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MAX Update No. 71: Suspension Streamlining


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 

jack mccornack
5/18/2011 11:48:27 PM

Hi TDeL, Man, great minds think alike! If the car has a full body, there's no reason the chassis has to narrow so much in the footwell. Mind you, right now I'm scrambling so MAX will make a good impression at the Mother Earth News Fair in Puyallup in (gulp) two weeks, but I do have a fat-footed conversion on the drawing board and it looks like an easy modification.

5/18/2011 12:36:22 PM

Hi Jack, I like the old Seven body style too, but your interpretation of a 50's sporty car and your eye for shape has always pleasantly surprised me. Hopefully an enclosed option will develop as well as a modded frame which gives a little more width for the legs ( I am not horizontally challenged). Looking forward to the new June unveiling. -Tom

jack mccornack
5/17/2011 3:36:57 PM

PyroTechie, I didn't compensate. In actual use, the difference in modulus of elasticity between PVC and steel is so immense that the stiffening effect of the fairing is insignificant. In this test, all I measured was drag, not stiffness...the plain pipe didn't flex back enough to make any difference in drag that I could detect.

5/13/2011 11:36:43 AM

I'm curous how you compensated for the added rigidity of the aero tubing in your measurement? The shape of the aero tube alone adds rigidity and having two layers (pipe + aero tube) creates an even more rigid structure.

jack mccornack
5/11/2011 12:39:40 AM

Ron, I did do a full underbelly, and someday I'll have to take a 4 x 4 down the fire trail I traversed last September and see if I can find it. I intend to make another one, and this time I'll take pictures. And Jeff, Duesenberg didn't do that but their chief body designer, Gordon Buehrig, did--google Tasco or follow the link below, and Aptera does it today. and will give you a peek.

ron garnes
5/7/2011 1:23:07 AM

Jack: Your latest experiment proves again that aerodynamics is critical even at low speeds (20-40 MPH). Have you done anything to the underside of MAX? A full length belly pan or relief vents on the back side of the wheel wells would probably get you over the 100 MPG mark. If you look at the old Formula V cars the SCCA used to race, many of them had fiberglass fairings molded around the old Type I VW front suspension. This not only made them look more exotic, but probably put some downforce on the front tires, as there wasn't much up fron weight-wise. Ron G

jack mccornack
5/5/2011 11:54:48 PM

Jason, it is indeed an arbitrary benchmark (100 mpg for ten grand) and in many ways, 70 mpg for seven grand would be more appealing...but I want to see if it can be done, and it's a number everybody can relate to. But I'm sure not going to be done designing efficient cars when MAX's mileage hits those magic triple digits. If you can get somebody out of a 30 mpg car and into a 60 mpg car, you save as much gas as if you go from 50 mpg to 300 mpg. In either case, the mileage improvement saves a gallon of fuel for every sixty miles driven.

jack mccornack
5/5/2011 4:58:37 PM

Jeff_58, To my perpetual humiliation, I grunched the streamlined body (see embarrassing photo on MAX Update #60) before I go the chance to test it thoroughly. I'm busy rebuilding it in time for the Mother Earth News Fair in Puyallup WA June 3-5 and should have some answers by then.

jason thatcher
5/5/2011 1:15:29 PM

Hi Jack, I am actually happy to hear that there are other folks out there who are taken in with the "old" look. In fact, I would be happy to settle for the gas mileage that you were getting before - I really liked the look of the car last year! Isn't 100 mpg somewhat of an arbitrary benchmark anyway? 60+ would more than double what i am getting now, and that would be a huge improvement in itself.. thanks for the effort that you are putting into this project. jt

jeff dean
5/4/2011 5:01:34 PM

Jack, How much gain did you get with the aero-body compared to the original? HP? MPG? can you calculate how much hp it takes at 55 mph? With the old body, the open wheels appear to be the largest drag. Maybe some fenders attached to the spindles or something. Didn't Duesenberg do something like that?