Impact of altitude on the aerodynamics of WRC cars

The coming rallies of Mexico and Argentina include some of the stages at a higher elevation of all the season. As it is shown in the excellent Rally-Maps.com website, a stage like Ortega (SS3 of next Rally Mexico) is held at an average elevation over sea level of 2500 meters. Something similar happens at Argentinian stages like El Condor or Mina Clavero, both ending at elevations around 2000 meters over sea level. On the other side, some special stages in Corsica, Catalunya or Australia are held very close to sea level.

It has been widely reported the impact of altitude on the engine (power loss due to the lower presence of oxygen) or drivers (higher fatigue). And, not to forget, the cooling problems due to higher temperatures (Toyota’s main concern for the last two years). But, has altitude any impact on the aerodynamics of the WRC cars?

picture by Michelin
S.Ogier/J.Ingrassia, VW Polo WRC, Rally Argentina 2016, 2nd

To analyze it, we start by considering the atmospheric pressure as the pressure caused by the weight of the column of air accumulated over us (over a car in our case). If the car is at sea level, the column is higher, and so the pressure. As soon as we get elevation over sea level, the size of the column decreases, and so does the pressure. The figure below show the extent of atmospheric pressure reduction with elevation.

figure extracted from engineeringtoolbox.com
Atmospheric pressure versus elevation above sea level

At an elevation around 2.000 meters, the atmospheric pressure becomes 80% of the pressure at sea level. We can assume now that air density is directly proportional to pressure: so, the lower the pressure (or the higher the altitude), the lower the air density, as can be seen in the figure below.

figure extracted from engineeringtoolbox.com
Air density versus elevation over sea level

The value of air density at 2.000 meters is 0,94 kg/m3, which represents 78% of the value of air density at sea level. This means that the amount of air molecules is smaller (for the same volume), compared to air at sea level. What does it mean, in terms of aerodynamics?

picture by Michelin
K.Meeke/P.Nagle, Citroën C3 WRC, Rally Mexico 2018, 3rd

The WRC car at high altitude is facing a less dense air, so the resistance generated by air against the movement is smaller (as it is impacting on a smaller amount of air molecules). This means the drag is reduced. So, on one side, high altitude benefits the aerodynamics of WRC cars, and cars could reach higher velocities on straight lines than at sea level.

To quantify the effect, we have to know that the relationship between drag force and fluid (air) density is direct: this means that, at elevations around 2.000 meters, the reduction in drag can then be estimated to be around a 22%.

picture by Michelin
D.Sordo/C.del Barrio, Hyundai i20 Coupé WRC, Rally Argentina 2018, 3rd

On the other side, a less dense air means that the force that air generates on the WRC car, for instance on the dive planes or the rear wing, is smaller, as the air is lighter. Smaller (down)force means that the aero appendices are less effective, and the result is a loss of grip. Again, the relationship between downforce and the fluid (air) density is direct, for what, at 2000 meters elevation, the loss of downforce is a 22% versus sea level.

Finally, a lighter air means a lower cooling capacity, as less air molecules are working on heat removal. To compensating this impact, internal airflow needs to be increased (both air inlets and vents) in order to force more air inside the car and increase the heat removal capacity.

In summary, the change in elevation has a triple impact: less air resistance (less drag = higher speed), lower downforce generation (less grip), and poorer cooling capacity.

Current regulations do not allow to make any change in the aero kit of the car (unless a joker is used). So there is no chance to modify the aerodynamics for high altitude stages for compensating the impact of altitude. But engineers still can modify other parameters: softening the suspension settings due to the smaller aero load or increasing air inlet/vent area to allow more air flowing inside the car, to increase heat removal.

picture by Red Bull
S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Mexico 2018, 1st

In addition to engineers’ efforts, drivers have to adapt their the driving style to these conditions, specially in terms of having less grip when cornering. Altitude conditions are the same for all competitors. Those who better adapt to them will definitively have an advantage over the concurrence.

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