Aerodynamics are a subdiscipline of fluid dynamics, which itself is a subdiscipline of fluid mechanics, so I get that this can be complicated stuff. Therefore it’s not surprising that at least a few times a week, I come across conversations where people are using the wrong terminology in relation to sports car aero. Usually, I don’t care. With this particular instance, however, it is important to know the difference because so many people are attempting to apply the science for their own purposes. I’m talking specifically about the Spoiler vs Wing conversation, and I am going to attempt to explain what makes them different (and what is so wrong about the way people describe each on a daily basis). I don’t want to get too technical here, because this is very complicated science, but I hope my basic understanding on how this stuff works can assist you on your next race car or street car project.
Spoiler vs Wing – Why Does It Matter?
Spoilers and wings, in essence, are completely different types of aerodynamic devices with very specific roles. By not understanding the difference, people will find themselves modding their cars incorrectly and getting the wrong results without knowing why. That’s no fun, and adding Aero to any build is supposed to be a great time! Before getting started here’s some terms I will use.
- Drag – This one’s easy, its force generated that pulls the car from the back and counteracts power
- Lift – Lift is the enemy of stability. If you look at a cross section of a modern car, you will notice that if you squint your eyes it resembles an airfoil in the fashion that air has to travel faster over the top (to clear the passenger compartment) than the bottom (which is basically flat). That means every car in its basic form will create lift that has to be handled.
- Laminar Flow – Laminar flow is when air flows completely over a piece of bodywork, and in general cars that have a lot of laminar flow will have less drag but more lift.
- Separation – There are areas on the trailing side of a car that will be too steep for the air to effectively stick to. When the angle is too steep for laminar flow, the air can’t stick to the panel anymore and separates. This is also known as stalling (when an airplane wing is at a too steep of an angle of attack, it stalls because the air can’t follow the airfoil). Separation creates a low pressure zone and a suction effect (lift and drag in most cases).
- Compression – At the leading edge of a body, the air must compress because it has nowhere to go. Specific areas of compression are referred to as high pressure zones.
- Expansion – At the trailing edge of a body, the air opens back up to decompress. Aerodynamically shaped vehicles have specific shaped trailing edges in order to promote expansion in specific areas as to provide minimum lift.
Spoilers are named after what they are designed to do, spoil airflow. Spoilers are generally small lips placed on leading and trailing edges that are designed to disrupt and/or modify the flow of air over that section of a body. The purpose of this is almost always to spoil lift and decrease drag. They do NOT produce downforce, they simply lessen the effect of the lift. Take the first generation Audi TT as an example. When it first came out, it was smooth and streamlined, and people loved it’s simplistic bauhaus-esq styling. Soon after, as customers started taking their new TT’s to the German Autobahn, complaints of high speed instability started coming in. After a number of high speed crashes, Audi recalled the new-at-the-time TT to fit it with a small rear spoiler (in addition to some suspension and electronic changes). Here’s a basic MS paint drawing of what was going on there:
With the front of the car nearly mirroring the back of the car, the areas of compression and the areas of expansion are nearly identical. This meant that the air was able to compress on the nose, and stick to the body (laminar flow) all the way down to the rear tail lights before separation. Because the laminar flow was so nice, once the air was forced to separate on the nice smooth tail, the tearing motion created considerable and organized upward suction on the rear of the car, making significant amounts of lift on the rear at high speeds typical of the speed limitless Autobahn.
The fix included adding a spoiler, a small lip made to distinguish the trailing edge of the body. The spoiler here did two things, it moved the separation point up the rear window glass, and filled it with swirling air air. That makes an air cushion that the laminar flow over the roof can’t act upon as efficiently. Effectively the small lip spoiled much of the lift created at the back, with minimal increase in drag. Some cars, like the BMW Z4, have a spoiler molded into the trunk lid to attempt to disrupt downward air over the center of steep rear window.
Take a look at this Porsche 907 below, fitted with a special ‘Longtail’ in order to gain extra speed at Le Mans and Daytona. Many consider it one of the most beautiful demonstrations of high speed aerodynamics. Notice the louvers fitted on the back slope. Because of the laminar flow over that section, the resulting low pressure sucks on the back window pulling hot air out of the engine bay and allowing the air-cooled engine to operate without major venting scoops (which would increase drag). It also reduces the surface area that the expanding air can generate lift on.
The air is then prepared to exit the back of the car by a short spoiler known as a Gurney flap. Its name is derived from Dan Gurney, who pioneered use of the device to add efficiency to rear aero elements and ultimately remove enough lift for the stability they needed.
You can see a large gallery of wind tunnel testing here: https://bigmike.marlincrawler.com/forum/index.php?topic=33.0
But what about a front spoiler?
Whenever laminar flow tries to separate on a flat surface on the back of the car, you get lift, but that doesn’t address the air flowing under the car, not very much anyway. A front lip spoiler, sometimes called an air dam, limits the amount of airflow underneath the car. In general, the more air flow you have under the car, the more lift the car will produce. Think of it like an airplane wing, which would remain stuck firmly to the tarmac if it was scraping along the surface. Front airdams increase the amount of frontal surface area, and thus drag. Modern cars commonly only have front spoilers ahead of the front tires, in order to provide pockets of turbulent air for the wheels to pass through, lessening the drag.
This is not to be confused with a splitter, which is fitted to the front bottom of race cars. A splitter is designed to provide downforce by blocking the downward pressure of air at the nose, and smoothing out airflow under the car by moving the leading edge forward of the front airdam. Splitters are designed to be used in conjunction with wings.
Wings are wrongly described as spoilers every day, and their effect is quite the opposite. Wings are meant to increase overall stability through the creation of downforce for racing applications. In addition to downforce, wings also create large amounts of drag, which is something to be considered when adding them to any racing vehicle. While spoilers are made to counteract forces that contribute to lift and drag on the body of a car, a wing is always meant to act completely independently of the airflow over the car.
Wings are specifically shaped airfoils, and wings have end plates.
In general, a wing will provide MORE downforce and disrupt the rear end airflow LESS if it’s hoisted high above the trunklid. Wings are not for street cars, they will only hurt your fuel mileage and make you look silly.
The wing on our Audi produces a ton of downforce, as well as a ton of drag. At high speed, it makes so much downforce that we can’t even use all of it. TCR class rules denote a front wheel drive layout, and because we don’t need the rear of the car glued to the ground, we will dial it back a little bit. Nearly all race car wings are adjustable in order for engineers to balance grip and straight line speed. The only time where the balance of downforce and top speed are thrown out the window is with series like Global Time Attack, illustrated below. This car is designed for unlimited class competition, and can have as much horsepower and aero as the race car designer sees fit to put together a blistering lap time on a short course. Notice ample use of the Gurney Flap on all trailing edges.
If you have any questions, comments, or pertinent information, hit the comment section below and let me know what you think! So next time when someone mentions what a cool spoiler your race car has, you can choose to correct them or just let it go. One thing’s for certain, you’ll never get my wife to stop calling them mermaid’s tails.