Racecars aerodynamics is an extremely complex field. Still, it is also one of the areas influencing car performance the most, together with tyres and powertrain. This is the reason why, especially in the sportscar world, manufacturers invest heavily in aerodynamics development.
Interestingly, this happens not only on the pinnacle of motorsport ladder, such as Formula 1 or LMP1 (where the rules also allow more freedom) but it is a determining factor also in “amateur” classes, like LMP2.
Who writes is by no mean an aerodynamics expert, but knows a bit about aerodynamics effect on cars performance and behavior and, in general, on vehicle dynamics. This is the main reasons why after a brief introduction about which are the most important aerodynamic forces acting on a race car, we will focus firstly on the effect of said forces on performance and handling. To better understand this, a basic knowledge about tyres would help and this is why this series of articles comes after our tyres introduction.
We will also go through a short description of the physics principles that allow a car (and, in particular, some of its components) to produce aerodynamic forces.
From a pure engineering perspective, racecars aerodynamics are very complex and highly non-linear: each single component can significantly influence how every other part works. Although the working principles of each aerodynamic part of a racecar is nowadays more or less known, the interactions between different areas are extremely intricate. This is why aero engineers mainly rely on wind tunnel testing (or CFD, Computational Fluid Dynamics, which is used as a kind of virtual wind tunnel) to develop components and the whole car.
Racecar Aerodynamic Forces
We will start our analysis with a short overview of the aerodynamic forces and the main components responsible for their generation. As aerodynamics have originally been a central element in building airplanes, much of the theoretical and mathematical basis today in use in the motorsport and automotive industry has been inherited by aeronautic engineering.
As we will see, the aim for engineers building airplanes is to create a force that lift the vehicle, allowing it to fly. For a race car, the opposite is true, with aerodynamic engineers doing all that they can to obtain forces that push the car down as it moves, to increase the magnitude of tyres grip forces produced and allow the car to negotiate corners at a higher speed or brake later and harder.
Physically speaking, we can actually describe all the actions experienced by a car, because of its relative movement with respect to air (not the ground! The speed we will have to consider is the relative velocity of the car with respect to the air) by using a single vector, with a generic direction and magnitude. We can call the resultant action on the car F and draw it as shown in the following picture.
As we have already seen during our tyres overview, we can decompose a force vector like F into two components, a horizontal one, and a vertical one:
The horizontal component Fx is normally called drag, because it brakes the car during its forward movement, while the vertical one Fz is normally identified as downforce because it pushes the car down.
The magnitude of these forces is a result of many factors and is strictly related to car design; in general, they depend on the square of car’s relative velocity with respect to the air. This means that, if we assume the air is not moving with respect to the ground (no wind), aerodynamic forces grow with the second power of car’s speed.
They also directly depend on air density and, hence, on weather condition (temperature, humidity, etc.). We can calculate them using the following equation:
- ρ is air density, normally measured in kilograms per cubic meter (kg/m3).
- C is a non-dimensional coefficient related to car aerodynamic properties.
- A is a relevant area, for cars aerodynamics normally associated with their frontal area and is hence measured in square meters (m2)
- v is car speed relative to the air (m/s).
Since car frontal area is often difficult to calculate or estimate, sometimes the above equation is used combining C and A together in a single factor (this can be alternatively seen as assuming the frontal area being equal to one square meter), that will now have a surface dimension.
As we saw already, we can decompose F in its horizontal and longitudinal components; we can then adapt the above equation to calculate drag and downforce separately:
The main design objective, in terms of aerodynamics, is to maximize downforce and to minimize drag. As we will see later on, more downforce means that the car will have the ability to negotiate corners faster, with its effects getting more and more relevant as car’s speed (relative to the air) grows (which means, corners radii gets bigger).
Less drag means higher straight-line speed and better fuel economy because the force acting against car forward movement will be weaker and the car will be able to advance more easily.
If the car does not travel on a straight line, it will also experience side forces, because it will move forward with an angle (often referred as yaw angle) with respect to the air (something similar to tyres slip angle, but considered for the whole car). For sake of simplicity, we will ignore these actions in our brief overview.
Also, it is worth mentioning that not every car produces downforce. Road vehicles, for example, normally experience lift (which means a force trying to push the car upward, or lifting it). Racing touring cars normally produce nearly no downforce or even small amount of lift. In the past, junior single-seater without wings (Formula Ford, for example) also produce lift.
However, both LMP and GT cars not only produce downforce but normally deal with a very high amount of it. LMP cars, in particular, are extremely effective in producing big amounts of downforce, allowing them to negotiate corners at very high speed.
Before we deal about how downforce and drag are generated, let’s try to understand in a bit more detail why and how they affect car performance.
It is pretty straightforward to understand drag’s effect: drag is a force that acts against car’s forward movement and that try to decelerate the vehicle. This means that, for the car to advance at a fixed speed, the powertrain will have to be able to transmit a net torque to the wheels (and, hence, a force at the road through the contact patch), in order to win drag’s resistance.
In the picture above, Fpow is the force that the road applies to the car (and that actually reacts to the torque produced by car’s powertrain) to let it advance at a certain speed (or accelerate, in case Fpow has a bigger magnitude than Fx). Aerodynamic drag is the element mainly influencing a race car top speed.
To understand the benefits of downforce on cornering speed, we will have to take a look again to one of the plots we analyzed when we dealt with tyres. We know that a tyre friction circle (or ellipse, or cornering potential; different definitions of the same concept) gets bigger, as the vertical load acting on the tyre increases (at least up to a point).
The advantage of having downforce can be understood by looking at the previous plot. If the load acting vertically on a tyre increases, the magnitude of the force the tyre can exchange with the road also grows, although at a slower rate as the vertical load gets bigger and bigger.
This means that, if the car produces downforce, the tyres will be able to produce a bigger force to accelerate the car or, in other terms, will have more grip. In fact, if the available tyre force is bigger, the achievable lateral/longitudinal acceleration will also get bigger, in accordance to Newton’s second law:
- m is car’s mass;
- a is car’s acceleration
In plain English, since we can assume car’s mass doesn’t change, this means the driver can negotiate a corner at a higher speed, brake harder and later approaching a turn or go easier on the throttle exiting a bend. The net force “F” applied by the road to the car will be bigger and, consequently, “a” will also have a bigger magnitude.
In a corner, our “a” will be the centripetal acceleration Ay. For a given turn radius “r”, Ay is directly related to cornering speed “v” by the following equation:
This equation tells us that, if for a given corner radius we can achieve a higher lateral acceleration (and this is what happens if the tyre forces are bigger), our cornering speed will be higher.
Since aerodynamic forces depend on the square of car’s speed with respect to air, we can deduce that this “magnifying grip” effect will get more and more important as we approach faster corners (bigger corner radius) or section of the track where the car travels at higher speed.