The Audi A4: Dynamic Suspension

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October 2, 2007

Source: Audi AG

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Dynamic suspension

The dynamic suspension on the new Audi A4 sets higher standards than ever before, combining precision and dynamism with a high level of stability and a supreme sense of control. It has been entirely redesigned, with suspension mountings, steering, wheels and brakes developed for top performance and many aluminium components used to keep unsprung weight to a minimum. Two innovative technologies make the driving experience even more fascinating: the Audi drive select adaptive regulating system and Audi dynamic steering. The new Audi A4 – the new way to drive.

The five-link front suspension makes a considerable contribution to the car’s dynamic character. By moving the differential to the front and the clutch to the rear, the design engineers were able to reposition the front axle 154 millimetres farther forward – a design approach that Audi introduced for the first time on the current A8 model. Together with relocation of the car’s battery in the boot, it optimises front and rear axle-load distribution.

Since the engine’s weight is on the front wheels, good traction is assured; front-wheel drive has proved its superiority in terms of safety, directional stability and performance in winter. It is only surpassed by the quattro versions of the new A4; since this special driveline concept includes an additional differential at the rear, weight distribution is even better balanced.

Even when seen at a standstill, the new Audi A4 displays the principal aspects of its character for all to see: the wide track (1564 mm at the front, 1551 mm at the rear), the large wheels and the short body overhangs are visual expressions of dynamic, sporty character. On the road, drivers will find that the A4 is the sportiest of the midsize saloons – an agile car that can be guided and positioned with effortless ease. The A4 responds willingly and eagerly to steering-wheel movements and handles in an almost neutral manner as the limits are approached. It exhibits very little understeer and maintains maximum traction when leaving a corner.

The hard core: the front-axle subframe

The largest and most complex component in the entire running gear is the aluminium support beam for the engine and the front suspension assemblies. It is bolted rigidly to the front end of the car and thus forms part of the body structure. Thanks to its high rigidity, steering movements are carried out promptly and accurately. On the new Audi A4, aluminium is also used for the bearing mount that links the upper suspension control arms with the body, and for the pivot bearing. The latter is manufactured by a combined casting and forging technique that gives it very high strength.

Each front wheel is located by five suspension links – a support link and a control arm at the bottom and two control arms at the top. The fifth link – the track rod –connects the steering box and the pivot bearing. All these links are made of forged aluminium, to keep unsprung weight low, provide ultra-precise wheel control and ensure a high level of crash safety. To save weight, the anti-roll bar is actually a high-strength tube.

The geometry of the upper and lower control arms enables the virtual steering axis – a value obtained from an imaginary extension of the control arms to the point where they intersect – to be close to the wheel centreline. This reduces the leverage exerted by driveline and external forces, so that the driver does not feel them at the steering wheel.

The front-axle kinematics on the new Audi A4 have a series of additional advantages. The camber and caster angles can be chosen for precise centring of the steering, which adds to the sense of steering accuracy around the straight-ahead position. Toe angle changes in response to lateral forces were chosen for a harmonious initial steering response even at high speeds. The generous diameter of 102 mm increases wheel-bearing life.

The five-link suspension with its tapering control-arm layout handles longitudinal and lateral forces separately when the car is in motion. The bearings are rigid in a lateral direction, to promote sporty precision and permit high cornering speeds. Longitudinally, however, they are softer and suppler; in this way the dynamism of a sporty saloon and the ride comfort of a luxury car are combined in the new Audi A4.

Design breakthrough: the low-mounted steering system

The steering system used on the previous model was also extensively revised, the decisive step being its repositioning. It was previously high up and well to the rear, above the gearbox and attached to the base of the radiator water chamber. It is now farther forward and lower, on the subframe just below the front axle.

Since movement of the steering wheel is transmitted to the wheels in a very direct manner by the track rods, steering response is immediate, and feedback from the road surface precise and well differentiated. Friction-optimised track rod joints, lubricated with a special grease, add to this desirable level of sensitivity.

The steering column has a torsionally rigid bolted joint at the cross-member below the windscreen and the bearing mount on the firewall, which also makes steering response more precise.

The standard power steering has a rack and pinion mechanism in an aluminium casing. With an overall ratio of 16.1 : 1, the steering is sporty and direct, but with no hint of nervousness. A controlled-output vane-type pump supplies the necessary hydraulic energy; unlike conventional power-steering pumps, which circulate a large volume of oil internally, it delivers only as much oil as is needed in any given operating situation. This reduction in energy consumption lowers the fuel consumption of the new Audi A4 by 0.1 litres per 100 kilometres.

