There’s a nerdy site and forum called Ecomodder. Most of the site is focused on fuel economy, but some of it can be applied more appropriately to racing. In this post I’ll see how we can apply Ecomodder’s aerodynamic template tool to a Miata.
The aerodynamic template refers to the shape of a car that has the lowest possible drag. It’s basically a tear-drop shape on wheels and looks like this.
The Ecomodder site has an online tool that allows you to superimpose the aerodynamic template over a picture of your car. I’ll do this with a NB Miata using a simple line drawing I found on Google images.
I’ll superimpose the aerodynamic template over the car, resizing one or the other so that they fit. The zero mark on the template should line up with the highest part of the roof, which I’m putting in the middle of the door handle.
While this is the ideal shape for lowest drag, it doesn’t mean that air will become detached if you don’t follow the shape! Sub-ideal shapes will result in a thicker boundary layer, or maybe even some turbulence, but lots of shapes will work just fine without separation.
Likewise, it isn’t necessary to extend the tail to a point. Wunibald Kamm found that cutting off the tail resulted in minimal increase in drag, because the wake region converges to mimic the effect of a fully tapered tail. The so-called Kammback shape is quite efficient at 50% of the cross sectional area, which in this case is the second vertical line past the rear bumper in the picture above. That’s way the heck back there!
A more realistic stopping point is at the end of the trunk, which has the roof at an angle of about 14 degrees. At that length, the roof is still quite flat, and has a shape like a Honda CR-X, CR-Z, or a Hyundai Veloster. And like a lot of SUVs, damnit.
Most cars these days have rooflines that slope gradually, just like the aerodynamic template, but if you look at older cars, the roofs are often flat, and the rear window comes back at a fixed angle. The backlight angle refers to the slop of the rear canopy, which is often the rear window. The ideal angle is around 15 degrees. It’s not intuitive, but the worst angle is 25-35 degrees. Guess what a Miata’s backlight angle is?
If you extended the rear window of a Miata to the trunk lid, this would make about a 20-degree angle. This isn’t ideal, but still a lot better than an OEM hard top. Adding a spoiler would be an easy way effectively achieve a better backlight angle.
Boat tailing the sides
So far I’ve only been considering the two-dimensional side view, but the aerodynamic template is a 3D shape that tapers on the sides as well. The side taper is referred to as “boat tailing”.
You can see that the Miata rear end fits pretty closely to the aero template. Extending the length of the rear quarter panels to the full length of the car (even with the rear bumper) would reduce drag. Although it wouldn’t be necessary to go any further than that. (Note that the end of the bumper is right at the 50% line.)
Streamlining the canopy
The Miata canopy is wide and short, and to reduce drag you need to make it longer. In other words, a fastback. If you’re thinking about building one, bring the sides of the fastback all the way to the end of the trunk lid and you’re at about the right angle. You’d have to figure out a way to relocate the fuel filler, as the fastback goes right over the top of it. But you would save some weight by getting rid of the trunk lid.
My fastback is considerably narrower in the hips, and this allows a more efficient rear taper. This wouldn’t work well on a street car (it creates big gaps behind the windows where water would get in), but a fastback built like this would have less drag, and uses the existing gas cap location.
The final piece of the puzzle is the air going underneath the car. You’ll see the aero template shows a 10-degree diffuser. I’m not sure why this is a fixed angle, I’d imagine a continuous curve would be better.
I had to shrink the aero template to fit the diffuser under the rear bumper, and so this is no longer the ideal shape for the roofline, as it’s too short and the angles are thus too steep.
Ecomod all the things
I also race 24 Hours of Lemons with The Awkward Corner, in a Honda Odyssey minivan. It’s had a few different themes, but if it were my car, the next theme would be a gigantic Honda CRX, following the lines of the aero template.
I race my brother Ian’s Yaris from time to time. I drew a version of his car with a really long tail that was inspired by the aero template and Porsche 930.
Ian also used to track a BMW E30, which is now with WinSome Racing. I’ve cropped the template to show what it would look like as a shooting brake.