You Suck at Aero

Guest post from Mario. Editorial comments in red.

Every year I make the same New Year’s Resolution: 1) drink more water, and 2) stretch. It’s simple, free, and I still can’t fucking do it. Same thing every year. At the start of 2019 I had a crazy vision to do a bunch of aero tests at Watkins Glen. I didn’t call it a resolution (because then I wouldn’t have done it), but I resolved to do the tests just the same.

I wrote the story of Real World Testing Miata Aerodynamics on my website occamsracers.com, and so I won’t reiterate that here, but I thought it would be good to give my brother’s readership a fresh angle on the tests, and recap some of the high points. Or low points, as it were.

The resolution I make every year is “talk less, listen more”.

You Suck at Watkins Glen

The three biggest obstacles to testing at Watkins Glen are weather, Armco, and the combination of weather and Armco.

Ian and I grew up 25 miles from Watkins Glen, and have suffered a lifetime of events that have been hampered (or is that hammered?) by weather. Upstate NY weather just plain sucks. Nevertheless, Watkins Glen is an historic track, attains high speeds, and has a great back straight for testing aero. So despite knowing better, that’s the location I chose. Of course I got bit by it.

The night before the test it rained, and the track was still wet track in the first session. I took the opportunity to rush home to get some more aero parts, while the rest of the test team sent the car on track for some initial shake downs. When I arrived back at the track, I see three guys literally hammering on the hood of my car. What the fuck did I just miss?

Apparently they left the hood pins unengaged, and as soon as Anthony entered the track, the hood smashed flat against the windshield.  Thankfully the window didn’t break, so he got to make a full parade lap looking through the gap under the hood. We borrowed a hinge from Evan’s Miata, but we got the hood fixed. (In the tests, the bent hood probably affected the coefficient of drag, but at least it was the same for all of the testing.)

After we got the hood fixed, we had a 90-minute delay due to thick fog. So it wasn’t until nearly noon that we got out first test done, and then the track closed for an hour to break for lunch. So here it is 1:00 pm, and we haven’t done shit yet.

The other thing that’s special about Watkins Glen are the steel guardrails, or Armco. You see these on the sides of roads, and on race tracks that don’t have sufficient runoff. Watkins Glen paints their Armco in a distinctive light blue color that’s not dissimilar from Gulf livery.

Cool little 914 in Gulf Livery at Watkins Glen

Whenever someone crashes at Watkins Glen, it’s a long delay. WGI doesn’t do hot tows, and so they have to close down the track to bring out the tow vehicle and clean-up crew. The Armco virtually guarantees that any crash is a wreck. I know this first hand – I came together with an E30 in an AER race and we bounced off both the inside and outside walls of Turn 6. Both cars were kaput for the rest of the weekend. Armco is not kind.

In a weekend that started with delays, we really didn’t need any more delays. But when you mix a wet track and steel guard rails with impatient drivers in race cars, you get more wrecks and more delays.

Those delays meant we didn’t get to test everything I brought. Most significantly, I didn’t test a stock front end vs R-package lip vs airdam vs airdam and splitter. I also wanted to remove the mirrors and see how much that affected drag, do more open-top tests, and other etceteras. Maybe next time. Maybe never.

If you want all the details, check out my site, occamsracers.com, but I’ll recap the high points of the test here.

Really, go check out the site. This post has only a small fraction of what is there.

Open top

I see a lot of convertibles with rear wings: Miatas, S2000s, Corvettes, etc. I’ve often wondered about the effectiveness of a wing with an open top, and now I can answer that question. On my Miata, the open top generated the least downforce and reduced the effectiveness of the rear wing a lot – to the tune of 2.5 times less downforce than an OEM hard top. But that doesn’t mean you shouldn’t use a wing with an open top. If you run simulations in OptimumLap using my data, the open top with a 9 Lives Racing wing beats any combination without a wing every time.

Chop top

The Treasure Coast “Chop Top” is a partial mold from the OEM hard top. It’s primary purpose is to enclose the cockpit so that you don’t have to wear arm restraints when racing. It also helps aerodynamics slightly by reducing drag and lift. When compared to an open top or OEM hard top, the chop top is slightly faster.

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.

I enclosed the sides of the Chop Top, and that makes it about the same thing as using an OEM hard top without a rear window. And so if you have a hard top and you’re not using a rear wing, removing the rear window will make you go a bit faster. Likewise, if you have an OEM hard top and a wing, don’t remove the rear window.

OEM hard top

The OEM hard top generated more drag and lift than 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 around .30-something, and we measured in the .5s again. Sucky.

All told, if you run simulations in OptimumLap using numbers from the test, the OEM hard top is only a bit faster than an open top, and actually gets beaten by the Chop Top.

However, once you add a wing, the OEM hard top wins by a lot. It’s all about getting clean air to the wing, and most importantly, beneath the wing, and the hardtop crushes them by a wide margin.

Adding AirTab vortex generators reduced the effectiveness of the wing by about 20%, and increased drag substantially. If I can save just one poor soul from adding vortex generators to their top, this test was kinda worth it.

