You don’t see a lot of Miatas with aftermarket roofs, so I thought I’d write an article and put them all in one place. There’s a lot of performance to be gained by changing the top, and yet very few people bother. They’ll throw thousands of dollars into time-attack aero, and then use the OEM hardtop. Why?
My DIY fastback reduced drag by 20% and increased rear downforce by 130%. Another way of thinking of that is it made my 60” wing into a 78” wing. Although more accurately, the fastback didn’t do anything at all, it’s the inefficiency of the OEM hardtop that’s the problem. Flow separations and turbulence of the hardtop effectively made my 60” wing behave as a 48” wing, and increased total car drag from .41 Cd to .48. Yuck.
I hope to inspire people to build their own hardtops, so this article ends with some tops I’ve built, and the different ways I went about it. But before I get to that, give me a minute to review the primary design considerations, and show some tops from the aftermarket.
To achieve the lowest drag, the canopy should be a continuous curve, gradually getting steeper over its length. The Ecomodder website has a neat tool called the Aerodynamic Template, which allows you to superimpose this shape over your car. I did that in a previous article, let’s see what that looks like.
A roof this long would be impractical, and you just don’t need to. Tapering to a point is not necessary, as there are diminishing returns after about a 1:1 ratio. Meaning if the roof is 44″ wide, the fastback can be 44″ long. This is often referred to as a Kamm back, and you’ve seen it on cars like a Honda CRX.
On many older cars, the rear window is just about flat, and it’s easier to think of the slope of a roof as a single fixed angle, rather than a continuous curve. This backlight angle should be around 12 degrees maximum, because air doesn’t like to change direction at more than that. It’s not intuitive, but the worst angle is 30 degrees (plus or minus 5). Guess what the backlight angle of a Miata rear window is?
Side taper is basically the same thing as backlight angle, just from either side. Each side should taper at a maximum of 12 degrees, any more than that and flow separations occur unless you use strakes, guide vanes or other tricks. If you think about a car going around a corner in yaw, you can imagine that flow separations will occur along the inside side of the car, and so an angle of less than 12 degrees is probably desirable.
Knowing that the top should taper to the trunk at no more than 12 degrees, and the sides should taper inward at the same amount, it gives you something to work with. Unfortunately, there are things on a Miata that make this difficult or at least impractical.
- Visibility – Miatas are short cars, and if you follow the teardrop shape, it won’t be easy to see out of the rear window. You may need to compromise with a steeper backlight angle (more drag), or do something like Honda did with the CRX and Insight, and put a small vertical window on the trunk.
- B-pillar – The width of the canopy at the B pillar needs to cover the gap where the convertible top goes. This increases frontal area, exaggerates the parachute effect with open windows, and complicates the side taper (boat tailing) of the canopy.
- Fuel filler The location of the fuel filler is annoying. If you taper the sides at 12 degrees, the sides go directly over the middle of the fuel filler. If the angle is greater than 12 degrees, air won’t stay attached. If you go outside the fuel door, you’ll have to figure out a new fuel filler location.
- Spoiler – Fastback roofs are longer, and so air stays attached longer, creating more lift. You need to balance that out with a spoiler, or if you also do front aero, a wing.
- Trunk – A Miata’s trunk is lower and wider than ideal for a fastback. If you want a functional hinged trunk, it leads to compromises, such as a steeper backlight angle, and a kink in the side taper.
Coupes and fastbacks
I’ll start by reviewing the Miata coupe, because it’s interesting to see how Mazda addressed these design considerations. Then I’ll take a look at a few fastbacks, a couple alternatives to the OEM hard top, and some tops I’ve built.
Mazda Miata Coupe
Mazda only made 179 of these for the domestic market. I won’t delve too far into this, because Motor Trend did a good write up. But I want to point out a few key details.
First, notice they raised the height of the trunk, it’s taller both at the front and the rear. Raising the trunk allowed them to achieve a better backlight angle. (The BMW E30 M3 did something similar, but much less gracefully.) As a practical matter, a taller trunk also means more trunk space.
