Once your car’s rolled off of whatever assembly line its final screw was tightened down on, its aerodynamics get about as much attention as the little light bulb in the corner of your trunk. Redirecting airflow someplace else isn’t as glamorous as manhandling an extra 100hp by means of turbochargers and dyno tuning. It isn’t as easy either. For years the aftermarket had a grasp on how to effectively upgrade engine, drivetrain, suspension and braking systems. Determining the factory’s shortcomings and improving upon them isn’t hard. Making a modern car’s aerodynamic skin better, though, is more challenging than you think.
The fact that your Subaru has some sort of fiberglass body kit on it and a double-decker wing doesn’t mean it’s any more aerodynamically adept than what Subaru had originally done to it. In all likelihood it isn’t any more aerodynamically adept than a 1963 Cadillac Biscayne. As it turns out, not all aftermarket aero components are designed or tested for improved airflow but rather in hopes that somebody thinks a mouthful of composite stitched to the front of an otherwise stock-looking car seems like a good idea. The popularity of German DTM cars, Japanese Super GT racers and American time attack events have changed all of that. Today, more manufacturers offer components that are designed to increase speed and grip by means of aero. Weeding those out from the ones found on the Subaru with the light-up undercarriage is the hard part.
Aerodynamics is based upon all sorts of intricate math and fluid dynamics that you have no interest in. The least of which you should understand is Bernoulli’s principle. Here, the old Swiss scientist who thought all of this up about 250 years before Civics and Integras were cool says that for any airstream against a body, an increase in its velocity happens simultaneously with a decrease in its surface pressure. It explains all sorts of complicated stuff, like how airplanes and perfume bottles work and how the spoiler on a Formula One car helps keep it on the ground.
To understand why all of this has anything to do with your car, you’ve got to understand how it relates to something exponentially more complex, like that airplane wing. An airplane wing’s rounded top and flat bottom force air across two entirely different paths. Because of the top side’s rounded shape, it takes longer for an equal body of air to pass by when compared to its underside. Since the wing is a single piece, air speed remains the same along its top and bottom but, just like Bernoulli told us, pressure doesn’t, resulting in more underneath, creating lift.
The same principle applies to whatever it is you’re driving. Like an airplane wing, the bottom of your hooptie is relatively flat and its top, with its passenger compartment, windows and roof, isn’t. And like the airplane wing, it takes longer for air to pass over the top of your car than its bottom, resulting in more pressure underneath, which won’t launch your hatchback into the air but can reduce the weight on top of its tires at high speeds, compromising traction, braking and handling. Tire traction is a function of the amount of weight placed on top of them; reduce that load and you reduce how well your car does almost everything it was intended to do.
Downforce is the phenomenon of increased air pressure applied to a given surface area. It’s what puts the air around you to work, pushing down on the appropriate areas, resulting in increased traction at the tires. Also known as aerodynamic grip, all of this is independent of the car’s suspension, tires and overall mass, each of which affect a different type of grip—mechanical grip. Most cars are designed to provide just enough downforce to prevent lift and provide reasonable stability. Generating enough to increase traction is a luxurious benefit of a thoughtful design. Creating downforce is a balancing act, though, and is almost always a compromise; add too much and drag suffers, which is simply the resistance of air against forward movement yet is absolutely necessary when creating downforce. But excess drag can slow you down. Consider the effects of releasing a parachute on a race car. The trick to all of this is generating the right amount of downforce to maximize traction but without limiting top-end speed due to increased drag.
Drag can be generated three ways, each of which you should know about if you plan on reducing it. Drag resistance is the first and is a function of your car’s shape. Here, the car’s overall frontal footprint, including its mirrors, bumper and roofline all affect how easily everything can slip past it. Smaller mirrors, skinnier tires and a lowered stance all reduce its overall footprint. Second, air friction against the car’s body due to appendages or surface coatings can increase drag. Smooth, under-body trays like you’ll find on high-end super cars reduce turbulence and drag, which also prevent lift by means of higher air velocity and lower pressure, just as Bernoulli said would happen. Third, air flowing through the car’s ventilation and cooling passages can also increase resistance. The negative effects of each of these multiplies exponentially as vehicle speeds increase, making proper aero even more important the faster you go. For example, as speeds double, drag resistance quadruples. In fact, your car requires nearly five times more power at 100mph than it does at 50mph. It’s no surprise that the secret is determining the magical compromise between downforce and drag, and is precisely what separates that goofy-looking fiberglass hunk of a bumper from properly designed aero that serves a purpose.
