In the last article, we discussed in basic terms how downforce and drag will affect the race car performance within the grip-limited and power-limited sectors. In this article, we will go into more detail about the aerodynamic performance, particularly downforce, across the car ride height range and how it will affect lap time.
Downforce refers to the total vertical aerodynamic force pressing the car down towards the ground. For the car to be driveable, the aerodynamic balance must be within an acceptable range. The balance describes the contribution of the total downforce acting on the front axle. For example, an aero balance of 60% means that 60% of the downforce is on the front and 40% on the rear axle. In combination with the weight distribution and front/rear tyre cornering stiffness, it will determine if the car has oversteer, understeer or neutral behaviour at a given speed.
A corner can be split into different stages such as braking, entry/turn-in, apex, exit and acceleration. And within a lap, corners can be classified as low, medium and high speed. The car ride height will vary significantly across those different stages and types of corners, and it happens that the aero balance can also vary significantly as a result.
If we take as an example a Formula 1 car approaching a medium-speed corner from a long straight when the driver hits the brakes the car will be at a speed above 300km/h, its DRS is closed or closing and therefore it will have near-maximum aerodynamic downforce. Its suspension and tyres will be compressed, and its overall ride height will be very low.
The deceleration will cause the front to dive further and the rear end to raise. This high pitch state will in turn mean that the front wing will get closer to the ground and will generate significantly more downforce while the diffuser will move away from the ground and will generate less.
The aero balance will therefore move forward at this stage, sometimes in a very drastic manner. This condition can sometimes become the limiting factor for the car setup, if the driver cannot cope with the level of rear-end instability during braking, then the baseline aero balance might need to move rearward with changes in the aero setup.
As the car speed drops through braking, so will the downforce which in turn means that the front and rear ride heights will begin to raise. As the front wing moves away from the ground again, the aero balance will move rearwards. By the time the car reaches corner entry and apex the aero balance will have migrated rearwards significantly.
When the driver finally presses the throttle, the rear end will drop while the front will raise, meaning that the aero balance will migrate further rearward. This tends to be a good thing since the driver would favour rear downforce to be able to accelerate quickly, however, the lack of front downforce can make the control trickier in certain conditions.
To complicate things further, the magnitude of the aero balance changes across the cornering stages will be different in low, medium and high-speed corners. Thus, when setting up the aero package for a given track, the driver and engineers will be faced with many trade-offs, they might need to accept medium-speed apex understeer to be able to control the rear end during braking or might have a car out of balance in low speed to maintain a good balance in high-speed corners.
The significance of these trade-offs will be somehow related to the variability of the aerodynamic performance across the map. A car which performs very highly within a narrow window, but its performance largely drops outside, will likely face larger compromises across a lap than a car with a “less peaky” aerodynamic performance distribution.
The overall aerodynamic performance of a racing car, therefore, needs to be judged not only by the absolute levels of downforce and drag at one or two operating conditions but also by the variability of its performance across its entire operating range.
In our next article, we will further discuss how aerodynamic driveability is affected by steer, roll, yaw and flow curvature.