Drive the car at different speeds and take video or high speed still shots as the car goes by, or from inside the car. Don't put the camera where it will disturb the flow. Maintain a clean surface free of dirt. You can video from other cars along side, both close to and farther away. Look at how the yarn changes with another car in certain positions relative to your car. Different cars will affect your airflow a little differently, but generally in the same way.

Ideally, every yarn will lay flat on the surface and point straight back to the rear of the car. High pressure areas will do one or all of the following: lift the yarn, flutter the yarn violently, or point it off to one side. If you see these characteristics, you may be making more drag and less downforce than optimal.

Low pressure may do one or all of the following: lift the yarn in a more limp action, cause more of a limp flutter, and/or keep the yarn moving all over the place. These could be areas of lift. These indicators show more drag than optimal. How much affect do you get from getting it right? From what I see on a Dirt Late Model, you may be able to cut the drag in half. I accomplished attached flow on a 180hp aircraft and nearly doubled the cruise speed, with everything else being the same.

Stall Angles of Spoilers and Wings
These are critical devices that need very small angular changes of 1 or 2 degrees at a time to see the results. Most airfoils stall at around 22 degrees. They still create lift or downforce beyond that angle, but it will make less than if you reduced the angle. As you approach stall or go past it, drag will go up considerably almost immediately. The gurney flap, wicker bill, spoiler strip, or whatever you want to call it can add downforce, but will also add drag.

Never position any of these devices angled forward beyond vertical. As the device points forward of vertical, you increase the stall effects negatively, and shorten the surface the force is generated through, and add a little lift. The steeper angles at which any of these devices are set will also move the downforce rearward, until stall. At stall, the downforce will move forward some.

Reducing Drag
Interference drag is drag caused by compressing air with a disturbance protruding out of a surface that air flows over or around. The more the surface is flat, the greater the interference drag. The cross-sectional shape of the protrusion can also dramatically affect drag. A round object, like a front axle on a Sprint Car, has five times its diameter in equivalent drag. You don't need to know just what that means as long as you understand that a round tube placed at 90 degrees to airflow has huge drag. If you shape the same tube like a wing strut on an airplane, you'll see huge gains in reduction of drag. Most Formula cars incorporate this design aspect.

Make the components really aero-dynamic and drag will go down at an amazing rate. I did extensive flight testing on a 100mph aircraft with wing struts. By only changing the shape of the wing strut I discovered the following through observing actual flight test data: A design with a round tube strut yielded a 100mph top speed. Using a standard aircraft strut yielded a 127mph top speed. Using an optimized aerodynamic shape for the strut of my own design yielded a 136mph top speed.

There are millions of different designs of airfoils out there and they all do something different. For optimum performance in a race car, we want high downforce with minimal drag--not too different from an airplane--but there are other differences in airfoils. They are: the stall angle, max lift over drag angle, pitching moment, and the percentage of airfoil to which the flow is attached. Pitching moment means where the net lift or downforce occurs relative to the front of the airfoil.