We have briefly analyzed what are the most important aerodynamic forces acting on a race car and why they exist. We will now look at some important parameters that relate to how these forces affect cars performance and behaviour on track.
As we said, we would ideally want to have a car producing a lot downforce and very low drag, thus combining the benefits of a very high top speed (or fuel economy) and very good grip during cornering or braking. As we will better see when dealing with wings, for a given car, increasing downforce normally leads to a bigger drag. In other terms, we can imagine drag as a price we need to pay to have more downforce or, alternatively, we can imagine downforce as kind of profit that we can obtain when investing in drag. For this reason, engineers normally define how effective the aerodynamic design of a car is using a ratio called efficiency and defined as:
Since, as we mentioned, aerodynamics theory has been initially developed with aeroplanes in mind (where the target is to have lift), efficiency is often defined also using the following alternative equation:
- L is car’s lift (downforce) force.
- D is car’s drag force.
Of course, the higher the efficiency the better for car performance.
As we mentioned already, the magnitude of downforce and drag depend strongly on car’s speed with respect to the air. Also, as we will see later on, downforce and drag also change depending on car ride heights: a vehicle’s ride heights vary not only because of road asperities but also because of downforce. Downforce is a force pushing the car down and, hence, bringing its components closer to the ground because of the deformation of elastic elements like springs or tyres, that deflects under load.
To have a less speed-dependent definition of efficiency, its equation is often rewritten as a ratio of CzA (downforce coefficient multiplied by car frontal area) and CxA (drag coefficient multiplied by car frontal area):
A car with a very high downforce but also a very high drag (hence, a car with a low efficiency), will probably not be that fast on a complete lap, because it will pay the advantage of a better cornering grip with low top speeds. On the other hand, a car with a very low drag but also a very low downforce (hence, again, with a very low efficiency) will have very high top speeds, but less cornering grip, thus not being able to obtain competitive lap times because of poor cornering performance.
Although certain tracks require very high downforce settings, even (up to a point) at the cost of efficiency, while others could indeed need lower drag (see Le Mans, for example), having a higher efficiency is always effective, in terms of performance. This is why aerodynamicists always try to design a car producing the highest possible aerodynamic efficiency.
Another important part of our discussion about the aerodynamic forces acting on the car is how these forces are distributed.
For a given car, travelling at a certain relative speed with respect to air, we can define a point called pressure centre, being the point where the resultant force of all the aerodynamic actions is applied. In the following picture, the pressure centre is identified as CP:
In terms of handling, the most important characteristic of the pressure centre is its position along car’s wheelbase, because it will define how much of the overall downforce will act on the front wheels and how much on the rear ones.
Looking at what shown in the picture above, said “l” our car’s wheelbase. The longer “a” is, the larger the portion of our downforce acting on the rear wheels. Conversely, the longer “b” is, the larger will be the portion of our downforce acting on the front wheels. In general, depending on the magnitude of Fz, the ground applies two reaction forces Fzf and Fzr respectively to the front and rear wheels so that:
We need to remember that Fzf is here the sum of the vertical forces acting on the two front wheels, while Fzr is the sum of the forces acting on the two rear wheels.
The definition of a car’s pressure centre leads also to the definition of another, somehow equivalent concept: the aero balance. The aero balance is the portion of downforce acting on the front wheels, with respect to the overall downforce acting on the car and is given by the following equation:
Where, beside the known Fz, Fzf and CzA, we have CzfA identifying the downforce coefficient relative to the front axle, multiplied by car’s frontal area.
Aero balance is normally expressed as a percentage and is a very important parameter, because it tells us which axle will tend to have more grip, as the aerodynamic forces grow (hence, in fast corners, where the aerodynamics effects are more dominant on grip). More rear downforce will make the car more stable and will generally produce a stronger understeering tendency in fast corners, because, as the speed increases, the rear wheels will get more and more load. On the other hand, more front downforce will lead to a bigger front grip in fast corners and to less understeer, but could also help in braking phases, as we will see.
Aero balance and center of pressure are related concept, as we said, but they don’t quite identify the same thing, as for the aero balance what matters is only pressure center’s x coordinate (hence, its longitudinal position).
Another important aspect about racecar aerodynamics is pressure center position relative to the center of mass (often identified as CG, as we saw in the tyres series).
Depending on CP distance to CG, the car will experience a moment around a horizontal (lateral) axis, whose magnitude will grow as the speed increases. Depending on CP being fore or aft the CG, this will influence both car ride heights and rake. With rake, we identify the attitude of the car or, in other terms, how far from the ground car’s underbody is at the rear axle with respect to how far from the ground car’s underbody is at the front axle.
If the center of pressure seats behind the CG, the car will tends to squat as the speed increase and, if other countermeasures are not in place, its rear ride heights will tend to reduce more than its front ride heights. In other terms, the rear end of the car will go down more than the front as the speed increases (again, assuming no specific countermeasures have been taken to avoid this specifically). The opposite happens if the CP seats in front of the CG.
In general, as we mentioned, anytime the x-coordinate of CG and CP will not be the same, downforce will create a moment that will tend to rotate the car around an axis parallel to the y one. This happens because, compared to the weight distribution between front and rear axle that the car has in static conditions (hence, with the car not moving and the only weight force acting on it), the added downforce will tend to load one axle more than the other. This means the dynamic load distribution (the portion of the overall load acting on the front axle, compared to the overall load itself) will not be the same as the static weight distribution.