· By Zebulon Motorsport

Aerodynamics Tuning: How to Tune Your Car for Aero, and with Aero

So you went and got yourself some Downforce, awesome! You’ll immediately reduce your lap times, right? The answer to that question is a resounding “maybe.” Bolting on an aerodynamic device is pretty simple, but extracting as much of the performance you can from them can be much more difficult.  Where do you start? As an example of how complicated this can get, let’s say you have added a splitter with 3 possible positions, a wing with 8 possible positions, and dive planes, where you have three options: 2, 1, or none. You now have 72 new aerodynamic configurations possible (3 x 8 x 3) that will all have different performance windows. This assumes you didn’t have the choice of locations on where to mount these devices, which adds even more complexity. 

Let’s uncover some of the basics of tuning your car for the newly-added aerodynamics, and then let's tune the aerodynamic aids themselves. Disclaimer: this post was written with the assumption that you already know and understand the fundamentals of tuning a race car and the technical language surrounding it. If you don’t have that knowledge and you’re interested in it, we suggest you grab some books to get up to speed. From Carrol Shelby's Tune to Win to Milliken and Milliken’s Race Car Vehicle Dynamics, there is a wealth of reading you can choose. But for now, feel free to read on and make a note of anything you don’t understand to go back and reference later.

The Basics

We’ll start by making the assumption that we are starting with a car that does not have any aero devices on it and is tuned well/to the drivers preferences. That means it can be driven fast and “comfortably,” i.e. you don’t feel like the car is going to send you off track at a moment's notice. 

So what then should our goal(s) even be?

  • There’s really only one: Minimize lap time

What about having the highest top speed? Fastest mid corner speed? Most downforce? Those are great to understand, but lap time is the "one true king," therefore it is what we will focus on here. Things can inevitably get more complicated, but minimizing lap time is still the main goal for car performance. As such, we won’t discuss race-ability, traffic performance or other points in this article. 

How do we optimize the car with aero to minimize lap time? It boils down to the two main aerodynamic forces: downforce and drag. One might think: “Well, it's simple, add the most downforce for grip,” right? Or do we go for a slippery low drag setup for the highest straight line speed? What we actually need to do is balance the aerodynamic loads properly for a given circuit - this depends on many things, but you can imagine it as some ratio of cornering to straight line at the given circuit, and how they relate to total lap time. In reality, there are very few low-drag circuits like Le Mans, Daytona, and Monza. Most circuits will fall into the mid-to-high downforce level range. The goal is to determine how sensitive your car is to added drag or downforce, and you will use these “sensitivities” to help determine your aero setup.

Aero Map

An aero map is a numerical approximation of the aerodynamic forces generated by the car. Usually, the main inputs are front and rear ride height, but there are other modifiers that can be layered in like wing angles, splitter angles, dive planes, roll angle, yaw angle, etc. A good aero map helps you determine a car's aerodynamic response and sensitivities. Things like the increase in drag per degree of wing angle or aero balance percentage change per degree of forward pitch.

Aero Balance vs. Front and Rear Ride Heights

You may be thinking right now “I don’t have an aero map, and it sounds complicated, time consuming, and expensive.” You would be right, but there is good news! You don’t need an aero map. Instead, having an understanding of what your car does as it moves around the track and as you adjust its setup will be your basis of tuning the car. If you’d like to know more about aero mapping, you can find some decent articles with minimal searching, or reach out and we can provide more information. 

So, here are some basic points that we would derive from an aero map. Understanding these concepts can directly be applied to tuning a car.

  • An increase in downforce level will result in an increase in drag, in almost every instance. There are times where replacing or covering a very draggy component can result in an efficiency increase, meaning your Lift over Drag (L/D) increases. Adding a floor under a road car floor pan is a good example of this.
  • Reducing the height of an aerodynamic device that is in ground effect (near the ground) will increase the downforce that component generates. This is usually not a linear relationship, and there is a limit where downforce will no longer increase and will likely decrease beyond that limit.
  • The relationship of front and rear ride height will cause the aerodynamic forces to change, which we can break up into total downforce, aero balance, and drag. 

Tuning Your Car for the Addition of Aerodynamics

Can’t we just bolt a splitter and wing on and get out on track? Sure, but also, not really. With all we’ve discussed above, you can see that it is imperative to think about what is happening with the aerodynamics as we drive the car around the circuit. Increasing the grip that the car generates increases the loads that the tires, suspension, chassis, and everything else must deal with. Specifically, that means we need to have control of the attitude of the car, also known as its “platform.”

 How do we control the platform?

