Many years ago (OK, 13), when the Mk1 Lotus Exige was new, everyone was very excited that it had downforce - you know, like a racing car and all. So we set off to do a feature on it. On the road, it gripped and gripped, but then we went to a track. The lateral g was too much for me to explore, but one of our testers was a bit handy on a track, so set off down the pitlane with a cheery wave. At the first corner, he decided to throw it merrily sideways, and we all stood by to enjoy watching the rest of his lap.
No sooner had the tyres started to squeal on that first bend, he was in the gravel. The Lotus engineer who'd come along to shepherd the car turned to us spectators, shrugged his shoulders and deadpanned: "That's the trouble with wings. They only work in one direction." Once the air was no longer passing exactly fore-aft over the rear wing, it became a whole lot less effective. So the car lost rear-end grip at the exact moment our man needed it.
Road-car aerodynamics, then, are no simple matter. And they can't even be directly translated from that other brain-scramblingly complicated matter, race-car aerodynamics. We mostly tend to think only of drag and lift - or even negative lift, aka downforce. But there's a whole lot more to say. Designers have to think about the airflow that cools the powertrain, brakes and, increasingly, high-power electrical parts, too. They have to keep the glass and mirrors clean. And control noise. While maintaining practicalities like safety and space.
For most of the automobile's history, all this has behaved like a hostile, zero-sum matrix, where winning one characteristic meant losing another. But, over the past couple of decades, the science has found ways to reconcile what had been mutually exclusive aims.
Gordon Murray has designed Formula One cars with clever aero, including the Brabham BT46B ‘fan car', which remains the only F1 car with a 100per cent winning record, having caused so much uproar among rival teams it was allowed just the one outing. He did the McLaren F1, which was about the first road car with ground effect, and also used clever movable aero devices that haven't been equalled since. And he designed the T25, a car that seeks ultimate energy efficiency. But he says of the T25: "It's a city car, and aerodynamic drag doesn't really matter until you get to 40mph or so - stick your hand out of the window then, and you can feel the force on just those square inches. The faster you go after that, the more it matters." Quite so - the force of aero drag increases with the square of speed, and so the required power to overcome it rises with the cube of speed.
Once it had got itself autobahns, Germany had acquired speed. And so, in the Thirties, the value of cutting drag became clear. Paul Jaray used his aeronautics background to design a series of streamliners for Maybach, Audi and others, culminating in the Tatra T77, which had a Cd of 0.21- pretty staggering even today. He also designed a small Tatra V570, which influenced the VW Beetle. The rear engine of the Tatras meant it made sense to have the long-tail design of your stereotypical low-drag shape, the teardrop. The opposite, the usual boxy pre-war front-engined cars, were often lower in drag going backwards than forwards.
So, it was reputed, was the early, spoiler-free Porsche 928 of 1976. The American TV show Mythbusters experimented by hacking off the body and actually building a rearward-facing 928. Itwas more draggy than the original, so they figured they'd busted the myth. But I'd say they hadn't, because they still had the underside of the car going in the normal direction.
And underbody aero is crucial in cutting drag, and has been vastly underestimated until recently. Or, at least, misunderstood. That's why from the Seventies onwards - see the hilarious BMW 3.0 CSL - go-faster merchants simply riveted a huge air dam onto the front of the car to keep the pesky air from getting under there at all. Air gets turned hugely draggy and turbulent as it wends its way around the bottom of the engine, the suspension, exhaust and so on.
You can't even use a conventional wind tunnel to investigate it. You need a tunnel where the floor is a giant conveyor belt, and the car stands on it with wheels turning. In the real world, the car is moving, but the road is not - and the air isn't, either, until the car arrives to disturb it. In a moving-floor tunnel (Murray built one of the first of them while at Brabham), the air and the floor move but the car stays still, so the relative motions are the same.
Next year's Ford Mondeo tackles the underfloor issue by, in effect, putting a bandage on it. Under the whole length of the car are uninterrupted flat shielding panels. Adrian Whittle, chief engineer on the Mondeo, says that while they used to keep air out of the underside: "We now want as much air under there as possible, because once you've got a shield the drag is tiny. It also improves stability - in the past, you had the asymmetric transmission and exhaust, and so on." There's now more drag reduction because it promotes a ‘clean' - less turbulent - departure of air from the tail of the car. And it cuts underbody wind noise, too. "Mind you," says Whittle, "that shield costs proper money - in materials and in development.
"The aerodynamicist who worked on the Mondeo actually used to work on the space shuttle," grins Whittle. The car is 10 per cent lower in drag than the current one. Its other win is active shutters inside the grille - the radiator is blocked off except when it's needed, which cuts out a whole lot of turbulent flow inside the engine bay. On most cars, that air then vents itself inside the wheelarches, causing more drag and lift. BMW was among the first with these shutters, as part of the EfficientDynamics initiative. But it helps when you design a car around them as, says Whittle: "It changes the whole airflow." Detail matters, too: "Moving the mirrors to the doors and spending ages reshaping them got their drag contribution down from 0.15 to 0.09."
It's often the details that matter. To get the diesel version of the new Honda Civic down to its 110g/km target, Honda added two features denied the petrol: radiator shutters and, on each side of the car below the tail-lamps, a tiny glued-on plastic fillet that tidies up the separation of air from the car. Not pretty, but effective. Reminds me of the little ‘ears' that were fitted behind the rear side windows of the jelly-mould Ford Sierra hatch part-way through its life - they did the same job and helped stability, too.