Based on my research, aerodynamics for stock car racing has never been adequately defined. Body shapes and methods that improved the aero efficiency of stock cars related to drag and downforce have evolved over the years, but no one explained why what works, works. I'm not sure any of us knew why.
Because of technological advancements that have become available during the past few years, we are now better able to define how downforce works to make our cars faster.
As short track racers, our interest is not so much with drag efficiency, but rather with making more downforce, especially in the front of the car.
Going back 30 years or so, we could see automotive manufacturers and many racers began to redefine the body shapes to make the cars cut through the air more efficiently with less drag. The research done for the space program, as well as in the design of modern aircraft, provided much of the early impetus for the factory teams and local racers to use to improve their cars.
The square-back sedans became "fastback" cars. Square corners became more rounded. Spoilers and even wings were added to production model cars to allow these configurations to be legal for competition use.
Issues such as drag coefficient, frontal area, and downforce were looked at closely. Teams on the cutting edge of this new area of racing technology were the ones who ran out front.
One of the early proving grounds for short track racing was the annual Daytona Speedweeks races at New Smyrna (Florida) Speedway. Every year, teams from all over the country would show up with "stock" bodied cars that adhered to the strict rules of their respective sanctioning bodies, only to discover there were no rules at NSS. After about the third night of racing in the nine-night series, most of the cars would be transformed into sleek, wedge-shaped oddities with huge fantail rear spoilers.
This is how we evolve in stock car racing--by going to the extreme. When the rule makers reel us in, we have to get more creative. In this modern day, we now have a better understanding of how aero works to provide additional downforce.
Downforce is a by-product of differences in air pressure on two sides of an object. Similar to how an airplane wing produces lift, a car can produce anti-lift, or downforce, so more load is placed on the four tires to provide more overall grip. The more grip we have, the faster we can drive through the turns. The bonus is that downforce provides added grip (traction) without the negatives associated with added weight or mass.
The "wing" area of our stock car is the hood and rear deck. We need to design our bodies to direct the air around the car in order to vacuum air out from these two cavities.
If we can use the swift flow of air flowing past the sides of the car to help vacuum air out of the engine compartment under the hood, we can lower the pressure along the underside of the hood as with the airplane wing. High pressure on one side of an object will push that object in the direction of the higher pressure, or high toward low.
To accomplish this, teams use wider, angled noses that will direct the oncoming air around the sides of the car to the wheelwells in such a way that a low pressure area is created just outside the front wheels. Air rushes out of the engine compartment to fill this "void," and the pressure under the hood is reduced.
The average atmospheric pressure at sea level is 14.7 psi of pressure on all sides of an object, even on our bodies. If we reduce the pressure under the hood to 14.5 psi, a drop of only 0.2 psi, over an area of just a square yard, we would generate about 260 pounds of downforce (0.20 x [362 or 1,296] = 259.2 pounds).
A counter effect to the air flowing out of the wheelwells relates to the air that flows under the nose of the car. This air will replace some of the air being suctioned out of the wheelwells and this reduces the low pressure and effect of downforce. That is exactly why we need the front spoiler, or valance, on the nose to be as low as possible. The popular soft spring setups tend to help this situation by allowing the overall chassis to compress into the track to help eliminate the flow of air under the nose.
Once we have modified the outside of our cars to provide the airflow to create the low pressure areas we need, the next step is to try to provide the greatest amount of surface area under the hood and rear deck that can be exposed to that low pressure to increase the downforce effect. We also want to try to limit the volume of air that could flow under the nose and make what does flow less able to diminish our goal of low pressure.
One way to increase front downforce is to open up the area under the hood that is available for pressure reduction. One area is at the top of the radiator shroud or airbox. Most designs offer a flat top built very close to the hood. We can re-design this structure so the top of the airbox is curved from a side view starting at the very top of the radiator. As it comes forward, it is at an angle of 70 degrees off horizontal and then curves forward to intersect the top of the openings in the nose. This shape creates a large cavity under the front part of the hood and nose that is now available for reduced pressure and added downforce.
