In a previous article that was published about four years ago, we offered an explanation of the way the aerodynamics of downforce works for a stock car, something that had never before been adequately defined. In today's racing circles, aero effects are discussed on a regular basis.
Body shapes and methods have evolved over the years that improved stock cars' aero efficiency related to drag and downforce, but until recently, no one was told why certain methods are effective. Let's readdress aero effects and downforce now that our knowledge has progressed.
The addition of a high wing to the rear of this Daytona prototype helps provide a lot of downforce at the rear of the car. NASCAR's newly designed Car of Tomorrow has a very similar wing attached to its rear for the same reason. These units look really out of place on a stock car. Courtesy Grand Am Racing
What A Coincidence
Just as I was about to write this piece, I read the morning Daytona newspaper and my friend Godwin Kelly's story about four crew chiefs being barred from the Daytona 500 and fined for illegally helping out their cars' aero balance situation by attempting to improve the rear downforce of the cars.
Those Nextel Cup cars are suffering due to having to run hard tires, and the stability factor is at an all-time low. Any help with rear downforce is a huge help. That sounds strange, but with all of the technology concentrating on the front downforce, the cars have become unstable because the rear loads are not enough to keep up with the front loads. We'll explain this in greater detail later.
Our interest as short track racers is not so much with drag efficiency, but in how to make more downforce, especially in the front of the car. Going back 30 years or so, we could see that the 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 improve their cars.
The wing on an airplane provides lift by causing the air passing over the top of the structure to travel farther and faster than the air moving 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 this provides lift to enable the airplane to fly. We use differential pressure in a similar way to achieve downforce in a stock car.
The squareback sedans became fastback cars. Square corners became more rounded. Spoilers and wings were added to production model cars to make these configurations legal for competition use.
Issues such as drag coefficient, frontal area, and downforce were looked at more closely. Teams that were at the cutting edge of this new area of racing technology were the ones who ran out front.
One of the proving grounds for short track racing is the annual Daytona Speedweeks races at New Smyrna Speedway (NSS). Every year, teams from all over the country show up with "stock"-bodied cars that adhere to the strict rules of their respective sanctioning bodies only to discover that there are no rules at NSS.
Years ago, we noticed that after about the third night of racing in the nine-night series, most of the cars were transformed into sleek, wedge-shaped oddities with huge fantail rear spoilers attached.
This is how we evolve in stock car racing-by going to the extreme. When we are reeled back in by the rule makers, we have to get more creative. In this modern day, we now have a better understanding of how aero works to provide additional downforce.
Because the air under the hood is at a lower atmospheric pressure than the air above it, a force much like the one on the airplane wing is created in reverse. We refer to this as downforce. By making changes to the shape of the front nose, fenders, and wheelwells, we can greatly increase the amount of low pressure and the overall downforce effect.
Downforce, or its close cousin lift, is a byproduct of differences in air pressure on two sides of an object. Similar to how an airplane wing produces lift, a car can produce antilift, or downforce, so that 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 of our stock car consists of the hood and rear deck areas. We need to design our bodies so that we can direct the air around the car in order to vacuum air out of these two cavities.
If we can use the swift flow of air that is flowing past the sides of the car to help vacuum air out of the engine compartment, 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 lower pressure, or high toward low.
To accomplish this, teams use wider, angled front noses that 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 wheels. Air rushes out of the engine compartment to fill this void, and the pressure under the hood is reduced.