In addition to the standard power steering, Audi offers a further technological alternative. The A4 3.2 FSI, A4 2.7 TDI and A4 3.0 TDI are equipped with servotronic power steering, with the degree of power assistance dependent on road speed. When parking the car, this system makes the operation more agreeable by reducing the effort needed at the steering wheel, whereas at higher road speeds the necessary effort is increased to ensure maximum precision.

Perfect ground contact: the rear suspension

Springing, wheel location, self-steering action – the rear suspension in the new Audi A4 fulfils all these tasks magnificently. The design engineers have based it on the toe-controlled trapezoidal layout chosen for the largest A6 and A8 model lines – with compact dimensions and excellent ride quality and comfort. The suspension kinematics for the A4, however, were computed without reference to any other models.

The backbone of the rear suspension is formed by a subframe resistant to torsional and bending loads. It is welded together from two longitudinal and two lateral tubes of high-strength steel, the longitudinal tubes being produced by the internal forming method using water at very high pressure.

This subframe is attached to the bodyshell by four large rubber mountings; these are extra-firm laterally for dynamic handling, but softer in the vertical and longitudinal directions for reasons of ride comfort. All the suspension control arms are acoustically decoupled from the axle subframe by means of elastomer bushings.

If sporty handling is to be combined with a high standard of ride comfort, the unsprung masses must be kept as low as possible. Audi has pursued this approach for all the rear-axle components. The two trapezoidal links are warm-hardened aluminium castings; the wheel carriers are of chill-cast aluminium. The upper control arms and the track rods are aluminium forgings. Their high rigidity ensures that toe and camber angles change very little when dynamic forces act on the wheels. The anti-roll bar, in fact a tube as at the front axle, also combines low weight with high rigidity.

The installed position of the suspension springs is entirely new: they no longer act on the trapezoidal links as on the previous model, but directly on the wheel carriers. The designers adopted this layout because it enables the new Audi A4’s ride comfort to be raised to luxury-car level and at the same time the road dynamics of this sports saloon to be improved still further.

Separate spring and shock absorber mountings improve initial response, and as a further contribution to ride comfort, suspension travel has been increased by 20 millimetres. The suspension bushings use special rubber mixtures that also enhance ride comfort. A novel type of elastomer element in the wheel carrier has a damping effect on vibration generated by wheel rotation. The rear suspension kinematics control brake dive closely and in this way also add to the car’s sporty, dynamic appeal.

Taut and designed for speed: the sports suspension

Audi also supplies the new A4 with sports suspension as an option; this has firmer springs and shock absorbers, and reduces the body’s ride height by 20 millimetres. The lower centre of gravity gives the car even greater visual tension and dynamism. Those who wish to intensify the car’s sporty character to an even greater extent will choose the S line sports suspension. This has even firmer damping, and the car’s body is lowered by a further 10 mm.

The compact trapezoidal-link suspension has notable packaging advantages: the floor of the boot in the new Audi A4 is low and flat, with a load-through width of 100 centimetres. Front-wheel-drive and quattro models have axles of almost identical construction, differing only in minor details: the subframes of quattro models, for instance, have an additional mounting for the rear-axle final drive.

A new dimension: Audi drive select

Audi drive select is not just a system that makes driving even more fun: it is a breakthrough in the midsize car segment and enables the A4 to explore entirely new dimensions. Audi drive select integrates the technical components that govern the driving experience into a single system: engine, automatic transmission, steering and the new adaptive shock absorber control; these modules, however, can be ordered separately.

By means of buttons on the centre console, the driver can influence the operation of the various components by choosing from three main modes, from ‘Comfort’ to ‘Sport’. If the full version of this system is ordered, the driver can even compose a specific profile that suits his or ideal personal preferences. Audi drive select makes up to four individual cars out of the new Audi A4.

Various versions of Audi drive select are available. Three systems are integrated into the first version: the engine’s response characteristic when the throttle is opened, the speed-dependent power assistance provided by the servotronic power steering and the automatic transmission’s shift points, as well as the dynamic steering and/or the damping control can all be determined in three main programmed ‘maps’ that are quite different from one another in nature.

The ‘comfort’ mode is ideal for relaxed progress on long journeys and over poor road surfaces; ‘dynamic’ is the most balanced setting and ‘dynamic’ the boldly contoured, firm stage in which the new A4’s running gear can demonstrate the full scope of its talents.

Changes from one characteristic to another are made at two arrow buttons on the centre console; the active mode is illuminated. All such changes are performed safely and smoothly: the driver can easily distinguish them but is not irritated when they take place.

