How to Calculate Wing Downforce

How much downforce does your car wing make? Mathematically, you can calculate downforce using this formula:
downforce = 1/2p * A * Cl * V^2.
If that means anything to you, then you don’t need this website. If that calculation scares the shit out of you, I provide a cheat code below makes it a lot easier.

However, if you’re not interested in math, and more interested in exactly how much downforce different wings make on different cars, I’ve tested several wings and spoilers in a wind tunnel.

The wings tests span two different wind tunnel reports, because the cars are quite different, and that ends up mattering a lot more than the wing itself. Click the button, fill out the form, and you’ll get a link where you can download the reports.

An easy way to calculate downforce

You can break this formula into four parts:

• 1/2p – I’ll simplify this as a constant value of .00119. That’s all you need to know about 1/2p. (It’s air density, which changes due to elevation, temperature, and humidity. Accept this static value and move ahead.)
• A – This is the area of the wing in square feet. If you have a 64″ 9 Lives Racing wing, then your area is about 4 square feet (64″ x 9.25”/ 144”).
• Cl – This is Coefficient of Lift. It’s a tricky value because it changes with different shapes of wings, how fast you’re going, the wing angle, and turbulence. For the time being, you can use a value of 1. Multiplying by 1 is easy, and I’ll explain later why this is also a reasonable value.
• V^2 – Velocity in feet per second, squared. For this you need to know that 1 mph = 1.467 feet per second.

OK, so let’s figure out about how much downforce a 4 square-foot single-element wing produces at different speeds. Three things will remain constant, the 1/2p value, the wing area A, and the coefficient of lift, Cl. Multiply all those factors together and you get a value of .00528836. Now all we need is to multiply .00528836 by the velocity in feet per second, squared.

Velocity	FPS ^2	Downforce
40 mph	3441.8	16.5 lbs
60 mph	7744.0	37.2 lbs
80 mph	13767.2	66.2 lbs
100 mph	21512.1	103.5 lbs
120 mph	30976.0	149.1 lbs

How Much Added Grip?

If we assume that tire grip is linear with weight we can calculate the amount of grip you gain at different speeds. (For an in-depth look at how tire grip and aero are related, see Tire Grip and Aero.) Downforce helps light cars more than heavy cars, because the percentage gain is greater. I’ll use the same wing and speeds to illustrate this.

Speed	2400 lb car	3000 lb car
40	.76%	.60%
60	1.70%	1.36%
80	3.03%	2.43%
100	4.74%	3.79%
120	6.82%	5.46%

You can see that at low speed, downforce isn’t very effective, and you might question its use for autocross (40 mph avg). To be fair, drag isn’t very consequential at low speed either, so as long as your aero parts aren’t heavy, or at the polar ends of the car (which they usually are), then low speed aero is somewhat useful.

At medium speeds, downforce is a noticeable advantage. If you’re looking at this table and thinking 3% grip isn’t that big of a deal, it is. At a high speed track like Watkins Glen, it can be the difference of 5 seconds per lap, or more. I go into that in depth in How Much is Aero Worth in Lap Time? .

Coefficient of Lift

In the above calculations, I chose a Cl value of 1.0. However, some airfoils can achieve over twice that much downforce, and you can read about that in Airfoil Comparisons, where I review the best airfoils for car wings.

But don’t get too excited about the high Cl numbers. There is always something creating turbulence at the front of the car, which creates downstream losses on the wing. This could be hood and fender vents, canards, sharp angles at your roofline, the wake of other cars in front of you, crosswinds, or all of these. Anything that creates turbulence destroys lift.

I saw this firsthand when I changed only the roofline shape on my car and back-to-back tested them: removing the top resulted in only 40% of the downforce as an OEM hard top; A fastback increased rear downforce by 130%. That’s a dramatic example, but you can see that it’s possible to lose or gain over three times the downforce by changing only the shape of the roof.

So this is a very long-winded way of saying that a coefficient of lift of 1.0 is fine for rough calculations, on a single element wing. For a properly designed dual element wing, you can use an equally rough value of Cl 1.5. My original dual element wing measured more like 1.3, but I’ve made more modifications since, and it’s probably closer to 1.5 now.

Since coefficient of lift is affected by roofline shape, use the following modifiers when you calculate downforce for a Miata (or other convertible).

• If you have an open top convertible, divide the wing’s downforce by 2.5.
• With a choptop (or a hardtop with the window removed), cut downforce in half.
• With an OEM hard top (also probably applies to a convertible with the roof up), use the figures in the table as is.
• With a fastback, multiply by 1.25.

If you want to nerd out on it, you can experiment with wing shapes, angles, turbulence, and lift using the NACA wing calculator or BigFoil. If you want real values, you don’t do rough calculations, you hire someone like Kyle Forster to do CFD on your car. Or if you want real-world data, hire Man and Machine Consulting, who did the testing on my car.

If you’re looking for more reading on car aerodynamics, check out my resources page. If you found this or other posts useful, consider buying me a coffee so that I’ll be fueled up to write more informative posts. Thanks!