There is no such thing as “good enough” for a racer. Every detail of our personal performance, and our machine’s performance, has to be refined to the absolute best it can be to win races. That’s because our competitors are engaged in the same quest, and the strong ones have created highly refined racing programs. Finding performance gains in high-payoff areas that your competitors aren’t even thinking about is a great way to produce a big performance jump over the field. If the tracks you run are fast and open, with long straights and big corners, then improving the aerodynamic performance of your car can produce a big performance gain.
To get a direct feel for how much airflow affects your car’s performance, try sticking your hand out of a car window on the highway, palm down. Then consider that the frontal area of your car is about 500 times larger than the edge of your hand. Then consider that your top speed on the track may be twice your highway speed, which generates four times as much drag as highway speeds. That’s 2,000 times the drag on your hand, which is a lot!
Because we cannot see airflow, not many racers have a solid understanding of where the air goes and what it does as a car travels through it. Part of that is because the shape of a car is highly complex, with constantly varying curves, edges, corners, junctions, free edges, open windows and wheel wells, every imaginable shape under the car, and airflow through the coolers, engine and cabin.
What if we could see airflow? That would answer a lot of questions about what is actually happening to the air while the car punches through it. It would tell us where the airflow is clean, efficient and logical. More importantly, it would tell us where the zones of trouble are, and how big those zones are. After making a change to the shape of the car, we would know whether that change improved the airflow in a zone of trouble, and if that change affected anything downstream of that new feature. In addition, it would define what the local direction of “downstream” actually is since a car pushes air in all sorts of directions that aren’t straight along the car’s path.
Well, there is a way to see airflow. Even better, all it will cost you is some time. It doesn’t get any cheaper than that. One way to see airflow on the surface of your car is called oil streak flow visualization, and preparing your car for it is as simple as anything ever gets. Save some used engine oil from your next oil change to use for this investigation. Used oil is a good choice for the dots because it is a lot darker than new oil, so the streaks are easier to see. It doesn’t cost anything either, which is great. New oil will work too, but it doesn’t show up as well in photos. Other fluids like spray lubricants and pink antacid liquids also will do the job, but antacids can stain plastic windows.
Just before a high speed trip on the track, put a series of oil dots on the body with a finger dipped in your used oil. Repeat as many times as it takes to cover the area of interest. Oil dot spacing can vary from 1” to 6” between dots depending on the level of detail that you want to see in the streaks. Dot the horizontal surfaces first, because the oil will run down angled and vertical surfaces slowly.
During the track run, airflow on the surface of the body will push the oil in the direction of local airflow. The length of each streak is a rough indicator of how fast the air is moving there. If the track run is fast enough and long enough, the car will carry a record of local airflow that will last long enough to get good photos before gravity takes over again and pulls the streaks toward mother Earth. After taking photos, cleanup will take several paper towels, spray glass cleaner and auto detailer spray.
Now the tricky part begins: interpreting what the oil streaks mean. All you get is surface flow direction and a rough indicator of flow intensity. Sometimes it takes an educated and experienced eye to tell what the off-body airflow is doing to cause the streaks that you can see. We will describe and illustrate a few common flow patterns here, but covering this topic fully is beyond the scope of this article. If you see something that you aren’t sure about, contact your favorite aerodynamic expert for a good bench racing session and an in-depth education.
The cheap and easy nature of oil streak flow visualization makes it a practical research technique for every track racer. Knowing what is really happening to the airflow on the surface of your car is the first step toward modifications that will improve your car’s performance. Retesting with your modifications in place will provide an indication of their effectiveness. If your oil streaks show the need for a modification that no one has ever done before, and it’s legal for your class, go for it. Stepping into the twilight zone of aerodynamic modifications is the essence of innovation.
Although these photos were taken at the Bonneville Salt Flats, airflow is airflow and cars are cars. Oil streak flow visualization works the same there as at your home track, but without the occasional effects on the oil streaks from cornering and braking.