Gone with the wind: How aerodynamics work on cars


How aerodynamics work on a car

Gone with the wind: How aerodynamics work on cars-wind

Tiny cameras instead of sweeping exterior mirrors: with tricks like these, the VW XL1 achieves a drag coefficient of just 0.189.

Source: picture alliance / srw

Air resistance, drag coefficient – every driver knows these terms. But what is really hidden behind it is hardly known to anyone. Because the aerodynamics hold many surprises.

E.There are questions for which the answer seems crystal clear: For example, whether a large truck or a modern Formula 1 racing car has a better drag coefficient and thus lower aerodynamic drag. Basically, a truck is certainly not in very good shape – the world’s most aerodynamic production car to date, the Mercedes CLA BlueEfficiency, has a drag coefficient of 0.22, heavy trucks come in at 0.6 to 0.8.

A Formula 1 car, on the other hand, looks and is sporty, but in terms of air resistance it looks like the proverbial wall unit. The drag coefficient of the starting field is currently around 1.2, which corresponds to the level of a classic car such as the Model T from Ford, which was built from 1908.

Reason for the bad values: Modern formula racing cars shouldn’t just slip through the wind quickly. Rather, they primarily need contact pressure. The air should therefore press with all its might on the spoilers and panels so that the drivers can get around corners as quickly as possible.

The requirements of the road car are completely different: Here you want to offer the wind as little resistance as possible in order to reduce consumption and allow higher speeds on the straight.

The drag coefficient is an assessment parameter

If you ask yourself what the much-cited drag coefficient actually is, the answer is, however, a point of reference, no more and no less. cW is made up of c for constant and W for resistance. It is a so-called “dimensionless measure”, i.e. a number that cannot be assigned any physical quantity.

According to Rupprecht Muller from the ADAC technology center in Landsberg, the value primarily describes "the quality of a shape" that is exposed to a resistance such as air. “The drag coefficient is above all an assessment variable to make things comparable.” A car driver knows that a car drag coefficient is 0.28 more streamlined than one with 0.4.

Therefore, although the drag coefficient is an indicator of the aerodynamics of a car, it is basically only part of the whole truth. If the flow resistance is actually to be calculated, i.e. the force that is opposed to the car by the air, then further knowledge is necessary: ​​The speed of the car must be taken into account, as well as other values ​​such as the density of the air.

If that in itself is complicated enough, there is an additional factor when calculating the total flow resistance of a vehicle: the frontal area. That is the area that appears as a shadow on a wall when the car is lit from the front with a headlight.

According to Daimler, the combination of the data of the drag coefficient and the frontal area (A) provides the most meaningful result when measuring vehicles: The size of the frontal area in square meters is therefore multiplied by the drag coefficient called the drag coefficient, and the value is then derived from the drag coefficient and A. "CW x A".

Mercedes was a pioneer in aerodynamic design

How the drag coefficient and the frontal area (A) influence each other is explained by Daimler based on the development of the company’s top model – the S-Class. The first model that was specifically developed here with a view to aerodynamics was the W 126 series, which appeared in 1979.

After long detailed work, a drag coefficient of 0.359 was achieved, which is a top value. The frontal area (A) of the car measured 2.1283 square meters, which multiplied by the drag coefficient resulted in a total drag (drag coefficient x A) of 0.7641.

The successor to the W 126 was the W 140 series in 1991, which was largely forgotten because its gigantic dimensions meant that it did not fit on a car train or in a garage. This was also evident from the frontal area, which was significantly larger at 2.39 square meters. So you had to fine-tune the drag coefficient again to reduce the flow resistance of the run.

A teardrop-shaped car would be ideal

The windshield and the front section were inclined more strongly, the underbody was clad and flat wheel trims were used. This reduced the drag coefficient to 0.30. Only these tricks eradicated the disadvantage of the large frontal area, the total resistance sank to 0.717.

The measures taken by Daimler may lead one to assume that improving air resistance is quite simple: the smoother and less rugged a body, the less the wind presses against it. In fact, that’s more complicated than that – a car will only ever represent a compromise when it comes to slipperiness.

Nature dictates what a perfectly aerodynamic body looks like. But these forms can only be transferred to car manufacturing to a limited extent. A penguin, for example, has an air resistance of 0.03, a drop with a value of 0.02 is even more effective.

So the optimum would be a teardrop-shaped car, with which a certain Edmund Rumpler was already experimenting in 1921. Its teardrop car had a drag coefficient of just 0.28, but it also showed that this shape can only be combined with the desire for a spacious interior to a limited extent, for example.

Very aerodynamic cars are difficult to drive

Overall, according to Professor Jochen Frohlich from the Technical University of Dresden, the more aerodynamic a car is, the worse it can be driven. Or vice versa: if it is to remain usable, it will never be aerodynamically optimal. The designers are therefore always looking for the best compromise between effectiveness and usability.

They smooth the fronts of the vehicle and mount bumpers as flush as possible so that the wind does not swirl there. In addition, they optimize spoiler edges at the rear, through which air disappears to the rear with as little turbulence as possible. At the same time, they have to make sure that passengers can get on with ease and that there is enough space for them.

The fact that you always try so hard to reduce air resistance is due to the advantages that are associated with it. The positive influence increases with the pace.

So the faster the car drives, the greater the effect. Even at 60 km / h, the air resistance has the same influence as the rolling resistance of the tires. At full throttle on the autobahn, 90 percent of the fuel is burned to hit the wind.

Aerodynamics of production cars largely exhausted

When it comes to optimizing aerodynamics, it’s not just about the sheet metal clothing itself, but also about numerous minor details. “If you hold your hand out the window while driving, you will notice how the wind is pressing against it.

If you pull your hand in again, this resistance is gone, ”is how Professor Jochen Frohlich sums up the basic principle of air resistance. The hand is also an image for various add-on parts that are a thorn in the side of aerodynamics. "Therefore, one would prefer to do without antennas and, above all, exterior mirrors."

Overall, according to Professor Frohlich, the aerodynamics for series vehicles are now pretty exhausted, and progress is being made in ever smaller steps. A Mercedes E-Class of the W 124 series rolled out in 1984 with a drag coefficient of 0.28, up to the W 212 series of the E-Class from 2009, this value fell to just 0.25. On the other hand, an Opel Calibra achieved the same drag coefficient as a Mercedes A-Class from 2012 with a drag coefficient of 0.26 as early as 1990.

Theoretically, a drag coefficient of 0.20 for a car that can actually be used is the limit of what is feasible. A significant improvement below this threshold would probably only be possible if one parted with well-known traditions: Up until now, a car has mainly been an object without moving parts that adapt to the driving situation.

Airplanes, on the other hand, use various movable flaps to adjust the flow of air to the requirements, which in the automotive world has so far only been known at best in the form of retractable rear spoilers. This principle would therefore have to be refined and used much more comprehensively in order to achieve further progress.

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