I’ve had an inbox full of suggestions for inexpensive streamlining of MAX, our 100-mpg, DIY car. Some of them are good, some are not so good, and a fair number of them are (to quote the Magic 8 Ball of my youth) “Reply Hazy, Ask Again Later.” I won’t call automotive aerodynamics a black art, but when you get to a specific car, you won’t find all the answers in the textbooks.
The problem is every part of the body influences every other part of the body. Like the parable of a butterfly flapping in Barcelona causing a hurricane in Costa Rica, a small difference here can make a large difference there. A small change in the radiator intake might generate a mild change in how air flows over the hood, which could make a moderate change in airflow over the windshield, leading to a significant change of airflow over the roof, causing complete flow separation at the rear window and a huge turbulent wake behind the car. The textbooks can guide you, but the only way you’ll really know what you’re getting is to test.
As you regular MAX Update readers know, we got an involuntary do-over on body design about a month ago and we don’t have much time to fool around. We’re doing rapid prototyping and rapid testing and going back to the basics, back to how aerodynamic testing was done in ye goode olde days.
Now I hate to oversimplify, but as a general rule of aerodynamic drag, turbulence = bad; smooth flow = good. One way to observe the flow of air close to the body is to tape tufts of yarn on the car and watch which way they blow. It sounds a bit like the old “weather string” joke (if it’s wet it’s raining, if it’s moving around it’s windy, if you can’t see it it’s dark) but tuft testing has a long and legitimate history. In the nautical world, a tuft of yarn has been called a “telltale” since about the time yarn was invented. In our case, the problem is with tufts on a car body, how do you watch them?
In a wind tunnel, you just stand there and look, but with a moving car it’s not that easy. You can’t drive alongside in another car because the wake of your observation car voids the test. So I asked myself, what would Wonder Woman do? She had an clear airplane. Why not a clear car?
I’ve taken out the passenger’s seat and paneled MAX’s right side in $10 worth of one-eighth-inch Vivak, which is transparent, thermoplastic sheeting and is clear, tough, and easy to work with. You can saw it, drill it, rivet it, bend it … and tape tufts of orange yarn to it. I’ll watch the tufts from the driver’s seat and see how changes in the front of the car (different fenders in particular) influence airflow in the middle of the car, and maybe I’ll learn something.
Photo by Jack McCornack