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