How very low ambient temperatures impact on the aero of WRC cars

Next week a new event will join the WRC Championship. Held inside the Arctic Circle, the Arctic Rally Finland will take place in the heart of the Lapland, around Rovaniemi. Very low temperatures are expected during the rally weekend, similar to those that teams have experimented with during the pre-event tests, most of them at -20ºC and with peaks of -30ºC. We already reviewed in a previous post the impact of a snow event such as Rally Sweden. But how such low temperatures can impact the aero of the WRC cars is what we will try to explain in the following lines.

The first impact on the aero is related to the air density. The lower the ambient temperatures, the higher the density of air. But, to what extent? The graph below shows how it changes from the standard conditions we can find in a summer event (25ºC) to the freezing temperatures that teams could face in the Arctic event (-25ºC).

Variation of air density due to ambient temperature

The air density increases from 1.18kg/m3 at 25ºC to 1.42 kg/m3 at -25ºC, which is a 20% increase*. As the density has a direct impact on the aerodynamic forces (the lift force in this case), a 20% increase in air density translates into a 20% increase in downforce. This means that the car will have to deal with higher vertical forces, so that suspension has to be set accordingly. Once this solved, the result is a higher grip. Together with the very high speeds expected on the Arctic stages, it means that corners can be done at a faster speed. To what extent? It will depend on the confidence of the drivers on the aero, as well as on their ability to keep the car inside the road, far from the snowbanks.

*Note that even in the case that temperatures were finally around 0ºC, the increase in air density (and downforce) would be still 10% over an event held at 25ºC, which is not negligible.

O.Solberg/A.Johnston, Hyundai i20 Coupé WRC, Arctic Rally Finland pre-event test, February 2021 – picture by Colin Clark

A second impact of the change in density involves the air resistance (drag): as the drag force is also directly dependant on the fluid density, a 20% increase in air density translates into a 20% increase in the air resistance*. The result is a reduced top speed. This effect will be compensated by the route and the road conditions, which will surely allow reaching very high speeds, placing the Arctic event as one of the fastest events of the year… or the decade.

*Note that even in the case that temperatures were finally around 0ºC, the increase in air density (and drag) would be still 10% over an event held at 25ºC, which is not negligible.

Top 10 of the fastest stages in the WRC Championship in the period 2017-2021 – data from ewrc-results.com

Just as a reference, the table above includes the 10 fastest stages in the last four years in the WRC Championship, by the current WRC car generation. Outside of the WRC Championship, the fastest average speed was set by Kalle Rovanperä and Jonne Halttunen driving a Toyota Yaris WRC in the 2020 edition of the Arctic Rally when they set a pace of 143,9 km/h in Untamovaara. A prelude to what we can expect next week?

E.Evans/S.Martin, Toyota Yaris WRC, Arctic Rally Finland pre-event test, February 2021 – picture by Henri Vuorinen Photography

The third impact of the very low ambient temperatures is related to cooling. As the cooling systems of the cars are designed to remove heat at positive temperatures, when the air is so cold there is the risk of cooling down in excess. The result is to keep working temperatures below the ideal range, thus reducing the performance of the engine, breaks and transmission. For this reason, in the winter events, teams usually include some elements such as radiator blankets, to reduce the amount of fresh air fed to the cooling elements (radiators, intercoolers, brake disks and transmission coolers).

In the case of the Arctic Rally Finland, we have seen already in previous editions how these solutions were brought to the extreme. The image below shows the radiator blanket disposition that Toyota installed in the 2020 Arctic Rally in the Yaris WRC of Rovanperä/Halttunen.

K.Rovanperä/J.Halttunen, Toyota Yaris WRC, 2020 Arctic Rally, 1st – picture by Taneli Niinimäki

In this year pre-event tests, as well as in winter events such as the recent Otepää talveralli, we have seen the different solutions the teams have been evaluating: different radiator blanket sizes, as well as blocked front and rear brake air inlets, to prevent brakes from working at too low temperatures.

O.Tänak/M.Järveoja, Hyundai i20 Coupé WRC, 2021 Otepää talveralli, 1st – picture by Hyundai Motorsport

Also, the use of mud flaps is expected in both front and rear wheels, in order to prevent icing on the car surfaces.

