Oregonians have been flooding the radio stations with calls about a large glowing disk hovering above the … that’s the sun, folks, we just haven’t seen it for a while.
A perfect day to take MAX out for some aerodynamic testing. The body is off for repairs, and one of my aviation friends had some related questions, so since it’s my car and I can do what I want with it, I bolted a test fixture to the chassis and built a simple drag comparitor device to show how much better a streamlined tube is than a round tube.
Yeah, yeah, I could look it up, this isn’t cutting edge research by any means, but I’m a DIY guy, so I went ahead and did it. Besides, not every effort at streamlining is as successful as every other effort, and I wanted to try an off-the-shelf product called Streamline. It’s a PVC sleeve material that’s made for sport aviation and its shape is slightly compromised so it can fit different tubing diameters (the small stuff, which I used here, fits 5/8″ to 1-1/4″ tubes, the larger size fits 1-1/4″ to 2″ diameter tubes) and as far as I know, there’s no wind tunnel data available for that specific product.
And why do I care? Because many folks like the Old MAX body better than the New MAX body, and like the visible suspension parts and other old-timey styling cues. Anyway, I measured and found that MAX has a total of eight feet of 1″ tubing in its front suspension, and a quick back-of-a-napkin calculation suggests the air resistance of that tubing costs MAX about a horsepower at 60 mph. But who trusts napkins any more?
So I got eight feet of the smaller Streamline stuff, to slide over 1″ outside diameter 3/4″ Schedule 40 PVC pipe, and made a fixture that had eight feet of pipe sticking out one side, and eight feet of Streamline sticking out the other. I stuck the fixture on the front of the car, stuck a digital postal scale on the dashboard, and tied the two together with some nylon cord.
I’ll spare you the details but the result was, the scale read the difference in air resistance between the sleeved tube and the plain tube (times four; I used leverage to make the numbers more definitive). Then I mounted my FAA-pleasing orange and white flag and my yellow flashing light on MAX, and had the airport’s UNICOM operator listen for air traffic while I took two passes down the runway.
I’d hoped to be able to get up to 50 mph but no dice. The first thing I learned was, PVC is pretty flexible, and when I went much past 30 mph the ends of the tube and the fairing started flapping up and down like some ungainly bird with a 16 foot wingspan (the wide span is why I didn’t take it on the highway, by the way). The results came out as the textbook would expect, 1-1/2 pounds of drag reduction at 30, which means we’d save 6 pounds at 60 mph (it’s the square of the difference in speed–double the speed, quadruple the drag) which means saving that horsepower that I mentioned before…which in MAX’s case means adding 4 or 5 miles per gallon.
Of course the new body takes care of streamlining the suspension too, but it will be nice to have an improved alternative for those who are into “the look.”
Photo by Jack McCornack