Amateur Aerodynamics

  In my previous post about measuring changes in drag , I discounted coastdown testing as an unreliable method. I called it that because I had tried coastdowns several times and never got usable results. There were several reasons for this that I have been thinking about recently as I search for a method of measuring drag changes on my car. First, I was not good about keeping test parameters consistent; second, the tests themselves were not constructed properly; and third, I was trying to use the test results to calculate things (like drag coefficient) that relied too much on assumptions and were too small to reliably figure from this sort of testing on public roads, which naturally has high variability.   I decided to revisit coastdown testing and see if I could design and execute a test in such a way that the results could be trusted and could show me whether the aerodynamic drag of my car has changed. Here's how I did it, and how you can too.   Concept   The concept behind coast

Fences are commonly affixed to racing cars; road cars, not so much. What will they do here? Only one way to find out. Last time , I wrote a little about the difference between steady and dynamic directional stability in car aerodynamics. I only scratched the surface of this very complicated subject, but I was able to find, through testing, the effect of adding rear fins on my Prius’ steady stability (as measured by steering wheel angle) in a constant crosswind.   After finding that I could add an X-gauge to my Scangauge computer that would read the car’s steering angle sensor, I decided to try some more tests to see what various modifications do to the crosswind stability. Fortunately, here in the Midwest it is often quite windy, giving lots of opportunity for testing.   I first spent a few weeks driving around with the STA X-gauge displayed and noted that when the car is going straight down a level, straight road, it reads +1.6 ° (that is, the wheel is turned slightly to the left, tow