Let's get this out of the way right now: Aerodynamics is voodoo. You don't get it and neither do the forum monkeys. Aerodynamics also isn't glamorous. Some canards and a carbon-fiber diffuser never got anybody laid. All of which is exactly why you've made all sorts of boneheaded moves when it comes to your car's mix of body panels, spoilers, and lips that the Internet machine told you would work.
DOWNFORCE, DRAG, AND WHAT YOU DON'T GET
You can't talk about aerodynamics without first understanding what in the world downforce is. Apply air pressure onto any surface and you've just showed everybody how it works. Explain how it's what puts the air around you to work, pushing down on things in all the right places, resulting in increased traction at the tires, and you've just described how it applies to your hooptie. Call it aerodynamic grip if you want to sound smart, but don't mistake it for mechanical grip, which has more to do with your car's suspension and tires and nothing at all to do with its shape.
Whoever manufactured your car designed it with just enough downforce for it to remain stable at the kind of speeds that won't put you in jail. Figuring out how to get any more of it for when you want to go faster is your job. Do it wrong, though, and you're bound to introduce a whole lot of drag, which is just another way of saying that you and the air around you will have a harder time getting along. The trouble with drag is that too much'll slow you down but not enough will keep downforce from ever taking place. Told you aerodynamics was voodoo.
Your problem is knowing what's more important: more downforce or less drag. As it turns out, what you plan on doing with your car will give you the answer. Spend enough time going around corners really fast and you'll realize downforce matters a whole lot. Spend even more time going as fast as you can in a straight line and, all of a sudden, drag matters more. That's because on the road course, for example, you can make up more time in the corners than you can in the straights, and that's exactly where more downforce and better traction can help. For a land-speed car, however, all you care about is making everything as sleek as possible. Drag racing's a compromise between the two; here, you need just enough downforce to retain traction and as little drag as possible to make it through the traps as quickly as possible.
THREE WAYS TO SQAUSH DRAG
Go twice as fast and all of a sudden drag resistance quadruples. If your car could talk, it'd tell you that it needs about five times more power at 100 mph than it does at 50 mph. But you can't give it five times the power, which means you figuring out how to get rid of some of that drag is now important. Lucky for you, there are three ways to do just that:
Make a smaller footprint. Stand in front of your car and imagine it as a silhouette. Now do whatever you can to make it smaller, like lowering it or sticking on some smaller mirrors.
Nix the turbulence. Do this by smoothing out things underneath with some sort of flat under-body tray.
Stop the resistance. You need your car's ventilation and cooling passages, but they create a whole lot of drag. In some cases, air can be diverted away from them or in a more efficient sort of way.
STREET CAR AERO PROS AND CONS
- Nothing looks better than functional aero.
- You'll be ready for the track. All the time.
- You'll reduce weight in some cases.
- Go fast enough and you'll increase traction.
- You've just attracted the attention of your entire city's police department.
- You've just attracted the attention of your entire city's auto theft ring.
- Good aero isn't cheap.
- Cheap aero isn't good.
- Wish ground clearance a proper sendoff.
SCIENCE AND STUFF
Study aerodynamics for years and you still won't get it all. But understand two simple things, like how an airplane's wings work and what a 250-year-old scientist has to say about all of this and you just might be OK. It turns out that that 250-year-old scientist, Daniel Bernoulli, knew a whole lot about things like spoilers and canards a couple of centuries before you got into Hondas. In fact, he's responsible for coming up with all sorts of smart things, like recognizing how surface pressure drops as air speed goes up. An airplane's wings work, for example, because of this. Here, a wing's rounded top and flat bottom mean it'll take air longer to pass over itself than under it. But air doesn't move in clumps; instead, a steady stream means the only thing that changes here is pressure or, in the case of the wing, more of it underneath.
WINGS AND AIR DAMS
That wing that Subaru wanted you to have on the back of your WRX does the opposite of what an airplane's does. That air pressure we just talked about here, the people at Subaru made it so it'd push down on that WRX instead of up. Step on the gas and, if you could, you'd notice air passing over the top of the wing at a slower speed than it does its bottom side because of its shape. Complicated science says that if you do that, then you've just increased downforce, and the faster you go, the more of it you'll get.
How big that wing is matters, too. Give it a whole lot of surface area or stack one on top of the other—like you'd find on a Formula One car and not on a pimped-out Hyundai Tiburon—and you've generated even more downforce. You've also got to consider how that wing is sitting or its angle of attack. Tilt it the wrong way and you've just made all of this about as worthwhile as sticking a double-decker wing onto a pimped-out Hyundai Tiburon.
You want to restrict airflow and keep air pressure low underneath and also high up above and along the sides of whatever it is that you're driving, which means you want some sort of air dam up front. A well-designed air dam—or spoiler—will extend off of the front bumper parallel and close to the ground. The right air dam will do all of this by disrupting the airflow transition where pressure is high at the front of the car and low underneath. Design and position it the right way and, no surprise here, you'll end up with more downforce.
