Aero add-ons, that is, supplemental pieces attached to existing body work in hopes of improving a car’s aesthetics or aerodynamics, come in three areas: front “chin” spoilers, rear spoilers, and side skirts.
Rear-mounted spoilers and wings
Popular jargon uses the terms spoiler and wing interchangeably, but they have different functions.
The ubiquitous spoiler is the little flap or raised protrusion on the rear decklids of coupes and sedans, or on the upper edge of the rear gate on hatchbacks. Although the spoiler is often a cosmetic upgrade designed to tell the world that your car is “sporty,” it also has a very specific role in aerodynamics.
As the name implies, this device spoils the airflow over the top half of the car at the trailing edge of the car’s upper surface. The spoiler can keep airflow from tumbling and creating a swirling vortex behind the car. This is important for a couple of reasons — without a spoiler, swirling air behind the car can create both
- Drag (which can keep the car from moving forward as quickly as possible). Drag is expressed as a numeric coefficient of wind resistance.
- Lift (which reduces the car’s grip on the road at speed)
Manufacturers have been known to add spoilers to cars that didn’t have them in the original design. The Audi TT was released first without any spoiler. It could only be purchased with a spoiler after reports began deriding its high-speed stability and inherent rear lift.
A wing on a car is an upside-down version of the wing on an airplane. Instead of lifting, the upside-down wing pushes the car against the ground.
Because the wing must catch an undisturbed flow of air, most wings are mounted on raised pedestals.
Many wings can be adjusted for rake (vertical angle) so that the amount of downforce (and corresponding drag) can be fine-tuned for a specific application. This is often accomplished with adjustment holes that allow you to alter the angle of the wing’s plane for adjustable levels of resistance.
A wing needs to be bolted into the rear trunk of your car. This means drilling holes. This also means that if you get tired of the wing and decide to remove it, you will need to pay a body shop to weld these holes shut and repaint the trunk for you. Choose wisely.
There are several materials from which wings are constructed. In addition to the plastic, fiberglass, and carbon fiber variants available for other body add-ons, wings can also be made from aluminum.
Although these aluminum wings tend to be functional — they are often tall enough to catch reasonable airflow at the back of the car, and many can be adjusted for rake — they also look utterly out of place on a street-driven car. Aluminum wings are fine if you’re building a track monster, but putting one on a street-driven car screams poseur.
A rear diffuser, otherwise known as a venturi, is designed to create a low-pressure or vacuum area under the rear of the car using a physics principle known as the Venturi effect. In essence, a diffuser is like an air channel that is designed to accelerate the air out from underneath the back of the car and help to both minimize underbody wind turbulence and to create negative lift at the rear of the car.
Rear diffusers are available in sheet metal, carbon fiber, and plastic. They are used either with or without a rear wing.
Although much of the performance aftermarket seems to be visually fixated on the back of the car, with spoilers and wings being the leading indicators of performance (or at least a shallow pretense), managing airflow at the front of the car, either by reducing lift or creating downforce (negative lift), is just as important. As a rule, the more work the wing is doing in the back, the more attention you will want to pay to the front of the car to ensure that the car moves at speed. The front (which is where the steering input is generated), provides as much grip and feedback as the rear.
The air dam is the front valence mounted underneath your car’s front bumper. If you have no idea what this is, that’s okay. Nearly all modern cars have integrated air dams in which the lower-front valance and bumper skin are one seamless piece. Back in the mid-1980s and earlier, in the days of the exposed steel bumper, this was not the case. The air dam’s job is to manage airflow at the front of the car, guiding air to the radiator/air-conditioning condenser and/or front mount intercooler and away from the tires, where it would cause lift. Many air dams also provide the mounting location for fog lights or driving lights.
Today, one of the most common ways to give your car a “face lift” is the addition of a new front bumper (with air dam) along with a matching set of side skirts (the plastic extensions that bolt onto your car’s side sills under your doors) and rear bumper. These body kits (consisting of the four pieces described and with a front lip sometimes added as a fifth component) can be mixed and matched from a number of different manufacturers for a unique look.
Several tuners have signature designs for front bumpers/air dams that they carry over to a number of different makes and models. For example, the distinctive (in a bad way) Veilside front end looks virtually the same regardless of the car it is found on.
When choosing a front bumper for your car, go with quality/durability (polyurethane excels here; fiberglass will crack, chip, and shatter), aesthetics (flows with the lines of your car), and try to find something that allows you to retain your factory bumper beam and foam support for safety.
Beyond these requirements, keep the air dam’s functional purpose in mind. If you are running a front-mounted intercooler or larger radiator, make sure that the bumper skin/air dam that you are buying will provide an oversized opening to give it all the air it needs.
If you live in a state that requires a front license plate, check whether the front bumper allows for a logical mounting location for this.
A splitter is designed to separate the oncoming air at the lower-leading edge of the car.
By presenting a planed splitting surface, the air doesn’t immediately encounter the car’s front valance and tumble in front of the car. Instead, the air is channeled
- Up above the car’s front lip to the radiator
- Below the car toward brake ducting or other air channels.
Splitters are available in a number of materials, including
- Carbon fiber
Splitters must be mounted securely to the car’s bodywork to perform as designed. If they move around at speed, they aren’t effective.
For most racers, a splitter is an expendable item that is often broken and replaced. However, weekend enthusiasts need to be careful when negotiating driveways and hills to avoid damaging the splitter.
Canards (bumper winglets) are designed to provide downforce on the front end of the car. Much like a wing on the rear of the car, canards add drag but help keep the front of the car planted when at speed. Made of plastic, carbon fiber, or fiberglass, canards are relatively inexpensive. However, they flex the paint on the front bumper skin, often resulting in stress cracks in the paint’s surface.