Understanding the basic principles and applications of automobile aerodynamics does not take an advanced engineering degree. However, developing cutting edge aero systems seen in the elite ranks of IMSA, Formula 1, IndyCar and several other premier racing series
, most certainly does. From the salt of Bonneville to the grid at even the most humble of club racing events, savvy racers seek out ways to cheat the wind and maximize their car’s aerodynamic efficiency and traction.
Improving your car’s aero comes down to a few very simple and readily available components and the act of tuning them to work in harmony. To find out more, we turned to the aero pros at APR Performance and G-stream for their help in getting you up to speed with the concept of aero parts.
The Basics of Airflow
In order to design a functional grouping of aerodynamic components for your vehicle, a vocabulary lesson is needed first. These subjects are some of the most common vernacular you’ll come across when shopping aero for your track car and having a basic understanding of their function is paramount to implementing the best aero system possible.
Downforce: Ah, the big kahuna of the aerodynamic world. It is only fitting that the first question to ask is, “what is downforce?” KC Chou of APR Performance describes it eloquently as the result of a “pressure differential.” Downforce is an aerodynamic force – duh, right – that results (or doesn’t result) from the airflow passing over a moving vehicle’s shape. As that air interacts with said shape
, and passive aero devices (if the car is so equipped) force is generated that compresses the vehicle toward the track.
The increased downward force on the contact patch of the tire increases their grip on the track surface. More available grip on the track equates to a higher potential cornering speed. It’s a pretty simple concept in theory; however designing the components to generate downforce is where people with much bigger brains than ours have an advantage.
Lift: Lift is defined as a force that is perpendicular to oncoming flow. In layman’s terms, relating to automobiles, lift is the force that causes the vehicle to raise off the ground at speed. It removes weight from the tire’s contact patch and is all-around something you want to avoid at all costs. It is caused by a pressure differential, much like downforce, only in this situation the differential is between the air on the underside of the car and the air flowing over the roof. Also, that differential is the reverse of what any racer would like it to be.
Angle of Attack: As a bored kid on a car trip, did you ever stick your hand out the window while the car was in motion? Remember how your hand could ride up or down the incoming stream of outside air depending whether your fingertips pointed upwards or downwards? While this is a crude example, if your hand where a spoiler, you were actively changing its angle of attack as you change the orientation of your fingers.
Air flows along a car at speed and, depending where and how high up the wing is placed, that air may flow cleanly, uninterrupted by the car’s shape, or it may be significantly influenced by the roofline (or many other factors). The air, which has a viscosity and stickiness to it, tends to cascade down the rear roofline at an angle to the wing; not flowing perpendicular to the ground, like we may imagine it to, but rather on a slight diagonal path. Altering the position of the wing (referred to as angle of attack) can improve the wing’s interaction with the air, generating more downforce.
However, unless you have access to a wind tunnel, this is very much a trial-and-error procedure. Make one adjustment to the wing at a time, logging seat-of-the pants feel, lap times, and in some cases, photos of the car on the straightaway are great visual indicators of increased downforce, as the suspension will begin to compress under load.
Aero Parts, A Component Break Down
Aerodynamic improvements for any particular chassis can range from mild to wild, all depending on how much cash you have to spend. Professional time-attack teams invest hundreds of thousands – sometimes millions – in wind tunnels and CFD (Computational Flow Dynamics) to optimize their racecars. For our segment, however, we’ll stick to the more commonly available and affordable side of the aerodynamic spectrum.
Ah the wing: the most misunderstood and overall important element in automotive aerodynamics. This device
, also known as an airfoil is typically attached to the rear of the car – though, in many instances, such as time attack, a wing can also be suspended over the front wheels as well. Its job is to make downforce; and, when properly implemented/ adjusted, it can make a lot of it. In extreme usages such as time attack and Formula car racing, wings can generate well over 1,000-pounds (sometimes approaching 2,000 pounds) of downforce.
The wing functions by interacting with air flowing past its surface. Essentially, an automotive wing is an inverted airplane wing – though this is a gross simplification of aerodynamic technology, it does well to explain what the wing is intended to do.
