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MAX Update No. 18: Defining Drag, Part 1

11/26/2008 3:00:37 PM

Tags: MAX, Auto X Prize, gas mileage, drag, 100 mpg

If you have a bajillion dollars or a well-equipped university, you can determine a car's aerodynamic drag in a wind tunnel. But you don't, and neither do I, so we'll have to do it on the cheap. Champagne science on a beer budget, that's my motto.

Automotive wind tunnels work by blowing air at a measured speed over a stationary vehicle (or model) and measuring the forces (drag, lift and stability ... doubtless the source of those tailfins that started appearing on American sedans in the '50s) acting on the car via a number of scales under the tunnel floor.

It's a fine way to do things, and is quite comfortable for the technicians, who sit in a room outside the tunnel, who can wander off for coffee, and who don't have to worry about their notes blowing out of their pockets during the test.

For the MAX project, however, we're using a moving car traveling through stationary air, which presents two obvious problems: How do we measure the speed and how do we measure the force?

 

aerodynamic drag 
PHOTO BY KATHERINE LOECK
 

A car speedometer isn't sufficient for speed measuring — too vague and too inaccurate — but modern technology has brought extremely precise speed-measuring equipment to the masses: the handheld hiker's GPS. Not only do they measure speed, they measure position and date and time of day, and best of all, they record it for you. This is important because one disadvantage of doing aerodynamic testing in cars (and trust me, this is also critical when testing aircraft) is that the technician has to pay attention to not crashing, along with attending to the test.

I quit trying to multitask during test flights (and drives) nearly 30 years ago, when I started using an Apple II for data acquisition. I mounted it on a plank (a literal “on-board” computer) along with a motorcycle battery to run it, and a 5¼-inch floppy drive. It was big and cumbersome and costly and fragile and only accurate within 3 percent to 5 percent, depending on what I was measuring.

Who would have guessed that future sporting goods stores would have speed-time-and-distance recording devices for under a hundred bucks, accurate to 1/10 mph, and small and handy enough to take in the shower with you?

That yellow thing on the dash in the photo is a Garmin eTrex GPS, and I leave it running any time MAX is running. It records my last 600 miles or so of driving, and every now and then I download it to my laptop and examine the data at my leisure.

So that covers speed, but how do we measure the force? I’ll tell you the answer to that riddle next week. But in the meantime, I'll give you a hint: See that yellow light on the driver's side of the dashboard? That's the oil pressure light.



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Post a comment below.

 

Jack McCornack
12/18/2008 10:50:08 PM
Thanks Mark, I'm glad you find MAX of interest. As far as the articles go, I try to adulterate the fluff with a bit of intellectual input, but I figure that anybody who's going to actually build one will dig deeper and go to our web site for specifics. As mentioned in the article, it's www.kineticvehicles.com The information is available, the KV site has links to a couple of enthusiastic users' groups, it has sources and part numbers for the Kubota conversion, and real info is indeed shared, and shared with great enthusiasm. I haven't been pushing the point because MAX isn't done yet (heck, the rules aren't even done yet) and we don't know exactly how MAX is going to end up when it makes it to the start line, but if its current style suits you and you're satisfied with 70 miles per gallon, you can build a car like MAX right now. But be warned: although we give away the information on how to convert a Locost to turbo diesel power, the folks who provide the how-to-build-a-Locost books (Amazon, for example) are going to try to SELL you a copy. I know there are pdf's of these books floating around the internet, so if you're somebody who feels that charging for intellectual property is unethical, you can probably print up a pirated version and keep the author out of the loot loop.

Jack McCornack
12/18/2008 9:32:13 PM
>Get on with it already! Thanks for the nudge, Mark, I do my best work under pressure. MAX will be ready for the first stage races, but Progressive Insurance and the X PRIZE Foundation say it'll be a year before they drop the flag, and they're the folks putting up the bucks (millions of them) so they get to set the schedule. This blog offers a window to the MAX development process, and right now that process involves endurance testing (and the how-I-logged-6000-miles-in-ten-weeks story is a bit of a yawner). I hope you'll find other steps in the process more interesting, but if all you want to see is the finished product, all you have to do is wait. From now through MAX Update No. 75 or so we'll probably keep improving, testing, demonstrating, and sharing what we're doing, and I'm sure that's not everybody's cup of tea. Meanwhile, anytime somebody says "good enough for me" they can jump in and build a MAX of their own. We'll endeavor to make further improvements retrofitable so nobody is punished for starting too early.

Jack McCornack
12/18/2008 7:53:47 PM
Hah! Looks like there's a limit to how long I can drone on in Comments. Anyhow, 100 mpg is a serious challenge, but it's not yet apparent we won't make the whistle. As far as helping other people working in the field, I do plenty of that when I'm not wearing my writer's hat, and MEN isn't the place for me to be advising other car designers/manufacturers. As far as admitting my mistakes...man, that would fill volumes, there's barely enough space for the stuff I get right. :-)