THE FWD DELUSION
You don't have to go far to find some kook who'll try and tell you that rear wings don't work on FWD cars. There's a reason he's a kook, and it's got to do with the fact that downforce doesn't care which end of your car your transmission's hooked up to. Set your car up right and a rear wing can do three things:
Increase traction in the rear. Yeah, your car's driven by its front end, but those rear tires have still got work to do. This'll come in handy when doing things like going around corners and stuff.
A rear wing can also make your front spoiler more effective by balancing things out. You've got a front spoiler, don't you? You track your FWD car, which means your spring rates are probably stiff, and which means rear lift has probably happened to you. That rear wing, it can help settle all of this down.
SPOILERS VS. WINGS
You think spoilers and wings are one in the same and you're wrong. A rear spoiler attaches directly to a car's rear trunk lid or hatch without any space between itself and the car's body. If there's a gap, then you've got yourself a wing. Spoilers spoil or upset whatever fast-moving air is blowing past the roof line, separating it, slowing it down, and increasing its pressure, which means rear lift just got a whole lot harder. Like a wing, the closer to vertical a spoiler is, the more downforce it'll provide—but at the expense of even more drag.
CANARDS, DIFFUSERS, AND SIDE SKIRTS
Those funny-looking little wedge-shaped pieces of carbon fiber you'll find on any serious race car and any showboat worth its weight in pie-cuts and fake titanium can also be good for making more downforce. Design those canards (also known as dive planes) right, stick them in the right places, and air vortices will be generated that'll travel along the sides of your car, keeping high-pressure air from flubbing things up underneath where air pressure is much lower. You'll need to generate a whole lot of speed to notice these subtle additions, though.
Like canards, a good diffuser can win you car show points, minimize pressure underneath your hatchback, or do both. Look for them underneath either bumper or below that front air dam you don't have. Out back, diffusers utilize low-pressure air to help release more turbulent air from underneath. Do it right and air speed underneath will increase, which can lead to more downforce up above.
It turns out that side skirts do more than complement whatever front and rear bumpers you've subjected your Civic to. Side skirts streamline air past your car, preventing it from making its way underneath and disrupting what's an otherwise calm situation. Good news for you: The lower they are, the better they'll work.
COOLING DUCTS AND VENTS
Good aerodynamics will accomplish three things: increase downforce, minimize drag, and allow for better cooling. While you've been spending all this time trying to direct airflow past your car's body, a well-designed air duct's doing the opposite. NACA ducts, for example, can direct air toward bits that'd otherwise heat up into smithereens, like brake rotors and radiators, but do so without interrupting airflow along your car's exterior. A proper air duct can also make use of wasted air that's exiting past the tires and would otherwise create drag, and if you're really smart, you'll integrate them to work alongside those canards and take advantage of those air vortices they've been creating all along.
Air ducts and vents can also do the opposite, like help evacuate hot air from an otherwise steamy engine bay. Position them right and air passing through the radiator can make a clean getaway from under the hood and without disrupting everything that's going on outside.
Aerodynamics is still voodoo and it still won't get you laid. Understand that it's a balancing act, though, where downforce, drag, and proper cooling all have to be considered, and that you sticking a massive front splitter onto your Integra without any sort of rear wing because you think it looks tight will never make sense, and you're on your way to all of this becoming less voodoo and more science.
Q&A WITH APR PERFORMANCE
SS: Can highly modified street cars benefit from aero modifications like air dams and rear wings?
APR: With the right combination of aerodynamic components, lift can be reduced (to help stabilize the car at freeway speeds), and drag can be reduced (to improve both acceleration and fuel economy). Body components such as (front) air dams have been making their way onto factory bodywork for quite some time already and are easy to identify. Other components, such as undertrays, underbody panels, and diffusers are more subtle, and may require trained eyes to identify. The thing is, not all production cars get the same treatment from the automaker as far as aerodynamics go. So, any improvements we can do with aftermarket components (i.e. air dams, rear wings, side rocker extensions, rear diffusers, etc.) can produce tangible benefits for not only street cars of the highly-modified variety, but also for any road-going street car.
SS: In which order would you recommend making aero upgrades for enthusiasts who choose to make changes one part at a time?
APR: 1: front air dam. 2: rear wing and front wind splitter. 3: rear diffuser. 4: side rocker extensions and canards.
SS: How easy is it for someone to actually decrease their car's performance by making incorrect aero changes or adding the wrong parts?
APR: Quite easy. For example, adding only a rear wing, without adding a front wind splitter, can cause aero balance to be biased too much to the rear and produce handling problems.
SS: What sort of things should be considered when upgrading a front-wheel-drive car's aero vs. something that's rear-wheel-drive?