[Piwoslaw]

In the September issue of Auto, Motor i Sport (Polish version of Auto, Motor und Sport) there is an interview BMW aerodynamicist Hans Kerschbaum. I've translated parts as it may be of interest to fellow Ecomodders:

The aerodynamics of cars is too complicated for pure theoretical reasoning, that's why the field is composed mostly of testing and research. Every detail has an effect on every other and there is no sense in changing something in the body, for example, without keeping an eye on its effect on the wheelwells, everything is interdependent. Let's say you start with cleaning up something in the wheels and wheelwells. This effects the airflow under the car, so you have to do some work there. This changes the distribution of pressure behind the car, so you have to do something on the body, which makes you return to the wheelwells. This is what happens every time.

Every car shape can be characterized by three parameters: coefficient of air drag and the lift force at the front and rear axle.

Getting the right lift is extremely hard and has a lot to do with suspension.

Today it is impossible to separate styling and aerodynamics. The interrelations between them are sometimes good, but often contradictory. For an engineer it is important to have good airflow on the corners and sides of the body, control the turbulence at the A-pillars and the mirrors (which are a small catastrophe from the aerodynamic point of view), and make sure that airflow detaches in the right places at the rear. At the same time you must minimalize the amount of dirt that sticks to the headlights and windshield and make sure that noise stays at an acceptable level. Of course, stylists want an nice body with a long front and short rear, because that shape is regarded as attractive and dynamic. On the other hand, it would be much more effective if the front was shorter and the rear longer.

The classic discrepancies between styling and aerodynamics show up in roadsters. The problem with a car such as the Z4 is the terribly short rear, but it's hard to imagine a BMW roadster of different proportions. This is regarded as beautiful, it's enough to see how people look at the car. But the aerodynamicists have a headache trying to raise the rear and shape it to reduce the lift there.

As much as 30% of drag comes from the wheels and wheelwells - this the critical area, still not understood well. One of the funny things is that the drag coefficient of a car is about 0.01 smaller when the wheels are spinning. The basic principle is to leave as little room as possible between the wheel and wheelwell. If you think that this shouldn't be a problem for stylists, then take a look at SUV's...

Another delicate problem is the wheel's rim. It would be best if it could be covered with something flat, but then the wheel wouldn't look so attractive and may have problems with brake heat, especially in the front. That's why we won't see flat, full hubcaps any time soon.

Airflow under the car is responsible for 1/5 of the drag. It should be smooth, so more and more often belly panels are used. Another problematic area is the engine bay. It can't be totally closed off, but there shouldn't be too much airflow either, so the grille holes can't be unnessesarily big. That's why stylist can change their shape, but their area gets determined in the wind tunnel. This illustrates why the same car with different engines may have different drag coefficients. For example, the Cx of the X6 xDrive30d is 0.33, while for the X6 xDrive50i it's 0.36, mostly because of a larger grille, but also because of wider tires. BMW uses venecian-blind style grilles that close when the engine doesn't need cooling. The wind tunnel also dictates where opennings should be placed. For example, in today's cars the air no longer exits around the transmission but through the wheelwells, where there is a lowpressure zone that sucks the air out.

So what does the future hold for aerodynamics? More research into wheelwells, which cause a lot of turbulence.

Air drag comes from:
40% shape and proportions of the body,
30% wheelwells,
20% under the car,
10% airflow through the engine bay.