The Audi A4: Body

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October 2, 2007

Source: Audi AG

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The new A4 is a large saloon car with a powerful road stance. Its body is strong in the truest sense of the word: thanks to new high-tech materials and processes, it is safer and more rigid than the previous model. It has also shed a significant amount of weight; with a drag coefficient of cD = 0.27, the new Audi A4 cuts smoothly through the wind.

By midsize standards, the new Audi A4 is an exceptionally large, spacious saloon. Its wheelbase of 2808 millimetres is not only longer than its predecessor but also distinctly longer than its competitors, and indeed approaches that of a model in the luxury class. The space for each of the five seat occupants is correspondingly generous, and the high standard of dynamic stability and calm, unwavering straight-line stability also profit from this long wheelbase.

The governing factor has been the repositioning of the front axle further forward. This in turn was made possible by moving the differential to the front of the driveline, where the clutch or torque converter was formerly situated. This layout, which has already proved successful in the large A8 luxury saloon, has a further major advantage too: it is a sound basis for the new Audi A4’s fascinating road dynamics.

Balancing act: the battery has moved to the boot

A further factor in the new Audi A4’s excellent balance is the relocation of the battery to the lower level of the boot. On both front-wheel-drive and quattro cars, the boot holds 480 litres, a volume that also puts the new A4 in the lead among its direct competitors.

Thanks to a loading lip that is only 673 mm from the ground, the low step when loading, the straight side walls and flat floor, the boot can be utilised in a most practical way. It is lined with high-quality carpet and, with a loading width of 1000 mm throughout and a length of 1067 mm, can easily accommodate two golf bags placed sideways. The optional luggage compartment package features a universal net, retaining hooks, a side net and a 12V power socket.

Among the many new A4 features that bring more than a hint of the luxury class with them is the boot lid opening system. There are three ways of gaining access to the boot: by radio-operated remote control, by pressing a button on the driver’s door or by touching the electrical switch in the handle recess. Any of these causes the lid to swing up automatically on its innovative hinged arms. Closure remains manual but needs very little physical effort.

The rear seat backs have been arranged to fold down, a useful feature for those who require flexibility and who have to transport bulky loads. The seat back catches can be released by handles from inside the car and folded down on to the seat cushions without the head restraints having to be removed. In this way, the luggage capacity is increased to 962 litres. Another optional extra is a load-through hatch complete with removable ski bag.

Intelligent weight-saving construction: basic weight only 1410 kilograms

A major strength of the new saloon model bearing the four-ring badge is its low weight, which adds to the pleasure of the sporty driver and also reduces fuel consumption. The A4 1.8 TFSI tips the scales at just 1410 kilograms; the figure for the 2.0 TDI is 1460 kg. Iron and steel account for 32.0 percent of the total weight, closely followed by aluminium and magnesium at 31.7 percent.

The low weight is also explained by the fact that the steel bodyshell is exceptionally light: although it is almost twelve centimetres longer and more than five centimetres wider than the previous model, it weighs about ten percent less. This was an ambitious target that Audi’s engineers solved by a total redesign of the bodyshell, starting from scratch on a clean computer screen.

Audi had been among the automobile industry’s absolute leaders for a long time in the intelligent weight-saving area. By adopting aluminium, for instance on its A8 and TT models, the company has kept the pressure on the steel industry for some years now, with the result that steel grades combining exceptional strength with low weight have now become available.

Hot-formed ultra-high-strength steels are an excellent example. They are the finest grades that the industry can currently supply. On the new A4, they are used to reinforce the centre tunnel and for sections of the side members, for the inner body sills, the firewall cross-member in the engine compartment and for the B-posts. If the same metal thicknesses are used as for conventional high-strength steels, the new grades possess much better mechanical properties. If on the other hand the previous mechanical properties are adequate, the gauge of the metal can be reduced.

