August 19th, 2016 by Matt Holden
Aerodynamics: the one word that spawns happiness in engineers and dread in race fans. In the past 50-or-so years, aerodynamics in motorsports have done nearly complete 180° turn, going from slippery, super-low-drag vehicles to high-downforce behemoths with the sole purpose of bullying air molecules into doing work that they really don’t want to do. Want to experience this yourself in the iRacing sim? Go run a few laps at any track in the Lotus 49 and then switch to the Lotus 79. You may notice that the 49 will not navigate a turn much faster than your bicycle is capable of, doesn’t believe in “braking,” and will immediately be somewhere else when you hit the gas. The 79 will do the opposite, and that is the power of downforce. To understand how it affects racing vehicles, we need to start with downforce’s opposite: Lift. For that, we need to look to the sky.
My father is a recreational pilot, something he started doing long before I ever came along. Sailplanes (also called “gliders”) are his wheelhouse, and growing up on the grass runways of the Carolinas bit me with an aerodynamics bug very, very early in life. “Why does this thing stay in the air?” When NASCAR began seeing the benefits of aerodynamics in the late ’90s and early 2000s, this question changed into “How is it stuck to the track?”
The airfoil shape is the basis of lift-based aerodynamics. If anything is producing a force to keep it aloft or press it into the ground, there’s an airfoil shape on it somewhere. The basic shape is pictured below, but modern wings have evolved into more radical shapes with curves, thickness changes, and tiny “flicks” at the trailing edge. In simplest terms, wings operate on a phenomenon known as Bernoulli’s Principle: As a fluid’s velocity increases, pressure decreases. The curved surface on the top of an airplane wing causes the air to speed up, thus reducing pressure. The air on the flat side doesn’t change speed much at all, and thus maintains a constant pressure. This pressure difference exerts a force on the wing in the direction of the lower pressure, and “lifts” the wing.
For racing cars, we’d prefer them to stay on the ground, so we flip-pver the wing. Now, the flat side is facing up, the curved side is down, and the high pressure sits above the wing and presses down. This is downforce. Road racing vehicles have gotten incredibly efficient at doing this, and can efficiently make tons and tons of downforce at relatively moderate speed. Stock cars don’t do this very efficiently at all, but still make it work.
The major paradox is that a stock car has an airfoil shape, yet still produces a net downforce to keep it on the ground. Because of this, they have a tendency to depart the racing surface if spun around at high speed, creating what is known as a “blowover.” Despite a lack of externally visual aerodynamic devices aside from a tiny splitter at the front and a near-vertical spoiler at the back, they still make a lot of downforce, almost exclusively from the car’s underbody.
The Splitter and Radiator Pan
Based on aero numbers, stock cars make the entirety of their downforce from underbody effects. Essentially, the downforce is generated from air going beneath the car instead of over it. In most oval series, cars use what’s known as a Valence or Air Dam at the front, essentially a strip of plastic at the bottom of the nose that scoops air up over the front of the car. NASCAR’s top-3 series use a splitter, a flat piece of composite material that “splits” the air stream and sends some up over the car’s nose and some underneath the car. It’s not a new technology, it’s been used for years in sports car racing and even open-wheel racing.
NASCAR mandates a flat-bottom splitter, which is bad in terms of aerodynamic shape. Teams will often angle the splitter very slightly so the leading edge is lower than the rear of the splitter, which creates a “diffuser” effect. Essentially, the space behind where the air finds the splitter is a larger volume than the air that enters, and produces a low pressure. Due to the angle of the splitter, that area gets larger as the air goes farther back, thus reducing pressure even more.
The same happens at the radiator pan, essentially a flat piece of metal sitting underneath the radiator. It has a built-in angle like the splitter, and operates in the same way. NASCAR usually messes with the size of the radiator pan to change the front downforce numbers because they’re very easy to produce and swap out.
While the color legend isn’t explained for obvious reasons, it’s easy enough to see what is happening. As the air approaches the splitter from the left, it’s compressed underneath the splitter’s leading edge and then expands quickly as it passes under the splitter and radiator pan (large rectangle behind the splitter) before hitting the sway bar tube and then going everywhere as it goes into the engine bay. Despite the initial compression, this expansion under the splitter and radiator pan produces a very low pressure, basically a vacuum, which we call downforce.
Due to how the splitter functions, it is necessary to have air passing underneath it. Otherwise, it would create a full vacuum, which pulls the splitter into the track, which unloads the front tires . . . and turns the car into a sled. Losing the air underneath is known as “stalling” the splitter, and it can end your day very quickly. In addition, the lower the splitter, the more downforce it will produce, and NASCAR’s splitter is almost linear in downforce numbers as it travels down towards the racing surface. As a result, it’s desirable to have the splitter as close to the track as possible without it touching, which prompted the return of bumpstops and extremely stiff front springs when the COT debuted in 2007.
