WRC cars’ brakes cooling airflow review: air removal

In a previous post, we reviewed how fresh air is forced into the brakes of a WRC car for cooling purposes, and the aerodynamic implications it has. Once the braking system cools down, the hot air accumulates in the wheel spaces and under the car, which then has to be removed. How each car designer team solved this problem and even took advantage of it is the topic of the present post.

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Deutschland 2017, 3rd – picture by Nacho Mateo – eWRC.cz

The addition of air into wheel spaces is beneficial in terms of cooling, but it has a negative impact on the aero performance of a WRC car. Firstly, because forcing air to enter inside the car increases the total air resistance (drag) of the car, in comparison with a car with no air intake (due to the loss of energy the air faces while traveling inside the car). Secondly, we are adding a huge amount of air inside the wheel spaces and also, under the car. The consequence is that pressure increases under the car, while airspeed is reduced, and downforce generation by pressure difference with the top of the car is reduced. So, the air has to be removed as quickly as possible from the wheel spaces. Each manufacturer has designed different solutions for that, which differentiate from front to rear wheels.

Front brakes’ air removal

The original design of the front fenders of the 2017 WRC car generation was simple. Toyota and Ford (left and center in the picture below) included a long, vertical vent, in order to take advantage of the air removal along the sides of the car. The Yaris only included two small separators, so air could flow in parallel to the car, while the Fiesta included some deflectors that sent the air upward. On the contrary, Hyundai included a small vent with four louvers. The addition of louvers contributes to getting a more stratified exit flow (less turbulent). The higher the turbulence, the higher the drag this flow would cause, and the higher the disturbance on the air flowing on both sides of the car.

From left to right: J.Hänninen//K.Lindström, Toyota Yaris WRC, Rally Catalunya 2017 – D.Sordo/M.Martí, Hyundai i20 Coupé WRC, Rally Catalunya 2017 pre-event test – S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Deutschland 2017

Citroën’s design followed a different approach to that of the other teams. Instead of a rear vent, they chose to include a vent at the top of the fender. The vent included a small lip designed to protect the hot air exiting from the wheel space (in yellow in the picture below) from the external mean airflow (in blue).

S.Ogier/J.Ingrassia, Citroën C3 WRC, Rally Catalunya 2019, 8th

Not surprisingly, all the other teams modified the front fender vent design in the next seasons. Since 2018, Ford adopted an even simpler solution, by removing the small louvers and reducing the vent to a single exit on top of the rear of the fender.

T.Suninen/J.Lehtinen, Ford Fiesta WRC, Rally Catalunya 2019, 7th

For the 2019 season, Hyundai redesigned the front exit, increasing its size and increasing the separation between the louvers. The result was a better distribution of the airflow on the sides of the car. Lower louvers were designed to send airflow down to the side skirts, which resulted in contributing to pressing them down to generate downforce.

D.Sordo/C.Del Barrio, Hyundai i20 Coupé WRC, Rally Catalunya 2019, 3rd

The most innovative design was implemented by Toyota in the interseason before 2018. While all cars used the front fender vents to remove the hot air from brake’s cooling, they decided to use it to also remove hot air from the engine bay. Due to the high area required to remove such a large amount of air, they split the exit into two parts and they extended the vent to the top of the fender: the air coming from the engine bay was removed through the top vent (in red in the picture below as it is hotter), while the air coming from the brakes was removed through the rear vent of the front fender (in yellow).

O.Tänak/M.Järveoja, Toyota Yaris WRC, Rally Catalunya 2019, 2nd – hot air exiting from the engine bay (yellow) and brakes (red)

The opening of a vent at the top of the front fender forced Toyota to include a small deflector ahead of the vent, with the same purpose of the lip included in the Citroën: to deviate the main airflow (in blue in the picture below) from the exit, thus generating suction that contributed to the removal of the hot air from the engine bay (in red), while hot air from brakes (in yellow) remained unaffected due to the location of the vent.

O.Tänak/M.Järveoja, Toyota Yaris WRC, Rally Catalunya 2019, 2nd – hot air exiting from the engine bay (yellow) and main airflow (blue)

In summary (and even though front fender vents are thin and they are located in a relatively smooth area of the car, limited by the main, external flow above and on the side of the vented airflow), some positives can still be taken from this airflow. In the Fiesta, the air is directed to the rear of the fender where the interaction with the external flow is minimal and the removal of hot air is easier. In the Toyota and Hyundai designs, the airflow is directed just above the side skirt, to get some additional pressure. Or, in the absence of mud flap, the air is dragged by the high-speed front-wheel wake.

T.Neuville/N.Gilsoul, Hyundai i20 Coupé WRC, Rally Catalunya 2019, 1st

Rear brake’s air removal

Although cooling requirements are smaller in the rear brakes (due to the lower demand and usage), cooling air still needs to be efficiently removed to avoid pressure increase in the rear of the car. The picture below shows the different designs introduced in 2017 by the four WRC manufacturers.