In the hot forming process individual blanks made of boron-alloyed steel are heated to approximately 950 degrees Celsius in a continuous furnace, then shaped and quenched at the same time in specially cooled tools. The resulting microstructure enables high dimensional accuracy to be maintained and above all high tensile strength values to be reached.

These are in the region of 1650 Megapascals – similar to the cables of a modern suspension bridge, in which a single wire with a cross-section of one square millimetre has to carry a weight of more than 160 kilograms. Seven Audi A4 cars together weighing ten metric tons could be hung from a strip of this material only two millimetres thick and 30 millimetres wide.

The aim: controlled deformation

This is not to say that a totally rigid bodyshell would be ideal for a production car. On the contrary, the aim is to control the dissipation of forces by means of specific levels of deformation, so that loads on the car’s occupants are kept low. A large element produced from a single sheet-metal blank must therefore be of lower strength in certain predetermined part-areas.

In order to achieve this, the wall thicknesses of the warm-formed boron-alloyed steel are partially reduced, or it is combined with materials of lower strength such as micro-alloyed steels. The welded elements produced in this way are known as tailored blanks.

An excellent example of this process is the B-post for the new Audi A4. Its lower zone is rather more ductile (deformable) than the upper zone, since this is where energy has to be dissipated in the event of a side-on collision, taking into account the fact that the human pelvis can withstand more severe loads than the rib cage. The B-posts and the rear sections of the side members are in-house products; Audi is in fact the first automobile manufacturer to establish its own production facility for warm-formed tailored blanks.

Hot-formed ultra-high-strength steels already account for 12 percent by weight of the new A4’s body-in-white, that is to say without doors, windows, bonnet and boot lids. 18 percent are accounted for by conventionally processed ultra-high-strength steels, 32 percent by high-strength steels and 38 percent by conventional deep-drawing steel grades.

The joining techniques used for the body panels also exert a strong influence on rigidity and crash behaviour. Audi has adopted state-of-the-art methods once again for the new A4. Compared with the previous model the number of spot welds has gone down from 6500 to about 5500; in contrast, the total length of bonded seams has gone up from 26 to 125 metres.

Structural adhesive, a ductile elastic material that is applied hot, increases the strength of the joint and guarantees that flanges are fully sealed at the many points where this is necessary. It also allows the electric spot welds to be more widely spaced. In many areas, the two joining methods are combined. At the most crucial zones of the body, integral camera systems monitor the robots as they apply the adhesive to a very high standard of precision.

Art in body construction: the zero-gap roof joint

In addition to MIG and MAG welding, Audi makes use of two other complex techniques. Laser-beam welding is used at the sills and for the doors. The seam between the body side and the rain gutter at the boot, and the zero-gap joint between side element and roof – an extremely difficult area on any car – are produced by plasmatron brazing.

Audi’s high-tech character and quality approach not only become visible in these areas, but in fact everywhere on the body where narrow, strictly parallel joint gaps can be seen. Further proof of this high standard has been provided for many years now by the fully galvanised bodyshell, which enables a twelve-year warranty against body penetration by rust to be offered.

The highest possible level of occupant protection

Occupant protection reaches the highest possible standard in the new Audi A4. For this design area the brand makes use of a variety of sources and possesses a broad basis of relevant knowledge from its own investigations: all over the world, the AARU (Audi Accident Research Unit) looks into actual accident situations and evaluates data banks as appropriate.

If a frontal collision should occur, the A4 protects its occupants with an exactly planned sequence of measures, the precision of which sets new standards for a midsize car. In the very earliest stages of the crash, two acceleration sensors begin to transmit their signals to the central control unit; they are located below the headlights so that no time is lost in securing this initial information.

As the impact progresses, the front cross-member distributes the forces to both side members, which thanks to their carefully computed geometry and choice of materials deform to a predetermined degree to absorb this energy. The aluminium frame element for the engine and front axle acts as a further force absorption level by diverting forces and moments in a controlled manner into the floor and tunnel structure of the occupant cell. The steering column can be forced back by a maximum of eight centimetres, and the main elements of the pedal assembly released from their mountings.

