A good, quantifiable value is a good selling point. It’s an even better selling point if few people know what it actually means, so they can’t really challenge it or compare it.
If you follow Formula 1 these days, you will hear a lot, an awful lot, about ‘aero’. Assuming the drivers don’t all have a fixation on the bubbly, chocolate snack, we can assume this means that the aerodynamics of the overpriced racing cars are very important. They are important for road cars too but, oddly, nowadays manufacturers don’t make a big deal of it in their marketing, leaving you to guess from the often excessively racetrack mimicked shapes of splitters, spoilers and diffusers we see on so many cars.
The fact is that today’s cars are generally far better behaved aerodynamically than their predecessors, but you can’t really quantify general behaviour.
The drag coefficient of a car is an indication of how little air resistance it encounters whilst travelling in a straight line – the lower the figure the better. In the early 1980s Audi were positioning themselves as the cooly, technical company and they weren’t shy in their marketing. The smooth looking 1982 C3 model 100, with its novel flush side glazing, was released with its then class-leading Cd of 0.30 emblazoned on its rear side window.
It’s not exactly the first time the Cd was used as a marketing idea. The 1974 Citroen CX model name paid service to an alternative expression of the value. Its Cx, or Cd, was actually 0.36, good but not fantastic, being the same as the deceptively less sleek looking Mercedes W126 S Class of 1979.
But drag just tells you how smoothly air flows over the surface of the car. It does nothing for stability and there’s much more to aerodynamics than a slippery shape, as all the tortuous little features on today’s Formula 1 cars testify. The Ford Sierra, introduced the same year as the Audi, also featured a low drag shape with a Cd of 0.34, though not as slippery as the Probe III concept that preceded it. But the car was hastily modified with a couple of small side spoilers on the C pillar, after reports of instability at speed.
The Audi just had a good Cd for a saloon. The 1970 Citroen SM’s Cd was 0.26 (note : see comments below) which, apart from the car’s obvious shape, was helped by paying equal attention to the car’s underside and having a full undertray to the engine bay. Since then designers have paid a lot more attention to detail, helped in a large part by improved manufacturing techniques.
So the bulbous looking 2005 W221 Mercedes S Class also had a Cd of 0.26 and we now find a good number of current models in the 0.24 to 0.28 range, with EV’s making more effort than most, helped of course by not needing to gulp a load of air into a radiator.
Over 30 years ago, the W126 Mercedes, the Sierra and the C3 Audi 100 made drag figures fashionable for a while, with journalists trying to tease out of manufacturers the Cd of any vaguely fast looking new model likely to break the Cd 0.29 barrier but, fickle as we are, interest soon moved on to other things.