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MAX Update No. 82: The Dark Underbelly

9/23/2011 8:15:45 AM

Tags: MAX, 100 mpg, belly pan, aerodynamics, streamlining, Jack McCornack

A few updates ago, a commenter asked if MAX has a full belly pan, and I’m pleased to say, yeah, pretty much.

A smooth and airtight surface under the car is nearly as important as a smooth and airtight surface on top of the car, but since we don’t often see what cars look like underneath (and since gas was cheap for a long long time) Detroit was pretty slow to streamline cars’ undersides. Some of the German cars have had flat bellies for a while…if I may quote a translation of a Paul Jaray patent application submitted to the Berlin office:

“The lower part of the body has the form of a half streamline body and covers the chassis with the wheels, the engine compartment and the passenger compartment. The lower surface is even and runs parallel to the floor space…”

You’ve never heard of Paul Jaray? That’s funny, maybe it’s because you’re too young. He was a well known aerodynamicist in both aviation and automobiles at one time, but that was almost a hundred years ago. That patent application I quoted is dated 1921, and before then he was designing dirigibles — he’s the guy that made dirigibles dirigible-shaped instead of cigar-shaped. He was one sharp cookie, he was right in 1921 and he’s still right: for reduced drag, the lower surface of a car should be even and parallel to the floor.

So back to MAX. MAX’s cockpit had a full belly pan right from the get-go, because there’s nothing else under the driver and nowhere else to mount the seats, so it was either give MAX a cockpit belly pan or have me stand on the ground like Fred Flintstone. That portion of the belly pan is a welded-in sheet of 16 gauge steel. Folks building MAX-like cars for racing purposes often use lighter aluminum sheet for the floor of the cockpit, but on the street, there are too many opportunities for Bad Things to Happen From Below, like debris on the highway or running off the road and onto a rock. The lower surface of the car is, of course, parallel to the floor because the top side of that sheet is the floor and the bottom side is the lower surface.

However, the belly pan under the engine is a recent addition. Here’s a photo of the current engine belly pan — it’s that black ABS sheet draped over the hood.


Shortly after I took that picture, I put MAX up on a ramp and riveted the ABS sheet to the frame surrounding the engine, which gave me a nice smooth transition from the air duct under the radiator to the belly pan under the cockpit. I didn’t bother taking pictures of it in place because there isn’t much room for a camera under MAX and it’s dark down there, but trust me, that’s where I put it. Aerodynamically, this belly pan covers a hole under the engine that’s nearly the size of the hole in the top of the cockpit.

ABS sheet has a lot going for it in this application — it’s light, it’s cheap, and though it’s not very attractive, who’s going to look? However, it may not be robust enough for long term use in normal driving, but time will tell. I know it’s not robust enough for abusive driving, because I had one just like this one on MAX a year ago (see MAX Update No. 59: Lies My GPS Told Me) and though I got back to civilization alive, and though the steel belly pan under the cockpit survived with a few dents and scrapes, my original ABS belly pan for the engine compartment is still somewhere deep in the Oregon Outback.

Browse previous MAX Updates.

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

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