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A spoiler helps direct airflow on the car, usually to increase downforce (with the side effect of increasing drag). They are often fitted to coupes. Spoilers were part of the Datsun 1200 FIA homologation. Sometimes they are called "wings" but a true wing is something different.

Overview

The motorsport definition of a spoiler is an aerodynamic device attached to an automobile, contiguous with the bodywork, and where only one surface (ie the top) is exposed to airflow.

The motorsport definition of a wing is an aerodynamic device attached to an automobile where both upper and lower surfaces are exposed to airflow.

Spoilers slow the airflow down to create and area of higher pressure on the rear of the car (downforce). Wings (on a car to make downforce) acellerate airflow on the underside of the wing and slow airflow on the top to create the pressure differential.

Also see: 
* Air dam (front "spoiler")
* Wing (rear spoiler)

Whale-tail
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Bobtail

The "FIA" style rear spoiler -- aka Bobtail, Ducktail, etc.

Classic old-school "bobtail spoiler"
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The classic old-school "bobtail spoiler" works by stablising and slowing the rotational velocity of the bound vortex at the rear of the vehicle. Effectively a vertical strip of material at the tip of the boot (trunk) will do the same. The result is reduced lift and reduced drag. In simple terms, a bob tail works by increasing the air pressure on the rear of the car relative to if it weren't there.

Tomei spoiler
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