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Aerodynamics of a DTM car - Part I

by

Antonio Eiras

The concept limitations

 

The race cars that participate in the German Turing Championship, the DTM, are vehicles that must give the impression that have been developed from Turing production cars of the respective manufacturers.

 

In truth they happen to be real competition cars, developed from the beginning for this specific Championship.

 

The Technical Regulation obliges the manufacturers to use 4 litres V8 engines, opened at 90º, allows them to use one of two homologated gearboxes, forbidden 4 wheel drive, imposes one type of fuel, one only carbon fibre brake supplier, AP, and one tyre manufacturer, Dunlop. Finally, to all the car manufacturers it is imposed that the superior half of the bodywork of their competition car must be like the one of one of their production Turing cars.

 

This last regulation demand, excellent from the point of view of the Marketing Departments of the different car manufacturers involved, by the promotion of their production cars, during all the Championship, raises a complex problem to their project engineers to solve.

 

As we can see in drawings 1 and 2, the side view of a DTM car, similarly to what happens with the production Turing car from it is supposed to derive, behave, when submitted to an airflow that is crossed by this car, like an airplane wing. This means that the airflow separates at the front of the car, like it happens in the leading edge of a wing, and the part of the airflow that flows over the car must be accelerated, to travel over a bigger distance than the flow that flew under the car, in order to reach the rear of the vehicle at the same time, as it happens in the trailing edge of an airplane wing.

 

The referred acceleration will decrease the pressure on this upper airflow, and, by this, creates a pressure differential with the air that flows under the car. This will generate a positive lift that, applied to the moving car, will tend to force it to separate from the ground, reducing its adherence and handling.

 

The engineering team responsible for the project of a Turing race car, will have to invert this positive to negative lift, downforce, in order to create a real competition car, with good handling and traction in every circuit in which this car will have to compete.

 

After a bad experience in the middle 90’s, when an a excessive rise in the budget of the Championship lead to the interruption of the DTM, the Organisation imposed a Technical and Sportive Regulation that searches for the best show at a reasonable price.

 

With this aim, it is forbidden any mechanical or aerodynamic evolution or development during the Championship, and so the cars must respect and maintain the same technical specifications presented in the first race of the Year.

 

Also, with the same goal, the different cars can use one standard rear wing, imposed by technical regulations. It is a biplane wing, with two upper profiles, a main wing and a flap, and a single inferior profile.

 

One of the factors that contributes to the drag of a production Turing car, is the irregularity produced in its underbody by the prominence of its mechanical components. We can easily understand that the air that flows under any production Turing car will be stopped and disturbed by the lower part of the radiator, engine, gearbox, chassis, suspensions and, of course, of the rotating wheels.

 

This disturbance will produce a turbulent airflow that will contribute to increase the drag generated by the moving vehicle.

 

With this airflow behaviour on the underbody, it would be impossible for the engineers to generate enough downforce, to create a real competition car.

 

Aware of this reality, the Organisation of the DTM allows some freedom to the car manufacturers in the aerodynamic development of the inferior half of the bodywork of their competition cars.

 

As we can see in drawing 4, all underbody of these cars is conceived to generate as much downforce as possible, using, to this purpose, the air that flows between this surface and the track.

 

To limit the performance of these cars, the Technical Regulation permits the panelling of the underbody, but imposes a flat surface between the front and rear axles.

 

In other way, as the Regulations allows the development of two diffusers, one in front, the other in the rear part of this flat surface, it will consent the creation of two throats on the air that flows under the car, and, by doing this, the generation of two low pressure areas, with the induction of downforce, that will be applied to the front and rear axels of the car.

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Aerodynamics of a DTM car - Part II

by

Antonio Eiras

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