If one were to design a car strictly according to aerodynamic guide-lines, it would have a great amount of influence on the form and on the cross-sectional surface. Ever since 1920, tests have been done repeatedly in this direction. In the above figure, an example of the long gone company of Borgward, which in 1954, tried to get very high performance out of their weakest model, the Lloyd with a 300 cm³ two-stroke engine.
The air-flow around the car body does not only influence the performance and the consumption, but also the chassis and the centre of gravity. The further the centre of gravity moves to the rear, the more unfavorable is the crosswind behaviour. In addition, a part of the drag coefficient is sacrificed to generate down-drive on the axles. The interior ventilation also plays a part. This has become easier with the continuous circuit for the electric fans. Even the possible influx of exhaust gases, perhaps through the air vents, must be tested and accordingly eradicated.A further subject is the noise development in modern vehicles. Here the body-work building also plays an important part. It depends on the optimising of details, which can even be influenced by the standard-spreading-variables. Apart from this, multiple door-sealings also play a part. A problem, still not optimally solved is, e.g., the noise development of partly open sunroofs. Generally speaking, wind resistance and acoustics are closely related.
An important aerodynamic component is the windscreen, not only because of it's incline. Opinions differ about the placement of the A-pillar. Not only because of the wind noises, together with those from the rear-view mirror, but because of the forward vision aspect. An aspect receiving less attention, is the flow of the rain water on the windscreen. Mostly drain-strips provide for the fact that it does not flow over the side windows, thus dirtying them as well. In adition, apparently peculiar wiper constructions are sometimes to be found, to avoid the so-called 'water dragging'. This develops from water swirling, e.g., next to the A-pillar, which allows the rainwater to reach the wrong side of the wiper blades.
As long as only very little wind is blown around the car body, the problems are relatively small. At higher speeds they increase rapidly, the wind excercises enormous forces, on certain points, driving force, and on other points, vacuuming off the air, on anything which is not solidly mounted, or must be movable to fulfill it's function. This is especially valid for roof racks and for spoilers, although, even very solidly built convertible roofs are not really silent during strong crosswinds. It becomes even worse if the wind conditions change abruptly. 10/09