Accurately matched: the occupant restraint systems

The task of protecting people of different sizes and statures in a head-on crash is a difficult one. Laboratory research carried out in Germany, the USA and Canada using dummies has shown that small people are still at considerably greater risk than tall occupants. In the new A4, Audi has therefore networked the occupant restraint systems more closely together than on any midsize car so far produced.

Sensors on the seat rails check the seated position adopted by the occupant and transmit these data to the control unit as information for the computing process. In this way the computer knows how far the seat occupant is from the airbag and can ensure that the amount of forward movement, during which the seat belt and the airbag can restrain the body, is optimally utilised.

The airbags in the new Audi A4 are adaptive and make use of an innovative, finely differentiated strategy capable of protecting the car’s occupants even more effectively and reducing the loads on their bodies. The conventional technique is for the two airbag stages to be triggered off one after the other; if the impact is not too severe, the first stage alone is sufficient, whereas in a more violent collision the second stage follows directly after the first. In other words, the only computing criterion is the severity of the impact.

In contrast, the airbags in the Audi A4 are always inflated fully. If the control unit assesses the situation as relatively harmless, that is to say if the impact is not too violent and the seat occupant is sitting close to the cockpit, part of the gas from the airbag is discharged again through valves, so that the head and chest of the occupant are restrained without undue force. If the crash is severe, the airbags remain fully inflated for longer. The true innovation, however, is that the same procedure is adopted after a less severe impact if the passenger is sitting so far back that his or her upper body moves forward with too much kinetic force.

For safety: adaptive seat-belt force limiters

The seat-belt force limiters also use an adaptive operating principle, with two integral torsion bars connected together by gearwheels. In a not too critical situation, the torsion bars are separated at an earlier stage in the accident. This gives the belt a greater range of movement – the seat occupant’s upper body plunges relatively deeply into the airbag and the load that has to be withstood in the chest area is reduced. In a severe accident, on the other hand, the torsion bars are either disconnected later or remain coupled together, so that the seat belt restrains its wearer more firmly.

In the event of a side-on impact, acceleration sensors in the C-posts and ultra-modern pressure-change detecting sensors in the doors trigger off the alarm. The B-posts, which guide part of the incoming forces into the roof frame, the sills and two cross-members in the floor between these, perform most of the deformation work, aided by the doors. The edges of the doors overlap the posts, sills and roof frame to a generous extent and can therefore bear against these elements and help to maintain the strength of the overall structure.

A positive feature is that the new A4’s doors are made in a single piece, unlike those on the previous model, which had a separate aluminium window frame. These frames and the impact-absorbing support members made of high-strength steel are now integrated into the body-in-white structure, so that the doors are lighter in weight but also more rigid. The close proximity of the support member to the door hinge enables forces to be deflected into the B-post.

All-round protection: up to eight airbags

Inside the car, sidebags with a volume of 13 litres are integrated into the front seats (12-litre sidebags are an option for the rear seats), and large windowbags (25.5 litres) cover the glass between the A- and C-posts. Furthermore, the driver and front passenger in the new Audi A4 are effectively protected against accidents of kinds by variable-height seat belts, head restraints with a generous amount of vertical adjustment and anti-submarining ramps in the seats – a solution that not all competitors offer as standard.

If the car reaches 25 km/h but the seat belts have not yet been fastened, a warning gong is heard. A seat occupation sensor identifies whether the front passenger seat is in use.

To protect its youngest passengers, the A4 can be obtained on request with two free-of-charge Isofix mountings at the rear seats, so that modern children’s seats can be attached quickly and safely. Also an option at no extra charge are Isofix retaining mounts for the front passenger’s seat, together with a deactivating function for the front passenger’s airbag.