Per Bernoulli’s principle, “An increase in the speed of a fluid occurs simultaneously with a decrease in pressure.” As air flows over a wing, it is asked to move quicker on the underside of the wing, due to a CFD (computational flow dynamics) or wind-tunnel-designed curvature. The air on the top of the wing has a much easier job, simply needing to flow straight. As the air under the spoiler speeds up, its pressure drops. This pressure differential (high pressure on top, low pressure on the bottom) creates downforce, which helps plant the rear tires and increase cornering speed.
A spoiler is not a wing and a wing is not a spoiler. Now that we’ve got that out of the way, we can continue with the definition. A spoiler is an aerodynamic element that is molded into the body-work of a car. Unlike a wing, spoilers redirect airflow rather than interact with it like a wing. Typically spoilers are great at reducing lift and drag. They, however, do not typically generate much downforce.
How To Adjust A Wing
Setting up a wing requires some diligence and trial and error – unless of course you have the bucks to rent a wind-tunnel for the day. However, few of us are in that tax bracket. Most wings come with a plethora of holes drilled in the mounts to give racers the opportunity to perfect their setup.
“All of our wings are adjustable, adds Chou.”
The process of setting the wing up, requires an open track and some patience. Here’s an important tip: only make one change at a time, regardless of how tempting it may be to do otherwise. After you have made a change to the wing’s angle of attack, note it in a logbook, and drive the car at a similar pace in the next session on track. “If the car feels loose, adjust the wing,” adds Chou. When the optimal wing angle has been achieved, the car should feel more settled and rear grip should be observably improved.
The popular theory is that spoilers redirect air upward to create downforce, while this may seem like a logical assumption, modern wind-tunnel testing has proven it to be completely incorrect. Truthfully, many OEM manufacturer attempts at utilizing spoilers to influence aerodynamics were resounding failures. For example, look at the spoiler on the Plymouth Superbird. It was one of the early attempts to cheat the wind in stock car racing, and while its streamlined front bodywork helped reduce drag, its spoiler was ineffective and generated very little downforce, even at high speed.
The splitter is a major element of any capable aero package. It is a flat plane of material that is attached to the front bumper of a car. It extends from the front of the vehicle low and parallel to the ground. It creates front downforce by creating a pressure differential at the front of the car and reduces the amount of air that enters underneath the car, a major cause of lift. “98 percent of production cars create lift,” adds Chou.
When a car travels at high speed, the air stream pushing against the front bodywork generates significant amount of pressure. Much of that air goes over the car’s hood and roof, which forces it to accelerate and reduces its pressure. The air that flows under the car has a comparatively straight shot, hence it can flow slower than the air above the car, which is being forced to bend twist and speed up.
Again, we’ll reference Bernoulli’s principle: “An increase in the speed of a fluid occurs simultaneously with a decrease in pressure.” Hence, the slower moving air underneath the car creates a pressure differential with the air above the car. High-pressure air below the floorboards, when combined with a low pressure cloud hovering over the roofline combine in the worst way to produce lift. Essentially, the car’s body acts like an airplane wing. By using a splitter, the amount of air allowed to pass under the car is restricted. This creates a low-pressure zone underneath the splitter as the airflow is forced to accelerate and shoot the gap between the splitter and pavement. That low-pressure zone sucks the front of the car downward, increasing the force on the front tires and the available grip. “The splitter begins to take effect around 40-mph,” adds Chou. “Drivers will feel the steering get heavier.”
Also known as dive planes, bumper canards are small triangular winglets that are mounted to the front fenders of a vehicle. Canards act like mini-wings, interacting with airflow to produce small pressure differentials across their surfaces. Cars equipped with a large rear wing that have a rearward downforce bias at speed, stand to benefit from canards in addition to the aforementioned splitter.
