• Dec 14th 2006 at 11:38AM
  • 5

Did you know that geese flying in formation inspired the 1934 Chrysler Airflow, perhaps the first streamlined car ever built? Aerodynamics didn't start playing a major role in automotive design until the '70s and the first gas crisis. The measuring stick for automotive aerodynamic is the coefficient of drag, which is engineering terms is expressed as "Cw." American automotive enthusiasts and manufacturers usually refer to the term as "Cd" or "Cx."

Today's cars have a Cd around 0.29, and designers are trying for 0.25. Radical concept vehicles have gotten as low as 0.19. Again, animals are being studied to help with design work. Mercedes used a boxfish when sketching the Bionic concept vehicle.

But cars can't be just aerodynamically smooth, they also have to be attractive. Hence the challenges of today's automotive designer.

[Source: Playfuls.com]

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    • 1 Second Ago
      • 8 Years Ago
      One of those "radical concept vehicles" with a drag coefficient of 0.195 was the General Motors EV1, which I believe may be the most aerodynamically efficient production car ever made.

      Unfortunately, aerodynamic drag is the last thing on the mind of most car designers. In a typical project the car is first styled for looks, then later tested in a wind tunnel to find out how badly it performs. Next the aerodynamics people usually suggest some changes. Then the stylists cry and whine that any modification, not matter how small, will ruin the visual experience they wanted to create. After going around a few times, the stylists will grudgingly allow the minimal changes necessary to make the car stable (never mind efficient) on the highway.

      There was an excellent documentary that chronicled the design and creation of the Lotus Elise, and you could see this sequence of events play out. I'm sure it's no different at the big car companies.
      • 8 Years Ago
      There's an execellent article by John Westlund who estimates that automakers can improve MPG by as much as 30% just by improving the aerodynamics and without any modifications to the engine itself.

      • 8 Years Ago
      When VW launched the B5 passat way back, they said 0.27Cw, and the B5.5 refresh comes about and VW then says 0.30Cd.
      What a shocker.
      • 8 Years Ago
      #1, Having worked in the automotive industry for 8 years now as a Design Engineer, although I agree with your somewhat tongue-in-cheek description of the automobile styling process, I would have to disagree that "aerodynamic drag is the last thing on the mind of most car designers."

      While it is true that pure car stylists are not degreed-aerodynamicists nor engineers, the design process never occurs in a total vacuum or creative isolation (anymore). And fundamental to the curricula of any design school is to make the students appreciate aerodynamic forms, if without all the math and fluid mechanics.

      Highly experienced designers and engineers can even guess the Cd of their work to a good degree of accuracy on just a glance.

      It should be noted that Cds below today's prevailing average of around 0.3 have been achieved in production cars for decades; the Audi 5000 from the early 1980s, for example, boasted a Cd of 0.26.

      Before this post gets too bloated, I wanted to point out that Cd itself is not the sole determinant for aerodynamic efficiency. It's actually the product of the drag coefficient times the frontal area Cd*A that is decisive. Unfortunately, while Cd have been trickling down, the upsizing of cars, including the current trend of SUVs and crossover vehicles, have pushed up frontal areas (not to mention weight), such that progress in Cd*A have remained static or even worsened since the oil crisis of the 1970s.

      Increasing energy efficiency, and therefore reducing the dependency on primary energy sources (particularly), can only make meaningful strides in combined efforts in putting lids in bigger, porkier vehicles; powertrain developments; and equally importantly solutions to reduce unnecessary and wasteful travel.
      • 8 Years Ago
      There is a difference between Cd and Cw.
      When you park a car in a wind tunnel and blow laminar air flow at the parked vehicle, that is Cw, which is always at least 10% lower than Cd. Rolling wheels and moving floor helps get that down from 15% below Cd.
      Cd is where an actual car hits stationary air in making forward progress. rolling wheels & fixed ground.
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