Relation Between Vehicle Dynamics and Aerodynamics

Last edited: 2024-10-30 14:10:54

Thumbnail

In this post, vehicle dynamics parameters, such as slip, pitch, roll, and ride height are described which can affect the aerodynamic performance of a car.

Steering Angle

The steering angle is the angle between the direction of a wheel and the car's longitudinal axis. The steering angle for each front wheel can be different if the car has Ackermann steering or anti-Ackermann steering. An illustration can be found in the figure above.

Body Slip Angle

Body slip angle

The body slip angle is a yaw angle meaning it is the angle that is created when the car rotates around its vertical axis. The rotational axis goes through the car's center of gravity. The body slip angle is the angle between the velocity vector of the car and the car's longitudinal direction. A visual representation of the body slip angle can be seen in the figure above.

Pitch Angle

Pitch angle

The pitch angle is the angle created if the vehicle's sprung mass (everything but the wheels) rotates around its lateral axis, as shown in the figure above. The vehicle rotates around what is called its pitch center.

Roll Angle

Roll angle

The roll angle is the angle that is created if the vehicle's sprung mass rotates around its longitudinal axis, see the figure above. The vehicle rotates around what is called its roll center.

Ride Height

Ride height

The ride height is the smallest distance between the ground and the car's floor when it is stationary, see the figure above.

Aerodynamic Influence on Vehicle Dynamics

Vehicle dynamics is a large and complex subject. Mass distribution, tires, suspension setup, and aerodynamics are examples of areas that greatly affect the dynamics of a car.

One of the main aims of aerodynamics on a race car is to increase its negative lift force (downforce). Greater downforce makes it possible for the tires to produce larger forces (grip) while accelerating/braking (longitudinal forces) and cornering (lateral forces). The basic formula describing this relationship is Flong/lat=FzμF_{\text{long/lat}} = F_z \mu, where Flong/latF_{\text{long/lat}} is the longitudinal or latitudinal force, FzF_z the normal force which is increased with increased downforce and μ\mu the friction coefficient. Consequently, the speed of the car can be increased in most driving scenarios or the same grip can be achieved with less tire slip, i.e. smaller slip angle, when compared to a car with less downforce. This helps reduce heat production and therefore preserve tire life. Furthermore, smaller slip angles are easier to control for the driver. The car's stability in this case is improved compared to larger slip angles and aerodynamic parts tend to work more efficiently, improving the balance.

In most cases, a weight and aerodynamic distribution close to 50/50 on the car's rear and front axle is desired. A car with the center of gravity (COG) closer to the front axle usually generates more understeer. Conversely, the aerodynamic center of pressure (COP or aero balance), the point at which the resultant downforce is located, closer to the front usually generates more oversteer. A car with a front-biased COG can therefore be counterbalanced by placing the COP forward. However, it is favorable in some cases to design the aerodynamic package with a COP slightly rear-biased. This can generate understeer, but understeer is considered more stable and easier to control when compared to oversteer.

The Front Wing

Vehicle dynamics parameters described above are closely related to the aerodynamic package on a given car. While the generated downforce affects body slip, pitch, and roll angles, these parameters in turn affect the aerodynamic efficiency. Pitch angles for example can have a large impact on vehicle balance. When braking and pitching forwards, the front wing's/front splitters's downforce is generally increased.

However, other factors such as already low static ride height or an extreme pitch angle might cause a decrease in downforce for the front wing. This also results in less airflow to the car's floor and diffuser, decreasing the middle and rear downforce. If the front wing downforce increases, the aerodynamic balance shifts forward, decreasing the stability of the car. While this is considered unfavorable, increased grip on the front axle also results in improved braking capabilities. On the contrary, if the downforce on the front wing decreases the balance shifts rearwards and the opposite occurs. It is more difficult to describe the influence of steering and roll angles. These affect the aerodynamics in an asymmetric manner and can cause unpredictable behavior in the car. The same is true for body slip angles, or yaw angles, where the airflow hits the car with an angle from the longitudinal axis.

The Diffuser

A diffuser expands the airflow underneath the car's floor. This expansion forces the flow upstream to increase in speed in order to fill up the larger volume in the diffuser, decreasing the pressure upstream. In motorsport, it has been observed that diffusers are particularly effective aero devices. The reason is that diffusers increase the vehicle's downforce under the whole floor. Downforce in this region is close to the center of the car and distributed over a large area. This results in a good balance contribution and makes the vehicle less sensitive to pitch or other phenomena that affect the balance.

In order for a diffuser to work efficiently, the front wing needs to supply the floor and diffuser area with sufficient airflow. Optimally the airflow's velocity is large and laminar. Another option for increasing the airflow is by feeding the diffuser with air from the sides of the vehicle. This can however interfere with the low-pressure diffuser region and increase the pressure, resulting in a decrease of downforce. A compromise between keeping the floor sealed and increasing the airflow is usually optimal. This airflow can also be used to induce vortices.

Was the post useful? Feel free to donate!

DONATE