By attaching yarn strips,...
By attaching yarn strips, we can visually see how the flow of air is progressing at the surface of the car. We can use these aides in our wind tunnel testing or in a more simple and practical way by just driving the car around the track and taking video and/or photos. These are pre-made yarn strips made especially for this purpose. This is showing more areas of detached flow than attached flow.
When you look up a laminar airfoil from, say, NASA to use it on a race car, odds are about 99 percent you won't actually achieve the laminar flow. You really don't want a laminar flow airfoil on a race car, because slight imperfections, dirt, rain, humidity can dramatically and suddenly reduce its performance. The surface must be clean, smooth and exceptionally accurate. If you put a pinstripe on some laminar airfoils, they will not make any lift or downforce. Laminar flow doesn't exist on any race cars in the U.S.
As a vehicle pushes through air, it creates an area of high pressure in front of it. This is called a bow wave. The bow wave on a race car will be somewhere between 10 to 20 feet in front of the car. This means that the car is affecting and changing the airflow that far in front of the car. The steeper the angle of the front of the car to the ground, the farther out in front of the car the bow wave will be. The farther out in front it is, the more drag the car will have.
On many race cars, there are...
On many race cars, there are splitters attached to the front bodywork that act to split the air that goes around and over the car from air that goes under the car. This part is adjustable and the height and angle relative to the ground plane determines how much effect it has on suction downforce. -Bob Bolles
If the bow wave is strong enough, it will detach the flow completely over the rest of the car, at the point where the hood blends into a more horizontal surface. As it detaches higher, the airflow is actually causing suction or lift, counteracting the downforce you thought you were creating. The two probably can't cancel each other completely, but it can significantly reduce downforce and increase drag.
Separation of flow at the front of the car will also reduce the downforce created at the rear. This is because the air of normal pressure can't get back down to the level of the rear spoiler. Now, up to a certain point, increasing the angle of the spoiler will reattach some of the flow in front of it. If a car is sliding at some angle to its actual direction of travel, as in a dirt car, the sides of the car will detach some or all of the downforce flow. This is due to the abrupt change in direction of the oncoming air up over the side of the vehicle and it can't reattach to the top of the car. You'll also reduce some downforce because high pressure air on the side of the car will get under it, causing lift, unless you can seal the side to the ground.
The Sprint Car wing could...
The Sprint Car wing could be one of the most misused aero components in racing today. The large side plates offer little advantage to the wing's function and actually create a ton of drag when the car is sliding sideways through the turns. It's generally thought that this configuration provides more of a braking effect than a tool for downforce. The high angle of the wing also creates a high degree of drag that may benefit load transfer to the rear wheels for added traction rather than by providing aerodynamic downforce. -Bob Bolles
Drag and Moment Arm
Surfaces of the car that create downforce also have a moment arm. That's the distance from the wheel to the force. With a spoiler far back on the car, you could change downforce on the rear and it will affect the downforce on the front in the opposite direction. The height of the spoiler or airfoil affects the length of the moment arm. A Sprint Car wing has the most radical moment arm of any aerodynamic surface used in racing.
The bottom of the car is also important. Air does flow under there and it's usually under some suction force, so it has much less affect, but things can be done here to gain speed. Lining everything up will give you an improvement over certain designs, but yet more can be had. Look at how flows are detached under the car. You have to know what to look for, and not everyone sees it. There are ways of obtaining more downforce and less drag under the car. Look also at what the underside is doing under roll, squat, and dive. Look at how these conditions disturb the airflow and downforce. The more suction you produce, the more air wants to go under the car.
Homemade Wind Tunnel
You can make your own wind tunnel to get real data. It takes some good cameras, some small diameter yarn, and tape. Ideally, paint you car white, and do a good job with minimal overspray or "orange peel." A bad paintjob will give negative effects on airflow. Tape a 2- to 3-inch length of black yarn every 6 inches or so all over the car, including the sides--just eyeball it. Make sure the tape is put down smoothly so it doesn't disturb the airflow over the yarn.