How to Test

Setting out to modify your car’s aerodynamics can be daunting. Where should you start? What should you do first? What changes will give you the most bang for your buck?

 
Unfortunately, you’ll find a variety of answers to these on the internet—many of which will consider themselves the definitive, correct response, and most of which aren’t based on actual data or first-hand experience. Ignore them.
 
Identify Goals
 

The first thing you should do is identify your goals. If you don’t have something in mind you want to accomplish, you can’t plan out a way to get there. And the more specific you can be about where you want to end up, the better ability you will have to make effective modifications that accomplish your goals. Take some time first to identify these. Then, make a testing plan: what changes you want to test, how you will test them, and what you will do if the tests are successful or not. This plan can change as you go along (my plans change every few weeks or so as I finish more testing), but it’s important to have something down on paper so you have some direction.
 
Test Effectively
 
To get trustworthy results from your tests, you will need to control as many variables as possible. You have some influence over these both in how you plan your testing and how you carry it out.

About as perfect a road for testing as you will find anywhere. This first mile has a slight downward slope to the next intersection, after which it is flat and level for nearly a mile.

First, identify a good road or roads on which you can test. Ideally, these should be flat, straight, and lightly travelled by other traffic. The influence of a passing car can be enough to mess up your results, depending on what you are testing and how sensitive it is to changes in airflow. Similarly, even slight road grades (up or down) alter the resistance force acting against a typical car much more than even large changes in aerodynamic drag, and the vertical forces more than even a significant change in lift—so be cognizant of this and find a flat road if you can. You will also need a long enough road that you can get up to test speed, measure whatever it is you are interested in, and still have enough room to stop and turn around to go back to your starting point and do it again. I suggest 1 mile at a minimum; some tests (like coastdowns or throttle-stop) I use a 2-mile stretch to give myself enough room to get up to speed, do the test, and then turn around.
 
Once you have identified a suitable road, you will need to watch the weather forecast to see when conditions will be appropriate for testing. Most often, this will mean stable temperatures and low winds, but sometimes you might want a windy day. Play it by ear; sometimes the forecast for a particular day changes by the hour in the days leading up to it.
 
If you have trouble finding days with good conditions, one thing you might try is to test at night. Winds are usually lower overnight, and temperatures are often stable for hours at a time. Additionally, traffic tends to be lighter at night, making it sometimes an ideal time to get tests done.
 
Be consistent and rigorous. If a car messed up a run, do it over. If the winds have changed, come back and try again another day. Testing is no good if you can’t trust the results so do everything you can to ensure they are reliable. Use road signs, telephone poles, mailboxes, or light poles to ensure you’re starting a particular test at exactly the same spot each time. Keep an eye on the outside temperature if your car displays it. Watch for the movement of trees and grass, which can indicate that a wind has increased or changed direction. Don’t be afraid to scrap it if circumstances beyond your control require. You can always come back and try again another day.
 
Analyze

Once you have good results, you will need to analyze them. Some tests don’t take much; throttle-stop testing, for instance, will tell you immediately that drag went up or down based on the speed your car achieves. Others will be more involved; for example, if you measured lots of pressures on the body and now need to average results and compare locations to see what actually happened, or if you have hundreds of tuft test pictures to sort through. Use programs such as Excel to chart data and plot it visually, which can sometimes make it easier to interpret. Identify new questions based on your results of previous tests—don’t throw out old data! Finally, lay out a modification scheme that fits with your goals for your car, based on the results of your testing. Remember, no one else has exactly your car and exactly your goals.
 
Just Do It
 
Most importantly, the only way to learn how to test is to go out and do it. The same as learning to throw a baseball or play a musical instrument, reading about how other people do it might give you a lot of knowledge about the process but it will not give you the skills necessary to actually do it. Just like music and sports, the joy is in doing and in cultivating your own savoir faire. Try it!