The Audi TT's brilliant aerodynamics

In 1982 the Audi 100 set up a new aerodynamic world record for saloon cars, with a drag coefficient (cD) of only 0.30. Since then, Audi has justly been regarded as a pioneer in the field of motor-vehicle aerodynamics. This know-how is a feature of the new sports car too. Its body was aerodynamically optimised in both the wind tunnel and the water chamber. Despite tyres of generous size and the additional air volume needed to cool a high-performance turbocharged engine, a drag coefficient of cD = 0.34 has been achieved ­ an outstanding value for a sports car. Together with a frontal area of only 1.99 square metres, the result is exceptionally low resistance to the airflow.

The underside of the car has a significant part to play in this. The benefits of the smoothest of bodies can be sacrificed if the under-surface is broken up or includes too many aerodynamically unsatisfactory obstructions. The underside of the Audi TT Coupé was therefore optimised with the aid of a wind-tunnel simulation technique originating from motor sport. The aerodynamic tests are not performed as in a conventional wind tunnel, but on a car that appears to be moving along a road. This phenomenon is achieved by placing the car on fast-moving belts that cause its wheels to rotate. A further belt passes beneath the car's body to simulate the road surface. This new method of measurement yields far more realistic results and increases the engineers' scope for optimisation.

In their striving for the perfect sports car, the aerodynamicists made use of several ingenious technical principles. For example, how can one supply a powerful engine with the necessary volume of cooling air, without spoiling the car's aerodynamics?

Audi has tackled this problem successfully on the TT Coupé and on other models as well by making use of an aviation-industry development: the NACA air inlet. This takes its name from the organisation which preceded the better-known space authority "NASA".

The airspace engineers' task was to develop new cooling concepts for aircraft, in other words to divert as large a volume as possible of air into a suitable duct in order to cool the engines and the oil. The best solution was found to be an aperture which increases in width according to a special pattern ­ a technique that racing cars have also adopted with success. NACA inlets are used on Audi models in the engine enclosure panels underneath the car.

On the TT, a nose-end enclosure improves the airflow in the front axle and engine areas; a large-area fuel tank cover panel performs the same function at the rear. Eddy currents round the wheels are smoothed by spoilers integrated into the underside of the body.

Low levels of wind noise at high speeds are a sure sign of good aerodynamics. In this respect, the Audi TT Coupé matches the level of comfort achieved by Audi's saloon cars. Special door seals, for example, prevent resonance from building up in the panel and door cavities. The frameless side windows generate only a minimum amount of wind noise right up to the car's top speed. To ensure that the doors can be shut without undue effort despite the side windows pressing firmly against their seals, the windows move down by about a hand's width whenever the door handle is operated, and slide back up again after the door has been closed.

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