A spoiler is an aerodynamic device attached to an automobile whose intended design function is to 'spoil' unfavorable air movement across a body of a car in motion. This can result in improved vehicle stability by decreasing lift or decreasing drag that may cause unpredictable handling in a car at speed. Spoilers are often fitted to race and high-performance sports cars, although they have become common on passenger vehicles, as well. Some spoilers are added to cars primarily for styling purposes and have either little aerodynamic benefit or even make the aerodynamics worse.
Spoilers are often incorrectly confused with, or even used interchangeably with wings. Automotive wings are devices whose intended design is to actually generate downforce as air passes around them, not simply disrupt existing airflow patterns.
or increase the amount of force pushing the vehicle's tires to the road surface (also called downforce).
Spoilers generally work by disrupting the airflow going over a car. This disruption has the primary purpose of reducing the amount of lift naturally generated by the shape of the car.
The result is increasing the contact between the tire and the road surface, thereby increasing traction. This increase in traction allows a vehicle in motion to brake, turn, and accelerate with more stability. Additionally, this is accompanied by an increase in aerodynamic drag.
In nearly all cases, drag increases as the speed of the vehicle increases. Thus, spoilers that are effective at very low speeds often generate excessive drag at high speeds, and spoilers that work well at high speeds are often ineffective while moving slowly.
Spoilers are almost always ignored in racing applications in favor of wings. Wings create a directed & deliberate effect on handling and downforce and typically by design can be adjusted to suit the needs of the vehicle. A notable exception is in NASCAR where the vehicles have a roof spoiler that deploys in the event of air flowing backwards across the car. This was added in recent years as a safety component to keep the vehicle from leaving the surface of the track if it spun out at high speed. This was likely to happen because the body of a NASCAR is designed to generate downforce, but if air flows in the opposite direction across something that generates downforce normally, it instead creates lift. Spinouts would typically result in the car facing backwards for a brief moment before lift took the car off the ground & sent it flying or rolling. A similar device is also adopted for drag racing funny cars.
Spoilers have become increasingly popular on all types of consumer vehicles mainly for styling reasons. Passenger vehicles, which are mostly front-wheel drive currently, have debatable gains from any theoretical increase in traction that might be provided by a rear spoiler simply because of the low speeds permitted on public roads. However, a well designed spoiler can greatly enhance a car's appearance.
Newer light trucks can sometimes be seen with spoilers affixed to either the roof of the cab, or the back of the bed near the tailgate.
Sports cars are most commonly seen with spoilers, such as Ford Mustang, Subaru Impreza WRX, and the Chevy Corvette. These vehicles typically have more rigid chassis and stiffer suspension to aid in maneuverability, but at high speed this can cause unsteadyness in handling. Professional racing competition versions of these cars use wings instead to improve stability by increasing downforce, but for safety reasons, the production versions of these vehicles are offered with spoilers instead. (Remember, if air passes the opposite direction across a wing it generates vertical lift.)