A car's profile is much like...
A car's profile is much like that of a wing, which causes lift. Different pressures are acting on the car as the air flows over it as indicated by the arrows (vectors,) which are normal (perpendicular) to the body's surface. When all of the pressure forces acting on the body of the car are added together, the net result will be more force pushing the car up (lift), and more force pulling the car back (drag).
You knew this day was coming. It was only a matter of time before we would have to add automotive aerodynamics to our long list of technical rants. But this isn't what you think. Although we wince at triple-decker aluminum wings and ultra-wide, overducted body kits, we'd be the last to tell you that such alterations are not important. We don't have a problem with the spirit of these modifications; it's simply the execution that leaves so much to be desired.
But before we can get into that, it's important to understand the fundamentals. Although complicated and steeped in theory and equations, common sense and a basic understanding of the mechanics of airflow go a long way towards providing an understanding of how aerodynamic aids work and how to utilize them.
We all know a little about aerodynamics. Stick your hand out of the window of a moving car and you can immediately feel the effects of airflow-lifting your hand up or pushing it down, in addition to moving it backwards. Place your hand perfectly flat in the wind, parallel to the ground, and you should only feel your hand being tugged backwards a little. That tugging force is drag. As you increase the angle your hand makes to the oncoming wind, your hand is forced upwards. This is lift. Decreasing the angle of your hand from the flat position will cause the wind to force it down, hence the term downforce. In both situations, the effects of drag increase because the surface area exposed to the airflow increases.
As applied to cars, the broad logic of these concepts is pretty easy to understand. Generate enough lift and you can fly, which is about the last thing you want a car to do. Turning lift upside down, however, creates downforce, which race cars use to corner faster. But all of this comes at the cost of drag, just like in the hand experiment. The trick is to find that magic balance of getting the most downforce with the least amount of drag. Doing so requires digging a little deeper into aerodynamic theory.
The point of automotive aerodynamics is to shape a car so that it disturbs the air it moves through as little as possible. Literally, aerodynamics means the study of the forces acting on a body moving through air. In our case, the body happens to be a car and the forces will depend on many factors such as the shape and size of the car, in addition to some of the properties of air.
Engineers measure power in units such as watts (W) or horsepower (hp). Torque is measured in Newton-meters (Nm) or pound-feet (lb-ft). Aerodynamicists determine how well an object slices through the air by using coefficients, which differ from other measurements in that they lack units. This is because unitless coefficients allow for the comparison of cars of different sizes and shapes. For example, a Honda Civic is shorter and has a smaller frontal area than a Mack truck, but by using the coefficient of drag (CD) as a standard of measure, the aerodynamic efficiency between the two can be fairly evaluated. In addition to CD, other commonly used measures are the lift coefficient (CL), and side force coefficient (CY).
By now you might be wondering what all the fuss is all about. The hand-out-the-window example is useful for explaining the concepts of lift and downforce, but doesn't do justice to the negative aspects of poorly designed aerodynamics. Aerodynamic drag hurts performance. It is chiefly responsible for the top-speed limitation on any car since it takes exponentially more power to maintain higher speeds. For example, it takes almost five times more power to maintain your car at 100 mph than it does to drive it at 60 mph. More drag (a higher CD) magnifies this effect, negatively impacting top speed and fuel consumption by requiring the engine to do more work.
There are three primary ways to reduce drag. One is to reduce the frontal or cross-sectional area of the vehicle. Think of a car punching a hole in the atmosphere while in motion. Cars with less frontal area punch smaller holes, thus creating less disturbance. Of course, there are few things you can do to reduce frontal area on a car beyond taking off the side mirrors and running on skinny doughnut tires.
A better way to reduce a car's CD is to make it more streamlined. This happens at the design phase and involves implementing smooth curves on body panels, rounded lights, soft windshield-to-roof transitions, smooth A-pillar-to-side window treatment, and more.
The third method is a modification of streamlining and centers around using a flat or smooth tray under the car to allow air to escape with less drag. Without using an underbody tray, airflow beneath the car is highly turbulent, due to the clutter of mechanical bits like the exhaust system, driveshaft, fuel tank, shift linkage, and suspension. A smooth underbody also reduces lift by allowing the air to move faster and therefore at a lower pressure. The added stability from the reduced lift is why most supercars and race cars now have smooth underbellies. It's the most practical way to improve performance compared to reducing frontal area or streamlining the body.