The New A6: Body Structure

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March 18, 2004

Torsional stability up by more than 34 percent: progress of this calibre versus the body structure of the former model already lauded for its unique strength and stiffness again clearly confirms the qualities of the new Audi A6.

These qualities also come out clearly in terms of the car’s vibration control and driving dynamics. Vibration amplitudes reduced to a minimum, for example, are a clear indicator of the high standard of stiffness and solidity achieved by Audi’s engineers. And precisely this kind of vibration control is the crucial prerequisite for excellent acoustics within the interior.

A further crucial objective in developing the body structure was – and is – the need to maintain the highest conceivable level of passive safety, that is to efficiently protect the occupants in a collision. And at the same time it is important to avoid any increase in weight, since this would merely mean an increase also in fuel consumption and emissions. The only way to meet these conflicting interests, obviously, is to use the most advanced materials and joining technologies.

Lightweight structure and solidity

Using high- and ultra-high-strength steel plate, Audi’s engineers have succeeded in reinforcing specifically those structural components that are absolutely crucial to superior body stiffness and stability. Lightweight aluminium components such as panels and extrusion-pressed profiles, on the other hand, help to save a considerable amount of weight.

Aluminium plate is the ideal material, for example, on the front lid and front wings, like on the former Audi A6. Aluminium can also be used, however, on large components subject to only moderate loads, such as the lower section of the rear bulkhead or the front panel upstream of the radiator. Used in the form of an extra-strong and stable extrusion pressing, aluminium can also be used for reinforcement purposes, for example on the side sills, where it stiffens the cell structure for extra safety in a side-on or offset collision.

The front roof frame is made of a hybrid structure, that is a combination of steel and aluminium. This blend of materials combines the positive properties of the two materials used, providing a lightweight structure with a high degree of solidity.

Tailored blanks or tailored rolled blanks are used at points subject to particular loads and strain, such as the spring strut crossbar or the front longitudinal arms. Such tailored blanks or tailored rolled blanks are, as the name indicates, rolled plates varying in wall thickness.

The various components and body panels are connected to each other by a number of different methods: apart from conventional spot-welding, rivets, spot-welding combined with bonding, laser welding and soldering as well as MIG welding help to provide a lasting, solid connection. This, in turn, serves to fulfil all kinds of different demands and requirements in each area and on each component.

Passive safety

One of Audi’s fundamental objectives in developing every model is to make the passenger cell very stiff. After all, this is the basic structure serving as the foundation for all of the car’s restraint systems and their protective effect.

One of the most important requirements in designing and building this cell is to focus on head-on collisions, by far the most common and significant type of impact in accident statistics. To put it in simple terms, therefore, the body development engineers have two main tasks in the area of safety technology: first, to convert kinetic energy into deformation energy absorbed by body components. Second, to ensure that such energy does not even reach the car’s occupants within the passenger cell built as robustly as possible.

In a head-on collision intelligent up-front sensors behind the radiator grille interact with further sensor and control units, registering within a few thousandths of a second that the car is involved in an impact. The belt tensioners are then ignited just a few milliseconds later in order to minimise any possible slackness in the seat belts.

Once the loads acting on the belts have reached a certain limit, a belt force limiter feeds out additional belt length, allowing the occupants to “fall into” the airbag after inflating. For the first time with Audi, the full-size front airbags operate in two stages: stage 1 activated in accidents at low speeds reduces forces acting on the occupants to a minimum. At higher speeds stage 2 is inflated to give the front airbags their full safety potential.

The particular position chosen for numerous components and systems such as the air conditioning, control units, the ignition lock and key is also an integral part of the car’s safety concept. All hard components and surfaces have been moved away from critical impact areas possibly endangering the occupants, other areas such as the footwells are cushioned by extensive padding. This significantly reduces the risk of injury for the car’s occupants.

The safety steering column comes with a double telescoping function, meaning that like the pedals it is disconnected from any intrusion into the front end of the car. A force limiter at the mounting point holding the steering column in position also ensures that the steering column will give in a defined process when reaching the driver’s chest, the steering wheel thus moving down to prevent injury.

