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
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.
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.
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.
