More than mere weather protection: the hood

The Audi A4 Cabriolet has, as standard, an electro-hydraulically operated hood that opens fully automatically at the push of a button - a process that takes less than 30 seconds and is the ideal way to enjoy the sun and open-air driving.

At the same time the new Cabriolet offers maximum operating convenience: a simple press of the button is all it takes to open and close the soft top. The side windows are lowered and raised again automatically; it is not necessary on the A4 Cabriolet to release a locking bracket manually or to undo a tonneau cover.

When the hood is being opened, the lid of its stowage compartment behind the rear seats is raised, and the hood is folded away out of sight. The flap then folds down flush with the body so as not to interfere with the dynamic shoulder line of the A4 Cabriolet.

The hood compartment itself can be varied in height from the luggage compartment: the driver can raise or lower the compartment as required using a lever. This increases the size of the luggage compartment considerably when the hood is closed.

The lined Cabriolet hood is also perfectly equipped for winter-weather use: it has a heated, scratchproof glass window, and its triple-layer structure and accurate fit shield the interior effectively against both adverse weather conditions and exterior noise.

The hood's linkage is barely visible when the hood is in the raised position; the tautness of the hood and the cushioning prevent any "tent ridge" effect. The shape of the roof as a harmonious component of the Cabriolet design is thus preserved at all speeds.

The hood is also completely smooth on the inside. Together with the high-quality appearance of the surface, this look underlines the luxurious overall impression conveyed by the interior ambience.

Aerodynamics: close to the wind

In the field of aerodynamics, absolute drag coefficient records long ago ceased to be the sole measure of all things. This is especially the case with convertibles which generally lag way behind saloon cars due to their lack of permanent roof. But all Audi vehicles are built according to the motto that optimised aerodynamics is one of the most important criteria for moderate fuel consumption and good economy.

With a drag coefficient of 0.30 (closed; open: 0.33), the new Audi A4 Cabriolet also complies with this requirement, improving on its predecessor by no less than 15 percent. And compared with the current competition in the four-seater convertible segment, the Audi has nothing to hide either: even with the top down, it achieves a better drag coefficient than its key competitors with the roof closed!

The significance of this is not just the impressively low value itself, but the way it was achieved. After all, the significantly more spacious interior on the latest generation model necessitates a much larger frontal area compared with its predecessor. And now that the standard equipment specification includes air conditioning, the new A4 Cabriolet requires much more cooling air, which likewise has an adverse effect on the drag coefficient. On paper this would actually add up to an increase in the drag coefficient.

So how can a deficit like this be turned into a genuine gain in aerodynamic perfection? The first area of concentration is known as "shape optimisation"; this begins in the early development phase with the help of computer-aided simulation and 1:4 wind tunnel models. Aerodynamics therefore accompanies the entire course of product development, from the first virtual and real models to prototypes and pre-production vehicles right up to the production launch.

The aerodynamic talents of the A4 Cabriolet are visible both at the front and at the rear of the vehicle. One result of the detail optimisation in the wind tunnel is the V-shape of the front end including the aerodynamic styling of the bumper and the front spoiler beneath it.

At the tail end, which differs from that of the saloon as clearly as the front end, it is the raised edge on the rear end of the luggage compartment lid that catches the eye: a detail that helps to reduce lift considerably without spoiling the streamlined silhouette of the Cabriolet in the least.

But that's not all. One zone which is generally not visible to the driver offers immense potential for optimising the vehicle's aerodynamics: the underbody. The target aerodynamic shape is seemingly very simple: a smooth underbody would represent the ideal.

Aerodynamic underbody

Faired underbodies have already proven just how effective they are, particularly in motor sport. However, considerable detailed work has to be carried out before they can be used on production vehicles. This calls for highly advanced wind tunnel technology.

The new Audi wind tunnel centre in Ingolstadt is specially designed for simulating on-road driving. Virtually no other wind tunnel is able to include in its analysis wheel rotation and the simulation of the road using moving belts under the vehicle. The aerodynamics engineers therefore succeeded in developing a largely smooth and low-resistance underbody which also satisfied practical requirements with regard to weight and ease of installation.

Putting on the pressure

The airflows to and from the radiator area, which are of considerable aerodynamic importance, can be analysed at the Audi wind tunnel centre. For instance, it was possible to move apertures for cooling air to zones where the aerodynamics create a positive pressure. Positive pressure also promotes the supply of fresh air for the air conditioning.

Conversely, the outlet apertures for the air extracted from the passenger compartment are located below the rear bumper on the A4 Cabriolet. This is because there is moderate negative pressure here. This careful choice of apertures results in a largely loss-free and therefore efficient cooling and air conditioning system.

All these measures taken in isolation appear to be no more than a drop in the ocean, but they all add up to an impressive overall drag coefficient for the new Cabriolet.

Calming the storm

Audi engineers also devoted considerable development time to the problem of wind noise. One example of many are the exterior mirrors, which have always caused aeroacoustics engineers in particular one of their biggest headaches.

The mirrors themselves do not produce an annoying noise. However, the wake behind the mirror housing must not strike the side window, for instance, as the front passengers would otherwise register a pronounced, high frequency airflow noise. This is a phenomenon that the new Audi A4 Cabriolet avoids thanks to its design. Even details such as a highly efficient mirror-gap seal are developed in the wind tunnel and consequently contribute to the car's good overall aeroacoustic impression.

The work that has been carried out on the door seals is equally important. Like their counterparts on the A4 saloon, these were developed in Audi's aeroacoustics wind tunnel. In the same way as the saloon, the Cabriolet therefore has two sealing lines in the lower door area (one line in the region of the window) that effectively block out outside noise. The new flat wipers also produce an exceedingly low wind noise level: this has already been proven in particularly impressive style in the A6 series.

The aeroacoustics of the hood were also optimised in the Audi wind tunnel in the course of a number of tests. An integrated insulating mat effectively absorbs sound. The surface and design of the closed soft top help to keep the wind noise level down. And, together with extensive sound-insulating measures in the body at the rear end, the glass rear window also reduces the wind noise level in the interior.

These are just a few of many measures which together make for impressive reading: compared with the predecessor, the sound-pressure level in the new A4 Cabriolet has been reduced by no less than five dB(A) or 45 percent. This adds up to a significant increase in comfort for the car's occupants and a clear edge on the competition.

Things get a little louder in the A4 Cabriolet with the roof open - this is usual for a car of this kind and is also all part of the fun of driving a convertible. The occupants, however, will hardly notice any turbulence.

If the rear seats are not occupied, an optional wind deflector can be attached in next to no time: this keeps the interior draught-free, all the way up to motorway speeds.

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