In a head-on collision at low speeds of up to 15 km/h the highly efficient bumper system serves to minimise energy right from the start, avoiding expensive repair of the welded body structures further back. Even the wings of the car remain unharmed in a head-on collision at 15 km/h thanks to the specific deformation of the bumper. Clearly, these are features duly honoured by insurers reducing insurance premiums to a minimum.

Outstanding safety also in collisions from the side

While at the front end of the car there is ample deformation length, the deformation travel available at the side for absorbing energy is reduced to a minimum. Only a particularly high standard of stiffness and stability, therefore, can protect the occupants – which is precisely why the new Audi A6 comes with a highly stable structure made in particular out of dual-phase steel for superior stiffness and protection against excessive loads and forces.

Within the side sill, an extruded aluminium profile ensures effective absorption of energy. Within the doors, in turn, tailored blank plates, together with special reinforcements on shoulder and sill level and an aluminium impact absorber, offer substantial resistance to deformation. Several support bars arranged perpendicular to the direction of travel likewise serve to stabilise the passenger cell, and two high-strength steel pipes also fitted crosswise in the front seats maintain the level of protection required.

Highly stable reinforcements also extend far into the roof area, serving to protect the occupants in a rollover.

In a collision from the side the occupants are protected by side airbags fitted as standard at the front and optionally available at the rear. The additional sideguard head/airbag system covers almost the entire window area at the side, offering occupants of all sizes a high standard of all-round safety. With the airbags being inflated quickly and efficiently by a hybrid gas generator and maintaining pressure for an adequately long period, the requisite protective effect is built up and maintained long enough to provide an adequate safety potential also in a secondary collision.

Sensors in the doors and C-pillars ensure reliable and rapid activation of the airbags in a collision from the side or in an offset impact.

The new Audi A6 is also excellently protected against collisions from the rear, already fulfilling the standards of the future today – which means that it offers adequate protection when hitting a deformable barrier at up to 80 km/h and with overlap of at least 70 percent.

In such a collision both the passenger cell and the fuel tank remain outside of the actual deformation area. With its extra-large support bars, the rear end structure deforms in a specific, predetermined process, reliably absorbing the impact energy.

Active head restraints in the front seats protect the driver and front passenger from the consequences of a rear-end collision. Released mechanically, the head restraints move forward whenever necessary and reduce backward acceleration of the occupant’s head at an early point in time.

Aerodynamics and aeroacoustics

More space, greater comfort, wider track: the new Audi A6 is significantly larger than its predecessor. Since this also means a larger frontal area, the task of the aerodynamics engineer is to set off this fundamental drawback by particularly intelligent design of the car’s outer skin. And Audi’s aerodynamics engineers have successfully mastered this challenge, giving the new Audi A6 2.4 a drag coefficient (Cd) of just 0.28.

Compared with its predecessor already developed to a very high standard of aerodynamic excellence, the new A6 comes with an overall drag factor (Cd x front area) improved, depending on the engine, by 9 – 13 percent. And compared with its current competitors, Audi’s executive saloon also ranks right at the top in terms of aerodynamic efficiency.

One feature particularly important in terms of aerodynamics is the fully covered underfloor. Encapsulation of the engine compartment and transmission at the front end of the car ensures a swirl-free flow of air to the central section, while wheel spoilers reduce flow loss effects around the front wheels. Large cover panels, finally, optimise the flow of air along the transmission tunnel all the way back to the rear wheels.

In all, the various improvements on the underfloor of the car reduce the drag factor by no less than 0.031 which, related to the car’s overall drag coefficient, means an improvement by almost 11 per cent. Travelling at a speed of 180 km/h, this means a reduction in fuel consumption by approximately six percent compared with a comparable vehicle without suitable modifications at the bottom.