From behind you can see that the tapering of the canopy doesn’t run the full length of the car, because the width of the trunk opening. This is a difficult area to design around, but they did a nice job fairing this into the fuel door and trunk.
While this isn’t a fastback, and the top is both wider and shorter than ideal, I really like this coupe. If Mazda made these for our market, I’d own one.
These seem to be the most popular fastback, and for good reason – they look great, are readily available, and function better than an OEM hardtop.
If I’m going to nitpick it, I feel like it’s too wide at the B pillar, but that’s always going to be the case because of how wide the convertible top is there. The side taper looks a bit steep, but that’s necessary to duck inside of the fuel filler. The side taper isn’t carried in a straight line, because it has to flare out for the trunk opening.
The backlight angle is too steep as well, but that’s because the trunk is too low. A higher rear deck (like the Mazda Miata coupe) would have been better.
All of these details amount to something that looks more kit-car than OEM. But this is about as well as a fastback can be executed on a street car, given the design elements. Well done CCP.
Renderos Racing Longtail
The longtail looks like a longer CCP. It’s not exactly the same, as I see some differences in the B pillar and the rear window, but the general shape and the way it dodges around the fuel cap look similar. I think I understand what they are trying to do, and I like the execution, but I would do it differently.
When using such a long rear extension, your wing overlap won’t be ideal unless you move the wing rearward, and that may create front balance problems. As is, you’ll probably lose some downforce in the shape of the rear wake, which you could get back by increasing wing angle, but that increases drag, which seems the exact opposite of what this is trying to achieve.
I mean, it’s a damn cool fastback, but I think you could achieve the same drag reduction using a short box cavity. That would not only integrate the sides better, but put wing overlap in the quarter-chord range. That would help extract air from underneath the car, which is especially important for a diffuser.
Hardcore Design Fastback
Panos of Hardcore Design is making new fastbacks in Greece. I’ve seen a couple of his designs, the first looks a bit like the CCP, but the trunk hinge is carried all the way to the roof. I like it better, it’s not only a cleaner look, but better for trunk access. You might also notice that it’s a targa top. So cool!
There’s a very aggressive side taper at the quarter windows, and I suspect there’s some turbulence here, right around the location of the fuel cap. Not much you can do about that without relocating the fuel neck. But the rest looks awesome, and I especially like the shape and size of the spoiler.
Panos was making these to order, and there was a group-buy at one point. Good luck to him, and I hope he can keep the customers that were invariably burned by global shipping during the pandemic.
The Lightyear fastback is nice looking, with a large trunk opening that breaks cleanly at the rear window. The side taper looks good, and the backlight angle is a bit too steep because they are trying to get the height of the trunk lid the same as stock. It looks very sleek, but needs a spoiler.
If you have a 3D printer or know someone who can print stuff for you, you can make a 3D printed fastback. The files will cost you $100, and you’ll need various other parts like fiberglass, epoxy, window seals, Lexan, and the know-how to put all that together. All told you’ll be $400 into it, but it’s the labor that is the killer.
I’d guess there’s at least 100 hours here, with lots of head scratching the small details. Because you’re glassing the topside and not working off a plug, there will be a lot of fairing. As I often say when I’m doing bodywork, “I’m 90% done with the sanding; I’m half way there.”
The top itself looks like a Lightyear, both in the way the trunk line comes to the rear window, and the way the roofline ends before end of the trunk lid.
If you’re considering building one of these, there are lots of details on the Hutchins Racing YouTube channel. If you haven’t worked with fiberglass and epoxy before, you’ll make some mistakes and go through probably twice as much glass and resin as a professional would. Still, this is a worthy project and I may yet build one for a street car.
This is a very rare body kit from Japan, there was one for sale recently in the US, but it might be the only one. This is a full body, not just a roof, and there are trick details all around it.
The roof is quite wide, as is necessary to seal the windows, and they’ve thrown in Opera Coupe style rear quarter windows.
This top shows some of the shortcomings that you need to design around if you want to build your own top. You’ll notice they had to relocate the fuel door, and the trunk does not appear to be functional.
There are a couple hardtop alternatives that are similar to OEM and use the stock trunk. They both have the same shortcomings as a standard hardtop being, short, wide, and with less than ideal angles for drag and flow separation.