If we build the bottom of the airbox wider at the front, less air will flow under the nose and upward to the low-pressure area. With less air being replaced, more low pressure is retained and more downforce is available.
At the rear of the car, we can manipulate the shape of the spoiler, the rear window posts, and the body just in front of the rear wheelwells. By routing the air that is flowing past the sides of the car out and to the sides of the rear wheelwells, a similar suction effect takes place to create a low-pressure area under the rear deck. There are obvious body shape limits in this area, but a little reshaping can help.
Racetracks that require less downforce and ones where we can benefit from reduced drag will cause us to rethink how the air flows past the roof (greenhouse area) and onto the rear spoiler. If we reshape the post that connects the rear window with the side window openings, we can direct air away from the spoiler and greatly reduce aero drag.
The rear spoiler produces two effects at the same time. It provides aero downforce generated from low pressure on the underside, as well as aero drag that creates a cantilever effect, distributing some of the weight of the car from the front to rear. Many racers mistake this effect for downforce because it will tighten a loose car by adding more weight to the rear of the car.
If we put the car on a set of scales and have four guys pull straight backward on the rear spoiler to simulate drag, we would see a change in the front-to-rear percentage of total weight distribution. This is what drag from the spoiler does. If we need the car to turn well (and who doesn't?), this area of drag will not be in our best interest.
It has been found that the optimum angle for the rear spoiler is between 55 and 70 degrees from horizontal, depending on the type of track. The longer and faster the track, the less spoiler angle we need to run.
Nose Shape Affects Downforce
The shape of the nosepiece can help determine how much air is directed under the car and how much moves up and over the hood. Obviously, the more air that passes over the top, the less that will "invade" the low-pressure area under the hood.
Vertically straight noses tend to build a pressure front that pushes the air in all directions and disturbs the orderly flow of air. This will force more air under the nose as well as disturbing the free flow of air around the side of the car and past the wheelwell. The suction effect that produces the low pressure is less effective and the car will have less frontal downforce.
If the nose were angled slightly with the lower edge farther forward, the oncoming air would be "cut" more cleanly and would not build up into a pressure front. Less air would flow under the nose, and the flow of air around the sides of the nose would be cleaner with less turbulence.
Downforce The overall balance of the setup in the car has an effect on downforce. A car that is set up with a rear suspension that is rolling over more than the front will cause the left front of the car to rise up. This allows more air to flow under the car and replace the low-pressure air, which causes less downforce effect.
In a test I attended at Daytona in early 1996, a team rearranged their spring rates so the left front was more planted as the car negotiated the turns. The left-front shock travel increased over an inch and the car picked up an honest 3 mph.
Short track teams are now using larger sway bars and softer springs so the body roll is reduced and the front end is more compressed into the racetrack to eliminate much of the airflow under the nose. These cars will turn better due to the increased downforce this creates.
Major league teams use wind tunnels to perfect aero efficiencies for both downforce and drag. Each session may be split to include a short/medium track car and a superspeedway car, because the goals for each are different.
In the wind tunnel, the attitude of the car is adjusted to simulate the way the car dives and rolls as it circles the track. There is also a feature at some tunnels that will yaw the car, or rotate it as if it were on a round table. The latest technology utilizes a moving platform under the car to better simulate a real race car driving into the wind.
The key goals with stock car aero design and testing is to create a body shape and inner construction that will provide more downforce to enhance turn speeds, produce less drag, and promote a more balanced race car.
If we work hard to develop 600 pounds of downforce on the front end and the rear is not able to keep up with that high amount of grip, the car will be loose and no one can drive a loose car fast. There are limits to how far we can go. It is possible to create so much front downforce that the car cannot be driven well.