DIY Fastback

My fastback uses the Chop Top for the roof, to which I attached a long sloping back. It’s quite light, weighing about 17 pounds less than the OEM hard top, and when you consider there’s no trunk, it’s lighter still. But weight isn’t why I made a fastback, I did it because it looks cool.

I really had no idea how well it would work, and I don’t mind saying I’m pretty impressed. The fastback beats up every other top and takes their lunch money. Compared to the OEM hard top, the fastback made 20% more downforce with the wing. I suspected that cleaner air to the wing would help, but I didn’t imagine it would be that much.

The fastback also reduced drag by 15%, which 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.

With the wing removed, the fastback was less impressive. It still beats all of the other tops in endurance racing simulations, but the lap times weren’t that much faster. Also, every top with a 9 Lives Racing wing beat the fastback without a wing.

9 Lives vs Cheap Wing

I tested two wings, a 9 Lives Racing “Big Wang” and a cheap Chinese double decker wing. I had to modify the cheap wing a lot to make it work. Nevertheless, the 9LR wing simply outperformed the cheap wing in every way possible, and it only contributed .03 to the coefficient of drag. The cheap wing, on the other hand, was like dragging an anchor.

There is some reason to use one of the cheap wings, however. At autocross speeds, where drag is inconsequential, the wing helps. I ran a simulation using the 2010 SCCA Solo Nationals West course, and the double decker wing was three-quarters of a second faster than without a wing. The 9LR wing was another .5 seconds faster than that, but still, a cheap wing is better than nothing.

Splitter

I intended to try four different front ends. OEM, R-package front lip, Supermiata style airdam, and the airdam with a 4” splitter extension. I already mentioned the many delays, and so the only front-end test I got to do was airdam vs airdam and splitter. The splitter made more downforce, adding .38 to the coefficient of lift, and reduced drag by .01. It’s clearly a win-win situation, use one.

Simulations

I keep referencing OptimumLap simulations because it’s the best way to use the comparative data. Nobody drives every lap exactly the same, the track changes ever lap, and so does the weather. In the end, it’s hard to quantify real-world lap times. Case in point: we had an 11 mph headwind at one point, and that totally skewed the data until we corrected for it. If we hadn’t run environmental sensors, I’d be telling you to buy a cheap double-decker wing. As it is, I’m telling you not to.

I put the various aero combinations into OptimumLap and ran endurance racing simulations at Watkins Glen. You can find those here. Watkins Glen is a high-speed track and drag matters more here than just about anywhere else. If I re-run these simulations at different tracks, the margin between the various configurations are a bit closer, because drag factors into it less.

You still suck at aero

If I come off sounding like I know a lot about aerodynamics, it’s just me regurgitating various things I’ve read. I still suck at aero, but I’m learning as I go. When I look around at other amateur race cars, I see a lot of other people suck at aero as well.

Here’s a quick recap of dumb shit I see all the time.

• Exposed front tires are a large source of drag. Cover them.
• Splitters without dams.
• Splitters that are too flimsy. They should be able to support your body weight.
• Wings with too much angle. The roof creates downwash, and if that angle combined with your wing angle is more than about 10 degrees, you’re making the wing stall. That means less downforce and more drag. A wing at zero degrees still creates a lot of lift and not much drag. Check it out, planes fly around like that!
• Wings assembled incorrectly. I saw a Lemons team that had the wing on backwards. Not intentionally, but because it came from China that way. For realz.
• End plates on the wrong way. The low pressure zone (the important part) is low and forward on most wing shapes. Cover that part with the end plates.
• Wings set too low. If air can’t get underneath the wing, it isn’t a wing. Get it roof height, at least.
• Dual wings with gaps that are too large, don’t converge, aren’t adjustable, or are otherwise defeating the purpose of the second element.
• Cockpit venting done wrong. This is things like installing vents at the base of the rear window, thinking that air will go out. It goes in.
• Removing weight at the expense of aero. This one is aimed at my brother, Ian. He was chasing weight and enlarged the openings in his front windows and removed the rear windows on his Yaris. This made his hatchback into a drag chute, and we lost 4 mph on the front straight at Thunderhill.

I admit to being (a) curmudgeonly and (b) skeptical about aero. The curmudgeon in me hates race-inspired cosmetic enhancements from fake factory air ducts to stick-on vortex generators. The skeptic in me wants someone to “show me the data”. It’s difficult to model the cost/benefit of various aero components when it’s so difficult to measure exactly what they are doing. You need really expensive equipment and someone who knows how to operate it. So Mario hired someone with his own money and actually got the data. Fuck’n-A.

I’m no longer a skeptic. I still hate ricers though. And while the benefits of a sorted aero package are absolutely clear, I prefer cars with terrible aero. Back in the 60s, sportscars looked like WWII airplanes, and in my mind that’s what they should look like. I don’t give a shit if the shape has a CoD of 0.5 and generates lift. Now you may be wondering why I race a Toyota Yaris, which has a pretty clean CoD and dog-shit looks. Half of that answer is that it’s the cheapest car to run. The other half is that beating the snot out of other racers is more fun when you have no business doing it.