Generally speaking, touring cars are limited by front downforce, and so optimizing your wing for more rear downforce isn’t super useful. Buy a decent wing and conventional mounts, set it at roof height and zero degrees, and experiment with wing angle to taste. You don’t need to overthink this. However, I do.
I know that by optimizing my rear wing, I can get the same amount of rear downforce while reducing drag, either via less wing angle or less wing area. Some of that can be done by choosing the correct airfoil and using a 3D shape, but it also comes down to how and where I mount the wing. Because I also know that by locating rear downforce further forward, I can get more total downforce with the same aero balance.
And that presents a lot of choices, because wings can be mounted from the bottom, top, or sides, and at different heights and set-back distances. What’s best?
In an ideal world you’d laser scan your car and do CFD testing to find out the wing location where you get the performance you’re looking for. Usually this would be where the car (not the wing) makes the most downforce, but you might also be looking for the car’s ideal lift/drag ratio for a particular race track, or for an ideal front/rear aero balance.
Most of us aren’t going to laser scan our cars, nor do we have the hardware and software to run CFD. More to the point, racing rules often restrict where you can put a wing. For example, some SCCA classes limit wings to 8″ above the trunk deck. Grid Life Touring Cup rules state that the wing can’t extend 5″ past the rear bumper. Some rules limit the wing to being entirely in front of the rear bumper!
But let’s start with a no-rules situation, what’s the best location for a wing? As a general guideline, wings should be mounted at about roof height. This provides enough area underneath the wing for the low pressure region to form, helps extract air from underneath the car, and is a good compromise of aero balance and center of gravity.
Mounting a wing higher than that will get the wing into less turbulent air, and the wing may make more downforce in that location, however, the car may make less downforce. Huh? When you place a wing above roof height, it loses interaction with the body of the car, and the rear wake doesn’t kick up as high. The net result is the overall Cl of the car (downforce) is reduced, even if the Cl of the wing is itself better.
In addition, the higher you place a wing, the more it affects rear aero balance, because it has more leverage. So unless you’re making an over abundance of front downforce, it’s not a good idea to get more rear leverage. Placing the wing higher also raises the center of gravity, which will be detrimental to handling.
Conversely, mounting a wing lower than roof height reduces rear leverage and lowers the center of gravity. In addition, a lower height can also help extract more from a diffuser. The downside is that the wing will make less downforce because there’s more turbulence behind the canopy, and there’s less area under the wing for the low pressure region to form. Indeed, the low pressure region under the wing may run into the high pressure area on the trunk lid.
The shape and angle of air also change as you lower a wing. If you read my post on 3D wings, you’ll know that if you mount a wing at roof height, there’s about a 5-7degree difference in angle between the ends of the wings and the center of the wing. If you lower the wing, that angle can be 10 or even 15 degrees. Which means if you mount a 2D wing close to the trunk, the wing is going to stall either at the ends or in the middle of the wing.
All said, mounting a wing at roof height is a good baseline setting for the vertical axis. The next issue is where to mount the wing on the horizontal axis. A good rule of thumb is to overlap the wing onto the trunk by 1/4 chord. Meaning, if you have a 12” chord wing, there should be 3” of wing in front of the trunk lid and 9” behind it. This baseline setting provides the ideal amount of extraction under the wing, which kicks the rear wake upwards and pushes the car down.
If you’re constrained by racing rules that don’t allow you to get a wing that far rearward, you’ll have to run more wing angle for the same amount of downforce. You’ll get more drag as a result of that, but rules be rules.
The only reasons I can think of to place a rear wing further rearwards than 1/4 chord overlap have to do with practicality or aero balance:
- If you need to be able to open the trunk and the wing is in the way of that, then you could place the wing further rearward. Although you could also mount it higher. Performance wise, I’m not sure which is better, but this is concessions for practicality anyway.
- If you have so much front downforce that you need to balance that with more rear leverage, you could mount the wing further back. But personally, I’d just add more rear wing, not a longer lever.
- If your car is a hatchback, you may not be able to get enough space under the wing for the low pressure zone to form. Again the options are higher or further rearward, and it would be hard to say which is better.
Twice now I’ve touched on aero balance, and so a few words on that won’t be out of place. Typically you set up a car with the same aero balance as its chassis balance. So on a Miata with equal 50/50 weight distribution front/rear, you’d put the same amount of downforce on the front and back. On a FWD car with 67% of the weight on the front wheels, you’d skew aero balance 2/3 to the front. And on a rear-engine Porsche 911 you’d put 2/3 on the rear.
That’s how an aerodynamics engineer would baseline the aero balance on a car, but that’s not how I like it. I want a car to oversteer in slow corners and understeer in fast corners. This isn’t difficult to do, you just remove rear mechanical grip (or add front grip), and use a big wing.
On most racetracks, the slow corners are maybe 40 mph and the fast ones are say 90 mph. If the car oversteers slightly in a 40 mph corner and understeers slightly in a 90 mph corner, then there’s a whole range of speeds in the middle where the car is perfectly balanced.
I’ll dive into the details of aerodynamic balance in another post, but the point here is that the type of wing mounts you chose will slightly affect the aero balance by providing more or less leverage on the rear of the car. Mount a wing further rearward or upward and it levers up the front of the car, adding understeer. Place the wing further forward, and the aero balance moves forward, creating less understeer (or more oversteer, depending on how you look at it).
With that see-saw effect of aero balance in mind, let’s take a look at some wing mounting solutions.
