PUT A WING ON IT...
An idiots’ guide to active aerodynamics on road cars. Warning: contains physics
Active aerodynamics is banned in most codes of motor racing. But in support of my long-held theory that the phrase ‘race-bred road car’ is usually a pile of marketing-led baloney, road cars now do often employ active aerodynamic devices, in the form of moving wings and other clever hidden flappery.
This month we experience what’s perhaps the most outlandish of all, as it gets jiggy above the tail of the Zenvo
TSR-S. The theory is solid. An aircraft turns by leaning. In that state, the wings’ force isn’t solely lift – there’s also a vector component that points inwards. Same with the Zenvo, except, of course, car wings are upside-down versus plane wings, as they’re aiming to keep you on the ground. While a plane banks with its inner wingtip down, the Zenvo’s is up.
So the wing is actually pushing the rear of the car toward the inside of the bend, independently of the tyres’ grip. But it has another efect too. The non-vertical force from the wing also puts more downward force onto the inner tyre than the outer. It’s levelling the car, and it’s helping the inner tyre fnd more grip, because the grip of a tyre depends on the downward force acting on it.
Asymmetric applications of wing are a modern thing, partly because they need a whole lot of computational power. The frst was the Pagani
Huayra, which has a pair of faps either side in both the nose and tail. As with most movable rear spoilers (including the Zenvo’s), the rear pair increase their angle of attack under brakes, adding drag and resisting pitch. But all four can also rise independently, controlling the force on each tyre. But as the faps are narrow and rather close to the centre of the car, they don’t give efective airflow underneath where it matters. Pagani seems to have gone a little quiet on the idea in the Huayra BC.
But they were theatre. You can’t even see the lateral aero on the Lamborghini
Huracán Performante. But, boy, it seems to work. The rear wing has hollow supports, and hidden inside are little butterfy faps. When the butterflies are closed, the wing acts as normal. But as for all wings, with downforce comes drag. So in fast straight-line running, the butterflies open, sending air up inside the wing pylons to emerge under the wing, reducing its drag. The faps are closed under brakes and the car makes seven times the downforce.
But there’s more with ALA. The left and right wing channels operate separately too. So into a right turn it’ll open just the left fap. This means downforce on the right side of the car, and (as with the Zenvo) adding traction to the inner wheel. More drag on the inside too, pivoting the car around.
One revelation about ALA is that these little faps and their actuators are small and light, but they have the same efect as angling the entire wing. If you’ve ever looked at the massive supports and actuators of a normal lifting-and-pivoting wing, you’ll see what a huge saving that is.
Maybe that’s why the McLaren
Senna’s wing pivots but doesn’t snuggle down into the bodywork like a P1’ s or
Bugatti’s: simpler mechanism. Also, because it doesn’t retract, the wing can use endplates, preventing the air from spilling of, and dramatically increasing its efectiveness. That’s why the loop wing on the Ferrari F40 was canny.
The Senna’s wing looks even more sinister because its pivoting supports
attach not to the bottom but the top. That leaves the bottom surface clear, as it’s that curved lower surface of a wing section that does most of the work. The Senna, like the Aston Valkyrie, has adaptive anti-roll, and also lowers itself in Track mode, allowing the underfloor aero to better do its stuf.
Another top-mounted pivoting double-deck wing rears out of the
Koenigsegg One:1. But its mounting arms take the form of fns knifing back from the cabin to add stability to the rounded shape, which is handed down from other Koenigseggs. In contrast, the Senna’s entire bodywork has so many edges it’s hard to see what a set of fns could possibly bring to the party.
The Mercedes-AMG Project One has a fn too. If the car’s standing still, the fn looks oddly tapered. It’s actually cut away to allow the rear wing to rise, and the second airbrake fap embedded in it. Yes, its active aero has active aero.
These cars’ far-out shapes are very diferent from the McLaren F1, which had no obvious spoilerage. It was sublimely clever, mind, not just in its basic shape but in the active detail. As well as a fan that controlled airflow under and through the body, it had a rear fap that popped up under brakes. That fap did two jobs: bolstering the rear difuser’s efect, and also opening cooling ducts to the rear discs.
Such subtlety is a long way from the wing that was on our bedroom walls as my generation grew up. The crazy V-wing on the back of the Lamborghini
Countach LP5000S. It added weight and drag and blocked the already terrible rear vision. It actually had no benefcial aero efect. But blimey it looked fast.
At this point we need to name another culprit. Those lovely S-shaped sections on either side of the rear cockpit of a BMW i8. Do they, I asked at the launch, reduce drag? I was told not. Do they add downforce? No. Keep the glass clean? Nope. What then? They look good to us, a designer told me, but “to the air, they’re absolutely invisible”.