If you’re serious about downforce, use a wing; it can generate more downforce, and is around 5x 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 (Supermiata S2, for example).
- 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 fixed-angle 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
As the name implies, a spoiler “spoils” the airflow coming over the top of the car, fooling the air into behaving as if the car has a different rear profile. This creates an earlier separation of flow, with consequently less drag and lift.
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 about 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 doesn’t get any better than about 3:1, whereas a wing can be like 15:1. Which is why you use a wing if you’re serious about downforce.
- Increasing spoiler angle to 60-degrees or more increases downforce, but at a diminishing return.
Spoiler height and angle combined
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.
Based on Katz and McBeath, here are some conclusions:
- Downforce increases fairly linearly with either spoiler length or angle. In other words, it’s all about the height of the spoiler.
- For any given length, increasing spoiler angle increases efficiency.
- Downforce tails off slightly at the steepest angle and longest length.
- There is no one answer for how high or at what angle you should run your spoiler, it’s all about tuning the aerodynamic balance. The easiest way to find out is use an adjustable spoiler built by Blackbird Fabworx and sold through Good-Win Racing, etc.
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. But will curved work better?
- 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 better at 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 more NASCAR; 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.
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. It’s also a lot more expensive, but you get what you pay for.
Spoilers by the inch
One of my favorite websites and resources is Ballistics by the Inch. They start with a long gun barrel and shoot different ammo out of it, keeping track of velocity. Then they cut the barrel 1″ shorter and repeat the process. In the end, they know exactly how different ammo performs, and which is best, from any particular length barrel.
And so I’m going to do… spoilers by the inch! Start with a short spoiler and keep making it taller, all the while keeping track of drag and downforce. In the end I should know what’s the best size spoiler for a standard Miata. And as I repeat this experiment with different front ends (R-package lip, airdam, splitter), I should be able to know what’s about the right size spoiler for different aero packages.
But first I need to make a spoiler with adjustable height. In fact I’ll probably make two, a small one that’s adjustable from 1-4″ and another for 5-12″ or so.
DIY adjustable spoiler
I like building things and I’ve never seen an adjustable-height fixed-angle spoiler, so I’m going to make one. My construction method of choice is a lightweight core (luaun plywood in this case) wrapped with fiberglass or carbon fibre.
- I started with an old fiberglass trunk lid I’d made previously. This was an easy project in itself, using release wax and a couple layers of fiberglass on top. I’m not using the trunk any more, so I cut the rear of the trunk lid off to serve as the base of the spoiler.
- Next I cut a 42″x8″ piece of 1/4″ lauan plywood, and held it up against the trunk at about 70 degrees so I could trace the rough curvature of the rear trunk using a sharpie.
- I then drilled holes in the center and ends of the plywood, and matching holes in the trunk lid.
- Next I put safety wire (zip ties also work great) through the holes and pulled them tight. This makes the plywood conform to the shape of the trunk lid.
- I then filled the gaps between the plywood and trunk lid using a mixture of epoxy and silica microballoons (a lightweight filler and thickener). The beauty of working with epoxy is you can be sloppy, and gaps up to 1/4″ are fine.
- After drying, the wire stitching can be removed and the rest of the gaps filled.
- I cut the top slightly convex so it matches the roofline. The spoiler is 5″ tall in the center, which seems pretty tall, but I’m going even higher with a Lexan extension.
- Next a layer of fiberglass on either side for strength.
- Finally I’ll add another layer of thickened epoxy to fill the fiberglass weave, and make it easier to sand.
I’m using a Lexan plate to change the height of the spoiler in 2″ increments. I think 12″ is as high as I need to go.
|None||Zero||4″||Base height of spoiler|
|10″||+2″||12″||13″ is max size for SM2|