The second major point of downforce production is the fender openings. The air that went under the splitter has to go somewhere, and if it fills-up the engine bay it will produce a high pressure under the hood and lift-up the car . . . which if you’ve been paying attention, you know will be detrimental to performance. By working air around the fender openings properly, teams can create a low-pressure zone just outside of the wheels, pulling air out from underneath the hood area, and causing the engine bay to have a low-pressure zone underneath the hood. Just like the splitter, this produces downforce. The same thing can happen at the rear fenders of the car, but usually to a lesser extent. This is why we saw teams pulling out the right-rear fender and bending the right-side skirts up in 2015.
Decklid and Spoiler
While the spoiler has basically been reassigned to sideforce production (later article), the shape of the rear of the car still produces downforce. Not all of the air that goes into the front of the car will escape through the wheel wells, so some will come out of the back. Similar to how sports cars and old Formula 1 cars utilized skirts to “seal” the sides of the car, NASCAR teams have done the same. Skirts are essentially walls that channel whatever air is left to the back of the car, where the angled trunk pan can produce a diffuser effect similar to the radiator pan. If you remember back in the 2011 and 2012 seasons, NASCAR made attempts to cut the skirt length by a few inches to reduce rear downforce and allow air under the car to “bleed” out of the side. This was almost a complete failure, since almost everyone (except NASCAR, apparently) knew that softening the rear springs would fix that issue basically overnight.
The spoiler also traps air on the top of the trunk area, known as the decklid. Stopping this air in place produces high pressure, which works with the low pressure under the trunk to produce downforce.
The Greenhouse Effect
The major aerodynamic challenge on a stock car is the roof, aka “The Greenhouse.” It has a curved shape which, as we know from airplane wings, produces lift. Below is another image from Stockcar Engineering magazine showing the wind tunnel numbers from the 2014 Sprint Cup aerodynamic package.
In the engineering world, it’s not proper to use two terms to describe the same force, and “Lift” is used for all aerodynamic forces acting up or down. Positive lift acts upward, negative lift acts downward creating what we know that as downforce. From these numbers, we can see the downforce produced by the car’s underbody exceeds the car’s weight at a staggering 3,817 pounds of downforce. That would be enough to do that “drive on the roof” thing, but that gets killed-off by the top of the car, primarily the greenhouse, which produces over 1,400 pounds of lift. All told, the two numbers resulted in just under 2,400 pounds of downforce in 2014, which has since been reduced tremendously as of 2016.
Interestingly, NASCAR’s R&D released information in early 2016 following the High-Drag package tests at Indianapolis and Michigan that showed cars trailing another car produced more downforce due to the roof being unable to produce the same amount of positive lift it creates in clean air. This basically confirmed that downforce was not the problem for trailing cars, and is the reason why we see a lot of effort going into reducing sideforce this season.
The Aero Package
With all that laid out, you should have a basic understanding of how stock car aerodynamics works. We essentially want the car as low to the ground as possible without anything touching the track, and we want it to stay that way for as long as possible. Since sim racers cannot do much to alter the car’s aerodynamic devices, most of the cars will have a very similar lift-to-drag ratio, and in most cases the effects of drag can be ignored in terms of chassis setup, unlike something like Formula 1 where a car with more drag needs to have that taken into consideration. In the mid-’90s, NASCAR teams wanted as little drag as possible, and relied primarily on mechanical grip, which resulted in cars that rolled a lot, had a large gap between the chassis and the track, and weren’t as fast as today’s cars. In fact, of the 23 tracks on the NASCAR Cup schedule today, only three tracks have qualifying records that were not set with a Generation 6 car: Daytona, Talladega, and Atlanta. Daytona and Talladega’s records obviously stand due to restrictor plates, and Atlanta’s record stands because it was repaved in 1997. The record there is a lap of 197.4mph, while the closest anyone got this year was Kyle Busch at 193.3mph. It’s safe to say that the 4mph would be made up easily by new pavement.
Looking through the past few years, it’s easy enough to see teams are still going for the same result in their aerodynamic platform each year. Despite rule changes to remove ride height minimums, the cars still look the same on-track.
Some images used in this article appeared in the 2015 edition of Stockcar Engineering, titled “NASCAR 2015: The start of a simulated revolution?” The issue can be viewed in its entirety through the link below:
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