DIY Miata Fastback

When I built my first fastback, the design was pretty organic, mostly a game of seeing what would fit with the Treasure Coast chop top. At the time it seemed a simpler way to define the roof shape, but it’s probably easier and cheaper to start from scratch.

Treasure Coast chop top.

I started by laying a piece of wood between the wing uprights and playing with the angle. In the pic below you can see I’m using a pair of vice grips clamped to the wing uprights. I moved them up and down until it looked right. I settled on around 15 degrees. The ideal angle is closer to 12 degrees, but that would create problems seeing out the rear window.

I cut silts in the wood to to fit the shape of the chop top. This was just to copy the shape of the roofline, I’d fill the sides later.

Then I built a wooden “transom” to hold the back end up. I also wanted to taper the sides in at the same 15-degree angle.

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Playing with angles and shapes.

I added a strip of angle aluminum along the body, which acts as the base of the fastback. These rails meant I could no longer use the stock trunk, which became a slight problem when I did the testing at Watkins Glen. Not a functional problem, but I wanted to test OEM bodywork, and that was no longer possible with this modification.

Mocking up the side shape with skateboard laminates.

The sides are made from skateboard laminates. They are thin and bend easily, and I had some on hand. Once I was satisfied with the basic shape, I glassed the outside seam to hold it together.

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Fiberglass on the outside seam to hold it all together.

Then I pulled it off and glassed the inside as well. You can see the graphics on the skateboard laminates, which are printed already.

Fiberglassing the seams inside.

I put in three Lexan windows. I filled the bodywork gaps with red race plastic, which was a mistake, as they expand and contract with heat. I’d eventually replace the red parts with sheet aluminum.

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The tapered sides may be a large source of the drag reduction, first because of the shape, and second, because the sides of the OEM hard top scoop air into the cabin, while the fastback does not. You can see how much narrower the fastback is in the hips, it’s all that red plastic.

To help deflect air getting into the cabin, I added small Lexan air dams where a B pillar would be, to help direct air going past the windows along the bodywork, and added vents at the roofline and at the trailing edges of the windows (these are not shown here).

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Finished, more or less.

How did it work? Better. In the race, the engine was running badly, to the tune of at least 5 seconds off pace, and yet I repeatedly passed a BMW e30 on the back straight at WGI. He would pull car lengths on me accelerating out of every corner, and I’d reel him in and pass him on the back straight. A year later I’d do aero testing at Watkins Glen and find out that the fastback is even better than I had imagined.

I recently saw a picture of the VW EV and from the back, the top looks somewhat similar to my fastback. I like the box cavity. Functionally, it should decrease turbulence and wake on the fastback. I may try that that on the next one.

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