The price of used OEM hardtops now regularly exceeds $2000, and so there’s definitely room for some alternatives tops, even if there is no performance advantage.
Smoothline makes a replacement top that’s priced economically. The shape isn’t much different than OEM, but there are two rear window options, and the smaller of the two looks pretty cool. Honestly though, one of the best things about the OEM hardtop is visibility, and the smaller window would be a lot like the convertible window. But they also have a vinyl top option, which is like a convertible top pulled tight. Neat.
Based on the dimensions of the Smoothline top, I’d expect it to function very similar to OEM. There’s nothing to be gained aerodynamically here, but it’s visually interesting.
I like this top. I don’t know what the backlight angle is, but they did their best to reduce it. The top of the rear window is dropped slightly, and they used as much of the rear deck as possible while retaining the stock trunk.
The sides are longer also have a more gradual taper, and I bet there’s less separation on the sides of the canopy. I was inspired to build my own version of this top, which I call the TT, more on that below.
I’m not a fan of vortex generators, because in my testing, they created drag and ruined downforce of the wing. So when I see a picture like the following, I die a little inside. You don’t need vortex generators on a curved surface! However, these vortex generators are fake and do nothing.
But I have to give them a shoutout for putting the rear window vents in the correct spot. I often see window vents at the base of the window, which is a high pressure zone. Placed there, air goes in the cockpit. Placed correctly at the roofline, air is extracted from the cockpit.
Tops I’ve built
I build fiberglass-wood composite boats, and building a hardtop is similar. When I build them, I like to experiment with construction methods, and so they are all a bit different.
Original chop top
I built a chop top on my first Miata. Construction was strips of wood, epoxied together which I covered with vinyl. Look closely and you can see the longitudinal strips of wood underneath the black vinyl.
The rear window was a clear vinyl sheet, grommeted and tied to the rollbar. The top didn’t keep the rain out, but that was OK because I was living in California at the time.
My first fastback design started as a Treasure Coast (CCP) Chop Top, to which I grafted on a rear canopy made from thin skateboard laminates and fiberglass.
The rear hatch was hinged so I could get to the battery, and the entire top weighed (I think) 14 pounds. The main problems were that the large Lexan windows got dinged for too many points in Champcar, and the whole assembly consisted of too many parts. It was rather complex to put on and off.
My second version is built on top of the first, creating one solid structure rather than a pivoting trunk. The battery has been relocated to the engine compartment, so there’s no need to get into the trunk now, anyway. The top uses the front bow from a soft top frame, and bolts down to the Frankenbolts in the rear, and so this top can be put on just about a NA Miata in a couple minutes. (NBs have a slightly longer trunk so I don’t think it’ll fit.)
Fabricating this as one piece meant using more fiberglass and metal than V1, and now it weighs about the same as an OEM hard top. But that includes the trunk, and so it’s about 12 lbs lighter than an OEM top/trunk combination. But light weight wasn’t the concern here, it was to simplify, reduce window size, and decrease drag.
Some of those improvements include rounded B-pillars like NASCAR stock cars, a drip edge above the window for rain and to keep air from curling under, a slightly smaller window opening for less air intrusion, and rivnuts in the trunk so that I can quickly attach spoilers of various heights.
I also removed the quarter window on the driver’s side, as it was useless anyway, and reduced the size on the opposite side. The rear window got 1/3 of it taken out. The window size reductions were done in case I ever race Champcar again, who penalize 3 points per square foot of plastic. As it sits now, the top comes in at 22 points, and half of that is the rear window.
<rant>C’mon Champcar, just make any roofline or rear window modifications 10 points, which is the same as other aero mods. This will speed up tech, and it’s just plain silly how many of your cars have no rear windows because of the points penalty.</rant>
Keen eyes will note that Fastback V2 has a lower trunk lid than V1, which is something I complained about on other people’s fastbacks. This is because V2 is designed to work with underbody aero, an area I’ll be testing at some point
Shooting brake (breadvan)
I like hatchbacks for their utility, and as a hunter, I like shooting brakes for their history. I’ve always wanted to build one and sketched up a plan.
