spoiler – Occam's Racer

The wind tunnel report

Back in 2002 a group of SCCA racers took a C5 Corvette to a wind tunnel and published a report on the results. It’s not a very long report, but the story is compelling, and the data speaks for itself. The report is available here for $37. I’m going to review some of what’s in that report, but without any specifics, because the author said not to reprint any of it without permission, and so I won’t.

The group did 26 runs in 10 hours, which is oddly the same number of runs I did at the A2 wind tunnel. They used a much larger wind tunnel at the Canadian National Research Center, in Ottawa, Canada, whuch measures 9 meters square by 24 meters long. This is quite a bit larger than A2 wind tunnel (which is 14 feet wide and 58 feet long), and so the Canadian results should be more accurate.

But how accurate is a wind tunnel compared to the real world? I don’t know. When I posted my wind tunnel data online, some internet pundit, without a shred of empathy or humility, puffed up said I made a major mistake in my report, because the wind tunnel optimizes to a constant Qrh, not V-100mph, so that my data was useless without the Qrh average for each run. I have no idea what that means, but I don’t see anything like a Qrh average column for this wind tunnel report either. And so I guess all this data from Canada is similarly worthless?

Well, I’m not a professional aerodynamicist, I’m a fuggin hack, but I’d have to think the differences from each test run are still important, even if the actual numbers aren’t 100% accurate. So let’s shove all the caveats and internet buffaloes aside and move ahead with what they tested, and the comparative data.

Drag – In the test they tried various things to reduce drag, from taping up the front grill to rounding the B pillar, to putting a hole in the license plate. Some things worked surprisingly well, some had no effect at all.

Rear wing vs spoilers – A couple different wings were tested, and since the baseline car used a spoiler, they included the data for that as well. But isolating the spoiler data is rather difficult.

Wings, end plates, and Gurney flaps – They tested three different end plates on the standard wing, and their results were somewhat similar to mine, which is that end plates are the least important part of the entire aero package.

Splitters – They tested a splitter with a flat undertray and one with diffusers. They call this a Laguna undertray for whatever reason, and I will say the design looks quite good.

Yaw – I didn’t test yaw, but they did, using both + and – 3 degrees for most of the runs, but they also tested higher yaw angles initially before settling on just 3 degrees for the rest of the tests.

Tire life – While tire life isn’t something you test in a wind tunnel, the report concludes with results from the race season, which showed tire life was considerably longer using downforce. This is something I wrote about before, that downforce increases tire life, and their experience was the same.

OptimumLap simulations

With all of this wind tunnel data in hand, I went into OptimumLap and built Luke’s exact car. I started with the basic specifications for a C5 Corvette, but used a 252 horsepower flat-tuned dyno chart instead. Detuning is what allows a Corvette to compete in GLTC, and a result of that is a very flat torque curve. This is recognized as an advantage, and flat-tuned engines are penalized for that. Cars are also penalized for aero.

To see which aero version was fastest, I created nine versions of his car, each with different aero parts. I used the coefficients of lift and drag from the report, and swapped out every version of splitter, wing, spoiler, etc. This may sound easy, but the the table that shows the coefficients has low resolution, which made isolating the individual aero components a little tricky. Anyway, I persevered and had my nine different cars, giving them different weights to match the rules.

Grid Life Touring Cup is a pounds per horsepower series, and penalizes cars for using aero, by making them heavier or less powerful. For example, if you use a spoiler or wing that’s larger than 250 square inches, there is a penalty depending on how large you go. Likewise, a splitter carries a penalty over an airdam, and there are penalties for various combinations of wings with splitters.

Because it’s easier to adjust a car’s weight than its engine tune, I simply changed the weight of each aero build to match the GLTC rules. Thus, the car would weigh between 3213 lbs (free aero) to 3371 lbs (splitter and big wing). Note that these weights may be off by a season, as GLTC again nerfed the flat tunes. And also, don’t take too much into the lap time itself, OptimumLap can’t really predict lap times without a lot of fudging, so this is just comparative data you’re seeing.

I then ran all nine cars around various the race tracks GLTC goes to, to see which would win. The winner wasn’t the same at every race track, but a few builds bubbled up to the top, and some sank to the bottom. The following image shows a speed trace and lap times of five of those builds at COTA. I’m not going to reveal which one was the fastest (that’s between me and Luke), but I will tell you which one was the slowest.

Speed trace of five C5 aero builds for GLTC.

See that black line that has the highest top speed? That’s the OEM aero version, essential a base trim model (BTM) off the showroom floor. It might have a 5-10 mph advantage on the back straight, but it posted a 154.44 lap time, which was the slowest at COTA, and also the slowest at every other track. In the end, cornering speed matters more than top speed.