Wings generate downforce along the entire surface, but the downforce is centered at about 25-30% of the chord distance from front of the wing. So when mounting a wing, center your supports around that location. Next you might be wondering what kind of mounting solution is best, conventional mounts, swan necks, or end-plate mounts?
Conventional upright mounts
Wing mounts cause turbulence. If you’re read my post or watched the video on Visualizing Airflow, you’ll know that there’s a lot of turbulence right around the wing mounts (mounts = pylons, stanchions, stands, uprights, or whatever you want to call them). Some of that is due to air wrapping around, which adds even more turbulence.
All of that turbulence is happening beneath the wing, which is where most of the downforce is made. So you can see this is the worst case scenario for downforce and drag. Still, conventional mounts are short, stiff, and simple. Like I wrote at the beginning of this article, it’s front downforce that is the main limiting factor in your aero package, so there’s nothing wrong with using conventional bottom mounts and moving onto more important projects.
Conventional mounts can attach to the trunk lid, trunk gutter, or chassis. If the trunk lid is rigid enough, that’s the easiest spot, as the wing pivots easily out of the way. On Miatas, the trunk lid isn’t that well supported, so we typically mount wings in the trunk gutter. The frame around the trunk is very stiff, and with just a little cutting around the trunk lid, there’s no interference when opening it.
Chassis mounting is for cars that produce a metric shit ton of downforce and can’t trust any part of the trunk or rear surfaces to mount a wing. Chassis mounts are heavier, and unless you need them, I don’t see the point; they just make you look like a poseur.
Swan neck mounts move the mounting and adjustment hardware to the top of the wing. The pressure side of the wing doesn’t matter much, and so adding turbulence here is of less consequence, which leaves the underside undisturbed. In addition, because there’s turbulence around conventional bottom mounts, you get earlier flow separation at steep angles of attack. Meaning that a swan neck mount allows you to run more wing angle before stalling.
Another thing I like about a swan neck mount is the ability to add in some calibrated flex. Normally I think of wing mounts as being rigid, but if the wing mount can flex a little, it allows you to drop a couple degrees of wing angle at high speed.
This is effectively an active aero system, or call it passive aero, but it would theoretically give you some of the benefits of a DRS system. A swan neck mount allows you to do this easier than other types of mounts. However, it’s of less benefit than you’d think, and you can read about that in my post on Active Aero.
The only real drawback of a swan neck mount is structural. It’s a bit more wear on the wing and wing stands, and so the wing must be stronger, and the mounts must be larger and heavier than conventional bottom mounts.
Reverse swan necks
One fault of conventional swan neck mounts is that the wing stands are still in front of the wing. Flat plates aren’t great aerodynamically, airflow along the plate begins to detach with more distance, and eventually separates entirely, creating a turbulent wake.
Reverse swan necks place the wing mounts behind the wing, removing all turbulence in front of the wing. This is an ideal mounting solution if you can find a good way to mount it. And therein lies the rub.
Let’s use a Miata as an example. We know that the wing should be mounted at roof height and overlap the trunk by about 1/4 chord. That puts the the trailing edge of a standard-sized wing about 7″ behind the trunk lid. The wing mounts need to be behind that, and so if they go in the trunk gutter as usual, that would make for a very large S-curved wing mount. Audi has done something similar mounted to the trunk lid, as you can see in the image below.
Now there’s nothing intrinsically wrong with S-curved wing mounts except for the weight of the materials and less rigidity in the system. But an easier solution on a Miata would be to come straight down and mount the wing to the chassis at the bumper supports.
A reverse swan neck wing mounted like that would be bomber strong. The mounts would weigh a bit more, but the real penalty is where the downforce is applied, very far rearward. However, I could see this being ideal for a time attack car, where you have huge splitters and hammerhead extensions that make a ton of front aero, and you need as much aero as you can get.
An end-plate mounted wing integrates the wing stands and end plates into one piece. There’s no additional vertical supports, so there’s no drag or turbulence generated from the wing mounts themselves, just the end plates, as it were. When I think of iconic cars that had end-plate mounted wings, I always think of a Ferrari F40. The wing didn’t have a very large chord, but it was a perfect marriage of form and function.
There are compromises with every mounting solution, and end-plate mounted wings require a stronger (heavier) wing. Wings generate most of their downforce in the middle of the wing, and when supported only by the ends, you need a very strong, rigid wing.
Another drawback is that end-plate wings are limited to the width of the car. You can’t easily put a 78″ wing on a 62″ wide car and mount the wing via the end plates. Well I guess you could, but the end plates would be angled heavily outward, not vertical. Most end-plate mounted wings are vehicle specific (like the F40, NSX, etc.), and so these are typically OEM offerings, not aftermarket.
9 Lives Racing makes an end-plate mounted Street Wang for Miatas that has angled wing mounts so that they can use a larger 48″ wing instead of a 40″ wing, which is the size of the trunk opening. The Street Wang is mounted a bit too low and aft for pure performance, but it looks good, provides trunk access, and as the name implies, is more of a street application anyway.
Every wing mount is a compromise:
- Conventional bottom mounts are the easiest, lightest, and strongest. They also make the most drag and least downforce. But like I wrote in the first paragraph, that rarely matters on touring cars.
- Chassis mounts are the strongest, but also heavy and usually restrict access to the trunk.
- Swan neck mounts have less drag and turbulence. The downsides are mostly structural, but could be turned into an advantage.
- Reverse swan neck mounts have less turbulence than conventional swan necks, but might increase rear leverage due to mounting location.
- End-plate mounts offer the best performance, but they limit wing span, and the wing itself must be very strong.