After seeing this picture of a Ferrari shooting brake, I wanted to build one for a Lemons theme. We were going to serve pizza out of it during the race.
The construction method was different than the fastbacks, being more like a strip-built canoe. I took a 2×4 and cut it into narrow strips on my table saw, then laid them down longitudinally to create the shape.
I then tacked it in place, filled the gaps with epoxy, and covered the top with fiberglass fabric. The sides were mocked up in cardboard and transferred to plywood.
I completed the top but never raced it because I got Lyme disease and shit all over myself.
Functionally, a top like this should have less lift than a fastback, but it might have more drag due to the larger rear wake. If you mount a wing, the wing stands need to be tall and/or rearward, as there will be interference with the low pressure zone under the wing. This is conjecture; I need to test this one against other tops.
I still have the top and want to race it at least once before I give it away to a Lemons team that wants to run that theme. Or I’ll turn the top into an enormous duct to feed a wing.
If you don’t know what an opera coupe is, then you’re probably from a later generation than I am. As a kid, I saw these everywhere and they are nostalgic. If you aren’t from my era, you can call them what they are: ugly.
I wanted to build one, not for the look, but to fit some design considerations. The NASA ST/TT rules allow you to change the hardtop on a convertible, but the rules state that the top must end before the trunk begins.
I thought about that and started drawing a top with a roofline that stopped just short of the trunk opening. I swept the sides longer with a gradual taper to the trunk opening. Stepping back I said, shit, this looks like a 70s Supercar. A Pantera, M1, or 512 Boxer had that shape because of a mid engine, but it might work on a Miata, and unlike the Monocraft top, left a functional trunk besides.
I got about 75% finished building it and then realized that some rules-lawyering jerk would say that the sides of the roof extend past the front of the trunk. So this top would be illegal. It would have been cool, tho, kind of like a Lancia Scorpion.
But to be compliant with rules, I cut the sides at the forward trunk opening and found I’d suddenly built an opera coupe.
I have a long write up on this one, and will share that in the future.
This roof is my second attempt to bend to the NASA rules, which state the top must end before the trunk begins. The problem with that stipulation is that the backlight angle is going to be about 25-30 degrees.
I measured the angle from my rollbar to the trunk opening, and it’s about 25 degrees. That’s as much as I can reduce the backlight angle, and I’d like to get the angle close to 20 degrees, as it would have a lot less drag.
To reduce the backlight angle, you have to figure out some way to drop the height of the roof or raise the front edge of the trunk. I did both.
I chose to drop the height of the back window by putting a vent at the roofline. My thinking was twofold: 1) the vent would extract air from the cockpit, and 2) the extracted air would then “fill in” the gap to the rear window.
I also created a small gap at the bottom, which should hold a recirculating air bubble there, making a taller transition to the trunk. (I hadn’t filled in the bottom gap in the following pic, I definitely don’t want a vent here!)
I haven’t finished this one yet, as I’m skeptical that it’s worth it. The top is a bit narrower than an OEM hardtop, and the sides should have less turbulence because they are more gradual. I have no idea if the roofline vent will work, because the cabin air is low velocity (and there’s a rollbar in the way), so it may not fill in that rear window region very well.
But it looks interesting, was fun to build, and is a good conversation piece. For someone who is above the pounds per horsepower limit and can spare the .4 lbs/hp penalty that a custom top brings, it might be worth trying.
If you know of any other aftermarket Miata fastbacks or hardtop alternatives, drop me a comment and tell me about it. If you make your own, I’d love to see it.
I’m hoping to be able to do a lot more testing this year, but hardtops are just one of several things I’m interested in. So if you’re doing a track day or racing at Watkins Glen and want to A/B test one of my tops, contact me, I need the data. I’m about 25 miles away from WGI and can set you up pretty quickly.
One thought on “Miata Fastbacks and Aftermarket Tops”
I’m super happy to have discovered this!
I used some of your research before while messing around with Fusion360 and a model of the NB I found, and I managed to design a form for a fastback style hardtop that managed to drop the drag coefficient by a bunch!
I’ll definitely be taking your further research into account!