T.Neuville/M.Wydaeghe, Hyundai i20 Coupé WRC, 2021 Otepää talveralli shakedown – picture extracted from Power stage video

Finally, an indirect consequence of the very low temperatures is related to the reliability of the aero parts. In the past, the snowbanks were the most efficient way to get through the corners fast, as the vehicle could lean on them without having any damage (most of the time). But now, the presence of multiple aero parts discourages this practice, at the risk of losing rear fenders or front splitters or dive planes, which would result in a sensitive loss of the aero performance. The most frequent is likely to be the loss of rear fenders, the main impact of which is an increased drag generated in the car wake, resulting in some loss of top speed, as well as impairing rear stability.

In summary, the very low ambient temperatures at the Arctic Rally Finland will facilitate WRC cars to turn faster, to be slower on the straights as well as to deal with cooling and reliability issues. A mixture of conditions that will bring additional interest to an already exciting event.


 

 

4 thoughts on “How very low ambient temperatures impact on the aero of WRC cars

  • 2021-02-18 at 05:08
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    Can’t wait. It’s been too long since the last proper snow rally owing to global warming, politics, and WRC’s general ineptitude of hosting and planning the ‘perfect event’. Some technical questions as usual, if you don’t mind:

    1. Is every car’s internal fluids homologated? If so, then do the teams have special dispensation to change them in extreme ambient temperatures such as the 2021 Arctic Rally?
    2. I saw Hyundai seriously chopping off their rear fenders/’DVD stack’ in place of mudguards. Do this change require a joker?
    3. What will the teams do against misting/frosting of their various windows and screens? Are there any regulations for this?

    Reply
    • 2021-02-18 at 19:32
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      Yes, it will be great to get back to full snow again! And great to have questions again, thanks!
      1. I don’t see any reference to cooling fluids in the technical regulations (while other fluids such as those from shock absorbers have some limitations), so I guess they can freely use the most appropriate for so cold conditions.
      2. Not sure if they chopped them off or it was most probably the result of impacting on snowbanks. In the case it was done on purpose, for sure it should be homologated. It’s only the stack distribution/orientation that do not need to be homologated with a joker, just a Variant Option (which is not limited).
      3. Regulations only say that protective transparent film can be added to the outer side of the windshield. Also, the removal of the original heating apparatus is allowed, provided that an electric or similar demisting system is fitted. Also, the original air-conditioning system can be removed. So I guess they will play with the roof scoop and any other demisting element they have. Plus the use of mudflaps to prevent ice formation over the external surfaces of the car, which might affect some aero part’s performance.

      Reply
  • 2021-02-18 at 20:05
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    Hello again !

    First, does the denser air also increase downforce under the car (diffuser and floor) not only over the wings ?

    Secondly as Finland has bigger snowbanks and the road surface is probably more icier than in the last years, does the car setup change more drastically ? For example compared to the last year Sweden rally. Ice provides more grip and (from this article 😉 the downforce will increase naturally by air temperature. Also the road gets narrower. Are the teams making cars more agile or stiffer as the road is narrower and more precise driving is needed.
    And finally, what do you think, are the aero attack angles going to increase or decrease to provide/reduce downforce as the air is so much denser ?
    Cheers,
    Laur 🙂

    Reply
    • 2021-02-19 at 17:13
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      Hello Laur, and thank you for your questions, they would deserve a new post.

      Under the car, due to the small space, the increase in air density will probably lead to thicker boundary layers (viscous layers the fluid generates close to the surface), thus the airspeed will be reduced. The result is higher pressure, thus less downforce will probably be generated there.

      The car setup has to be different, last year Sweden Rally was more of a gravel event. The goal has to be to avoid as much as possible contact with snowbanks, so they should be as precise as possible… even though they’re on ice with studded tires.

      Regarding the angle of attack: an increase of this angle usually gives you more downforce and more drag. In the cold temperatures, as the downforce is higher, you can reduce the angle of attack and still get enough downforce, while reducing the drag at the same time. So this is one of the advantages of these conditions. Although the angle of attack can only be modified with the car’s rake, that is the inclination of the car, by adjusting the suspension set-up accordingly.
      Cheers, Lluis

      Reply

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