NASA defines aerodynamics as the study of forces and the resulting motion of objects through the air. Since we are talking about cars here, and how car designer make sure our cars donít move through the air, but rather how car designers help our cars stick to the road through aerodynamics.
The effects of aerodynamics are best demonstrated when you stick your hand out the window. With the heel of your palm or your thumb facing the wind, you will be able to feel your hand slicing cleanly through the air. Face your palm to the wind and you have to use your arm muscles to keep your hand steady. Now imagine how strong the forces are on an object as big as your car. The force that manifested itself with your hand-out-the-window experiment is called drag. Car designers and Engineers use the term coefficient of drag (or drag coefficient for short) to measure a car's ability to slip through the wind. The CD equation is CD = D/(1/2xpV2xA), where D is drag, p is air density, V is velocity, and A is the car's frontal area. Obviously, you need to be an aerodynamics engineer to make sense of that equation and assign the proper values to it. Lowering a carís CD and reducing drag are the main goals of aerodynamic design, since it has been proven that getting a reduced CD gives substantial improvements in terms of horsepower requirements, fuel economy and road holding. Reducing the drag working on a vehicle reduces the horsepower needed to run a car at a given speed. An engine that doesn't have to work as hard has better fuel efficiency, which is why car designers from automobile manufacturers nowadays spend so much time in the wind tunnel with their new car designs. Conversely, reduced drag will enable a car to go faster for a given horsepower output.
In terms of road holding, adding down force combats the effects of lift, which results when air moving under the car acts on the underbelly of a moving vehicle, much like the action of air on an airplane wing. By adding down force, you manipulate the movement of air over the car, which will increase the friction between the tires and the ground. The effect of this is that you can brake and corner harder. You can also create down force by lowering a car. Bringing the bottom of the car closer to the ground makes the air flowing beneath it, move faster. An aerodynamic principle called Bernoulli's Equation states that faster air speed results in lower air pressure, and low air pressure under the car helps the shape of the car, plus any aero aids it has installed, to create down force.
When discussing down force, one principle that experts maintain is important is aerodynamic balance. Just because a humungous rear wing works on a particular car, it doesnít mean that it will work on your car. For example, drag-race carsí aero aids are useless on street cars. The forces acting on a drag racer are completely different because of its weight distribution and optimization for working is a straight line only. If anything, street cars should follow the example of cars built for road racing. Tips for improving street car aerodynamics should put as much emphasis on air flowing under the car as well as over it. First of all, lowering the car will, as stated above, help create more down force. Second, mounting an air dam or chin spoiler will minimize air going under the car and becoming turbulent when it hits the pipes, sub frames and suspension linkages under the car. Another good aero item to install are side skirts, which, when run as low as possible to the ground, will seal the sides of the car, reducing turbulence beneath the car and smoothing airflow on the external surfaces.
At the rear, running a wing instead of a spoiler will provide the most benefits, but in a street car, you will have to balance function with aesthetics. The choice is up to you. Bear in mind though that aero devices really start working at higher speeds (around 90 kph) so if all you do is cruise the urban jungle, you can get away with the much cheaper aesthetic bits that havenít been really tested in the wind tunnel or designed using computational fluid dynamics.