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

Browse previous MAX Updates.

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

The new MAX body is getting a new nose. Cardboard and computers are working hand in hand on this aspect of the design. Don’t worry, the finished nose won't look like this, we’re just seeing how small an air inlet we can get away with.

For drag reduction, the less air that goes through the car, the better. Properly guided, air that goes around, over and even below the car can have relatively low air resistance, but air that goes through will always be a big drag, to coin a phrase. If you assume that all the air that goes through a radiator is converted to drag, you won't be far wrong. If you go back to Update No. 16, you can see that MAX’s radiator opening is substantial — a full square foot of MAX’s frontal area. Is that enough area to be worth messing with?

Dynamic pressure (shown by the letter ‘q’ in the aerodynamics biz, for reasons shrouded in mystery) is the pressure of air in motion. For horseback calculation of car performance you can use q = 1 pound at 20 mph. Dynamic pressure (q) increases at the square of airspeed (if you double your speed, you hit twice as many air molecules and you hit them each twice as hard, etc.) so q at 40 mph (20 mph x 2) is 4 pounds per square foot (2 squared). At 60 mph (20 mph x 3) q is 9 pounds per square foot (3 squared). I’ll spare you the math, but it takes 1.5 horsepower to exert 9 pounds of force at 60 mph. If we could reduce the size of the radiator opening to one-third of a square foot (as shown in the photo), MAX’s “cooling drag” would only be 3 pounds, would only take one-half horsepower to overcome, and would save us one full horsepower at 60 miles an hour.

Testing will show how little air MAX needs for cooling, but my guess is not very much. While Kinetic Vehicles encourages even our high performance customers to build their cars with four cylinder engines, we have a few who have put Chevy V-8s in cars that look just like MAX — same nose and everything — and they cool just fine. Still, one test is worth a thousand guesses, all it takes is some cardboard zip-tied to the grill and a keen eye on the temperature gauge, and when we’re done we can recycle the cardboard!

Photo by Jack McCornack

Browse previous MAX Updates.

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

Don't pay the ransom, I escaped! Wow, it's been a busy month for me, and I apologize for the lack of updates recently.

The last weekend in May was MAX's farewell public showing in its Escape from Berkeley trim, at a fun, wonderful event called the Maker Faire.

We were there to show the flag and drum up business for Escape from Berkeley II. I figured if the Escape folks were willing to give us five grand for winning the event last year, the least we could do is encourage others to join in the fun this year. So we pulled off the streamlined body bits and put back the stylish-but-slow fenders and lights and all, and drove down to San Mateo, Calif., for one last hurrah.

Now we're back, and we have to knuckle down on the streamlining. In order to reach 100 mpg, we’ll have to get MAX down to the drag coefficient of a typical modern sports car.

The drag coefficient (aka “coefficient of drag”, abbreviated Cd) is a comparison of the drag of an object versus a flat plate of the same frontal area. A Mazda Miata, for example, has a Cd of 0.38.

MAX gets better mileage than a Miata because (among other things) MAX is so small it doesn't have a lot of frontal area. But MAX’s Cd is about 0.7, which is pretty terrible. That’s about the same drag coefficient as a shoe box.

Obviously, we have to make a lot of improvements, and one feature we can improve is a curved windshield. Hey, if you were making a windshield for a shoe box, it would look a lot like MAX's windshield does now — a flat panel right across the front of the cockpit.

But unfortunately a curved windshield will blow our $10,000 budget, because there's nothing off the shelf that will fit MAX (the windshield is only 33 inches wide — more than a foot narrower than the Miata windshield, for example) and custom-curved windshields cost a bundle.

So how about a split windshield, with a steep rake and a deep V to emulate a curved windshield? It'll be pretty cheap, and it would add to MAX's old-timey personality. I think I like it ... though I wasn't willing to commit the glass cutter before I saw how it looked.

Cardboard is a nice medium for conceptualizing design features, but it has its limitations. It should come with a sticker that reads, Warning: Remove Cardboard Before Operating This Vehicle. Nowhere would that be more important than the windshield …

Photo by Jack McCornack

Browse previous MAX Updates.

Read the introductory MAX article, Here Comes the 100-mpg Car.

Visit the Kinetic Vehicles website for more technical details on MAX.

I've been on the road for a couple weeks, laptop by my side, weighing the suggestions about pumping up MAX's performance (see Update No. 26 and the numerous comments attached to it).

One that especially sparked my interest was the idea of relocating the turbocharger to the back of the car — the commenter explained the idea in detail and it makes sense. The comment, from Glen2Gs, ended with this:

“… The "Still Born" Top Gear USA television program took a VW Rabbit Diesel (AKA The Sipster) and were able to "tweek" it to 84mpg with a 0-60 time of 7 SECONDS!

http://www.topgear.com/us/features/more/project-sipster-sipster-indeed/  

May be time to send the Kubota ... packing.”

