The impact of ice build-up on the aero of WRC cars

We reviewed in our last article the impact of the very low temperatures on the aero of the WRC cars. We have also seen in previous articles the different solutions the teams have designed to prevent snow accumulation on certain parts of the car, such as the front grills or the rear fender vents. But there is one additional factor that it is worth covering, due to its impact on car performance and aerodynamics: the presence of ice.

E.Evans/S.Martin, Toyota Yaris WRC, Arctic Rally Finland 2021 pre-event test – picture by Tapio Lehtonen/Rallirinki

The main problem of the accumulation of ice over a rally car is the extra weight: the ice and snow accumulated on the car at the end of a stage can add an extra 30-40 kg weight. This is higher than the weight of carrying an extra wheel (estimated around 23 kg), so any measure to reduce this penalty is really welcome.

But there is also a significant impact on the aerodynamics of the rally cars: the presence of ice (also frost or snow) disturbs the airflow around the car, resulting in aerodynamic performance degradation. On one side, the icy surfaces of the car become rougher, and the result is an increase in the air resistance (drag), limiting the top speed of the cars.

Ford Fiesta WRC, Arctic Rally Finland 2021 pre-event test – picture by M-Sport

On the other side, the ice accumulated on the surface of some aero parts renders them ineffective. The air velocity through an icy rear diffuser, such as the one in the pictures above and below, is significantly lower than under normal conditions. The result is that pressure increases under the car, due to the lower airspeed, and less downforce is generated, thus reducing the grip of the car and, by extension, the speed at the corners.

Ford Fiesta WRC, Arctic Rally Finland 2021 pre-event test – picture by M-Sport

So, once the enemy is identified, what solutions do teams use to prevent ice build-up? There are preventive and corrective solutions, and, although all of them work, any of them is totally effective, for what there is still room for improvement.

Citroën C3 WRC, Rally Sweden 2018 – picture by Michelin

The first (mechanical) solution to prevent ice accumulation is the use of mudflaps. The result is that the wheels do not spray water upwards, and less ice is formed on the sides and the rear of the car.

A second (chemical) solution is the use of deicing/anti-icing products. These are added to the parts most exposed to ice accumulation, such as the rear wing, the dive planes, the winglets, the fenders and the rear diffuser. They can be sprayed or washed before leaving the service park, to prevent ice accumulation. Typical deicing products are water-repellent, such as lubricants or waxes, or antifreeze, that is, water-based solutions containing organic products that freeze at very low temperatures, such as ethylene glycol (freezing point -40 ºC for a 52% solution) or propylene glycol (freezing point -48ºC for a 60% solution). The water-repellent products are the most commonly used in the WRC, while the antifreeze products are typically used in aviation, to prevent ice formation over the wings, propellers and control surfaces of airplanes, especially before takeoff.

T.Neuville/N.Gilsoul, Hyundai i20 Coupé WRC, Otepää talveralli 2021, 2nd – picture by Hyundai Motorsport

In spite of the use of these solutions, still, some ice is accumulated over the car at the end of a stage. In case there is no scheduled service before the next stage, there is only one (manual) solution to be taken: the use of ice scrapers, snow shovels and other manual tools to remove ice and snow from the car, thus minimizing the problems of extra weight and poor aerodynamics performance for the next stage.

The stages of the next Arctic Rally Finland will be a good opportunity to see how effective these solutions are and how teams deal with ice, in the search of optimal performance.

Thank you José Frade for inspiring this article.


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6 thoughts on “The impact of ice build-up on the aero of WRC cars

  • 2021-02-26 at 04:02

    I’m back! What I mean is my crappy ISP finally allows me to view your site normally without the need of VPN or proxies. Not sure what the original problem was; I’m just glad I can comment easily again. A few questions regarding this article, as usual:

    1. Does ice/snow physical accumulation impact the first or the last car worse? For example, does Ogier have something to worry about given he’s practically the entire field’s snowplough?
    2. I’m not too clear on the impact of differing snow, gravel, permafrost, and ice combination affects each car in regards to their running order. Which one benefits the first car, and which one is most feared by the first car? I was too absorbed by the latest DirtFish videos’ physical harming of Colin to notice.
    3. Does Pirelli have anything published regarding the maximum (or minimum) condition their latest snow/ice studded tires can handle? What I mean is sort of the deepest snow they can safely travel through, or the barest gravel they can drive over without the studs getting ripped off.

    Once again, thank you for the great coverage of this sport.

    • 2021-02-26 at 21:16

      Hi Kevin! Glad that you solved your ISP problems, and glad to have more questions, thanks!
      1. Sure, the first cars will probably accumulate more snow… about the ice, it can possibly depend on the ambient temperatures: more ice for the first cars early in the morning, while less in the afternoon until temperatures fell down.
      2. Same as in gravel stages, the first cars have to clean up the fresh snow (like the dust on gravel). Once ice covers the road, the studs do their job…. until ice is broken, and gravel appears. You can easily see that due to the presence of sparks (like today in SS2).
      3. I’m sure they know it but I bet they don’t publish it, as there is only one studded tire available, so… no alternative. But, from today’s images at the end of the day, studs seemed to remain in place, so congrats to them!
      Thanks for your interesting questions!

  • 2021-02-26 at 07:57

    Espectacular!!!! como siempre!!

    • 2021-02-26 at 21:17

      Nuestra audiencia, esa si que es espectacular. Muchas gracias!!!

      • 2021-02-27 at 11:35

        Looks like studs held in pirelli tyre, but studs collapse whith too soft rubber and too soft tyrebody

        • 2021-02-27 at 11:48

          Yes, Ari, but with only one studded tire available, it is the same for everyone. We’ll go into further detail on tires soon.


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