Testing Miata Tops

The first thing I wanted to test was the largest knowledge gap, roofline shape. This meant I had to have different options, that would come on and off quickly, using the same brackets. The four options were an open top, an OEM hard top, a Treasure Coast Chop Top (which should approximate a hard top with the window removed), and a fastback of my own design. 

I built the fastback before I had the notion to do this test, or I would have built it differently. The main problem is that I made long brackets along the bottom edge, and this required removing the trunk lid. This meant I didn’t get to test any of the other tops with an OEM trunk lid. Instead, I bolted a plywood cover over the trunk cavity. This new trunk lid is about 3.5” taller at the back than a stock trunk. It’s hard to say exactly what the effect of this was, but it’s likely a reduction in drag and lift, akin to adding a spoiler. So when you look at the data later, note that none of the tops used an OEM trunk.

Open top results

Miatas are meant to go topless, let’s start there and address some burning questions: 

  • What happens when you use a wing with an open top? 
  • How much does an open top affect a wing’s performance? 
  • At autocross speeds, is it better to remove the top or leave it on? 

Take a look at the following table, and you can see that an open-top Miata generates about 40% of the downforce as one with an OEM hard top (Total Cl field). Of all the options, this was the worst at creating downforce.

It might be a little confusing that the coefficient of drag (Cd) is better with an open top than with a hard top. This is likely the result of two factors. First, all tests were run with the windows open, which turns the hard top cabin into a parachute. Second, the hard top allows the wing to generate 3.65 times more downforce (Rear Cl), and with that, it creates more drag.

TopFront ClRear ClTotal ClCdL/D %HP @ 100mph
Open top-0.20-0.23-0.430.431.0147.16
Hard top-0.17-0.84-1.010.482.1152.78

Let’s plug these numbers into OptimumLap and see what happens. I’ll use three different tracks to represent a range of speeds. These tracks are already in OptimumLap.

TopWatkins GlenWaterford Hills2010 SCCA Nationals
Open top2:24.021:20.971:03.88
Hard top2:22.961:19.951:03.34

The hard top is worth about one second at both Watkins Glen and Waterford Hills, and just over a half second on the autocross course. 

OEM hard top with plywood trunk lid, a concession to the fastback.

For these simulations, the car weight was kept the same. Someone will point out that the top weighs 45 pounds, and that OptimumLap doesn’t factor in the change in center of gravity. Both true. But I can calculate how much weight you’d have to remove from the open top car to match the autocross time of the hard top, and it’s 210 pounds. I’m not sure how high you’d have to place 45 pounds above the car to equal 210 pounds, but it’s probably pretty far up there!

But running an open top car with a wing has two advantages. One, it looks cool. Two, an open top car with this wing beats any top without a wing, every time. That’s kind of jumping ahead in the data, but it’s worth noting.

Let’s get back to the real world and the test at Watkins Glen. Evan reported that the car was more difficult to drive with the open top. He had to brake before entering Turn 10, and then had to manage a car that was oversteering badly. With a hard top, he could mash the throttle from the exit of Turn 9 to the exit of Turn 10. That kind of confidence over an 8-9 hour race can mean a lot more than a second per lap.

Chop Top results

Treasure Coast Miata sells their “Chop Top” for budget endurance racing. It’s an economical and lightweight top that does the job of enclosing the roof. This has two benefits: better aero, and you don’t have to wear arm restraints when racing (the car is no longer considered a convertible). I fabricated mounts that attach to the hard top brackets, and with those the total weight of the Chop Top was a scant 7 pounds. 

There is a persistent myth in Miatadom, that removing the rear window from a hard top is aerodynamically better. So I put two small Lexan covers on the sides of the Chop Top, closing in the sides. This made the chop very similar to a hard top without a rear window. Let’s add this data to the open top and hard top. 

TopFront ClRear ClTotal ClCdL/D %HP @ 100mph
Open top-0.20-0.23-0.430.431.0147.16
Chop top-0.20-0.33-0.530.451.1949.40
Hard top-0.17-0.84-1.010.482.1152.78

As you can see, the chop top allows the wing to work a bit better than an open top, with an increase in downforce. But it’s not as much as you’d think.

However, once you add a wing, the Chop Top performs barely better than an open top. This is interesting, because you’d think airflow over the roof is considerably smoother than an open top. However, it’s what’s happening on the underside of the wing that’s more important, and the Chop Top roof can’t defeat the turbulence coming from the open sides of the cockpit and going beneath the wing.

Chop Top with plywood trunk cover. Note clear lexan and clear gas line, so we can see if the gas is about to overflow.

Next I’ll do the same track simulations, and what I find interesting here is that the Chop Top isn’t really that much different than an open top at any of the tracks. Not enough to really make a difference.

TopWatkins GlenWaterford Hills2010 SCCA Nationals
Open top2:24.021:20.971:03.88
Chop top2:24.041:20.821:03.79
Hard top2:22.961:19.951:03.34

Nevertheless, for those racing with an open top and a wing, the Chop Top is worth a look for a bit of weather protection and not using arm restraints. In addition, we’ve finally dispelled the myth that removing the rear window is more effective. It isn’t. At least when used in conjunction with a wing. 

OEM hard top

All along I’ve been citing the data for the OEM hard top without really discussing it. It’s the status quo in racing, looks great, and performs its duty.

In the data, the OEM hard top generated more drag and lift than what I expected from published data. This is likely due to the open windows and wide canopy, which turns the cabin into a parachute. The drag is supposed to be around .38 with closed windows, but we measured over .5. Lift is also supposed to be the high .30-somethings, and we measured .55 (with vortex generators, I don’t have the raw data without).