For those unfamiliar with the Sipster project, they took an '81 Volkswagen Rabbit and put in a power plant from an '03 Volkswagen Jetta TDI (diesel). The word “put” is a bit of an oversimplification and waaay out of my skill set, but that's the basic idea. They backed up the engine swap with aerodynamic modifications and got the results quoted above. It's a good story and worth the read, and it got new folks interested in the subject. All in all, I think it's pretty cool.

But before I toss the Kubota and trade MAX in for a Rabbit, we're not really talking apples and apples here. For example, they used different cardboard-and-duct-tape aerodynamic modifications for the performance runs and the economy runs. But the X Prize Foundation isn't going to let us change MAX’s body depending on which task we're facing, so that's apples and oranges at best.

Maybe not even apples and oranges, since they were “hypermiling” — drafting, coasting, and generally making life rough for the surrounding traffic— for their economy run. So we're comparing driving techniques as much as we're comparing cars, so maybe it's apples and donuts.

But still, 84 mpg is pretty spectacular. But then I read how they measured the mileage and said, “Whoa, that's how the hucksters did it back when they were selling high mileage refrigerator magnets to tie on your fuel line.”

In brief, one morning they filled the tank all the way up to the gas cap (diesel cap?), drove 70 miles, and filled it to the gas cap again. The pump stopped at 0.833 gallons, less than a gallon by a pint and a third. Pretty simple, so what could be wrong with that?

What's wrong is that as the day heats up, the fuel heats up expands, filling the tank from the inside. A factor in this case is the TDI system re-circulates fuel through the fuel pump (which heats the fuel), into the engine compartment (which heats it further), and back to the tank. Diesel expands at roughly 0.05 percent per degree Farenheit and...

Well, I haven't a clue what the difference was between starting and ending fuel temperatures in Top Gear's mileage test. But neither do they, so I'll guess a number that makes the math easy: 25 degrees. That would give an expansion factor of 1.25 percent; times 10 gallons (the capacity of the Rabbit tank) is 12.5 percent of a gallon, or one pint. That’s not a factor to ignore when the line between success and failure is a pint and a third (as was in their case, where they were shooting for 70 mpg). Now we're comparing apples and honey bears.

By the way, this fuel expansion thing is not just theoretical. When Sharon Wescott and I won Escape from Berkeley (see Update No. 14), supporters had brought us extra fuel at the finish line. So we filled our under-the-hood tank to the top before we headed out of town. The fuel was veggie oil — I don't know its expansion rate, but it was greater than our fuel consumption rate. In Vegas traffic, we couldn't burn it as fast as it expanded — 15 minutes down the road we stopped to see if we'd sprung a leak, but no, oil was overflowing from the top of the tank. Imagine what fabulous gas mileage we could claim if we used the fill-drive-refill mileage measurement technique.

Browse previous MAX Updates.

Read the introductory MAX article, Here Comes the 100-mpg Car.

Visit the Kinetic Vehicles website for more technical details on MAX.

OK, fun's over, time to get back to work.

I am getting so spoiled. When MAX's gas mileage drops into the 50s, I get grumpy.

I wanted to put 5,000 miles on MAX before the weather got wet (next project — a convertible top), a somewhat arbitrary figure, but it's been my experience that 5,000 miles (or 100 hours) is enough to say, “Yeah, it looks like this is going to work.” So for me it's a benchmark. I made it, but it involved a lot of road trips, and those involved a lot of freeway flying.

The freeways are not my favorite environment — one reason is they tempt me to go too fast for fuel efficiency. At 70 mph, MAX is no more fuel efficient than a ... well, actually, there aren't any production cars that get better than 50 mpg at 70 mph.

MAX does (barely), so I guess I shouldn't kick myself too hard. But heck, MAX is supposed to be spectacular, not just good. With its upcoming streamlined body (currently in that gray area between the drawing board and the highway), it's going to be spectacular. But gosh, can't it be kinda spectacular and still have the look-and-feel of a classic sports car?

I sure hope so, because — I know, I know, I should be ashamed of myself — I'm greatly enjoying driving MAX around in its Vintage/Prisoner/Escape from Berkeley regalia. Everybody digs “Classic MAX”, and I love all the attention. Total strangers say, “Ooooh, that car is so cute, can I take a picture, can I get in it, can you take a picture with me in it?” and I know I'm going to lose some of that attention when MAX goes all streamlined and serious.

Sadly, the lovely, swoopy front fenders are part of MAX's visual charm, I say “sadly” because recent fuel consumption figures indicate that those fenders have increased MAX's drag at cruise by about 10 percent.

As you can see, those fenders make up a lot of MAX's frontal area, and that has to translate into lots of drag. I don't know how much drag, but there are ways of finding out. Tune in next week, and I'll show you how to do wind tunnel testing when you don't have a wind tunnel.

Photo by Jack McCornack