The goal is to work toward a good balance of front-to-rear aero downforce to help produce more grip. Make sure the basic chassis setup is balanced, also, and then the combination of both will enhance your on-track performance.
 In the past, racers have pushed...  In the past, racers have pushed the limits of aero efficiency. Modern rule packages limit the amount of bodywork that can be done. Nonetheless, added grip can be achieved with a little understanding of what is needed to produce downforce. |
 Winston Cup cars of the '60s...  Winston Cup cars of the '60s and '70s showed signs of change. Manufacturers leaned toward more aero-efficient roof designs with sloped rear windows, rear spoilers, and slicker body shapes. |
 These same influences can...  These same influences can be seen in modern stock cars today. |
 The addition of a high wing...  The addition of a high wing on this Dodge provided a lot of downforce at the rear of the car. This is an example of the manufacturer going to extremes to stay ahead of the competing carmakers. Modifications such as these were eventually ruled out, and more uniform rules were established so the competition would be more even. |
 The wing on an airplane provides...  The wing on an airplane provides lift by making the air passing over the top of the structure travel farther and faster than the air that travels underneath. Because of the increased speed, the air on top is under less pressure than the air under the wing. The higher pressure under the wing pushes up on the structure and provides lift to enable the airplane to fly. We use differential pressure in a similar way to achieve downforce in a stock car. |
 As the car travels through...  As the car travels through the air, some of the airflow is directed around the nose and to the sides of the front fenders and wheelwells. If the sides of the nose are angled from a top view, the air will flow out away from the wheelwells and create a very low-pressure area just outside the wheels. The air under the hood is mostly stationary, and some of that air rushes to the wheelwells to fill the void created by this suction. This creates a low-pressure area under the hood, and the pressure differential between the top and bottom creates the downforce needed to provide more grip in the front of the car. |
 Because the air under the...  Because the air under the hood is at a lower atmospheric pressure than the air above it, a force is created much like the airplane wing in reverse that we refer to as "downforce." By making changes to the shape of the nose, front fenders, and wheelwells, we can greatly increase the amount of low pressure and the overall downforce effect. |
 This team built the airbox...  This team built the airbox in front of the radiator with the top angled down as it runs from the radiator to the front grille opening. The old box ran straight from the top of the radiator to the top of the nosepiece and did not allow enough air space for a low-pressure area to exist. The gain in downforce from this design is estimated at over 180 pounds. |
 We can redirect the air that...  We can redirect the air that flows along the sides of the car so that an effect similar to what happens at the front of the car takes place at the rear wheelwells. If the wheelwells are boxed in behind the tires, there will be very little chance of creating a low-pressure area under the rear deck. Most late model cars are very open in this area. Stock-type cars can be modified to open this area within existing rules. Teams must always make sure the passenger compartment is sealed to prevent entry of flammable materials and fire in case of an accident. |
 We can modify the shape of...  We can modify the shape of the window posts to either promote the flow of air to the rear spoiler for added downforce at smaller racetracks... |
 ...or to route the air away...  ...or to route the air away from the spoiler to decrease drag for larger and faster racetracks. |
 Rear spoiler angle will produce...  Rear spoiler angle will produce both downforce and drag. The greater the angle from horizontal, the more drag and less downforce is created. |
 A very low angle on the spoiler...  A very low angle on the spoiler offers less drag, but also very little downforce. |
 The ideal nose for creating...  The ideal nose for creating downforce in a stock car would be one that is narrow at the front with flat sides angled out beyond the tires. The radiator box intake would be wide and low to prevent the air coming in under the nose from mixing with the low-pressure air under the hood. This nose has all of those features. |
 A flat nose will create a...  A flat nose will create a pressure front much like a bubble. This jams up the air trying to flow over and around the car and causes turbulence. More air is forced under the nose of the car, and the air flowing around the sides will be disturbed. |
 A slightly angled nose tends...  A slightly angled nose tends to cut the air more cleanly and direct more air up and around the nose with a smoother flow. |