From left to right: E.Lappi/J.Ferm, Citroën C3 WRC; O.Tänak/M.Järveoja, Ford Fiesta WRC; D.Sordo/M.Martí, Hyundai i20 Coupé WRC; J.Hänninen/K.Lindström, Toyota Yaris WRC, Rally Catalunya 2019 (Lappi) and 2017 (rest)

Citroën again came up with a different solution, with the introduction of a single vent on the top of the rear fender. As with the front fender, a lip was again included to prevent the interaction of the hot air from the wheel space with the external, main flow. The airflow leaving the wheel space was then liberated without any restriction, and there was no intention of redirecting the flow in any direction. Also, an additional vent was positioned behind the wheel, although this exit became blocked in some events (as in the picture below).

S.Ogier/J.Ingrassia, Citroën C3 WRC, Rally Catalunya 2019, 8th

Hyundai redesigned the rear fenders at Wales Rally GB in 2017. The area of the vents and the number and width of louvers were increased. Note that the louvers were repositioned to facing the outside of the car, allowing that part of the air was oriented towards the external side of the car, in interaction with the main, external flow.

D.Sordo/C.Del Barrio, Hyundai i20 Coupé WRC, Rally Catalunya 2019, 3rd

Ford also introduced a new design of the rear fenders’ vent at Rally Finland in 2018. The whole fender was modified: the vent area was increased and flat louvers were included, following Toyota’s proposal. For the start of the 2019 season, the lower, external side of the fender was cut, to allow more effective removal of air from the wheel space.

E.Evans/S.Martin, Ford Fiesta WRC, Rally Catalunya 2019, 6th

The use of wide vents and louvers allow a stratified distribution of air into the car’s wake. The goal is to reduce the amount of turbulence that a nonstratified exit would add to the car’s wake. A turbulent wake adds drag to the car, so any contribution to reducing turbulence is welcome.

The geometry of the louvers determines the direction of the air when it leaves the fender: in the case of the Fiesta and the Yaris, the air is sent upward.

Toyota Yaris WRC (left) and Ford Fiesta WRC (right) rear fender vent, Rally Catalunya 2019

Images from damaged fenders allow us to see their internal geometry, such as in the case of the Toyota in the picture below. The image confirms that mud and snow can easily accumulate inside the vent due to the shape of the louvers.

O.Tänak/M.Järveoja, Toyota Yaris WRC, Rally Australia 2018, ret. – picture from WRCplus images

In the case of the Hyundai i20, the air is sent into different directions: upward from the upper louvers, horizontal from the center louvers and downward from the lower louvers. Such heterogeneous distribution allows for better air removal. Also, the image of a damaged Hyundai rear fender shows the internal geometry, where the probability of mud and snow accumulation is smaller than in the Toyota or in the Ford.

D.Sordo/C.Del Barrio, Hyundai i20 Coupé WRC, Rally Argentina 2019, 6th – picture from WRCplus images

To minimize the risk of blockage of the rear fender vent due to the accumulation of mud or snow, Toyota introduced a simplified design with a drastic reduction in the number of louvers in 2019. Since then, it has been only used sporadically, in Sweden 2019, 2020 or Argentina 2019.

S.Ogier/J.Ingrassia, Toyota Yaris WRC, Rally Sweden 2020, 4th – picture by Toyota Gazoo Racing WRT

To get the same effect (minimizing the louvers’ blockage), Ford intentionally blocks the lower louvers in some events. The higher airspeed in the remaining louvers allows the removal of any accumulated material, thus the risk of blockage is minimized.

E.Lappi/J.Ferm, Ford Fiesta WRC, Rally Sweden 2020, 5th – picture by MSport

The goal of the louvers is to send the air into the most adequate section of the car’s wake, where three different areas can be identified.

E.Evans/S.Martin, Toyota Yaris WRC, Rally México 2020, 3rd – picture by Toyota Gazoo Racing WRT

In the image below the three different areas of the wake are delimited with green dotted lines: an upper section originated by the flow over the rear wing, a central section with lower airspeed, and where some recirculation loops can be identified (which generate additional drag), and the lower section (high velocity and turbulence) generated by the rear wheels and the rear diffuser. Forcing the air from the rear fender into the central part of the wake ensures more efficient removal as well as adding some energy to this portion of the wake, contributing in reducing the amount of recirculation in order to reduce drag.

E.Evans/S.Martin, Toyota Yaris WRC, Rally México 2020, 3rd – picture by Toyota Gazoo Racing WRT

The video contained in this tweet from WRCMotorsport visually confirms where the air is sent from rear fenders vent.

In conclusion, removing the brake’s cooling air is necessary in order to not reduce the aero performance of the WRC cars (to keep downforce generation intact), and the teams have found the way to take advantage of it by improving the aero.

