A spoiler is intended to control the way the air streaming over the car is affecting the rear of the vehicle. When speeds get up to 90-100 mph a spoiler can make the difference between life and death. A correctly designed spoiler can counteract the lifting forces that most cars experience when speeds get high, and keep the car on the road. The absence of a spoiler or a spoiler of faulty design can actually cause the car to lift from the road and throw it into the greenery (or worse).
Fortunately, aerodynamics are of little importance at legal speeds off the German autobahn. But it can be critical on the racetrack or if you happen to test the limits of your car on a vacation in Germany.
Don't let the spoiler spoil the performance. Rear spoilers cause drag, and actually slow the car down or make it unstable, if not properly designed. If you choose an adjustable spoiler you can test its angle at successively higher speeds, to find out what the optimal angle is. If you go for a fixed spoiler, choose one that is is tested to work well with your car. If you buy a spoiler purely for looks, don't go really fast until you have checked how it changes the behavior of the car. It might be lethal. A good spoiler is shaped and angled to create a controlled air vortex behind the car. Controlling how the air leaves the surface of the vehicle minimizes drag and increases stability.
A proper spoiler works as a compact and refined version of the Kammback, which is an abruptly cut off drop shape that was found to reduce drag and give better stability than drop-like tapered ends on cars. One big and controlled vortex behind the car causes less drag than a number of small vortexes that occur when the layered airstream from the vehicle joins the surrounding air.
Spoilers don't have to be big
Spoilers don't have to be big to have the desired effect. When the Ford LeMans-winning racer first was tested on the Mulsanne straight of the track, driver Ken Miles discovered that the cars were wagging their tails down the straight, in spite of all the windtunnel testing and all the Ford Parts engineering expertise that had gone into the development. A wagging tail is pretty scary when you go 200 mph. Carroll Shelby ordered the master tinsmith of the team to add an aluminum spoiler to take care of the problem. A couple of inches of aluminum was shaped into a spoiler and riveted to the rear of the car, which solved the problem.
Many spoiler designs are inspired by the high and big wing featured by the classic Plymouth Road Runner. While this is a design that is very distinctive and fun, it is not really practical. The real reason for the wing to be placed on those high pods was that it was difficult to find good mounting points for the spoiler. The high pods were chosen as they didn't interfere with opening of the trunk lid, a fact that wasn't much publicized at the time.
What looks fast sometimes isn't
There is nothing wrong with high pods and big wings, if done right. They certainly make for a radical change of the design, making very clear that the car owner loves speed. Just set the wing at the right angle, so that it creates the amount of downforce you want. If angled wrong it can act as an air brake or, even worse, lift the rear of the car from the road when you speed up.
Aerodynamics is a field where a very dangerous devil is lurking in the details. A well respected German auto manufacturer discovered this when it was too late. The design department had vetoed the engineers demand for a ground effects package, including spoiler on their sleek new sports car. A number of people were killed in accidents on the autobahn before a 2" high spoiler was added to prevent the deadly rear lift that the original body shape caused.
For most practical purposes off the racetrack, a properly designed lip is sufficient and adds a performance touch to the design without being ostentatious. The choice of spoiler style is yours. And as always - to come up with a really good-looking and well-working solution that people admire, you need to know what you are doing. What looks fast and aerodynamic often is not, and real car people know what is for go and what is for show.
by Peter Altrem on