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MAX Update No. 91: The Light Underbelly

4/30/2012 11:46:00 PM

Tags: MAX, 100 mpg, fuel economy, streamlining, belly, bellypan, diffuser, Jack McCornack

MAX has a smooth belly pan under the engine compartment (it’s the black ABS sheet shown in MAX Update No. 82: The Dark Underbelly) and under the passenger compartment (the cockpit floor is a sheet of 16 gauge steel) and last year I completed the belly-smoothing operation by adding a sheet of quarter inch plywood between the rear of the cockpit and the rear of the body. The modern term for this tapering-upward-in-the-back-bellypan is "Diffuser".* The wind tunnel folks have determined that minimum drag comes with a diffuser angle of six degrees from horizontal, so that's what I used with MAX.

Here’s a video of the air under the rear third of the car, shot with the remarkable** GoPro HERO2 sport video camera, and edited on my Mac laptop with iMovie. I’m starting to have some fun with this setup.

 

It makes obvious sense that making a car smooth on the bottom will reduce drag***. Half a century ago, most cars in America looked pretty terrible from the bottom, with all kinds of things hanging in the breeze and no money wasted covering them up. New cars are much smoother underneath than old cars, and I think there are three main reasons for that improvement:

A) Cars are a lot more sophisticated and a lot more expensive nowadays, but a smooth belly doesn’t cost much more than it ever did, so percentage-wise, a smooth belly doesn’t increase the cost of a car as much as it used to.

B) Cars last longer than they used to (I expect 300,000 miles for a new car today, I expected 100,000 miles circa 1960) so the cost per mile for a smooth belly is greatly reduced.

C) Fuel is now the biggest expense of owning/operating a car, so people today will pay a little more for a car that burns a little less fuel. They may never see how pretty their car is from the bottom (I painted my plywood panel white, but that was 7000 miles and a trip to Pennsylvania ago, so it’s looking a bit smudged under there lately) but they’ll see the mileage on the window sticker and that’s enough to sway most of us.

Here’s some cocktail napkin math for you: a 1960 economy car that cost $2000 and got 25 mpg on thirty cent gasoline for 100,000 miles cost $1200 for fuel in its lifetime; the fuel cost 3/5 what the car cost. A 2012 economy car that costs $20,000 and gets 25 mpg on $5 gasoline for 300,000 miles will cost (gulp) $60,000 for fuel in its lifetime; the fuel will cost 3 times what the car costs. If belly smoothing increases the cost of a new car by $200 but improves mileage by 2%—1/2 of a mile per gallon—it will save a lifetime $1200 at the pump.

As you can see in the video, MAX hasn’t achieved a baby-butt level of smoothness yet. There’s still a good deal of flutter in those tufts of yarn. However, additional streamlining improvements may not be worth the time and money. MAX is getting its 100 mpg just the way it is. Now it’s time to make MAX more versatile, so we can drive it 12 months a year. I'll be developing a fully enclosed cockpit before I do any more streamlining work.

* [dih-fyoo-zer], noun, a device for utilizing part of the kinetic energy of a fluid passing through a machine by gradually increasing the cross-sectional area of the channel or chamber through which it flows so as to decrease its speed and increase its pressure.

** Small, lightweight, inexpensive, robust, and cranks out excellent videos for internet use.

*** I’ll acknowledge there are shapes which are improved by unsmoothing them—dimples on golf balls reduces drag, as does fuzz on tennis balls—but the reason doesn’t particularly relate to car body design. Yes, if you want to make a spherical car, dimples will help, but dimples aren’t good streamlining; dimples are a band-aid on bad streamlining.


Video by Jack McCornack 

Browse previous MAX Updates. 

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



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