Automobile aerodynamics is a subscript of the broader science of aerodynamics, the study of air and the interaction with solid bodies moving through air. Aerodynamics is itself a part of fluid dynamics, which is the study of the properties of a solid object moving through a fluid such as air. An automobile is being used here as a general term describing any motorized vehicle, including cars or trucks.
The aerodynamic properties of an automobile are fundamental to the performance of the machine. While the engine, suspension, transmission, and tires are the first structural components of a motor vehicle considered when automotive performance is assessed, the efficient performance of the automobile requires optimum aerodynamic performance.
Aerodynamic principles are most often considered in the context of racing cars, where slight adjustments to the profile of the vehicle can affect both speed and performance characteristics such as handling and braking. For the performance of a typical passenger car, aerodynamics are an important consideration in the achievement of maximum fuel economy, as well as in creating auto body styling that is visually appealing.
The creation of desired aerodynamic effect in an automobile begins in a testing facility known as a wind tunnel, where models can be subjected to varying types of wind effect. The test results will include an assessment of the fundamental components of auto aerodynamics, drag and lift.
Drag is the combination of all of the aerodynamic forces that act on an object as it moves through air. The force of drag operates in an opposite direction to the motion of the object. The friction created by the surface of the automobile as it moves through the air is one of the separate types of drag forces created. The principles of drag that apply to an automobile are identical to those created by the hull of a canoe or kayak in the motion through water, as air and water are both fluids for the purpose of the application of the principles of physics in determining drag forces. As a common sense proposition, drag force may be understood through the comparison between a sleek racing car and a large transport truck; the truck will more affected by drag forces than the racing car.
The effect of drag on an automobile increases as a square of velocity. The power (the rate of work) required to propel the automobile through the air increases as a cube of velocity. The drag coefficient may range from a factor of 0.2, for a very sleek and highly buffed race car, to 0.4 for a standard passenger vehicle, to 0.6 or more for a pickup truck, a more angular shape.
To counter the effect of drag, automobiles designed for performance will maximize down force. The faster a vehicle travels through air, the more it becomes affected by the forces of lift.
The Bernoulli effect is a physical principle applicable to lift and down force. The Bernoulli effect is observed when any fluid (including air) flows around an object at different speeds; the slower fluid imposes greater pressure on the object than does the faster moving fluid. As a result, the object is forced toward the faster moving fluid.
The force of lift acts on the moving automobile in a direction perpendicular to the flow of air over the vehicle. The faster the vehicle travels, the greater the effect of lift, and the more inherently unstable the vehicle becomes. Down force is achieved to counter lift by designing the bottom of the vehicle to imitate the shape of a wing of an airplane; when the air flowing below the vehicle is moving faster than the air above, the vehicle moves closer to the ground, as the air pressure there is less than above the vehicle: the reverse of the principle by which an aircraft takes off from the ground.
Down force is enhanced in performance vehicles such as racing cars through the addition of ground effects packages, specialized components that are built into the chassis or body. One such device is a "venture," a tunnel-shaped addition to the body to create further localized low air pressure. The greater the vehicle's down force, the more maneuverable the vehicle will be at higher speeds, especially in cornering.
The wings that are attached to race cars are also designed to increase down forces. The wings are angled to create a faster airflow below than above the wing. The larger the surface area of the wing, coupled with the tilt of the wing, creates down force. The wings, or "spoilers," sometimes seen on passenger vehicles usually do not create an significant aerodynamic effect.