Understeering and aerodynamics of a WRC car

Understeering was the second issue most cited during the last Tour de Corse… the first being puncture. Aerodynamics have little to do against punctures, but a lot to do on minimizing understeering. Let’s see how.

First, let’s understand the phenomenon. A good starting point is the explanation offered by Škoda Motorsport in one of their articles of the highly recommended series Drive like a Pro.

picture by Škoda Motorsport

According to Škoda understeer happens when there is either too much load on the front tires, or they don’t have enough traction. To correct it, you can either decrease the forces applied on the front tires, typically by easing off the accelerator or increase your traction by moving the weight of the car forward.

Some key factors are definitely involved in this phenomenon: the car weight distribution itself or the amount of acceleration/braking at the corner entrance, all having a significant impact on the weight/load distribution of the car. During the design process all of them need to be carefully considered and optimized.

picture by Vladislav Maschl – eWRC.cz
O.Tänak/M.Jarveojä, Toyota Yaris WRC, Tour de Corse 2019, 6th

Once all this is defined, aerodynamics play an important role in moving the weight of the car forward or backward: depending on the aero design, the distribution of load generated by air flowing over the car will be different, with the result of improving or worsening the car balance.

To determine the quality of the impact of the aerodynamics of the WRC car, we need to review two simple concepts:

– one is the Centre of gravity, or the point on a car where it can be assumed that the entire body weight is concentrated,

car picture by http://www.christiancazalet.com

– the other is the Centre of Pressure, or the point where the resultant force of all the aerodynamic load acting upon a car moving through a fluid (air in the case of a WRC car) is applied.

Depending on the amount of downforce generated at the car front and rear, the center of pressure will be located at a different location. The next three figures below show the three possible cases we can find.

Case 1

In case 1, both centers of gravity and pressure are located at the same point: the result is a neutral car, as the pressure distribution and the weight distribution are the same, and all axles receive the same load. This means the wheels will turn at exactly the angle determined by the driver (unless other factors appear, such as tire degradation or suspension problems).

Case 2

In case 2, the center of pressure is located behind the center of gravity. This means the rear axle suffers a higher load than the front axle. The lower grip at the front causing that the wheels are reluctant to turn, and the car becomes understeering.

To solve or minimize this problem, the aero package has to be designed to increase the amount of downforce generated ahead of the center of gravity, in order to move the center of pressure ahead, until the balance is reached.

Case 3

In case 3, the center of pressure is located ahead of the center of gravity. This means that the load on the front axle is higher than in the rear axle, which means lower grip at the rear. Lower grip means that rear wheels will find less resistance to turning, and the car will become oversteering.

picture by Michelin
E.Evans/S.Martin, Ford Fiesta WRC, Tour de Corse 2019, 3rd

How front downforce generation can be increased, to reduce understeer? The most appropriate seems to generate additional downforce at the car front, by using some of the solutions already available, such as more efficient dive planes, front splitter or winglets on top of the front arches.

There is an alternative solution, although less attractive: to reduce the amount of downforce generated at the car rear, in order to balance the car, and make it more driveable. Not the optimal solution, but sometimes is the most effective. At least since new regulations (introduced in 2017) allowed the use of bigger rear wings and rear diffusers, and WRC cars boosted the amount of rear downforce generation.

picture by Michelin
T.Neuville/N.Gilsoul, Hyundai i20 Coupé WRC, Tour de Corse, 1st

Ogier and Neuville both complained several times about understeering and lack of grip during the Tour de Corse and after (such as in this recent interview to Ogier in Autosport). Coincidence or not, Citroën and Hyundai have announced modifications on the aero package for the coming months. Most probably, they will both include modifications to increase the amount of front downforce generation. Hopefully, the cars will be less understeering in Germany and Spain, the next tarmac events. But only time will confirm.

picture by Michelin
S.Ogier/J.Ingrassia, Citroën C3 WRC, Tour de Corse 2019, 2nd

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