The two main aerodynamic forces imposed on a vehicle are lift and drag. Drag is the force that acts opposite to the path of the vehicle's motion. Lift is the force that acts on the vehicle normal to the surface on which it is driving.
The drag on a car is a major concern when factors such as acceleration, top speed, and fuel efficiency are important. Any car must overcome the drag forces on it to accelerate or even maintain a constant speed. This forces robs power from the motor which could be used to accelerate the car. Lower drag coefficients on the car correspond to lower drag forces. C_D can be lowered by designing a more streamlined model or by eliminating windows, door handles, and other gaps.
The lift force on a vehicle can either be beneficial or hindering. If a car has too much positive lift force on it, it will act like an airplane wing and lift off the ground, causing a great loss of control. However, negative lift, or down force, can be very beneficial to a car's handling. A higher down force acting on a car model will cause more friction between the tires and the ground. The more friction there is between the car and the ground, the better the car can corner and accelerate. This does come at a price, however. A car designed to create a huge down force will also produce a large amount of drag. Therefore, like everything in engineering, a good car should have a compromise of high down force and low drag.
Theory
In this experiment, force and wind tunnel frequency are measured. The frequency is converted to velocity according V = √2ΔP/ρ, derived from the Bernouli Equation. The forces can tell how lift and and drag are related to air speed. While these numbers are somewhat informative, they are specific to our model car. Far more useful are C_L and C_D. These dimensionless quantities are used to relate our measurements to a car of actual size and to compare with other models. We expect the coefficient of drag to remain constant at all speeds.