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Aerodynamics of a DTM car - Part II

by

Antonio Eiras

The Audi car

 

For the third consecutive year, Audi engineers used the A4 sedan as production base model for its DTM race car.

 

After the successful Championship of 2004, the Ingolstadt car manufacturer has been overlapped by the rival Mercedes in the next year, and reacted, developing an impressive new car for this year’s Championship.

 

For this study of the Audi, we have used the photographic material available and the DTM Technical Regulations.

 

By doing this we could appreciate not only the high level of technological refinement of these cars, but also the great distance between these race cars and the production models from whom they should derivate.

 

We can start this analysis by noting, in drawing 2, the front central position of the engine, moved backwards in relation to its production car original position, to allow a better weight distribution. This also justifies the retreated position of the driver, which is involved in a carbon fibre safety cell of the chassis.

 

In the same drawing we can appreciate the rear axle transmission, the exhaust of the gas escape from the engine, at each side of cockpit, and the radiators, displaced, in a V open to the front, just ahead of the front wheels (best noticed in drawing 5).

 

As the Audi moves, the air that flows around it, will be confronted, for the first, with the front surface of the car, which presents, from the top to the lower part, the air intake to the engine, the diffusers that drive the airflow to cool the radiators, the air intakes to the cooling of the front brakes, and, at the end, a splitter.

 

This last one will generate downforce as it separates the upper high pressure airflow, that is stopped by the vertical bodywork, from the low pressure air that rapidly flows under the splitter, and that is going to suffer additional acceleration as it is aspirated by the small diffuser developed just ahead and between the front wheels.

 

This airflow will be drained by the tunnels that develop behind the front wheels, at each side of the cockpit.

 

In the beginning of these tunnels, this airflow will be confronted with a second splitter, as we can see in drawings 2 and 3. Here part of this air will flow over the splitter, to be drained by the refereed tunnels, and the other part of it will flow under the splitter, accelerated by the suction effect that the rear diffuser will induce in the air that flows on the Venturi tunnel throat created, by this suction effect, between the underside of the car and the ground.

 

With the combined effect of these two front placed splitters, and the effect of the two diffusers that accelerates the air that flows under these splitters, a downforce will be generated in that front area, and will be applied on the front axle.

 

We can also note the fine tuning, in drawing 4, of the small turning vanes, that, placed on the front diffuser, will canalize in the most effective way the air that flows in that diffuser.

 

To obtain additional downforce to the front axle, the Audi engineers placed two small winglets in the front of the car.

 

Disposed immediately ahead of the opening of the wheels cover, they act like small delta wings, like we can see in drawing 3, generating downforce by the induction of small vortexes of air accelerated at incredible high speed, and, consequently, with very low pressure.

 

In drawings 2 and 5 we can observe that the warm air from cooling the radiators will pass between the front wheels, protected by a carbon fibre shield, from this flow, to be drained by the lateral tunnels that develop at each side of the cockpit.

 

By the middle of these tunnels, we will find the exhaust of the gas escape from the engine that will boost the air that flows on those tunnels, and that, by doing this, will increase the efficacy of the tunnels draining.

 

As we can see in drawings 2, 3 and 4, the rotation of the wheels can have a positive effect on draining of the air that flows on their proximity, under the car. As they rotate, the wheels will drag the air that flows near them, and will take it to the inner upper part of the wheels cover, from where it will be aspirated, through the fences opened in that part of the bodywork, by the suction effect of the low pressure high speed air that flows there, over the car.

 

On drawing 4 we can see as the air that flows under the car, between the underbody and the ground, will be aspirated, as well as the air that flows lower along the sides of the car, by the rear diffuser, that develops from the rear axel level to the most rear part of the bodywork.

 

Using the suction effect of the rear diffuser, the engineers will create a Venturi throat near the rear limit of the flat underside surface, immediately ahead of the diffuser. With the acceleration induced in the air that flows in that area, and consequent drop in pressure of that airflow, there will be generated an intense downforce, that will be applied over the rear axle of the car.

 

Not satisfied with all these forces of negative lift, the technicians disposed five small wings immediately up and behind of the rear wheels covers, that will not only produce more downforce, acting as delta wings, by the induction of small vortexes of air circulating at incredible speed, but also will increase the efficacy of the rear wheels in draining the air that flows under the car and near them.

 

This airflow will be drained by the louvers opened in rear part of the wheels covers, the same bodywork structure that supports the five small delta wings (drawings 2, 4 and 5).

 

In this area we can also note, in drawing 2, the air intake to the cooling of the rear brakes, opened immediately above the rear end of the lateral tunnels.

 

And, the last but not the least, we can appreciate, in drawings 4 and 5, the standard rear biplane wing, contrasting with the rest of the car by its simplicity, and that not only will generate considerable downforce, but also, as it will interact with the rear diffuser, increasing its efficacy in the extraction of the air that flows under the car, and, by doing this, contributing to generate more downforce.

 

With clear and intelligent technical regulations, perfectly adequate to the confessed aim of being able to proportionate a high emotion show at very reasonable costs, the DTM Organization induced the car manufacturers to conceive and build very high level competition cars that, strong of their 475 hp engines, and with a chassis and aerodynamics that allows them to use that power, have been protagonists of highly disputed races in an open and unpredictable Championship.

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Aerodynamics of a DTM car - Part II

by

Antonio Eiras

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