Spoilers work like an upside down airplane wing. We will now take this very complex subject and simplify it. Obviously there are many, many complex factors to take into account but we are dealing with the basic principles of spoiler design here and discussing whether there is actually any benefit of fitting them to your car.

In short, fast moving air creates pull if the other side of an object is subject to slower moving air. To test this theory take 2 sheets of A4 paper and hold them between your thumbs and little fingers so there is a gap between the sheets of about 1 inch. Then blow between the 2 sheets of paper and you will notice that they pull together, which is not actually what you expect to happen. The fast moving air creates a low pressure region between the paper causes them to pull together.

So to create lift or "down force" you need air moving faster on one side of the aerofoil (wing/spoiler) than on the other side - creating a pull towards the fast moving air stream. Obviously on a car you will not want a huge airplane wing (although with what we have seen driving about the streets, some people obviously do!) so we use simple plane wing deflection to create the down force required. The angle of deflection will have a bearing on the amount of down force generated, the larger the angle the more down force but the price you will pay is increased drag which, will slow you down.. Most spoilers have a two angled surface creating a two stage deflection forcing the air over the spoiler to take a shorter route than the air going under the spoiler. This brings us to look at the whole car and its effect on down force/lift.

If you look at the basic shape of a car the air will travel further and faster over the top of the car than it does underneath, this creates lift. Opening a sunroof at speed can make this problem even worse. So, an unmodified car, traveling at high speeds becomes very dangerous, effectively losing grip, traction and control. This is reffered to as the Coanda effect. So once deflected, the stream of air will continue on the deflected course after the end of the spoiler. Already we start to realise that just sticking a spoiler on the boot of your car is not addressing the whole issue of aerodynamics!

If you imagine the car as a see-saw when you press down on the rear the front will lift slightly. When you fit a spoiler to the rear you have to expect that at high speeds the front of the car will become lighter. In a front wheel drive car this is very undesirable as the front wheels lifting off the floor creates a period of reduced performance, never good in any race! In a rear wheel drive car you still need the front wheels to steer so although traction is improved a degree of lift at the front is also considered undesirable.

To counter the front lift you can fit front splitters, and front and rear skirts, which create down force at the front of the car and control the amount of air rushing under the car to create down force. When you start redirecting the air rushing over the car you will create an amount of drag which will reduce your speed and economy so there is a very fine line between an efficient spoiler design and an inefficient one.

Sometimes in Formula 1 and other motorsport's you will have noticed spectacular crashes where the car has lifted due to a bump and the spoilers have effectively helped the car take off like a plane, and then crash. Some cars generate more down force than they actually weigh, creating the possibility of driving upside down on the roof of a tunnel!!

The best spoilers are adjustable, allowing the driver to set his preferences to whatever the track conditions may be. For example a slow track with many bends will need a different setup to a fast track with lots of straights. They can also be mounted high up on the rear so they have an effect on the air flowing over the car. Vertical slats in the spoiler help to increase a cars straight line stability, something you tend to see when watching motorsport on TV!

Side skirts and sills help to cause the air moving under the car to move faster creating down force and enhancing cornering stability. For the best effect we recommend that you do not just go out and buy the first thing you find that fits your car. Instead go to a proper tuner and get a spoiler spec'd up to your car. A single rear spoiler is a pointless addition as a performance enhancement as you need to address the nose/front of the car, get some suitable side skirts and have the underneath of the car designed to create down force as well.

Spoilers provide down force at speeds of around 50mph but it's not until about 70mph that this becomes significant and offers a benefit to the handling of a car.

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