The Diffuser Belly Pan Project - Funky Fresh Flow

More and more, cars are being designed with aerodynamics under the car being considered as an opportunity for efficiency, not just the air flowing over the top of the vehicle. In the 1970s, the underside of a car was full of voids, gaps, and lumpy mechanical stuff. But now, even ordinary commuter cars have, at minimum, a large plastic inspection panel or belly pan of some kind under the engine between the front wheels.

Apart from reducing aerodynamic drag, these panels often are designed to help direct air into the radiator or intercooler, etc.

The underside of the BMW Z series of vehicles is pretty well flat under the nose, with the inspection panel in place. Under the middle of the car, there's the underside of the passenger compartment with the exhaust and drive shaft in between. But, behind the rear axle, there are some large spaces, with the rear bumper forming a kind of huge parachute. Now, I do not have access to a wind tunnel, and I have no illusions that I can perform CFD analysis in my head (Computational Fluid Dynamics). But still, a large gap with a large concave bumper cover right behind it has got to be an opportunity for air to swirl around and get all sub-optimal with its bad self. Surely, flattening the floor there can't be a bad thing, right? I've never read any forum posts explaining "I flattened the floor of my car and it ruined the handling. Don't do it."

Note the leading edge if the bumper cover, which curves forward, into the wind. The box between the mufflers is where the battery lives.

So, in June of 2024, I embarked on a mission of experimentation, with the potential for glory and radness. I reckoned I could shape some aluminum into a three-section belly pan, directing air out the back of the car, instead of into the bumper.

Disclaimer: Yes, anyone who's ever messed with CFD software will tell you that the dynamics of moving air can be counterintuitive, and can do really weird stuff that you wouldn't have guessed, just from staring at the car and frowning for a while. I did see the Mythbusters episode where the found out that driving around in your pickup truck with the tailgate down is actually worse than keeping it closed. A horizontal vortex forms there and loops around and around, while other air sails over the top of it, surprisingly improving aerodynamic drag.

But still I wanted to try making a flat belly pan. One in the middle, reaching from the differential (the box of gears between the rear wheels) all the way just past the edge of the bumper, and smaller ones just outside the mufflers and behind the wheels

Directive: No new holes. Find existing hard points to fasten the belly pans without making any new holes in the body of this already-rare car, for which parts are increasingly difficult to replace. That way, if it turned out to be a terrible idea, I would remove it and throw it into the recycle bin, with the car none the worse for wear.

The parts, modeled in CAD (Cardboard Aided Design).

There is another type of thing that's getting to be increasingly common cars: Rear diffusers. These are complex, trumpety-looking aerodynamic forms usually visible at the bottom of the rear bumper, ostensibly meant to keep air from forming drag-producing vortices as it exits from under the car. They have neat-looking strakes and fins on them, and, it must be said, are, in the vast majority of cases, very fake and non-functional. They first appeared on racing cars (as things tend to do), but now they're very much in fashion on any car that even pretends to have sporting ambitions.

My belly pan thingy would be made (mostly) from 1/16" thick aluminum, and would definitely need some stiffening fins on it. It just can't be helped if they happen to make it look like a diffuser.

The center section is about 22" deep, and reaches as close to the rear differential as I dared. So, at the very least, it's not just a decoration on the rear bumper that pretends to be a diffuser. I'm looking at YOU, nearly every car I see on the road these days.

The corner sections have a little 2" tall vertical part where they meet the inside of the rear wheel wells, in another attempt to keep air from getting directed into the bodywork... hopefully

The 2" fold at the leading edge of the pan hopefully directs air down and back, instead of up and into the convolutions of the mechanical bits down there. The fold looks like a 90 degree bend here, but it's really closer to a 45. Also, the fold helps to keep the thing from getting floppy. You'd be surprised how rigid a thin panel can be, with the addition of a few folds.

Was it necessary for the center section to curl up and follow the shape of the bumper? Nope, but it definitely looks more like it works with the shape of the car, and looks less tacked-on.

That was summer. Fast-forward a few months to drive around, do a bunch of autocross events, and go on a long trip or two, to see if it completely ruined the car, test it out, fix any annoying rattles, and make any necessary tweaks.

Verdict: I can't tell it's there. It doesn't make the car into a rocket ship. It doesn't seem to effect the handling in any way that the butt-dyno can detect. Does it effect gas mileage? Dunno. Does it look cool as hell? One hundred million percent fuck yeah.

Then came December, and I put the car into winter storage. I removed the contraption, took it all apart, and dropped the whole thing off (well, most of it) at a local anodizing shop. They said they could do orange - or at least something like it - as a custom color, if I wasn't in a hurry to get it back. I was not in a hurry. That's the benefit of winter car projects. You can be all relaxed and cool when asking shops to do stuff for you. You don't want people to be annoyed with you when doing a job for you that's so small, it's barely worth their time to bother with it.

The parts were finished this week. I put it back together, and it's now back on the car and looking not at all like some knucklehead bashed it together in his garage, which is exactly what happened.


Behold the majesty.

Orange? Well, kinda, I guess. This shop mostly does industrial application anodizing, and doesn't specialize in decorative finishing. They can do green, gold, blue, and red. Orange was a special mix, and they don't pretend to be artists with color-mixing expertise. In any case, this surface should be nearly as hard as diamond, and more resistant to dings and scratches than powdercoat or paint. Plus, I won't have to be constantly buffing the thing when it gets dull looking.

What's anodizing? It's an electrochemical process involving acid, dye, and electricity that opens the pores of the aluminum, bathes the metal in a dye solution, and then uses an electrical charge to seal the surface with an oxide layer that's 5 mils thick and actually goes, not just onto the metal, but into it. I have in the past buffed anodizing off of an aluminum part with a woven nylon wheel, and it really, really puts up a fight. An anodized surface is really hard.