For individualists: the personal profile

If the A4 is also equipped with MMI and therefore has a navigation system on board, the driver has access to a fourth set-up level known as ‘individual’. This can be configured via the control terminal and afterwards activated at any time. Within practical limits, this personal profile can be assembled from a wealth of possible settings. With this development too, the engineering team complied with the basic principle of simple, intuitive operation. Audi is convinced that the vast majority of drivers would find it confusing and unnecessary to have to choose from hundreds of different parameters.

The central element in electronic shock absorber control is the new high-performance control unit. It has the imposing data spread of 32 bits and operates at high speed. The computer analyses the signals from 14 sensors continuously, and calculates the current for the electrically controlled shock absorbers at each individual wheel a thousand times a second.

Continuous control: the CDC shock absorbers

The CDC (continuous damping control) shock absorbers, similar to those that Audi already uses on the luxury Q7 SUV with air suspension, are twin-tube gas-filled hydraulic units with an additional external valve and connecting pipe. Their operation can be continuously varied: an electromagnetically energised proportional-action valve opens against the force exerted by a spring and regulates the flow of hydraulic fluid between the inner and outer shock-absorber tubes. A smaller flow cross-section makes the damping characteristic firmer, a larger one makes it softer.

Within the mode that the driver has chosen at Audi drive select, the control unit employs adaptive characteristics. It modifies its command signals to match the driving style and road conditions. Even from the ‘comfort’ mode the shock absorbers can be switched in an instant to a firm setting if required, though not to the absolute limit. They are installed in conjunction with sports suspension springs, which have the effect of lowering the body by 20 millimetres, but still provide a good standard of ride comfort – on poor roads, as good as the standard suspension.

The control unit selects the optimal damping force for the actual driving situation, for instance firmer damping to prevent body roll when cornering fast or braking, lower damping force if the surface under the wheels is extremely rough or average settings on poor-surfaced country roads.

Tested in space: Audi dynamic steering

Sporty driving pleasure and a high standard of safety have been two typical Audi core values for a long time on all its cars. The dynamic steering system, now premiered in the new A4, is a new departure for the Audi brand. A superimposed zero-play transmission, of a type already used successfully on astronauts’ vehicles, varies the effective steering ratio according to road speed. The dynamic steering system moves sporty, safe driving up into a new dimension: together with the ESP electronic stabilisation system, it keeps the new A4 stable by means of slight, extremely rapid steering movements.

The superimposed transmission is located in the steering column and combined with an electric motor. It is of the ‘Harmonic Drive’ type that has already proved highly successful in robotics and space travel. In 1971, a transmission of this type voyaged into space for the first time on board Apollo 15: it was part of the individual wheel drive for the ‘Lunar Rover’ car used on the surface of the moon. In 1997, Mars was its next destination, on the ‘Sojourner’ expedition vehicle used during the spectacular ‘Pathfinder’ mission. This type of transmission is also operating on the Hubble space telescope, which was launched in 1990.

Proved in space, the strengths of the harmonic drive principle are applicable in all important areas of automotive technology as well. The transmission is extremely compact, light in weight and resistant to torsional stresses; it operates without play and is therefore extremely precise, with low friction. It can transmit immense amounts of torque and achieve high efficiency levels. Audi is the first automobile manufacture in the world to make use of this brilliantly conceived device, the features of which taken as a whole are far superior to anything that competitors have to offer.

Ingenious design: only three main components are needed

The harmonic drive transmission has the simplicity of genius: it has only three main operating components. An electric motor turns an elliptical inner rotor which, by way of a ball bearing, alters the shape of a thin sunwheel connected to the steering input shaft. At the vertical axes of the ellipse, the sunwheel meshes with the teeth on the inside of an annulus or ring gear, which acts on the steering output shaft. When the inner rotor is turned, the main axis of the ellipse changes, and with it the extent of gear tooth meshing. Since the sunwheel has fewer teeth than the annulus, there is relative movement between them – the superimposed movement used to alter the effective steering ratio.

The specific demands of the dynamic steering system have led to the appropriate optimisation of the components that form part of the system. The power steering pump supplies a larger volume of oil if necessary, and the servotronic operating characteristics have been revised. The control unit is an extremely robust unit, secure against outside interference. The dynamic steering system is available for all versions of the new A4 from 140 kW (190 bhp) upwards that do not already have servotronic as standard equipment.

Wide operating range: the ratio can be doubled

The dynamic steering ratio can vary by almost 100 percent, depending on the car’s road speed and the chosen Audi drive select mode. Changes in the ratio take place continuously and are unnoticed by the driver: here too the Audi system demonstrates its superiority over its competitors.

