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.

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