The wing on the back of your coupe was designed to do the exact opposite of an airplane’s. Here, the pressure difference is flip-flopped, pushing down instead of up. When air hits the wing, a portion of it travels above at a slower speed while the rest goes underneath at a higher velocity. Since the slower airflow up top results in more pressure, the wing generates downforce. Wings that are made up of multiple elements produce even more downforce, often two or three times as much when executed properly, like you’d see on a Formula One car. Also important is the wing’s angle of attack, or the angle at which it’s mounted in relation to the car, which can also affect downforce and lift considerably. All of this makes perfect sense when considering RWD applications where a rear spoiler increases downforce at the drive wheels. Understanding why this works on FWD applications is slightly more complex. First, despite being driven by their front wheels, FWD cars still need rear-wheel grip when cornering. Rear spoilers on FWD applications also work in conjunction with whatever aero is positioned up front. Increasing downforce up front without doing so in the rear can significantly upset the car’s balance. Finally, the rear spoiler counteracts rear lift that can occur due to higher spring rates that are typically used to decrease understeer. FWD track cars like stiff suspensions, but such rear ends can be unpredictable at high speeds and downright scary when braking. Rear wings help stabilize all of this.
Diffusers exist to minimize pressure underneath the car by giving airflow an easier path to escape. Once again, Bernoulli’s logic doesn’t escape you: increased air velocity means reduced air pressure. Diffusers are typically found out back at the lower portion of the bumper, but can also be stationed below the front air splitter, among other places. Rear diffusers make use of low-pressure air behind the car to help pull more turbulent air out from underneath the vehicle. Once air speed increases underneath by means of a properly designed diffuser, additional downforce can be achieved up top due to an increased pressure differential.
Canards also help generate downforce, only differently. Airflow reacts against them, creating thrust against the canards, but when designed and placed appropriately, something even more important happens. Here, vortices are generated that travel along the sides of the car that prevent high-pressure air along the outside of the car from colliding with low-pressure air underneath. The results are also more downforce up top since the low-pressure air underneath isn’t disturbed. Canards are a pretty subtle aero addition, though, of which their effects are likely to go unnoticed unless strategically placed and utilized at high speeds.
Side skirts do more than just streamline the side of your car, earning you car show points; they help prevent airflow from making its way underneath the chassis from the vehicle’s sides, which disrupts the low-pressure front. The closer they are to the ground, the better they work.
Front Air Dams And Splitters
Air dams positioned up front restrict airflow underneath the vehicle, forcing high-pressure air along the top and sides of the car, which increases downforce. Splitters work similarly, breaking the air apart at the naturally high-pressure area at the front of the vehicle from the low-pressure area underneath. When strategically placed, all of this does a whole lot of good for the low-pressure area underneath and, as a result, traction.
Although typically found on higher-end street cars and race cars, side ducts help keep brake and engine components cool, even when under-body diffusers are used that can block off all sorts of critically needed airflow. Fender-mounted ducts typically release turbulent air that’s exiting the back of the tires from within the fender-wells that would otherwise create drag. When properly implemented, such ducts do so in ways that work effectively with canards and the vortices they generate, releasing airflow along the sides of the vehicle for a smooth transition.
Aero tuning is still relatively-new to the automotive tuning world, despite the fact that a 250 year-old scientist conjured up mathematical formulas three centuries ago that had everything to do with you, your Evo and a rear spoiler. Still, aero tuning isn’t glamorous. It probably never will be. But it’s every bit as powerful as you think it is.