Aerodynamics: slippery is sexy

As engine efficiency gains in importance, so does that odd little 'Cd' figure

Ever noticed those Cd figures often mentioned in car write-ups? It stands for 'coefficient of drag' -- a numerical value given to the efficiency with which a vehicle body slips through the surrounding air.

Drag is the product of air density, the car's frontal area and the speed at which it's travelling. All of which conspire to make it the single biggest effect on the efficiency with which your engine carries you from A to B, and therefore the costliest.

So the Cd figure is an important pointer to your car's potential operating efficiency. The lower the Cd figure, the better the car is slicing the air. Which, by implication, means the higher the operating efficiency, the better the performance and fuel economy.

To give you a perspective on Cd figures and their meaning, here are some current or recent examples...

Lexus LS460 - 0.26
Toyota Prius MkII - 0.26
Mazda6 - 0.27
Nissan GT-R - 0.27
Toyota RAV4 - 0.31
BMW Z4 M coupe - 0.35
VW Tiguan SUV - 0.37
Standard housebrick - 1.00

Current Aussie big-six Cd figures are noticeably hard to find, but a VT Series II Commodore was rated around 0.32, while Ford's controversial 1998 AU 'cheese grater' Falcon broke the 0.30 barrier with 0.295.

The Prius Mk1 achieved 0.29 and the original 1999 Honda Insight 0.25. Hummer's beefy H2 cut 0.57 (GM's coy about the brickish H3, which means it, too, probably shares more aerodynamically with a government bus than an Airbus).

Low Cd figures -- the result of millions of dollars and thousands of hours of windtunnel work -- are important in keeping cars like Lexus's LS, Benz's S-Class and BMW's 7 Series whisper quiet even at autobahn speeds.

Lower Cd isn't always better

Which of these two cars is aerodynamically superior -- a Dodge Viper or a 1995 Range Rover? It's the slabby Range Rover -- and at 0.38, by a sizeable margin. The Viper's Cd is jacked up to 0.55 by all those airdams and spoilers and side-skirts. All of where are there intentionally to generate drag, in the form of downforce, to keep the car bound to the tar at high speed.

Drag increases exponentially with acceleration. That makes it very handy as a tool for keeping cars stable and earthbound at speed, but it also means that doubling your velocity doesn't merely double the drag to which the car is subjected. Indeed, it more like quadruples it.

The result? The best example of the principle lies in the classic Formula 1 factoid: that an F1 car travelling at a steady 180km/h generates enough drag to keep it glued to the ceiling of a tunnel, at least until it runs into an air duct or a light fitting.

For most of us non-Hamiltons, however, it's not so useful. Accelerating from 90km/h to 110kpm/h, for example, increases drag by something close to 50 per cent, which in turn increases fuel demand by 10 to 20 per cent.

Minimising the grille's adverse effects on fuel economy

One of the biggest contributors to drag is the grille, which long played an essential part in keeping the engine cool. But now, with many ultra-modern engines running efficiently and reliably enough to render temperature gauges redundant, they don't need perpetual blast of air coming directly in the front end.

Some of the more advanced marques are addressing this. As part of its EfficientDynamics (link to EfficientDynamics feature) program, BMW installs sensor-driven flaps in some models that close off the grille when it doesn't need the air.

How can you minimise drag?
>> Keep windows rolled up at speed
>> Keep your sunroof closed
>> Keep the roof up on your convertible
>> Keep a tonneau cover on your ute
>> Remove roof racks when you're not using them
>> Ride bicycles -- don't mount them on your car
>> Do you really need weather shields?
>> And that bull bar -- what kind of bull's it there for really? And the bank of driving lights?
>> Lose those decorative air dams, skirts and tailwings
>> Ditch the fat tyres, too. Especially if they're sitting under bolt-on wheelarch flares

In short, the news is bad for the road-rage boys: the faster that ageing Excel or Pulsar looks, chances are the slower it's going and the more bolt-on accessory purchasing power it's guzzling in the process.

Aerodynamics: slippery is sexy

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