  • Main suspension springs
    • Suspension vertical stiffness is usually controlled by your main springs on each corner of the car. Increasing your spring rates may be necessary to control the car’s platform to both reduce your aerodynamic load variation and also protect your nice new aerodynamic devices.
    • It is important to protect components that are close to the ground. If your splitter or wing is designed specifically to work in ground effect, wearing anything away from that device will decrease its ability to generate downforce, reducing performance. It has been proven over and over in wind tunnels; minimal damage creates measurable losses to the aerodynamic performance of a race car.
    • Aerodynamic load variation can cause the car to become difficult to drive. An increase in spring rate will reduce heave, roll, and pitch, which will aid in the drivers confidence due to reduced aero load variability. More confidence in the car usually translated directly into reduced lap times.
      • Increasing spring stiffness is the main method for holding the car in a more defined operating window in an effort to optimize the aerodynamic loads. This is why Prototypes and Formula cars are sprung so stiffly:
        • When adding aero to a non-aero car, you should expect wheel rates to require being increased by anywhere from 20% - 60% depending on the amount of downforce and how soft the car was run pre-aerodynamic aids.
      • An increase in roll stiffness may also be explored. Increasing grip will increase your lateral loading of the car, and create more roll. This is usually used as a driver preference tuning device, but it is notable to remember if the car starts with minimal roll stiffness. Low roll stiffness can cause both aerodynamic load variation and damage to components through contact with the road.
      • Additional components for platform control and damage limitation
        • Bump rubbers (also known as bump stops, jounce bumpers, etc.) can be used to limit suspension travel. Bump rubbers are usually a tunable item on race cars, where you can choose their stiffness, and adjust where in the suspension travel they start to engage. They can be very useful for tuning a car with aero, as we can limit how close ground effect components get to the ground, both decreasing aerodynamic load variation and limiting damage while even enabling slightly softer main springs
        • Rub blocks are a necessity for ground effect aero devices. When the component eventually does touch the ground, rub blocks are meant to wear instead of the part itself. These can then be replaced as needed. It is worth noting that constantly running on or crashing into the rub blocks will reduce aerodynamic performance and can eventually cause structural damage to most aerodynamic components.

      Tuning your Aerodynamics

      Now that we know more of the basics, and have a car that has enough platform control, how do we start optimizing our aero? There are two major areas that we need to consider: downforce level and aero balance.

      Downforce Level

      -At each circuit, you will have to choose how much downforce you want to run. This comes from experience, simulation results, or empirical testing. In the upper levels of motorsport, where there is access to high quality vehicle models, simulation tools, and professional drivers, simulation is the method of choice to figure out the optimum aerodynamic window. When you are missing even one of those things, it's best to focus on testing. This is because the driver(s) will require the setup of the car to be pushed a certain direction for minimizing lap time. For example, turns 3 - 5 at Spa Francorchamps (known as Eau Rouge/Raidillon complex) are very tricky. If a car has the performance envelope required to go through that complex without lifting, but the driver can’t drive to that limit, he will lift and lose a large amount of time. It would be better to increase the aero level to make sure the driver can get through the complex at full throttle and accept a little more drag. We can find that information by testing.

      Simulation of Spa Francorchamps showing differences in throttle lifting

      The previous example establishes a good rule of thumb: it is generally faster to run higher downforce levels than one would expect. This even shows up in the “perfect world” of simulation where most circuits have their optimum lap times being run when the car is set to mid levels of downforce or higher.

      • For these reasons, we start at a higher downforce level and trim out to test for a reduction in lap time.
        • Remember, changing the aero level with something like the rear wing angle will change your aero balance, which should be tuned as well.

      Aero Balance

      The other aerodynamic quantity that you can tune is the distribution of the aerodynamic loads. This is called the Aero Balance. If you haven’t read our post on it, stop here and check it out:

      What in the World is Aero Balance

      Now, why tune the aero balance, and how should you do it?

      Aero balance will affect the car in a similar way to your mechanical balance (the balance from your springs, bars, tire pressures, etc.)  However, we have to remember that aero loads change with the attitude of the car, and that they increase non-linearly with speed. 

      So let's think back to the aero map discussion (again); we know that as ride height decreases, the aero forces increase. This means that as we get on the brakes the front ride height reduces and the rear does the opposite (the car pitches forward), the aero load increases on the front and reduces on the rear. This added load variation may have made the car quite unstable and difficult to drive through the entry at the limit. We can perform this same thought experiment for mid-corner and corner exit as well.

      With that in mind, the best method of finding a good aero balance is starting rearwards and moving your balance forward until you are able to drive into the corner without too much instability, near the limit of what you like to drive.  While testing your car, this is a safe and quick method to go about this.

      Tuning Aero Balance

      There are many ways to tune your aero balance, though it depends on what devices are installed on the car. All of the following examples are for moving your aero balance forward:

      • Front wing or Splitter
        • Increase the angle
          • Small changes can have large effects: a 2mm change in splitter height is likely to be felt at the limit. This is due to ground effect aerodynamics
        • Move device forward
          • Generally not used or available due to rules limitations
      • Rear wing
        • Decrease angle
          • Most common tuning method
        • Move wing forward
          • Usually constrained by rules
          • This does not decrease downforce it only changes the balance
      • Ride height
        • Reduce front ride height
          • This creates more ground effect downforce, but can also increase pitch sensitivity
        • Increase rear ride height
          • This both moves the front splitter/wing lower like above, but also can reduce rear underbody downforce if there is a decent floor and/or diffuser
      • Other components
        • Add dive plane(s)
          • Many prototypes have different dive plane configurations to tune with

      That was a lot. We hope that this post will be a help to you in your quest for faster laps. Thanks for reading!