Interview with Markus Schadow, BMW M aerodynamics development. 5. Nov. 09
MPW: Mr. Schadow, the BMW M3 GTS has clearly visible dynamic measures with its front splitter and rear wing. The aerodynamics of the BMW M3 GTS is clear for all to see. Optimised for downforce. Why?
Schadow: The power which can be transferred from the contact area of the tyres to the road depends on tyre properties, the position of the tyre on the road - i.e. axle kinematics - and the wheel vertical force. Our measures are all about increasing the wheel vertical force. The higher the wheel vertical force, the higher the level of longitudinal and transverse acceleration which can be achieved.
MPW: An increase in vehicle weight would also increase wheel vertical force ...
Schadow: (laughs). That's certainly true. But more mass also generates greater forces of inertia and centrifugal forces. So this doesn’t do any good. On the contrary: the tyre-specific maximum load is reached earlier on, and the longitudinal and transverse acceleration can only be raised to this point, since above this point grip is lost. So what is required is low weight combined with maximum wheel vertical force. Which is why we generate downforce. However, the measures to increase downforce also increase aerodynamic drag. This is why they are adjustable in their effect in the BMW M3 GTS.
MPW: At the front axle, the downforce is generated by means of a longitudinally adjustable front splitter ...
Schadow: Yes, we chose this solution so as to limit the everyday suitability of the vehicle as little as possible and have sufficient ground clearance in racing set-up on the Nordschleife.
MPW: How does the front splitter actually work?
Schadow: While the car is moving, the pressure which builds up in front of the vehicle above the panel of the front splitter which is parallel to the ground is much greater than in the free stream of air below it. This creates a downforce. The lift force which you get in the standard BMW M3 is thus turned around.
MPW: And why the impressive rear wing in the BMW M3 GTS?
Schadow: The rear axle has a very important role to play in terms of drive stability. In order to be able to achieve a high level of drive stability at high speeds, we need at least as much downforce on the rear axle as on the front axle - ideally it should be significantly higher.
MPW: Why don’t you use a diffuser?
Schadow: A diffuser which would meet the space requirements here would be far less effective than the rear wing. We would never be able to reach a level of downforce required for a reasonable aerodynamic balance. This is also why in every racing series which allows the use of a rear wing, the cars are fitted with one. The rear wing of the BMW M3 GTS generates a high level of downforce and therefore drive stability.