Will Adding a Spoiler Make My Car More Aerodynamic?

The simple answer is yes, but maybe not in the way you think it will. When most people think of automobile aerodynamics, they envision a car in the wind tunnel with a smoke wand showing how the air flows over the car. They think in terms of coefficient of drag (Cd), and it's been ingrained in us that less resistance and a lower Cd number are always better. Well, that depends. Automobile aerodynamics is the study of how the motion of air interacts with an automobile, and while reducing drag for higher speeds gets the most attention, there are body modifications that are intended to actually resist airflow. All spoilers and wings produce negative lift, or downforce, and very often actually increase drag, but in many instances this is desirable. Since most aftermarket body parts have their roots in body modifications made to race cars, it may be helpful to further explain this by looking at 2 types of racing to see the goals of each and how the cars are modified to achieve those goals.

For cars at the Bonneville Salt Flats, the highest speed possible is the goal, so modifications here are for the least amount of drag. Anything that can interfere with the free flow of air over the vehicle is eliminated and a front air dam would be added to reduce drag (more about how an air dam works later). Only enough downforce for the light alloy wheels to maintain traction is needed. At the opposite end of the spectrum is the dirt circle track Sprint car. Top speed is not an issue here, but putting as much power down and getting the best drive out of the corners is. For that these cars need tremendous downforce, which is provided by the exaggerated wings that are customarily installed on these racers. The drag they produce is inconsequential. As you can see from these examples, there is no one monolithic goal toward which all body modifications are made. Different parts provide different effects. For most other types of racing, a balance is needed: downforce without too much drag for traction and steering, and low drag with enough downforce for top speed. Now that we've covered some basics, let's look at the individual pieces more in-depth to see how they can affect your car.

Air Dams

Air Dams

Air Dams, also called Front Valance Panels, Front Spoilers or Front Lips, are mounted on the front of the car below (or in some case in place of) the bumper. They vary in size according to vehicle and manufacturer, but all are used to reduce the amount of air flowing under the car. This reduced airflow benefits the car in 2 ways. First, the many parts on the underside of the average car (exhaust, drivetrain, and suspension) create a lot of drag, so the less air they come in contact with, the better. Second, reducing the air going under the car forces more air around the sides and over the top of the car; the limited amount of air that does go under the car travels at increased velocity, creating a low pressure area and, in effect, "sucking" the car toward the ground. Visually, an air dam will also make your car look like it's closer to the ground, as if it were lowered; just be careful of driveways and those parking barriers!

Front Splitters

Splitters work in conjunction with air dams and are used to create downforce. They extend horizontally forward from the bottom of the air dam. When a car travels at high speed, the blunt front end creates a high pressure zone in front of the car. As their name implies, the splitters "split" this high pressure air from the high velocity air flowing under the car, creating downforce.

Road race cars often have adjustable splitters to control the amount of downforce, in an effort to maintain an aerodynamic balance.


Canards are usually positioned at an angle to the ground on either side of the front of the car, in front of the wheel wells. They create downforce on the front end of the car by taking the oncoming air and redirecting it upwards.


Rear Wings

The wing on an automobile is flat on the top and curved on the bottom, the exact opposite of the way a wing is configured on an airplane. The reason is that the airplane needs lift while a car needs downforce. The downforce is created because of the pressure difference between the top and bottom wing surfaces. As air flows over the wing surfaces, the air on the bottom must move faster than the air on the top to go around the curvature of the wing. This creates a low pressure area at the bottom of the wing. The higher pressure at the top of the wing pushes down on the wing, creating the downforce. The wing can also be angled to resist airflow, to create even more downforce.

The wings on race cars are usually adjustable for different conditions. While most wings will increase drag, there are some that have been integrated into the design of the vehicle so well, that they cause little or no drag.

Side Skirts

Side Skirts also work in union with air dams, in an effort to minimize the amount of air that goes under the car from the sides. In fact, without side skirts, an air dam's effect can be negated because the low pressure area created under the car by the air dam would cause high pressure air from the sides to come rushing in. Accordingly, side skirts are more effective when they are closer to the ground. Side skirts also create the visual effect of making your car appear lowered, as if it were "hugging" the pavement.

Rear Diffuser

This is another component that is dependent on the presence of other parts to function effectively. The rear diffuser works with the air dam and side skirts to create a condition under the car where there is low pressure caused by high speed airflow. The rear diffuser will redirect this airflow to transition smoothly into the slower airstream at the rear of the car. It does this by providing space for the underbody airflow to decelerate and expand.

Rear Spoiler

Rear Spoiler

Since we'll be covering them separately, before we go any further we should clarify the difference between a spoiler and a wing. Many manufacturers refer to each indiscriminately, but for purposes of our discussion a spoiler mounts to the car's upper rear surface (the trunk in most cases) and there is no gap between the spoiler and the body. If there is a gap it's considered a wing. A spoiler's function and effectiveness are dependent on its size, angle and the shape of the rear of the car. If the spoiler's primary function is to provide downforce, it will be angled more vertically. In this position, the high speed airflow coming off the roof toward the back of the vehicle will be disrupted, reducing its velocity and creating high pressure. Downforce may be needed for rear wheel traction but it may also be needed if the shape of the vehicle causes it to lift and be unstable at speed. The aforementioned type of spoiler will usually increase drag, but spoilers can also be designed to reduce drag. On some cars the distance between the rear edge of the roof and the tail end is very steep, disrupting the airflow and increasing drag. A properly designed spoiler can, in effect, lengthen the distance or make the slope gentler, preventing airflow separation and reducing drag.

In Conclusion

We know that for most of you the way your car looks with these parts installed will be the most important consideration, but we hope we've been able to provide you with some helpful practical information about the functional aspect. Also, keep in mind that we've explained how these parts work on race cars at high speeds; their effect on passenger cars at legal highway speeds will be reduced.

by on