Rear-end collisions also lose much of their injury potential in the new A4. The multi-section side members and the rear-axle support frame deform as a means of absorbing kinetic energy; the rear wheels make contact with the body sills if necessary. The package consisting of the car’s jack, battery, temporary spare wheel and hi-fi components does not form a single block. If its components are displaced by a severe impact, they remain clear of the tank filler pipe, and the fuel flow to the engine is interrupted; safety valves prevent the fuel tank from emptying if the car should overturn.

The fuel tank, with a capacity of 65 litres on versions of the car with front-wheel drive and 64 litres on those with the quattro driveline, is constructed from six layers of plastic and can withstand very severe loads. After an accident, the control unit releases all the door locks, and at the same time the high-current line between the battery, starter motor and alternator is disconnected. However, the electrical energy supply to important components such as the occupant restraint systems, the hazard warning flashers and the door electrics remains active.

With Audi backguard, the seats and head restraints in the new Audi A4 are designed to provide reliable protection against injuries to the upper bodies of the front passengers in the event of a rear-end impact. This special ‘Whiplash’ system has obtained “good” ratings in independent tests. If the rear-end collision is severe, the seat-belt tensioners are triggered off in order to hold the occupants in a favourable position in their seats.

Pedestrian protection: new solutions

The new A4 is one of the very first cars to comply with all the new Japanese and European regulations concerning collisions with pedestrians. Foam material ahead of the bumper cross-member, ample deformation space and sheet-metal zone designed to deform effectively are among the specific features, together with special front-wing mountings designed to yield on impact.

The most frequent accidents, fortunately, are the more harmless ones: light front-to-rear contact or minor bumps when parking the car. In the standard insurance-category crash test, with the car striking a barrier at 15 km/h and 40 percent overlap, the aluminium front cross-member with its mountings keeps damage low. The radiator and oil cooler remain intact, another reason for this being that a programmed breakaway point enables them to move back by 20 millimetres. In another standard insurance-category test, the rear bumper moves down and prevents damage to the sheet-metal body panels. In neither of these tests does the body structure suffer any damage.

The bumpers are bolted to the body and can be replaced at only moderate effort and expense. They make contact with the adjacent side panels at accurately matched zero-gap joint lines. To achieve this, the engineers have used special reamed bolts and quick-release fasteners. Here too, Audi’s quality principles can be seen – and felt.

Rigidity: the basis for comfort and dynamic handling

A factor that influences the occupants’ well-being to a decisive extent is the rigidity of the car’s body. It is the technical starting point for the refinement that they experience in the new Audi A4, undisturbed by unpleasant noise or vibration. At the same time, the rigid bodyshell is the key to the car’s outstanding road behaviour – ensuring smooth passage over all surfaces and sporty precision in its handling.

The engineers’ task was to develop a body with minimum weight but high dynamic rigidity in all areas. Compared with the already excellent values achieved by the previous model, they achieved a five percent improvement in this respect. Two factors govern dynamic rigidity: the position of the natural global frequencies and the quality of the vibration patterns.

In the case of the natural frequencies, the aim is to separate them clearly from those emanating from the axles and driveline. All these vibrations occur in the range below about 40 hertz. The human ear does not register them, but they can be felt at the car’s so-called ‘comfort points’ – the floor pan, the steering wheel and the seats. If the interior mirror trembles, this is also a sign of body vibration. As on the previous model, Audi’s development engineers have confined body frequencies on the new A4 to the calmer corridors remaining available between the engine and axle vibration frequencies.

The qualitative form taken by these vibrations are described by the engineers with the aid of torsion and flexing lines around the longitudinal axis. The flexing line for the new A4 has been raised by just under ten percent. Any further increase would have been pointless, but would have resulted in additional weight. The gain that was in fact achieved comes from the more homogeneous transition between the front end of the body and the floor pan. The two torsion rings in the structure – behind the rear seats and around the boot aperture – also have an important contribution to make. The firewall, a floor-level cross-member and the roof enhance the effect of these rings by acting as thrust fields.