10 thoughts on “WRC cars’ brakes cooling airflow review: air removal

  • 2020-04-04 at 09:12

    Thanks again for a great content in this downtime! While I didn’t get the chance to comment on your previous few posts, I decided to do it now while I have the time. A few technical questions and requests, if you don’t mind:
    1. In other (unfortunately more popular) championships, staff movements are rather exaggerated in terms of their influence on the ‘DNA’ of the car. While for some figures this is true (Newey, Dall’igna, etc.), I feel WRC doesn’t have the same focus on this issue. Seeing some of the teams following each other and some goign their own ways, do you know of any transfers between the 4 factory teams (ca. 2019) to allow them to follow or reject others’ philosophy? Or is the aero in WRC so uncomplicated they’re able to work from photographs alone?
    2. In single-seaters (and some extreme prototype bikes), airflow at the rear of the car tends to be manipulated in a single direction (upwards and outwards). Why does Hyundai insist on diffusing the pathways while others don’t? Do you see another advantage other than a cleaner excavating airflow? I don’t think that’s contributing too much to performance.
    3. I greatly enjoy your more recent historical articles. Going back to my first (1) point, do you see a ‘linage’ in how factory teams built their cars, similar to how other technical journalistic experts coin the term in other championships?
    4. Do teams take samples from stages to gain a better understanding for kinematic set-ups, like how tarmac racers 3D-scan the circuits?

    Thanks for the hard work, and sorry for the long post!

    • 2020-04-06 at 17:22

      Thanks for your interesting comments, Kevin. Here’s what I think about each of them:
      1. Engineer’s transfers between teams have been rather unusual in WRC in recent years, while it was very common years ago. Each team started with his own philosophy in 2017, and results forced all of them to constantly improve. In some cases, they tried with original solutions and in some others, they got inspiration from other teams. But even in this case, it is not easy, as you have to adapt these designs to your car characteristics, and this stills requires hours of CFD, wind tunnel and test validation.
      2. I’m sorry I don’t get you, what do you refer to by diffusing the pathways?
      3. If we talk about linage, M-Sport would be the reference. Not only because of the years of experience but for their ability to create fast and effective cars in short periods of time, far from the massive resources from car manufacturers (just punctual support from Ford Performance). Citroën also had a very good reputation behind, while Hyundai is building it year by year (consolidated last year with the manuf title), as well as Toyota with the new structure (lead by Tomi Makinen).
      4. No. They are not allowed to test on real stages, and the amount of kms. is so big and varied that it would not be practical. They keep doing pre-event test before each rally in similar surface stages (tests are resticted only to Europe), where they constantly measure the ride height of the car (see a light pointing to the road under the front of the car in test pictures).
      Thanks again!

      • 2020-04-07 at 12:45

        Regarding Point (2), I’m speaking of the general direction the airflow is directed, as indicated by your graphics in the photos along with other tech journalists in other championships. Hyundai is manipulating the air in so many different directions when compared to its contemporaries, who mostly choose to deflect it upwards and outwards only – more in line with single-seater rear bodywork. Why do you think this is, and what’s the dis/advantage it can give the car? I seem to notice the Hyundai drives with quite a bit more understeer than the other cars; is this the result of the aero and chassis philosophy they’ve chosen?

        • 2020-04-13 at 17:38

          Thanks, Kevin! They have probably found that sending the air into different directions helps in some sense; otherwise, they would follow other teams’ trend. But I don’t see how do they take advantage from it. Regarding the understeer, it is possibly the consequence of a not completely balanced car, and still more load needs to be generated at the car front to equilibrate.

  • 2020-04-10 at 11:11

    Muchas gracias Lluis.
    Siempre tan claro.

    • 2020-04-13 at 17:31

      Moltes gràcies! Una abraçada!

  • 2020-05-08 at 13:51

    This may be a stupid question… But how is the air getting from the brake disc/wheel well, to the louvers? In photos I’ve seen in service parks, there does not seem to be any obvious openings in the fender liner to provide an escape path for the air, especially in the front leading to the fender vents: https://www.wrcwings.tech/wp-content/uploads/2019/06/C3-Bruce-Thomas-Rallysportmag.jpg

    Unless only a small opening is required along the side of the liner. In the case of the C3 it almost looks like the entire wheel well was sealed off.

    • 2020-05-13 at 15:17

      No question is stupid…some answers are. It is true that in some (gravel) events the air exit to the louvers can be partially or totally blocked, but the exits are there. In the case of the C3 WRC, the exit is in the inner part of the fender, hardly visible.

  • 2021-08-04 at 04:11

    Great to see your article about how teams take the air from the break ducts away!
    I have a question:Since the 2022 regulation has banned all these fenders,will it create more drag?

    • 2021-08-04 at 15:08

      Glad that you like it, David. The 2022 regulations seem to ban the rear fender vents, as confirmed by the images of the new cars during the tests. The result is that air fed to brakes for cooling will not be removed through the fender vents in 2022. This will cause air accumulation under the car, and the result is an increase in lift (which is the opposite to downforce), thus resulting in a loss of grip. Also, front fender vents were used to force hot air to flow towards the rear wing or towards the side skirts, which in both cases had a positive effect on downforce, while rear fender vents were used to redirect air to some areas of the car wake, to minimize air recirculation behind the car, resulting in less draggy wakes. In conclusion, removing the fender vents will have a significant impact on downforce and drag in 2022. This, combined with the removal of the rear diffuser, means that the loss of grip will be important, and speed on corners will be significantly lower, in my opinion.


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