Aerodynamics: Only the aerodynamicist knows the answer

Dec 15, 2009
  • Efficient: the more streamlined the design, the lower the fuel
  • 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 (MBVT) 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 behind the radiator grille 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 0.01 reduction in the cd figure
  • Small spoiler lips on the tail lights homogenise the airflow at the rear. The airflow is forced to break 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, giving the E-Class one of the lowest-noise interiors 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 dreaded "shoulder-knocking" effect has been reduced substantially at speeds of up to 120 km/h.