maneuver in considerably less time than the brake system needs to generate braking pressure. These rapid maneuvers eliminate the need for frequent braking and generate extra smoothness and dynamism. Despite their enormous effect, drivers generally do not notice the corrections.

Fishtailing – a classic critical situation – is triggered by counterforces produced in response to sudden evasive action. Audi dynamic steering can correct for small to medium fishtailing by countersteering. Braking is not required unless fishtailing is severe, and even then it largely serves only to provide damping. Dynamic steering also provides assistance for understeering (when the car slides toward the outside of a curve). Steering briefly becomes less direct, very likely preventing the driver from steering beyond where the tires have adequate contact with the road surface. This function is available exclusively from Audi. None of our competitors offer it.

Braking on surfaces with different coefficients of friction (known as μ-split braking) is another situation that can be difficult to manage. Higher braking forces pull the vehicle toward the side with the greater coefficient of friction. Audi dynamic steering in the new A4 resolves this problem – largely on its own. Just about all the driver needs to do is turn the steering wheel in the desired direction of travel.

Uncompromising: The brakes

A braking system designed for uncompromising performance was a point of honor for the sportiest midsize wagon. The new Audi A4 comes with completely redesigned brakes that have grown by an order of magnitude over its predecessor. The deceleration potential and precision of these brakes are improved greatly. The driver senses a brake pedal whose response is both firm and precise, allowing the perfect amount pressure with very little effort.

The A4 leaves the factory with minimum17-inch wheels (or larger) in U.S.A. The brake pads are around 20 percent larger; the new high-performance pads combine high coefficients of friction with very little fading. The internally ventilated front disks were trimmed down for maximum heat dissipation – a feat accomplished by ideal integration of frictional surfaces and a
new design that manages without traditional cooling channels. The two halves of the disc are joined by hundreds of small metal cubes that can disperse a great deal of hot air quickly.

The V6 versions of the new Audi A4 feature larger front brake disks (diameter: 320 mm/12.60 in). The brake calipers have a composite design based on the floating-caliper concept. High-strength spheroidal graphite iron is used where a high level of rigidity is required. The piston housing – bolted into place and made of aluminum – conducts heat admirably. Taken together, these features yield brake calipers that are both lightweight and extremely rigid.

Other areas where Audi has saved weight include the brake disks (up to two kilograms (4.41 lb) when installed) and aluminum cover plates (1 kg/2.20 lb). These reductions further reduce the weight of unsprung components, improving handling and comfort. An additional benefit for customers is the design of the calipers and wheel rims. When a car is brought in for servicing, mechanics can measure the strength remaining in the brake pads without even having to remove the wheels.

New design: ESP

ESP plays a major role in the agile, sporty impression made by the new Audi A4. This new generation of the drive stabilization system operates on the basis of highly exact hydraulic valves that manage braking pressure with unusual precision, regulating the brakes with no detectable vibrations and without the usual, notorious shuddering.

The operating concept also is new. ESP still can be completely switched off by pressing and holding a button. In this case, ESP is deactivated in all operating situations. The only systems that remain active are EDL and ABS, as well as support of μ split steering stabilization by the optional dynamic steering system, to make it easy for the driver to control the situation. In the instrument cluster, the message “ESP switched off” appears. If the button is pressed a second time, ESP is switched back on again.

Now, however, a second, lower level is available. Briefly pressing the ESP button at speeds below 100 km/h (62.14 mph) allows the driver to deactivate anti-slip control, which essentially switches off engine involvement and applies the brakes at a slightly reduced level. The stabilizing steering maneuvers provided by the dynamic steering system are not affected. A monitor light switches on in the cockpit as a safety feature.

“ASR-off” mode on the A4 with front-wheel drive is a huge advantage when driving with snow chains. It allows the wheels to spin – which can often be helpful – without having to switch ESP off completely. At speeds exceeding 70 km/h (43.50 mph), front-wheel-drive models automatically switch back to full ESP mode. In quattro models, “ASR-off” mode remains activated at all speeds. In the latter case, this mode works with the vehicle’s power distribution (which favors the tail) to help experienced drivers when drifting on surfaces with low coefficients of friction.

ESP in the new Audi A4 also has further capabilities. On wet surfaces, it wipes the film of water off the brake discs with brief, imperceptible applications of the brake pads. (Audi was the first to offer this feature within its segment on the previous generation A4.) ESP also compensates for the fading effect that can arise during repeated emergency stops.
Audi engineers designed ABS in close cooperation with the developers of the brand of tires used in the original equipment. The brake control strategy was finely tuned to allow the new A4 to take full advantage of the frictional potential of its tires.

The assistance systems

Calmer, safer driving – the innovative assistance systems on board the new Audi A4 help to make your trip more relaxed. Audi is the engine driving progress in this area, equipping the Avant upon request with high-tech systems taken directly from the luxury class. These systems regulate the distance to the preceding car, help the driver change lanes, and assist with parking.

Over the past decades, passive and active safety systems have significantly reduced the number of accidents and lessened their effects. And Audi is taking this progress to new levels. Its novel support technologies can, like human beings, register aspects of a car’s surroundings and use that information to make correct decisions and take appropriate action. Audi systems make for a relaxed, safe drive. Although responsibility still rests with the driver, these systems support the driver’s ability to concentrate by alleviating some of the stress.

ACC maintains the distance

Another assistance system that comes from the luxury car class is radar-supported automatic cruise control (ACC). It not only keeps the car traveling at any chosen speed from 30 to 200 kilometers per hour (18.64 to 124.27 mph), but also maintains a safe distance from the vehicle in front. The system reacts to vehicles up to 180 meters (590.55 feet) away: its radar sensor detects any change in the distance the car is maintaining from the vehicle ahead.

If the driver wants to overtake quickly, ACC is overridden by pressing the accelerator down hard, but it stays active and returns to the previously chosen road speed and distance from the vehicle in front. If the brake pedal is pressed down, ACC is switched off completely, and then has to be selected again at the control lever (Reset). The last speed setting before it was switched off is recovered.

Doppler effect and signal transmission/reflection time: how ACC gets its measurements

The radar sensor is in a casing with a plastic lens that is installed in the decorative grill at the right of the single-frame radiator grill. The sensor scans an angle of eight degrees each side of its centerline, and it has a range of about 180 meters (590.55 feet). The four radar transceivers operate in the 76.5 Gigahertz frequency band, which is licensed for radio transmissions. The transmission is of the Frequency Modulated Continuous Wave type. The measured values that the system needs are the frequency differences between the transmitted radar signal and the one reflected back from the vehicle ahead. Using the Doppler effect and the time it takes the signal to return, the speed of the vehicle in front and how far away it is both can be calculated. The preceding vehicle’s angle from the sensor’s center axis also can be determined by comparing the signals received at the four antennas. But before the relevant vehicle can be isolated from the other objects detected by the sensors, the car’s own path (the ‘tunnel’ within which it is moving) has to be determined as well.

This ‘tunnel’ is computed from signals received from the ESP yaw rate sensor and the steering angle sensor and from radar information derived from road boundaries (for instance crash barriers) and the movements of other vehicles. The system is integrated into the CAN network that extends through the whole car; it can, therefore, communicate with the other control units (for example engine management, transmission or brakes) within a few thousandths of a second.

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