What Is an Auto Spoiler?

Spoilers are aerodynamic devices found on the trunk lid or rear hatchback of a car, and serve to alter the airflow going over and behind the vehicle. Many of these wing-like add-ons are merely cosmetic in nature, but a properly designed spoiler can enhance high-speed performance significantly.

Function

• Air traveling over a car separates into two parts, a slow-moving "boundary layer" that sticks close to the car's body and the high-speed atmosphere that flows over the boundary layer. The boundary layer acts as a sort of "lubricant" between the car's body and the air around it.

Shape and Drag

• As air flows over the back of your car, it whips downward toward the rear bumper (and trunk lid if you have an older notch-back vehicle) to fill the vacuum pocket formed by the car's movement. The more severe the drop at the rear of the car the larger this vacuum "drag" pocket becomes. For instance, a boxy hatchback will have a much larger drag pocket than the same car with a trunk.

Spoiler Function

• The spoiler acts like a sort of wall to catch the boundary layer, causing it to build up and increase in pressure. The boundary layer gets thicker and "harder," causing the air above it to begin wrapping around the back of the car closer to the bumper than it otherwise would. This reduces the size of the vacuum pocket and the resistance to forward movement that it creates.

Other Functions

• Spoilers also help to increase high speed stability by creating down force. The high pressure boundary layer just ahead of the spoiler pushes down on the trunk lid, helping to push the car to the road.