Even more dynamic

Reduction of lift forces at the rear is crucial to a car’s driving dynamics. Conducting a long series of tests on this requirement, therefore, Audi’s aerodynamics engineers have developed a special spoiler edge integrated in the rear edge of the boot lid, offering not only technical qualities, but also attractive design merging smoothly into the rear end of the car. The result is a significant reduction of lift forces placing the new Audi A6 at the top of the league also in this respect.

Silence in the storm

Particularly at high speeds, the wind becomes the dominating factor in the medley of driving sounds. And this is no surprise, considering that, at a speed of 130 km/h, the wind rushes by the car at gale force 12 on the Beaufort scale, giving fast and comfortable cars a double problem: the first point is that the high level of acoustic comfort required particularly in such a car is crucial to the occupant’s well-being, the second consideration is that such cars are driven at high speeds more often than other vehicles.

This makes it particularly important to effectively dampen frequencies between 1000 and 8000 Hertz in a sporting long-distance saloon like the new Audi A6, since noise of this amplitude is most annoying to the human ear.

Precisely this is why the four doors on the saloon come with a triple sealing system effectively keeping out noise and closing even the smallest opening or hole.

The windscreen wipers benefit from the same kind of refinement, moving down whenever required to a particularly low rest position in order to avoid wind noise. And even when in use, the wipers on the A6 keep wind noise to a minimum thanks to their new flat wiper technology which, without detracting from the wipers’ performance at high speeds, ensures a lower noise level and a more pleasant frequency curve than with conventional wipers.

This wiper technology featured for the first time on a luxury performance car benefits in particular from the carefully conceived flow of air around the wiper arms and the shape of the wiper blades with their integrated spoiler.

Adaptive headlights and daytime driving lights

Audi offers no less than two features beneath the clear glass cover on the A6’s headlights: dynamic cornering lights (adaptive light) and daytime driving lights in conjunction with xenon plus technology. The combination of these systems is an absolute world premiere, a USP offered only by Audi.

Like other Audi models, the new A6 also comes with light units in Audi’s typical “tube” look. Featuring anthracite-coloured trims, the tubes for the high and low beams appear to hover almost freely in the air, giving the front-end design of the car its very own touch of high-tech.

The headlights on the new A6 come in three different versions: standard halogen headlights in free-form technology, xenon plus headlights, and the xenon plus system with dynamic adaptive lights able to literally “look around” a bend. In the latter case the entire ellipsoid module comprising the low and high beams follows the course of the road and the bends ahead, significantly improving illumination of the driver’s direction of travel as well as the side areas and making it even easier to look ahead.

Applying several parameters such as road speed, the steering angle and yaw rate, the system control unit calculates the required change in headlight position and gives the drive motors the necessary commands.

Daytime driving lights for extra safety

The significant improvement of safety provided by permanent operation of the headlights also during the day used to mean extra energy and fuel consumption amounting to approximately 0.2 litres per 100 km. This is why, with the exception of a few countries, this technology has so far not been introduced Europe-wide. And this is a shame, considering that daytime operation of the headlights helps to enhance driving safety, particularly when the sun is at a low angle, when driving on the motorway or in town.

With the low and high beams in the xenon plus headlights being integrated in one and the same – the outer – unit, the inner reflector remains free for daytime operation of the headlights. A dimmed lamp serves as the source of light, while the other lights are switched off. The result is virtually no increase in fuel consumption whatsoever.

LED brake lights

Apart from the basic version with conventional bulbs, the brake lights on the new Audi A6 are also available as an option in conjunction with xenon plus with an LED light unit. Over and above its long service life of at least 10,000 hours and energy consumption reduced by up to 80 percent, this technology offers the advantage above all of a much faster response time: while conventional bulbs take about 200 milliseconds to develop their full brightness, LEDs build up their full intensity in less than one millisecond, enabling drivers following from behind to recognise the brake lights a lot more quickly.

Yet a further advantage of the LED lights unit is the significant design highlight at the rear end of the car, the slot-like arrangement of the lights creating a wide, low and sporting brake light signal.

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