To get back to what I was saying earlier, Luke uses a spoiler and not a wing. GLTC allows you to use a small wing (less than 250 square inches) for free, and so wouldn’t this be better? Not always, and it’s actually quite close. I go through this investigation in my own wind tunnel report (a $25 bargain), showing that there are times when the wing is faster, but sometimes the spoiler wins.

If you’re serious about downforce, use a wing; it can generate more downforce, and is more efficient than a spoiler. It begs the question, why would anyone want a spoiler?

Spoilers are usually cheaper than wings.
Some racing rules don’t allow wings, but allow spoilers.
A small spoiler can reduce both drag and lift.
Wings are often gaudy on a street car, but spoilers almost always make a car look cool. Not only my opinion, but NASCAR fans as well.

I’m going to build an adjustable-height 70-degree spoiler so I can find out what’s ideal on a Miata. But before that it’s worth looking at the existing literature and products.

How a spoiler works

Cars are basically shaped like airfoils, and as air moves over them, it creates lift. The faster the car goes, the more lift and instability is generated. A spoiler, as the name implies, “spoils” the airflow coming over the top of the car, fooling the air into behaving as if the car has a different profile. This cancels some lift, and often reduces drag as well.

Image result for with and without spoiler airflow — How a spoiler works.

A spoiler also concentrates high pressure air on the rear deck lid. Pressure is akin to weight, and so this adds downforce to the rear of the car.

A spoiler also moves the center of pressure rearwards, and like a streamer on a kite, this promotes stability.

Spoiler height

How high should a spoiler be? Let’s take a look at what the pundits say. In Race Car Aerodynamics, Katz shows two different graphs for spoilers. The first is based on spoiler height alone, at a fixed angle of 20 degrees from vertical, or what I’d call 70 degrees.

I’ve put some pencil marks on the graph and drawn some conclusions.

A low spoiler about 1″ tall reduces drag the most. It also adds a bit of downforce. From a drag and downforce perspective, it’s a win-win!
A 3″ spoiler doesn’t add any drag, and doubles the downforce of the low spoiler. In other words, you get something for nothing!
A taller spoiler adds downforce and drag, but downforce increases more rapidly than drag. The gift that keeps on giving!

So no matter what height spoiler you chose, it has a benefit. Based on theory alone, we should all have low spoilers on our street cars, and taller spoilers on our race cars (rules permitting).

Spoiler angle

Katz includes another graph on spoiler angle, this time using a fixed-height spoiler. Confusingly, this time the angle is measured from horizontal, not vertical, and the 70-degree angle from the previous graph isn’t included.

Some observations of this data:

Drag increases fairly linearly with angle.
Lift-drag ratio seems best at a very shallow angle, but this may simply be the low overall height of the spoiler. Also note that L/D ratio is at best 3:1, whereas a wing can be 12:1 or more, which is why you use a wing if you’re serious about downforce. (If you look at pressure plots of wings, you’ll notice they have about 3x more suction under the wing than pressure on top. Spoilers only make downforce from the top side, and so that’s why they are a lot less efficient.)
Increasing spoiler angle to 60-degrees or more increases downforce, but at a diminishing return.

Spoiler height and angle combined

Next I’ll look at my other favorite reference, Competition Car Aerodynamics . McBeath cites CFD work done on NASCAR spoilers, in which they changed both the spoiler height and angle. Now we’re getting somewhere.

I’ll use the above results to compare spoilers of different lengths and angles that result in a similar total height above the deck. Which in turn allows me to figure out the most efficient spoiler angle.

160mm spoiler, 20 degree angle, 54.7mm total height
80mm spoiler, 40 degree angle, 51.4mm total height
60mm spoiler, 60 degree angle, 52mm total height

It’s a bit difficult to see in this graph, but a 60mm spoiler set at 60-degrees is slightly better than a 160mm spoiler set at 20 degrees, even though the longer spoiler is a little bit taller. In other words, a higher angle works better. But it’s only by a small amount.

Based on Katz and McBeath, here is my simplified conclusion: The total height of the spoiler is all that matters.

NASCAR spoilers

NASCAR used rear wings for a short period of time and then switched back to spoilers. Not because they could get better performance from a spoiler, but because the series is always looking for ways to make racing both closer and safer, and the wing did neither. In addition, the fans didn’t like the look of a wing. To be fair, the CoT wing was hideous, see for yourself.

So we can’t look to NASCAR for the most effective spoiler design, because we know their priorities lie in close racing rather than outright speed. But it’s worth noting a few things about NASCAR spoilers.

