Review of the Citroën C3 WRC aero test development in Catalunya pre-event tests

All pictures included corresponding to the Rally Catalunya pre-event tests held in the second week of October 2019.

Citroën Racing Team continues with the development work of the full aero package of the C3 WRC. Same as they already did last December when they tested new aero parts in French stages, they continued to evaluate modified aero parts in the Rally Catalunya pre-event tests carried out last week. Again, they tested multiple modifications, some of them developed enough to be included in the new car aero package (which could be introduced since Catalunya or left aside until Rallye Monte Carlo 2020). But other parts seemed just development parts, conceived to push the aero to the limits while evaluating its impact on car performance in real stages. Testing them on stage allowed the team to obtain very useful information, better than any wind tunnel tests or Computational Fluid Dynamics (CFD) result, that will be very helpful in the refining of the final aero package.

This is our review of all the parts evaluated in these recent tests, held in tarmac and gravel. Very significant changes in almost all areas were evaluated, except for the rear diffuser and rear fenders; all the rest had been modified.

Starting from the car front, a completely new design of the front bumper was evaluated. The two original dive planes on both sides included significant modifications.

The first and most visible dive plane modification was the addition of external vanes, kind of endplates (A). Endplates are usually designed to prevent low-pressure air from below a wing to mix with high-pressure air from above the wing. In this case, it is not a wing but a dive plane, with the shape of an inverted wing. By keeping these two airflows separated, the pressure difference was higher, so higher downforce would be generated.

But also, endplates would be useful for helping to direct airflow towards the desired location: the top of the front fenders and then to the rear wing side winglets. So, in the end, they would contribute to the generation of downforce at the car front and rear.

Original dive planes were enlarged for the test. The upper plane (A) had a wider and longer front end, while the rear end was prolonged upwards to move air almost vertically towards the top of the fender. The second dive plane (A’) was cut at the rear end, so it would no longer be connected to the fender, allowing air to flow upwards through the space generated.

The reason for that modification would be the addition of a third dive plane (B), in the space between the lower (now central dive plane) and the splitter. Smaller in size, due to space limitations in that area, the new plane also included an external endplate, to channel air upwards.

The addition of a third plane would be a novelty, as all other WRC cars include only two dive planes. In fact, it would represent a more developed solution with respect to the other cars, which obtain a similar effect with a small fence on the splitter at this point. Citroën solution would have the advantage of getting additional downforce (thanks to the plane) while also channeling air upwards. And it would increase the chances of retaining at least one dive plane in case of damaging the upper planes, so improved reliability.

The addition of endplates (A) in the dive planes would be also a novelty. Toyota and Hyundai included small lips on the external side of their dive planes, although Hyundai removed them in their last aero modification introduced in Finland 2019). Citroën would do a significant step ahead in this sense, in case they implement it, as the size of the endplates is much bigger than that of the lips.

Channeling the air towards the upper part of the fenders would have a clear goal: to send all air possible to the upper part of the front fenders, to obtain maximum advantage of another of the new modifications, the new winglets.

On one side, they added a small winglet (C) over the top of the fender, similar to the solution used by Toyota since 2018, which Citroën had already tested in December 2018. On the other side, top of the fender shape (D) would be modified, similar to the original design of the Hyundai, but prolonged by the rear, with the shape of an inverted wing. They both could be included to generate maximum downforce over the front axle, in order to contribute to solving traditional Citroën problem of lack of grip (understeering). Airflow would be sent from there towards the rear of the car while avoiding to impact with side mirrors, which is always good in terms of drag.

In consequence, the location of brake cooling air vent should have to be moved, from the top of the fender (where it would disturb the air flowing onto the new wing) to the rear of the fender (G), under the shape of a thin, vertical exit.

The modified front bumper evaluated included a bigger air intake (E), that would allow a higher amount of air entering into the engine bay for cooling purposes. The bigger intake would also allow the front radiator to be in an almost vertical position, in opposition to its inclined disposition in the original configuration (see dotted lines in pictures below). Such modification would lead to better cooling, as the pressure distribution of air across the radiator would be now more homogeneous, allowing a better heat transfer from the cooling circuit to air.

Note at the top right side of the new intake, two pipes were visible (F): one to send air to the engine and the other for additional cooling purposes. They would replace the original intakes located beside the car headlights (N). Also, brake cooling intakes were not visible in the modified design, although they would be probably located in the main intake, on both sides of the radiator.

The other big area where Citroën had been devoting multiple efforts in the last month is the rear wing. In fact, it is the origin of a tough controversy generated (before Rally Finland), when supposedly Citroën engineers detected that the Toyota Yaris WRC’s rear wing was not in agreement with regulation and homologation rules. This caused the Japanese team to quickly design a new rear wing, which was introduced in Rally Turkey, while the French team continued with the development of their new package.

During the Catalunya pre-event test, Citroën evaluated a new wing which was very similar to the Yaris new wing design, to evaluate its performance in comparison with its original design (unchanged since 2017). Their final goal would be to determine the optimal design of the wing, by trying different options. Same as they did in December tests when they evaluated a rear wing located at a much higher location than the original (and possibly outside legal dimensions).

The rear wing evaluated included significantly wider side winglets than those of the Toyota, with a smaller endplate in the lower, external side, only in the front part. Both the winglet and the lower endplate would be designed to take advantage of the airflow coming from the front fender, which would be used again to generate additional downforce, now in the rear of the car.

Other differences of the evaluated rear wing with respect to the Toyota’s were

  • the smaller size of the vertical fins (K) located on both edges of the top wing, making it less effective in taking advantage of the pressure from air impacting when the car is sideways,
  • a bigger separation between these fins and the wing supports, giving more space to air for side impact, making them more effective and thus compensating the previous disadvantage over the Toyota
  • the differences in the lower wing: while Citroën maintains a similar shape of the original (flat), that of the Yaris decreases on both sides until is connected with the rear of the car, leaving no room for airflow to enter laterally under the lower wing. Both alternatives have advantages: that of the Toyota uses pressure from air impacting when sideways, while in the case of the Citroën, air flowing under the plate increases wing efficiency on downforce generation, by removing pressure from below.

Which of these parts will be included in the final aero package of the Citroën C3 WRC is something we will know in the next weeks. What it seems to be clear is that not all of the modifications seen in these tests will be included, but testing them is really valuable for enhancing the final design. In any case, the new package will definitely contribute to the significant increase of downforce generation of the car front the drivers long time ago were demanding, while improvement in the car rear will contribute to maintaining the aero balance between the front and the rear of the car, which is crucial to keep the car driveable.

This post includes the opinions of the author, which are certainly not always right. If you detect any error or you disagree, do not hesitate to contact us, by sending an email to wrcwings@gmail.com. We love to learn!

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