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Spoiler Alert: Some Simple Spoiler Tips

So you’ve got the biggest possible tires, a ton of horsepower and a fully tuned suspension. What’s next to make your car faster? Likely it’s time for some aero modifications–assuming, of course, that your racing regs don’t prohibit them.

“Aero mods,” you say, “doesn’t that require lots of development and expense while not helping much at low speeds, anyway?” Luckily there is a type of aero device that is simple, inexpensive and effective. That would be a rear spoiler. Something not much more elaborate than a piece of flat sheet material can add measurable downforce to your car.

What Is a Spoiler?

A spoiler is basically a flat-ish panel placed at the back of the car in order to provide downforce. These rear spoilers can be massive or tiny. They can be tuned for high speeds or low ones. The shape and angle can be adjusted to tune the tradeoff between drag and downforce. They can help balance chassis setup.

How Does a Spoiler Work?

The basic operation of a spoiler can be explained in three basic steps:

Step 1: The first step to understanding spoiler operation is a quick primer on flow direction. The spoiler is ultimately just a flap of bodywork located on the back of an aero device-and in this case, we’re referring to your entire car as an aero device. The spoiler’s job is simple: change the airflow around that device.

The spoiler’s job is to elevate the low speed/low energy area usually found behind the car. Elevating that area creates downforce upon the car itself.

Unfortunately, there is a tradeoff: a drag penalty as it takes work to redirect the airflow. However, swirl around the ends of the spoiler can also help the flow under the car and generally works to push the wake higher.

Step 2: The second step is understanding the changes in the air pressures surrounding the spoiler. The pressure behind the spoiler will not change much since it is already in the wake of the car. The pressure ahead of the spoiler, however, will increase significantly.

Adding a spoiler to the back of the car results in a pressure increase that’s located further forward than the height of the spoiler itself. That means that the downforce is a function of the increased pressure over a much larger area than just the spoiler. The pressure change will increase with speed, making the spoiler more effective at higher speeds.

The pressure can be measured with a simple differential pressure gauge with a low enough range-like a magnehelic gauge. These gauges can be used to record pressures at different locations. If you have a data system with a spare manifold pressure sensor, you can easily record these readings.

The pressures that we are talking about are relatively small. The range is typically a few inches of water: 1 psi is equal to 27.7 inches of water, or 1 inch of water equals 0.036 psi. Using that conversion and approximating the area affected by the spoiler (in square inches) times the increase in pressure (in psi) gives you the local downforce increase.

To figure out the drag increase, you need to take the area of the spoiler itself and multiply it by the pressure differences from the front of the car to the back. (This will give a decent approximation, but is not the exact spoiler drag.) A typical car has 10–15 pounds of drag at 60 mph, so you can figure out how much you increased drag in exchange for the extra downforce.

Step 3: The third step is understanding the geometry of the spoiler. It is basically a flat panel, but its performance is controlled by three factors.

• Size: More area equals more downforce (and more drag). The taller the spoiler, the further forward the pressure changes. The wider the spoiler, the more consistent the forward pressure effects will be across the spoiler–and, of course, the pressures fall off at the edges. Both the height and width control the area needed for some basic force calculations.

• Angle: The next important factor is the angle of the spoiler. This is usually measured with reference to the ground, but a more accurate measurement is against a strong or straight part of the car that is not likely to get moved or damaged. The reference measurement guarantees that the setup can be duplicated, even if the suspension is changed or the car is not on flat ground. The spoiler angle to the local body work–usually the trunk or hatch-should be in the 60-to-80-degree range for best downforce. Reducing the angle gives less downforce but does not really reduce the drag effect very much; more angle than that doesn’t really increase downforce.

• Speed: Since we are dealing with an aerodynamic device, we need to address the old rule of forces increasing at the square of the speed increase. If you figure out your numbers at 60 mph and you typically run closer to 120 mph, those forces will be four times larger than your calculations. At low speeds, the downforce will be noticeable even as low as 30 mph but drag will not be significant until you start to get north of 60 mph. Watching a car with a large spoiler on a dusty track can show you how well a simple spoiler can work at low speeds.

More Practical Tips

In addition to those basics, here are a few more tips that can help you easily build an effective spoiler at home:

• The spoiler overhang beyond the back and sides of the car should be maximized. If the rules prohibit the spoiler from extending beyond the car’s bodywork, make every effort to take it right to the absolute limit. More overhang acts like a longer lever, putting more downforce on the rear tires.

• Where to place the spoiler depends on the car’s shape and general handling characteristics. If the car is an understeering hatchback, for example, then adding a spoiler at the back of the roof can add some downforce right at the middle of the car. A rear-drive, oversteering car will benefit from a trunk- mounted spoiler.

• The shape of the spoiler can matter, too. If the back of the car is square or the rules allow overhang, make the spoiler flat, adding small end fences to help capture the high pressure. If the car is curved along the back and the rules disallow overhang, then a curved spoiler will allow for the tallest splitter across the width. Curved spoilers will also provide a potential downforce gain if the car gets a little sideways.

“The pressure behind the spoiler will not change much since it is already in the wake of the car. The pressure ahead of the spoiler, however, will increase significantly.”

Applied Sciences

Before installing the splitter, we saw turbulence right here. Additionally, air exiting the bottom of the engine compartment interrupted airflow beneath the car.

Before installing the splitter, we saw turbulence right here. Additionally, air exiting the bottom of the engine compartment interrupted airflow beneath the car.

Putting our new knowledge into action, we added our aero tweaks.


Air pressures above the hood were higher than atmospheric at the leadingedge, dropping to neutral toward the middle, then rising again at the base of the windshield.

The Mustang’s airfoil shape greatly reduces drag. However, that shape also increases the speed of the air passing above the car, creating some lower-than-atmospheric pressure areas toward the tail.

Pressures at the trailing edge of the trunk were near atmospheric, but the air coming off the tail was turbulent.


After installing the splitter, the air passing beneath the car encountered less resistance while also creating downforce.

Since the splitter initially trapped air inside the engine compartment, we installed a pair of hood vents. Installing these openings in a low-pressure area ensured that the hot air would escape.

The rear spoiler deflects air upward, slowing the airflow above the car while also creating downward leverage. As a result, we replaced a low-pressure area with a pocket of higher-than-atmospheric pressure.

The rear spoiler also calmed the turbulence originally seen at the tail, somewhat reducing unnecessary drag.

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View comments on the GRM forums
9/27/17 12:36 p.m.

Good article!

Do you have the data on the performance difference with the aero and without?

Thank you