The development team also attached great importance to local rigidity. All points at which force inputs taken place when the car is in motion were reinforced to the necessary extent. The aim was to prevent as much energy as possible from being transmitted further into the structure as a whole, and to distribute unavoidable force inputs among several noise and load paths.

For example, the front axle beam distributes the impulses it receives from the wheels into a multi-element structure consisting of the side-members and tunnel element. This greatly reduces tyre noise in significant body areas. The larger body cavities such as the sills and posts, which could also generate undesirable vibration, have been divided up by inserting a series of partitions.

The quiet interior: detail aero-acoustical work

The new Audi A4 is an extremely quiet car that pursues a smooth path through the air much as one would expect from a saloon in the large luxury category. Intensive detail work was carried out to reduce wind roar – the biggest source of noise above 120 kilometres an hour – significantly below the level encountered on the previous model. This work was facilitated by a special tool – the Audi aero-acoustic wind tunnel.

Wind tunnel testing, for example, determined the shape of the rain gutter strips on the A-posts. Their rounded section avoids objectionable turbulence despite their height. When parked, the windscreen wipers are shielded in a non-critical zone behind the edge of the bonnet, where they do not interrupt the car’s elegant outline or obstruct the driver’s field of view. The doors, with their integral window frames, remain firmly against the body sides even at top speed, and have two main seals to isolate the interior of the car hermetically from the outside world.

The basic version of the new Audi A4 achieves an exemplary drag coefficient of cD = 0.27 – an unsurpassed value for a midsize saloon and, depending on which engine is chosen, between five and eight percent lower than the previous model. Although the wider body increases the frontal area from 2.14 to 2.19 square metres, overall drag has been reduced by three to five percent.

Virtual tools: airflow around the body

As in all development areas, Audi made extensive use of virtual tools and high-speed computers when shaping the body. During the concept phase, the very earliest stage in the process, numerical 3D flow simulation has now almost completely replaced the previous use of quarter-scale models, and makes aerodynamic conditions visible even in largely concealed areas of the body. This procedure rapidly established the difference in drag coefficient (cD) between the draft designs with the best and worst airflows: in this case it was 0.025.

In the development work that followed, the aerodynamic engineers refined a number of details on the basic body. Among their most important work areas were the front apron, the side panels, wheel arches and wheel transition points and the exterior mirrors. Because of the larger field of view now called for by law, the mirror housings had to be distinctly larger than on the previous model, but by reducing their depth, the losses caused by airflow around the mirrors was kept to the same level as before. The standard mirrors on the new A4 have remote electrical adjustment, heating and integral LED flashers.

The development team added small spoiler lips to the rear lights: by causing the airflow to break away at the sides at a precisely defined point, they further improve directional stability. The boot lid was optimised in length, lateral curvature and shape in a complex series of work stages, so that the airflow remains consistently high when the car is being driven.

60 percent of total aerodynamic drag comes from the car’s body: the majority of what remains is due to the underfloor area, with a major contribution from the wheels and wheel arches. By providing a smooth underfloor over almost the entire area, the aerodynamicists succeeded in reducing the drag coefficient (cD) by 0.039. This result also makes itself evident in the form of higher performance and lower fuel consumption: CO2 emissions are reduced by about three grams per kilometre. The plastic underfloor panels also protect the sheet metal of the bodyshell and the mechanical assemblies against salt, moisture and stone impact, and allow the previous PVC underseal to be dispensed with.

To check that all the tough quality targets had been achieved, development work on the new Audi A4 included an intensive testing phase in all kinds of climatic conditions and on a wide variety of roads. During summer-season testing in particular, body quality was given close attention – good sealing to prevent sand and dust from entering, strength and overall behaviour on bumpy roads. The new Audi A4’s test-driving programme amounted in all to more than 15 million kilometres, including a million in city traffic and 1.6 million over rough surfaces.

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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