Jack McCornack
12/18/2008 7:50:53 PM
Hi William, sorry for the delayed reply, I've been driving my impractical car home through 500 miles of snowstorm--very odd weather for the west coast--but I'm thawed out enough to hit the keyboard. I agree (with reservations) with some of what you say, but not all of it. There are guys who tell me I should be working on something that will tow their Bayliner, and guys that tell me I should be working on something that will carry their kid's T-ball team, and the last thing I'm likely to do is work on improving an Insight. I'm confident that Honda is working on that, and they can do a better job on the Insight than I can. The money? We're shooting for a 100 mpg car you can build for ten grand. If very few people had ten grand for a car, then the automotive industry would fall flat on its face...oh, right, the bailout...it looks like you've got a good point there. The skill? I don't know how many Locosts have been built worldwide, but I'd guess thousands; really, it's not that hard, and to date the only thing different about MAX is the powerplant. A hundred or so have been built as high school shop projects so your "academic exercise" comment is spot on! But the "no value" comment cuts deep. Are MEN articles only of value if the average reader does what the article says can be done? Does the "average Mother Earth News reader" make cheese, or build a solar collector for the south wall of their home? MEN provides options and inspirations, and while the average reader may only do one or two things found in an average issue, it's hard to predict who will do what. Also, though I suspect more MEN readers will make cheese than make themselves a MAX, having the option may make them think again before dropping forty large on a 14 city/20 highway ponycar. As far as dragging it out is concerned...to quote the Dread Pirate Roberts, This isn't as easy as it looks. 100 mpg is a serious chal

david lively
12/18/2008 6:17:47 PM
william and mark, do you even know what the xprize is?

Mark_4
12/12/2008 12:29:23 PM
I agree with William in part. This car is interesting as all get out. The articles and writings on it are pure fluff with no intellectual input or help. Nothing mechanical is shown. Poof, it's a miracle car! except for the fact that we can't have one or build one like it or know much about it! I can only surmise that no real info will be shared so that the Information can be SOLD at some future point in time. Get on with it already!

william_1
12/12/2008 11:38:36 AM
The Max project is not practicle. You should be working with improving the MPG of a production car like an old Honda Insight. That would be something that readers could copy to improve their cars gas millage. Very few people have the money or skill to build a Max even if you do get to 100 mpg or better. This makes your project an acedemic exercise of no value to the average Mother Earth News reader. You are dragging this thing out over a long time because it has probably become apparent to you that you will not be able to get to 100mpg. Doing this is not helpful to other people working in the field. Admit your mistakes and make plans to remedy them.

ScottInIowa
12/3/2008 6:56:05 PM
Love the Trail Tech Vapor speedo - have one on on my motorcyle and like it so much I'm getting one for my Locost. Really like the diesel engine you're using too - I'm on the lookout for one for my Locost.

Jack McCornack
12/2/2008 12:19:43 PM
Hi John, good guess and that's one way to do it. I'm coasting down a hill in this case, but you have the basic principle figured out there. Speaking of figuring things out, future MAX Updates may get a bit heavy into math, because the way to account for rolling resistance (sans wind tunnel) is to measure coasting rates of decelleration at different speed ranges, and since wind resistance declines at a different rate (a square function) than rolling resistance (a linear function) one can calculate which is which from a group of samples. For now, I'll take the college professors' escape route: "I leave that calculation as an exercise for the student..." :-)

Jack McCornack
12/2/2008 12:00:28 PM
Thanks, wannaBgreen. Regarding the part after the "but..." you make a good point. It's the same point as the folks who don't like MAX because it won't tow their speedboat or carry plywood to their work site. In MAX's defense, MAX is not designed to replace every vehicle on the road. MAX will meet most driving needs of most people. but even I wouldn't want MAX to be my only wheels. I too have a minivan, and I drive it when I need it. It gets about 22 mpg. Since MAX meets the great majority of my driving needs (solo personal transportation, or going somewhere with one other person, or a solo trip to fetch stuff like groceries, mail, paint...stuff that'll fit in the passenger compartment or on the luggage rack) and I only use the minivan when I have a group of people or a load of lumber to transport, my all-my-trips-added-together mileage is in the 60s.

jmsiowa
12/2/2008 9:32:35 AM
So, Jack, you cut the engine and put it into neutral on a flat part of the road and measure how long it takes you to stop? But that would also include rolling resistance from the tires. And the wind resistance would decline as your speed declines. John

WannaBgreen
12/1/2008 11:18:23 PM
This car is nice, but it won't hold two kids (in their carseats - required by law) and groceries, plus two of their friends (in their carseats). I need a minivan that gets at least 40mpg. It needs to be safe for kids during a crash. I don't care how fast it gets from 0 to 65mph as long as it gets there eventually. No extra gizmos needed.

sherry_2
11/26/2008 6:38:39 PM
We seriously need to get on with the business of becoming energy independent. While we are doing the happy dance around the pumps with the lower prices OPEC is planning yet more production cuts and will not quit until they achieve their desired price per barrel. The record high prices this past year have done serious damage to our economy and society. WE must move forward with energy independence. We have the knowledge, we have the technology, what America lacks is a plan. Jeff Wilson has a new book out that is beyond awesome. The Manhattan Project of 2009 Energy Independence NOW. He walks you through every aspect of oil, what it is used for besides gas, our depletion of it. The worlds increased need ie 3rd world countries becoming more modernized and consuming more. He explains EVERY alternative energy source and what role they can play to replace oil. His research is backed up with hard data and even includes a time frame and proposed legislative agendas to wean America off oil. www.themanhattanprojectof2009.com







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