The motor industry has long played the numbers game: price, fuel consumption, top speed, 0-60mph time, and servicing costs.
But there is another very significant numbers category that every new car buyer needs to consider.
It is called Cd – but it’s nothing to do with the Diplomatic Corps or compact discs. The initials Cd stand for co-efficient of drag, which provides a measurement of aerodynamic efficiency.
From a marketing standpoint, though, it is more than just a measurement; it is a grading that places cars in the league table of slippery shapes because efficient aerodynamics play a major role in cutting fuel consumption and CO2 emissions while enhancing performance.
In 1982, Audi achieved a Cd of 0.30 for the base version of its 100 saloon, at that time an amazing figure for a production car. The 100 looked a little odd, with its long, rather narrow body and wheels and tyres of modest dimensions tucked away well inside the car’s wheelarches.
But 0.30 was a significant barrier to be broached and Audi was very proud of its achievement. They, and other manufacturers, slowly whittled down the figure still further but the standard wisdom of designers at that time, and for years after, was that 0.25 was the likely limit for a practical, affordable car that didn’t look like a weasel on wheels.
In the world of aerodynamics design and engineering though, barriers on land and in the air are there to be broken.
In the late 1940s and early 1950s, military aviation was dominated by the challenge of breaking the “sound barrier” – exceeding Mach 1. US test pilot Chuck Yeager rocketed through that in 1947 in the Bell X1 and by 1969, the Mach 2-capable Concorde airliner was flying and mankind had taken a giant leap forward on the moon.
Aerodynamics played a very significant part in these huge technology strides. But for down-to-earth production cars that have to be aesthetically acceptable, functional and competitively priced, the 0.25 Cd breakthrough has taken hugely longer to achieve.
General Motors did it with their highly specialist, limited range, two-seat EV1 electric model in 1996 which had a Cd of 0.195. To some it looked wonderfully futuristic, but to others more than a shade odd, and hardly practical.
With about 1,100 built it could, arguably, be classed as a production car although it was only leased and never sold. However, as the EV1 faded from the scene in 1999, the Honda Insight arrived with a 0.25 Cd, which was a very good try at slipperiness, but it, too, looked a little odd.
In 2001, Audi was back in the Cd picture with the aluminium A2, with a vertical back end and shallow-sloping rear window, to achieve 0.25. The 2010 Toyota Prius also managed 0.25.
Meanwhile, Mercedes-Benz was taking a very determined approach to aerodynamics, steadily pushing Cd numbers lower. Its latest C-Class Coupé achieves 0.26 and the E-Class Coupé, 0.25.
Then last year along came the latest generation A-Class and B-Class, the latter in BlueEfficiency trim breaking that elusive barrier and getting down to 0.24. That has been quickly followed by the Mercedes CLA-Class with an extraordinarily good Cd of 0.22 in 1.6-litre CLA 180 BlueEfficiency Edition form. The regular model, with fatter tyres, records a still excellent 0.23 though.
The car has a squared-off Kamm-type tail with high boot lid, but the CLA remains a very good looking, totally practical car. How was it achieved?
Norbert Fecker, who led the CLA’s aerodynamics team, said the program to drive down its Cd was more intense than for any previous Mercedes, with designers, engineers and aerodynamicists all pitching in together: “To get the Cd value down as low as possible, we looked at saving thousandths – 0.0001, 0.0002, 0.0003 – which, when combined, added up to a big difference.”
Mercedes-Benz designer Mark Fetherston underlined the effort: “We worked together on every single exterior aspect of the car. It is about individual consideration – and then integration into the whole design.”
Mercedes’ aeroacoustic windtunnel at Sindelfingen, Stuttgart, with its 265km/h capability played a major part in the car’s Cd achievement. Fecker added that an improvement of Cd by a factor of 0.04 would see fuel consumption of a car cruising on an autobahn at 130km/h reduced by 0.5 litres/km, which approximates to 13g/km of CO2.
To achieve those figures via weight saving instead of refining aerodynamics would demand slimming down by 35kg; not an easy task. The interesting thing about the production CLA is that despite its slipperiness, it looks very much like the Concept Style Coupé unveiled at the Beijing motor show last year. So the design was always close to an optimum aerodynamic shape.
And then there is the diesel-hybrid Volkswagen XL1. Now described by VW as a production car (two a week!) but currently with no price tag, it is very low and narrow, but is practical with a fair-sized boot – and it achieves an extraordinary 0.189 Cd.
But hang on a minute; what about the long-forgotten Volkswagen V2 Sagitta (Arrow) that was being sketched towards the end of World War II?
The one-off streamlined version of the Beetle looked a pretty slippery customer for its time. So check it out, in January this year, it was placed in VW’s wind tunnel for a spot of testing.
The result was extraordinary: its Cd was a mere 0.217 – way beyond the 0.25 barrier and far better than even the 0.27 of the BlueMotion version of the new Golf.
Coincidentally, the Sagitta was unveiled in 1947 – the very year that Yeager took the Bell X1 through the sound barrier.
It’s a funny old world.