Operation
Spoilers generally work by disrupting the airflow going over a moving vehicle. This disruption's primary purpose is to reduce the amount of lift naturally generated by the shape of the vehicle while it is moving. This is accomplished by increasing the amount of turbulence flowing over the shape, "spoiling" the laminar flow and providing a cushion for the laminar boundary layer.[citation needed]
In cars, 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, some 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.
Racing
Spoilers are almost always ignored in racing applications in favor of wings. Wings create a directed and 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 stock car 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.
Passenger vehicles
The main design goal of a spoiler in passenger vehicles is to reduce drag and increase fuel efficiency. While many often imitate wings and airfoils, these serve mostly decorational purposes. Passenger vehicles can be equipped with front and rear spoilers. Front spoilers, found beneath the bumper, are mainly used to direct air flow away from the tires to the underbody where the drag coefficient is less. Rear spoilers, which modify the transition in shape between the roof and the rear and the trunk and the rear, act to minimize the turbulence at the rear of the vehicle.
Sports cars are most commonly seen with front and rear spoilers. Even though these vehicles typically have a more rigid chassis and a stiffer suspension to aid in high speed maneuverability, a spoiler can still be beneficial. This is because many vehicles have a fairly steep downward angle going from the rear edge of the roof down to the trunk or tail of the car. At high speeds, air flowing across the roof tumbles over this edge, causing air flow separation. The flow of air becomes turbulent and a low-pressure zone is created, increasing drag and instability (see Bernoulli effect). Adding a rear spoiler makes the air "see" a longer, gentler slope from the roof to the spoiler, which helps to delay flow separation. This decreases drag, increases fuel economy, and helps keep the rear window clear.
