New rear aero design for the Ford Fiesta WRC 2018 for Rally Finland

Updated: August 10th, 2018

This post includes our review of the modifications evaluated in the Ford Fiesta WRC during the pre-event tests and used in Sebastian Ogier’s car aligned in Rally of Finland 2018.

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Finland 2018, 5th – picture by M-Sport

As Seb Ogier already announced in an interview in early June, M-Sport used the pre-event tests of Rally Finland to evaluate an almost completely new aero design for the rear half of the Fiesta WRC, with Elfyn Evans and Sebastian Ogier at the wheel. Teemu Suninen also participated in these tests, but only with the old aero package. The final decision was to install the modifications only in Ogier’s car, together with a change of bumpers, now from Sachs (same supplier of Ogier’s former team, VW), for Rally Finland 2018.

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Test in Finland, July 11th, 2018 – picture by Henri Vuorinen

The modifications designed to give the French driver an even better car to fight for the title were concentrated both in the sides and the rear of the car.

The first visible modification was the use of the new side mirrors, similar to those used by Toyota and Hyundai, but with a bent support, instead of a 90-degree angle. As we already reviewed during the last winter tests, when it was first seen, the new design allows a better airflow into the rear wing, which contributes to a higher downforce generation in the rear of the car. All three cars used the new side mirrors in the Rally Finland 2018, as no new homologation is required for replacing the side mirrors.

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Test in Finland, July 2018 – picture by Henri Vuorinen

The second important modification was the modification of the rear wheel arches, in the case of Ogier’s car. With a design very similar to that of the Yaris, the exit area was increased in height and width, through new extensions. Also, a higher number of louvers were added (initially up to 10) to redirect hot air into the required direction (upwards in this test).

Note that two louvers were located on the rear top of the arch, to better send air upwards. Also, note that the shape was thinner on the lower part, probably to minimize damages on rough terrains. Even so, Ogier could not avoid to damage it in the very last corner of the Rally Finland, and he lost it during the last jump.

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Finland 2018, 5th – picture by AKK/Taneli Niinimäki

During the rally, two of the lower louvers were blanketed, probably to increase the speed of air to be sent upwards, while generating smaller disturbance on the turbulent flow behind rear wheels.

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Finland 2018, 5th – picture by David Williamson –

Finally, the shape of the bottom seemed to be designed to act as a prolongation of the rear diffuser. Further explanation of the reasons for the new design of the rear wheel arch extensions can be found here.

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Test in Finland, July 11th, 2018 – picture by Henri Vuorinen


S.Ogier/J.Ingrassia, Ford Fiesta WRC, Test in Finland, July 12th, 2018 – picture by se_rallyFI

A new rear diffuser was designed, with four vertical strakes on each side, and a central space, wider, with only two small strakes. Also, it uses now all the space below the rear bumper, so with a higher angle of attack. This is important in terms of higher downforce generation in the rear of the car, due to better removal of air coming from underneath the car, but also in terms of reliability: due to the higher distance to the road, it is expected to be less sensitive to rough terrains. Remember Ford’s one of the most affected diffusers in rough rallies such as Argentina or Sardinia.


New (on top) and old (at the bottom picture) Ford Fiesta WRC rear diffuser

The strakes of the new diffuser are not completely vertical as in the old one (or the diffusers of the Ce WRC, the i20 Coupé WRC or the Yaris WRC). They are shaped to send the airflow to the external sides of the car, while only the central part of the diffuser allows air to flow straight to the rear of the car.

Also, note that the shape of the rear bumper was redesigned, and the bottom is now a prolongation of the rear diffuser on both sides. The exhaust pipe was raised, due to the diffuser size increase, while at the same time it interferes with the airflow from the rear diffuser, still an exhaust blown diffuser, as we already reviewed in a previous post.

Original and new Ford Fiesta WRC rear diffuser

The only part that remained unchanged in the rear of the Fiesta was the rear wing. Although a new wing design was also developed, modifications on the rear diffuser and bumper offered a better return than a new rear wing, according to Massimo Carriero, M-Sport’s Head of Rally Engineering.

These were the modifications evaluated in the Rally Finland pre-event tests carried out by Elfyn Evans (July 9th and 10th) and Seb Ogier (July 11th and 12th), as well as during the Rally. All these modifications had their origin in the evaluation performed in the wind tunnel of Ford Performance last March, which we deconstructed here.

Looking back into the origin of the car, it is worth remembering that the Ford Fiesta WRC was the latest car to be presented in 2016 when all teams were preparing their assault to the 2017 title with the new cars. With a considerably smaller budget than VW, who could afford to start testing by mid-2015, and taking part in the 2016 season while developing the new car (not as Citroën did), M-Sport engineers, led by Chris Williams, made a considerable effort to put the new car on the road. The delay in the definition of the new Fiesta by Ford also contributed to such delay, which prevented the team from doing any wind tunnel test, and the car was solely designed by Computational Fluid Dynamics (CFD), as we reviewed then.

Later on, in December 2016, Seb Ogier joined the team, with no time for modifications, and he perfectly adapted his driving to the car as he was able to win his fifth title in a row, also contributing to the first Ford Manufacturer’s title in years.

Ford Fiesta WRC at Windshear wind tunnel, March 2018 – picture extracted from Ford Performance video

For the season 2018 to continue in M-Sport, Ogier claimed for higher implication, both financial and technical, from Ford. And he got it. The first example was the involvement of Ford Performance, the division of Ford where NASCAR or GT cars are prepared. In March 2018, after Rally of Mexico, the car from Ogier was directly sent to Concord, Charlotte, NC (USA), for a full wind tunnel session, in order to validate the design of the car, as well as to evaluate different modifications, some of them have been now evaluated in the Finland pre-event test.

The new version of the car finally incorporates both the advantages of wind tunnel testing and CFD design, as well as some of the suggestions Ogier made to improve the car handling. If we add those to the current World Champion car, the result has to be even better. Or not? Time will tell.

Note the shape of the bottom of the rear fender, it is designed as a prolongation of the rear diffuser.

In an interview published by Autosport before Rally Finland, Ogier recognized the new aero was giving “more stability at the rear and it gives you more potential [set-up opportunities] with the rear of the car. The disadvantage is that the nose is a little bit higher over the jumps – this happens when you have the aero mainly in the rear. But it’s manageable“. A similar problem has also been seen in the tests of the other teams, so it was in the hands of the drivers to control the balance of the car while jumping, as Mads Ostberg explained to us in a recent interview.

Massimo Carriero, M-Sport Head of Rally Engineering, declared to German magazine that an increase of 30% in downforce was expected with the new aero. And that level was reached, but this had, as a consequence, a loss of aero balance of the car (higher downforce in the rear than in the front) for what new modifications are expected in the next rallies. Also, in a recent interview by Martin Holmes published at, Carriero recognized the advantages of improving downforce through the underbody and diffuser, due to minimal impact ion drag, while improvements in any other area need to reach a compromise between downforce and drag. He also gives new details on their current collaboration with Ford Performance engineers from Charlotte, NC, USA.

Pictures by M-Sport from Rally Finland Parc Ferme show in more detail the new aero appendices.

The picture below shows the differences between the original (left side) and the new design (right side) of the rear of the Ford Fiesta WRC.

This is the first post we devoted to the new rear aero package of the Ford Fiesta WRC introduced in Rally Finland 2018, you can read the other two here:

Why the new Ford Fiesta WRC 2018 have such big rear wheel arch extensions?

Review of the Ford Fiesta WRC new rear diffuser

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 We love to learn!

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