We’ve all seen them. Some of us own them. Honda Civics, Nissan Sentras, and Toyota Corollas with gigantic, GT-style wings bolted onto their trunk lids. Some of us think they look cool, and some of us find them ridiculous. But despite how they look, what do they do? Is there a function? Do they actually make cars faster?
Yes and no. To clear things up in the realm of spoilers and splitters, wings and diffusers, we’ve poured a thin concrete foundation for the study of car aerodynamics. Keep in mind that we are not physicists, and that actual physicists have studied fluid movement for thousands of years. Aristotle was among them. We’re just a bunch of grease monkeys.
We start with some basic principles of aerodynamics. Air always moves from an area of high pressure to an area of low pressure. When it piles up, it seeks to release itself and pushes against whatever object is containing it. This is how a hydraulic jack works.
And that’s why the front of your car produces drag. As you speed down the highway, the front of your car pushes against the air, creating an area of high pressure. In response, the air pushes back. For a more visual example, picture a snow plow struggling through a January blizzard.
Now, you’re thinking that drag is only what occurs behind a car. That’s a bit of a misnomer, but it happens there, too. The areas behind your rear windshield and trunk produce negative air pressure which creates a vacuum effect. As air seeks to enter these areas, it curls over your roof and trunk, creating turbulence, which causes resistance.
Drag zones are like taxes. You never want to have any of them, but you always will, to some degree. Conversely, downforce is your friend, especially in racing. Downforce is created when, at higher speeds, the air pushes your car down toward the ground. This improves traction and stability, and is eagerly sought among performance car designers.
Unfortunately, since most cars on the road are roughly shaped like the cross section of an airplane wing, they oppose downforce. Yes, your car can actually generate lift. But you won’t take off, since your car is too heavy, and since there is some natural downforce to be had. Air piles up against your grille and windshield, and as it seeks an area of lower pressure, pushes down and back. The underside of your car can also produce downforce. Air is pulled out from under your car, which creates another area of negative pressure, pulling your car lower.
So with the science somewhat explained, what about those Civic wings? Do aerodynamic mods do any good?
Spoiler/Diffuser- These are the small fins you see on car trunk lids. Most of them are factory installed, and provide a clever way to break up the turbulence behind your car. They can reduce drag, making your ride faster and more efficient. You can find one on the back of a Toyota Corolla S.
Wing- A rear wing is large and swooping, commonly called a spoiler, but something very different. It is not meant to reduce drag, but to create downforce. Since it is essentially an inverted airplane wing, it uses the same principles airplanes use to lift off, to shove your car against the pavement. That’s why rear wings are essentially useless on front-wheel-drive cars. At speed, a wing pushes the rear wheels down, which reduces pressure on the front wheels, killing traction. For a car with wings on the front and back, see Walter Rohrl’s AWD 1987 Audi Quattro S1, which he used to set records at the Pike’s Peak International Hill Climb.
Bug Shield- It seems odd among this expensive, carbon fiber race equipment, but some say that the bug shields you see on trucks every day can actually reduce drag and improve overall aerodynamics. A bug shield can help prevent air from getting caught in the gap between your hood and windshield, reducing turbulence.
Air Dam- An air dam blocks air from getting under your car, which improves the vacuum there. This, as mentioned above, will increase downforce and can help reduce drag, as well. Many modern body kits include air dams for this purpose.
Splitter- This is a plate that runs horizontal to the ground in front of your car. It works like an air dam, preventing air from making its way to the underbody. It also increases the surface area of the underside of your car, which means a better vacuum. The bigger the splitter, the better. The 2012 Ford Mustang Boss 302 Laguna Seca Edition is a good production example. Unfortunately, most states have outlawed splitters over a certain size, citing pedestrian safety.
Lowering- Another way to improve underbody downforce is to simply lower your car. The less air that can get under your car, the more downforce you’ll generate. That’s why tarmac race cars seem to barely clear the pavement. For examples of this, see any of the cars of the modern British Touring Car Championship.
Racing machines, from F1 screamers to the Diesel mills at La Mans to dirt kicking rally cars, are designed to generate the least amount of drag and the most downforce possible. Adding similar mods to your car can do the same, but only under the right conditions. And aero modding won’t just improve your speed. It will also boost your fuel efficiency, since your car won’t have to work as hard to slip through the air.