When parking the car, the dynamic steering system is extremely direct, with only two turns of the steering wheel from lock to lock, and very little effort needed thanks to a high degree of power assistance. At typical country-road speeds, the direct response and level of power assistance are reduced slightly, but even then the driver will not have to move his or her hands to another point on the steering wheel when negotiating a sharp corner. At high speeds on motorways or similar roads, a more indirect steering ratio and less power assistance ensure that the car can be driven in a relaxed, confident manner and has a high degree of straight-line stability; in theory, four turns of the wheel from one steering lock to the other would be necessary.

The dynamic steering system cooperates closely with ESP in the vehicle dynamics and safety areas. It relieves the burden on the electronic stabilisation program for the simple reason that a steering correction can be performed about three times faster than the time needed to build up pressure for a brake application. This ability to intervene so rapidly makes many a brake application superfluous, and the journey proceeds more dynamically and smoothly. Although the steering corrections are highly effective, the driver remains unaware of them in most cases, especially since no noise is generated.

Corrective action by dynamic steering

A ‘classic’ critical situation is caused by oversteer following a sudden load reversal. The rear end of the car may for instance slide outwards if the driver is obliged to change direction in order to avoid a collision. If the car’s angle of float is small to moderate, the dynamic steering system can rectify the situation alone, by turning the steering in the opposite direction. A brake application will not be made unless the angle of float becomes too great, and in most cases is only needed to suppress residual movement.

The dynamic steering system also lends a helping hand in the event of understeer; that is to say if the car tends to run wide on a corner. The steering ratio is made more indirect for a short time, so that the driver is unlikely to turn the wheel beyond the limit of good tyre grip. The steering angle remains small and easily controlled, and understeer is almost completely suppressed. This function is exclusive to the new Audi A4 and not available from its competitors.

Braking on surfaces with different friction coefficients under the left and right wheels, also referred to as µ-split braking, can give rise to situations that are difficult for the car to master. The side of the car with the better tyre grip and braking force pulls the car away from the chosen line. The dynamic steering on the new A4 can cope with this problem almost unassisted: the driver simply turns the wheel in the direction he or she wishes the car to take, and can then concentrate on more important matters. If a µ-split situation arises, the system takes action even if ESP is switched off.

No compromises: the wheel brakes

Audi would not be living up to its reputation unless it supplied a brake system of no-compromise high performance for the new A4, the sportiest car in the midsize market segment. The wheel brakes for the new Audi A4 are an entirely new development and larger than those used on the previous model by a whole order of magnitude. Even cars with a four-cylinder engine (1.8 TFSI and 2.0 TDI) have a 16-inch system at the front (with discs of 314 mm diameter) and rear (300 mm); the front brake discs are ventilated. V6-engined cars have composite aluminium brake callipers and larger (320 mm diameter) front discs.

Even on the standard brake system, the pad surface area has been increased by about 20 percent. The newly developed high-performance pads combine high, stable friction values with low susceptibility to fading even when called upon to sustain severe loads. The new brake discs also increase the system’s retardation potential significantly. The ventilated discs have been modified in detail to improve heat dissipation; this is achieved by optimal friction surface contact and a new design principle that dispenses with the conventional cooling channels. The two halves of the disc are connected by hundreds of small metal cubes, between which a high volume of heated air can dissipate in a very short time.

The brake callipers for the more powerfully engined cars are also totally new: they use the floating-calliper principle and are of composite construction. Areas calling for maximum load resistance are made from high-strength spheroidal-graphite cast iron. The bolted-on piston housing is made of aluminium, and conducts heat away most effectively.

These brake callipers are light in weight but exceptionally rigid. The driver of the new A4 will experience this as firm, precise pedal movement and accurate feedback that enables the brakes to be controlled sensitively and with low pedal effort. The brake-servo operating characteristic has also been modified with this aim in mind.

Audi has achieved weight savings with the new type of brake disc (up to two kilograms per car) and by using aluminium cover plates (one kilogram). Since this is unsprung weight, every reduction achieves a direct improvement in the car’s sporty handling. Calliper and wheel design also have specific advantages for the customer: in the dealer’s service reception area, for instance, the mechanics can measure brake pad thickness reliably with a simple tool; the wheels do not have to be removed.

New valves: ESP

ESP makes a considerable contribution to the agile, sporty impression that the new Audi A4 makes. This vehicle stabilisation system, supplied by specialist manufacturer Bosch, is now a new-generation version coded 8.1. It incorporates high-precision hydraulic valves of a new type that control pressure build-up with great accuracy, without unpleasant but all-too-frequent jerking or noticeable vibration.