The overall effectiveness of canards is somewhat limited though, which is often why vehicles utilize two-three on each side of the bumper. As previously mentioned; air, being a fluid, has a viscosity/stickiness to it. Because of this, the air closest to a vehicle in motion, sticks to the body surface, moving slower than the surrounding, outside air. This air is referred to as the boundary layer. Because canards are limited in their size, they often don’t protrude much past the stagnant boundary layer. This limits their ability to interact with the ‘clean’ air flowing around the car.
Drag is not your friend. However, drag is always a side effect of adding wings and other downforce-increasing components to a car –though, well designed components will work more efficiently, producing less drag per unit of downforce. Defined, drag is the resistance a vehicle creates as air flows over its surface. The higher the drag, the more horsepower it will take to accelerate a vehicle. For that reason, low-powered cars may exhibit impaired straightaway speeds when equipped with aero systems. The tradeoff is that their cornering speeds will be substantially increased due to the improved grip at the tires.
For this exact reason, many race teams utilize aero packages that are unique to the track they will be racing at. High-speed tracks with long straights tend to favor lower-drag aero components that subsequently produce less downforce. Ultimately, the war between drag and downforce is a balancing act that requires significant experimentation.
Balancing Aerodynamic Forces
We’ve all heard someone make a sarcastic quip about a rear wing on a front-wheel-drive car. Take everything that person says and file it in your mental trash can. Downforce inherently benefits all drive layouts. Unless we’re talking about a Segway, all four wheels of a vehicle stand to benefit from increased grip – the end result of downforce.
However, changing the balance of downforce between the front and rear of a car can drastically alter the way it drives. For example: If you were to add a front splitter and dive planes to the front of your track car, at speed, that car may begin to exhibit oversteer mid-corner, even if it had a tendency toward understeer previously.
Why is that?
Think of downforce like an invisible lead ballast. It’s easy to imagine how your car might handle if you bolted 200 pounds to the hood; the front end would have tons of weight smashing the tires into the pavement while the backend was light and simply along for the ride. Now, if the front of a car generates 200 pounds of downforce, while the back generates zero, or perhaps even negative downforce (lift), the front wheels of that car are seeing more load (and grip) than ever, while the back are seeing significantly less. When the traction threshold of the car is eclipsed, which set of tires will be the first to let go? Obviously, the pair with the least grip; in this case, the rears.
For this reason, it’s important to plan your aero strategy according to how your car already behaves and to make subtle adaptations to coax the desired effect from the chassis. For example: a car that is already biased toward oversteer will likely benefit from a rear wing. As a rule of thumb, a car that has a gentle tendency toward understeer will be much easier to drive fast than one that steps the backend out at every possible opportunity.
When To Add Aero?
Unlike suspension modifications, engine upgrades, and safety equipment, there isn’t as clear-cut of a time to add aero components to your car.
However, Dave Martis of G-Stream, cautions that lower-horsepower cars may suffer from radical aerodynamic improvements such as wings and splitters. “Lower power cars can’t handle as much power loss [drag],” he cautions. “If a car does have power, the aerodynamics will help plant the car and increase cornering speeds.”
Aero On The Street
There’s no argument that aerodynamics and the downforce they create is a force to be reckoned with at the racetrack. But, on the street it can actually do more harm than good. “It [downforce] can make the car undriveable on the street,” says Martis. Downforce can increase tire wear, since there is more force driving the tire into the pavement. And, in situations where the downforce is considerably more at the rear of the car, it can cause understeer in slick weather conditions such as snow, ice or rain. Lastly, low-hung front splitters that peel away air trying to enter the underside of the car work great on track. On pothole-ridden pavement, they tend not to fair quite as well.
Aero Is A Common Goal
When planning out a host of aerodynamic modifications for your car, approach the process not as a checklist of shiny carbon parts
, ready to attach to your virgin bodywork, but as a team of related components all working toward a common goal. There are far too many cars out there to provide one catchall recipe for aerodynamic success; hence, some experimentation will be required. Make improvements one at a time, taking note of what helps the car to perform better on track. And, most importantly utilize the above-mentioned components to achieve balance and high-speed drivability.