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 vehicle of some kind 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 for automobiles 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.
Spoilers generally work by disrupting the airflow going over a moving vehicle. This disruption's primary purpose is to reduce the amount of lift or turbulence naturally generated by the shape of the vehicle while it is moving.
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.
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 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.
This Toyota MR2 sports car has a factory-installed rear spoiler.The main design goal of a spoiler in passenger vehicles is to reduce drag and increase fuel efficiency. Spoilers which are often confused with wings or air foils (raised detached surfaces) are frequently used in modern passenger vehicles. Actual spoilers decrease drag and lift whereas the more commonly known commercial "spoilers" (see the picture) often imitate wings and air foils and are purely decorational. 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, at high speeds, 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. 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 eliminate flow separation. This decreases drag, increases fuel economy, and helps keep the rear window clear. The spoilers on "serious" sports cars, are designed to specific tolerances in contrast to the more general designes of pedestrian cars, which act generally to reduce drag and in a few cases provide a modest amount of downforce. The Toyota Prius has this type of spoiler, as well as some SUVs (especially hybrids) and minivans.
Heavy trucks, like long haul tractors, may also have a spoiler dome on the top of the cab in order to eliminate drag cause from air resistance from the trailer it's towing, which may be taller than the cab and provide a very non-aerodynamic effect. These spoilers primarily increase fuel economy instead of improve handling, however.
Trains may use spoilers, both as a way to reduce or induce drag (like an air brake). A new prototype Japanese commuting train, the FASTECH 360 is designed to reach speeds of 250 mph. Its nose is specifically designed to spoil a wind effect associated with passing through tunnels, and it can deploy 'ears' which act to slow the train in case of emergency by increasing its drag.