The hard top with airdam, splitter, and wing made a killer combo: .48 Cd and 1.01 Cl. Those are good numbers. Racing numbers.

Fastback results

The front of my fastback uses the Treasure Coast Chop Top, and the rear fastback section bolts on and slopes back at about 14 degrees. So essentially the roofline is the same as OEM to about the rear window. Starting with the Chop Top made building the fastback fairly easy, and it was also easy to add and remove for this test. 

An older photo, but the top is the same.

The Chop Top plus fastback weighed 17 pounds less than the stock hard top, and to equalize the two I bolted 8-pound lead weights to the top of the seat belt towers. This was the only time I made adjustments to the weight of the car, and so the open top and Chop Top configurations were a bit lighter. 

The fastback significantly reduced drag, and helped the wing create more downforce. Compared to the OEM hardtop, total downforce increased 120%. Another way of thinking of that is that the fastback turned a 60″ wing into a 72″ wing.

The large gain in rear downforce was offset by a small loss in front downforce. Essentially, the wing was so effective with the fastback that the front end lifted, changing the height and angle of the splitter, reducing its effectiveness. 

TopFront ClRear ClTotal ClCdL/D %HP @ 100mph
Open top-0.20-0.23-0.430.431.0147.16
Chop top-0.20-0.33-0.530.451.1949.40
Hard top-0.17-0.84-1.010.482.1152.78
Fastback-0.12-1.09-1.200.412.9744.81

In addition, the fastback reduced drag by 15%. This in itself is pretty surprising, and not only helps top speed, but fuel economy. Combined, the downforce and drag created a lift/drag ratio that was 50% better than the OEM hard top with a wing. Astounding.

Note that the .41 coefficient of drag is actually quite good when you consider that the wing made the most downforce in this configuration, and just as in all of the tests, the windows were open.

But all was not rosy with this setup. Both Anthony and Evan commented that the car understeered badly in this configuration, and was boring as shit to drive. Given time, Jeremiah would have changed the mechanical grip by adjusting the front roll couple, by means of spring and/or stabilizer bar. This would have helped balance the vehicle at speed. 

Let’s do another simulation in OptimumLap. The fastback gains 1.9 seconds at WGI, and about half that at Waterford, which is pretty spectacular. The Parts Badger team that’s been upsetting the balance in Champcar for 2019 has been the subject of much scrutiny. Where does their speed come from? Part of it is surely the fastback on their Miata.  

TopWatkins GlenWaterford Hills2010 SCCA Nationals
Open top2:24.021:20.971:03.88
Chop top2:24.041:20.821:03.79
Hard top2:22.961:19.951:03.34
Fastback2:21.061:19.021:03.12

Vortex generators on an OEM hard top

The shape of the Miata’s canopy is abrupt, and if you look at wind tunnel tests, you can see smoke trails that are turbulent, and then separate, as air moves over the top. Vortex generators (VGs) create a thicker turbulent layer of air, which keeps air from separating completely. This should result in less drag, and may also help interaction with a wing.

Most vortex generators you see are cosmetic and don’t create vortices. We bought the real deal from AirTab. Made of thin plastic, they go on with double-sided tape, just peel and stick. The manufacturer says they should be mounted no closer than 4” apart. We set them at 5” on center, and so that made 9 for the roof.

If you do some research on VGs, there’s good data that they work. They’ve been used on semi trucks, RVs, the underside of race car wings, and many places where flow separation can occur. For cars, take a look at the four-part series on Autospeed, where they tested VGs on a Prius and Insight. Even better, check out Hi-kick Racing’s blog on adding VGs to a Miata. VGs decreased his lap time from 1:02.8 to 1:02.1. Here’s a photo from his site.

Nevertheless, my expectations were low. If vortex generators are the cat’s meow, then every cat would have them, right? As you can see in the table below, VGs made things worse. Total downforce decreased by about 20%, and drag increased substantially. Take a look at HP consumed at 100 mph and you’ll see you have five less at that speed. 

ConfigurationFront ClRear ClTotal ClCdL/D %HP @ 100mph
Hard top, splitter, wing-0.17-0.84-1.010.482.1152.78
VGs, splitter, wing-0.20-0.61-0.820.521.5757.58

And this is how those values affect lap time in OptimumLap.

Vortex GeneratorsWatkins GlenWaterford Hills2010 SCCA Nationals
Hard top, splitter, wing2:22.961:19.951:03.34
VGs, splitter, wing2:24.321:20.421:03.54

We placed the VGs at the trailing edge of the hard top, but it’s possible that moving them forward may have helped some. Or perhaps we used too many? We followed the instructions and they were supposed to work. 

The double-sided tape was difficult to remove, and so experimentation with the number and location of VGs wasn’t possible. The only lasting impression the VGs made was the adhesive, it was a bitch to remove and so all further tests would use the OEM hard top and VGs combined.

Test conclusions

Different tops change airflow over the roof, and this affects how a wing works. It’s unclear how much of this is is based on turbulence, or because of a change in downwash angle as air hits the wing. It’s likely a combination of both, but we didn’t have time to experiment with wing angle and this mystery remains. 

In addition, our data revealed a rolling rake angle that changes ~ ½ degree depending on rear wing configuration, and this impacted front downforce and distribution more than expected. By adjusting chassis rake and splitter angle, it’s likely the total downforce and Lift/Drag efficiency would have been higher, and this may have reduced rolling-rake changes as well. 

We can make the following general conclusions about using a wing with different tops. 

  • An open top reduces a wing’s effectiveness by about 2.5x. 
  • A Chop Top performs marginally better than an open top.
  • A wing is much more effective with an OEM hard top, but don’t remove the rear window. And don’t use vortex generators.
  • A fastback allows a wing to perform the best, increasing downforce, while also decreasing drag.