The new Mercedes-Benz E-Class Cabriolet: Four seasons, four passengers
Stuttgart
Mar 08, 2010
Aerodynamics: Only the aerodynamicist knows the answer
  • Efficient: the more streamlined the design, the lower the fuel
    consumption
  • Quiet: low air swirl means low noise
  • Exemplary: the E-Class of 1984 – and today's model
In 1984, the E-Class (model series W124) achieved an aerodynamic landmark, posting a cd figure of 0.29. It became, and remains, the benchmark against which all saloons have to be measured – a benchmark that very few manage to match. Design elements such as smooth surfaces, an inwards-drawn rear end and a clear spoiler lip on the boot lid remain at the heart of good aerodynamic design to this day.
Since then, Mercedes has been working tirelessly to reduce this figure by yet more crucial hundredths. After all, lowering the cd figure by 0.01 is equivalent to a reduction of one gram of CO2 per kilometre (NEDC), or two grams in the case of average real consumption, and as many as five grams of CO2 per kilometre at 150 km/h.
The new E-Class family is the new benchmark in the automotive world. Although the base tyres are becoming increasingly wide – not necessarily to the delight of the aerodynamicists – and the wheels are becoming increasingly large, the Saloon version is one of the world's most streamlined four-door models, with a cd figure of 0.25. And the Coupé model's cd figure of 0.24 is a new record for production cars.

The detailed work behind this development is highlighted by the following examples:
  • The louvres of the cooling air control system behind the radiator are mainly closed when there is no particular demand for cool air, thus reducing pressure losses at the front of the vehicle and air swirl on the underbody. Here the payoff is an improvement in aerodynamic drag to the tune of five percent or a reduction in the cd figure of 0.01
  • Small spoiler lips on the tail lights homogenise the airflow at the rear.
    The airflow breaks away at a clearly defined point. There is therefore a uniform spoiler lip across the entire rear end
  • The contours of the spokes and the rim flanges have been optimised to ensure levels that were only previously achievable by using smooth-surfaced hub caps
  • The underbody panelling has been optimised, while the spare wheel well is designed as a diffuser
  • In isolation, tweaking the shape of the spoilers in front of the wheels, the rubber sealing sections or the underbody panelling only brings about a minimal improvement in each case; however, when combined, these measures contribute to the world-leading cd figure
The new E-Class Cabriolet also benefits from all of these measures. Naturally, the fabric soft top cannot quite match the closed sheet-metal design of the Coupé. But the fabric and the contours of the folding top have been optimised to such an extent (see the section entitled "The roof" in this press kit) that the Cabriolet likewise achieves the best aerodynamic performance in its segment with a cd figure of 0.28.
Calmness itself: acoustic optimisation right from the start
Wind noise is another discipline of aerodynamics. Key requirements for a low wind noise level in the interior include draughtproof door and window seals. This requirement especially applies to cars with frameless side windows such as the new E-Class Coupé and Cabriolet.
Measuring tools such as dummy heads and directional microphones enable even the slightest weakspots to be pinpointed. These can then be eliminated by implementing the best possible technical solutions. At a very early stage in the development of the new, sporty E-Class model, a three-metre concave acoustic mirror was used to optimise the exterior shape of the A-pillars and the shape of the exterior mirrors in the wind tunnel.
The Cabriolet model marks the debut of a new acoustic soft top, which is fitted as standard, meaning that the E-Class interior has one of the lowest noise levels in the segment for four-seater premium cabriolets with a fabric soft top. It is therefore possible to have a perfectly normal phone conversation in hands-free mode at speeds of over 200 km/h. Further details can be found in the section entitled "The roof" in this press kit.
A further innovation is likewise designed to enhance comfort: now the front seat belt straps no longer run horizontally but, instead, are turned 35 degrees towards the occupants' shoulders. The advantage of this modification is that the wind pressure on the outside of the belt strap prevents annoying belt flapping when driving with the roof down. This "shoulder-knocking" effect has been reduced substantially at speeds of up to 120 km/h.

Under the microscope: aerodynamics
Technology for efficiency
Resistance that does not have to be overcome requires no power and, therefore, does not cause fuel consumption. As motor vehicles increase their speed, wind resistance above all becomes a factor, as it increases at the square of the vehicle speed: at a speed of around 80 km/h, it becomes greater than the sum of all other driving resistance, making it the key variable when determining the overall resistance. But even at lower speeds, the 1.2 kg or so of air per cubic metre that the car has to get through must not be neglected: in a modern car, assuming a typical customer driving cycle, around two litres of fuel per 100 kilometres are required just to overcome the wind resistance.
Wind resistance is determined by the vehicle speed, the air density and two
further factors: the frontal area of the vehicle and the drag coefficient (cd figure). Whereas there is a significant difference between the frontal area of a roadster and that of a SUV, for instance, the difference between two vehicles in the same segment is far less pronounced: comfort and safety requirements determine the dimensions of the car, especially its height and width. Mounted components such as the exterior mirrors cannot simply be reduced to any size, since statutory requirements have to be met here.
Hence the main focus when developing a vehicle is to split the air ahead of the vehicle so that the flow is as unrestricted as possible and then bring the air together again as smoothly as possible behind the vehicle. The cd figure indicates how successfully this is achieved. But here, too, practice has shown that there are limits. Fully closed wheel covers, for example, are neither visually appealing nor beneficial for brake cooling. The aerodynamically ideal droplet shape with a long and tapered rear end, for instance, can be discounted immediately because of the length of parking spaces and garages, not to mention dimensional concept requirements such as the rear seat width, sufficient boot width and boot loadability. What's more, the air has to be channelled around the vehicle so that the side windows do not become dirty and hamper the driver's view of the exterior mirrors. A cooling air control system ensures that air flows through the radiator to provide the required degree of cooling here. Another important criterion in aerodynamic development work is driving stability, which is influenced by the front and rear axle lift.
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