The ESP operating concept is also new. As before, it can be switched off completely by holding the button pressed in, but a second, less complete shut-down stage is now provided as well. If the driver presses the ESP button once briefly at a speed below 70 km/h, the wheelslip control function will be de-activated. This largely prevents engine-management intervention and weakens the system’s brake applications slightly.

If the dynamic steering option is ordered, the stabilising steering impulses remain active. A warning light comes on in the cockpit as a safety precaution.

For the A4 with front-wheel drive, the “ASR off” mode has useful benefits when snow chains are fitted: the wheels can be allowed to spin, which is frequently an advantage – without having to switch ESP off completely. The “ASR off” mode is effective at any road speed on the A4 with permanent four-wheel drive, whereas cars with front-wheel drive switch back automatically to the full ESP mode above 70 km/h.

Versatile: new ESP functions

ESP can do even more on the new Audi A4, for instance stabilise a trailer that starts to snake by applying the wheel brakes of the towing vehicle separately in a rhythm opposed to the outfit’s swinging movements. If a panic brake application at a rate of retardation exceeding 0.7 g is made, it switches on the hazard warning flashers automatically. In wet weather, it clears the water film off the brake discs by applying the brakes briefly at intervals to an extent that remains unnoticed by the car’s occupants. If braking loads are very high, for instance when an enthusiastic driver is descending a mountain pass, it compensates for the fading effect caused by heat build-up that can occur if a full brake application has to be made.

During development of the new Audi A4, there was close cooperation between the engineers responsible for ABS and their colleagues at the tyre manufacturers who supply their products as initial equipment. Their work was closely coordinated and yielded a suitably modified ABS strategy, as a result of which the new A4 can make full use of the high friction ratings of its tyres when the brakes are applied.

Visual distinction: the choice of wheels

The range of wheels also emphasises the character of the new Audi A4. The various patterns of wheel, with sizes from 16 to 18 inches available, create a strong visual impression, and their large diameters of 660 mm or more fill the wheel arches in an imposing way. Audi is launching the new A4 with four patterns of elegant, high-quality wheel to choose between.

The sizes are 7.5 J x 16, 7.5 J x 17, 8 J x 17 and 8 J x 18. The 2.0 TDI is shod with aluminium wheels and 205/60 R 16 tyres; from the 1.8 TFSI upwards, 7.5 J x 16 alloy wheels with 225/55 tyres are fitted. Alternative wheels are available in the same size, but with different spoke patterns.

Three types of 17-inch wheel can be ordered, with 225/50 or 245/45 tyres. At the top end of the range is an 18-inch wheel with 245/40 tyre, but quattro GmbH is able to supply 19-inch wheels. Three different wheel types between 16 and 18 inches in diameter are available for use with winter tyres.

As a further option, Audi can supply 8 J x 17 wheels with 245/45 run-flat tyres. The tyres have special rubber elements inside that enable the car to be driven in safety for at least 80 kilometres at a speed not exceeding 80 km/h, even if air pressure has been totally lost. Since run-flat tyres are always supplied with a pressure monitoring system, the driver receives a warning in this situation. For the cold season, a 225/50 R 17 winter tyre suitable for use with snow chains is included in the range.

More precise measurements: the tyre pressure monitoring display

The innovative second-generation tyre pressure monitoring display that Audi installs on the new A4 is both intelligent and reliable. It detects rapid loss of air from the tyre in the same way as a conventional indirect-measuring system, but unlike such systems also indicates which wheel is affected.

The tyre pressure monitoring display also detects situations – caused by diffusion – in which all four tyres lose their pressure very gradually. This may not amount to more than 0.1 bar per month, but if disregarded can lead to severe damage. The system does not only guard against this situation in the conventional way by comparing the four road wheels’ speeds of rotation, but also registers the torsional vibration generated by the road surface. If there is a loss of pressure, the tyre’s sidewall stiffness changes and with it its characteristic natural frequency.

Since the tyre pressure monitoring display is a software system integrated into the ESP, its readings are not falsified by external circumstances – a heavily laden boot, snow chains on the tyres or loose stone surfaces. It re-analyses the frequency patterns continuously and can identify disturbance factors. After correcting the tyre pressures, the driver has to recalibrate the system. Unlike direct pressure measuring devices, which use battery-powered sensors at the wheels, The tyre pressure monitoring display needs no routine maintenance or replacement parts throughout its life. Here too, Audi customers profit directly from ‘Vorsprung durch Technik’. the tyre pressure monitoring display is standard equipment on cars with V6 engines.

The equipment, data and prices stated here refer to the model range offered for sale in Germany. Subject to amendment; errors and omissions excepted.

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At-a-GlanceExteriorBodyInteriorEnginesTransmissionsSuspensionAssistance SystemsMultimediaEquipment

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