NASCAR probably knows more about spoiler design than any other race series, and they still don’t settle on one design. In fact, the regulations change almost yearly. Looking only at the height, in 2016 it was 3.5″, in 2017 2.375″, and in 2019 8″.
Some years the spoilers were adjustable for angle, some years they were fixed, and there have been different heights, widths, and shapes throughout the years.
NASCAR uses the spoiler to balance not only the overall aero package, but as a way to balance the performance between different cars, and at different tracks.
When NASCAR reverted from rear wings to spoilers, they set the spoiler angle at 70 degrees. In 2019 the fixed angle remains 70 degrees. Interesting.

Here’s an excellent article on A comparative look at NASCAR’s new spoiler, old spoiler, and wing.

Nscs-newspoiler2010hi_medium — Click image to enlarge.

NASCAR spoiler shapes

The 2019 spoiler is flat across the top, but different shapes have come and gone.

Image result for nascar spoiler shape — Curvy, almost bat-wing style.

The size and shape of Miata spoilers

So now that we’ve looked at spoiler theories and real-world examples from NASCAR, let’s get down to what matters: Miata spoilers.

Miatas have a roofline that is peaked in the middle, and you might imagine that the ideal spoiler shape has a matching convex arc to it. Although like all things aerodynamic, this could be totally false, and maybe the sides should be taller.
The rear edge of the trunk is curved and so a curved spoiler would look more natural, and could be an easier DIY project as well. Also, a curved spoiler would be more rigid than flat. However, some race series say that the spoiler must be flat, with no curvature. Booo!
There’s no reason to “spoil” the air coming along the sides of the car, and so a spoiler much wider than the rear canopy seems like a waste. Although the exposed spoiler ends are probably adding downforce. Albeit not very efficiently, and at probably a different angle than is ideal for spoiling the roofline shape.

Miata products

This IKON spoiler is an attractive design, with a convex top edge and curved profile. It would be neat to see something like this with a flat extension that’s adjustable for height.

The Rocket Bunny spoiler is flatter across the top, taller, and with a steeper angle. I’d guess it’s slightly more effective than the Icon, but it has a tacked-on look that doesn’t really appeal to me.

And then there’s this JSP spoiler that looks like a wing, but isn’t (air isn’t going to flow under it, hence not a wing). The shape follows the curvature of the sides and roof, and this may be an efficient design. But meh to the looks.

Of course all of these spoilers have a fixed height and angle, so there’s no way to adjust the aerodynamic balance. On the other hand, the Blackbird Fabworx spoiler is large and adjustable for angle. I’m also not a huge fan of the way this one looks, but the beauty lies in the function.

DIY spoiler, testing height

I made my own spoiler, it’s about 3.5″ tall and has some curvature to it that follows the trunk shape. It’s made of plywood and fiberglass, and there are 6mm T-nuts so I can add an extension.

With the low spoiler (without any extension), I ran very consistent 1:22s at Pineview Run. And by consistent, I mean 1:22.03, 1:22.05, 1:22.07, and in my second run, 1:21.99, 1:21.99 and 1:21.93. This was a hot day, and if I compare the times to previous ones, the track was definitely slower than normal.

With a 3.5″ extension (total 7″ height), my lap times were less consistent, most of them around 1:21.5, but my fast lap was a 1:21.03, almost a full second faster. But that one was an outlier, and if I average the five fastest laps, the taller spoiler was about .55 seconds faster than the lower spoiler.

The following table is an average of four back-t0-back runs, two with the spoiler extension, and two without. I’ve averaged the top six fastest laps.

Configuration	Avg Lap	Simulated	HP	Lbs	Cg	Cd	Cl
Low Spoiler	1:22.0	1:21.11	112	2400	1.00g	.44	-0.25
Tall Spoiler	1:21.45	1:20.63	112	2400	1.00g	.45	+0.20

I added .01 to the Cd as a guess, but drag isn’t that consequential anyway. I came about the Cl figure by changing that value in OptimumLap until I got the .55 delta in lap time. It seems absurd to think a spoiler can make a .45 swing in Cl, but that’s what the simulation says. Interestingly, this is also the value cited for a 8″ tall spoiler in MacBeath’s Competition Car Downforce.

In Race Car Aerodynamics, Katz cites several examples of spoilers, but none that go as high as 7″. In his examples, the relationship between height and coefficient of lift is nearly linear, and from 0″ to 4″ there’s a change of about .4 in Cl. So if I extrapolate those values from a 3.5″ spoiler to 7″, I’d only expect to see a change of .4 Cl, which is again pretty close to the test result.

Whatever the case, a 7″ tall spoiler works on a Miata. Now I have to make a taller one and test that.

Tag: spoiler

From Corvette C5 Wind Tunnel Test to GLTC Win