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MAX Update No. 43: Testing Aerodynamics With a Clear Car

4/6/2010 10:57:01 AM

Tags: MAX, 100 mpg, aerodynamics

So, what did Wonder Woman's car (Update No. 37) have to teach us?

Some of it is a bit embarrassing, like slap my forehead and say “D’oh!” But at the time, it seemed like a clear car was a good idea. Tuft testing is a good idea — it's a bunch less expensive than wind tunnel testing and it gives immediate feedback — and I figured clear panels would let me observe the tufts while I drive. Pretty clever, eh?

Tuft testing lets the tester see what the air is doing on the surface of the object (MAX, in this case). Logically enough, smooth airflow is (usually — we'll talk about the exceptions in a different update) a good thing and turbulence is not, and tufts of yarn are excellent indicators of airflow quality (much like the traditional West Virginia Weather Rope: when it's wet it's raining, when it's swinging it's windy etc.).
But — big surprise — it turns out that a clear car does not improve tuft testing. Here's why:

• Observing while driving is not an optimal use of one's time. At the beginning of this year, Oregon passed a law forbidding the use of handheld cell phones when driving. I suspect tuft testing is equally distracting. Better to have somebody else take photos (or, even better, video) and do the analysis later, in the comfort of one’s own home, with a crackling fire and Bach in the bachground.

Aerodyanic Tuft Testing• If somebody else is doing the visual documentation, you don't need them in the car, so clear panels aren't going to help. A skilled photographer can pan the car as it goes by, a less sophisticated photographer can sit in the back seat of another car diving alongside. I had thought the wake of the camera car would be a problem, but we tried it with various cars at various distances. With a small camera car and a four lane road, when each car hugged the outside of our lanes, there was no noticeable aerodynamic interference. 

• For best results, the yarn should be a color that contrasts with the background color. Uh ... what contrasts with clear? I figured anything would, but found it depended entirely on what was in the car — my toolbox, my passenger, whatever. These bright orange tufts, which were so perfect against MAX's forest green paint job, are nearly invisible in this photo (which is why I photoshopped blue arrows in there). Adding red seats didn't help, either.

This doesn't mean the exercise was a waste of time. The tuft tests confirmed that the clamshell fenders (see Updates No. 12No. 16 and No. 22) are absolutely horrible. Not only do they have drag of their own, they scramble the air behind them and eliminate any chance the air will regroup smoothly after MAX goes by. With the front fender and headlight removed, the tests also showed the expected light turbulence along the side of the car, but it started just behind that little 'kink' at the front of the cockpit (see Update No. 22; the kink is about 6 inches behind the yellow diamond-shaped sticker) and the airflow was smooth and attached forward of that kink ... I guess I need to do more tests to see if the air reattaches itself after the front suspension goes by, or if somehow it's staying attached through the snarl of control arms and shock absorber.

The one genuine surprise was how radically the airflow splits in front of the rear fender. It spills nearly vertically, over the side and into the cockpit. All the more reason to block that flow with a window.

There is one more effect from having the side of the cockpit clear: From the passenger's perspective, it makes MAX even more of a four-wheeled motorcycle and less of a conventional car. I guess if I wanted to hammer the point home, I could give MAX a clear floor.

Photo by Jack McCornack 


Browse previous MAX Updates.
Check out the 100-mpg Car page for all things MAX.

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