Aero upgrades for the Subaru WRX STI in the 2022 ARA Championship

In the last years, summertime was the part of the year when Subaru Motorsport USA used to implement upgrades into the aero package of the Subaru WRX STI competing in the ARA Championship. It happened in 2021, when a new package, including dive planes, fender winglets and a new rear wing was introduced, and it happened again in 2022.

The modifications introduced this year are not as drastic as in 2021, but an improvement in the performance of the previous upgrade. The first change arrived in the New England Forest Rally. Under the pressure exerted by rivals driving ex-WRC cars, such as Ken Block (Hyundai i20) and Barry McKenna (Ford Fiesta), the Subaru Motorsport USA Team, together with Vermont Sports, made a deep read of the current ARA Rally Technical Rules to identify some room for some improvement, and they found it.

T.Pastrana/R.Gelsomino, Subaru WRX STI, Ojibwe Forests Rally 2022, retired – image by David Cosseboom (@davidcosseboom)

Effectively, the ARA 2022 Rally Technical Rules allow the rear wing to protrude up to 76 mm from the roof line, while the previous design of Subaru’s rear wing was way too below that line, as shown in the image below.

B.Semenuk/K.Williams, Subaru WRX STI, Olympus Rally 2022, 1st – image by Subaru Motorsports USA

What’s the problem with the low, original rear wing? The small space between the upper and lower wing. As in any inverted wing, the airflow under the (upper) wing is a low-pressure flow (due to its high speed). But, at a very short distance, we have the lower plate, which needs a high-pressure flow to generate downforce. The result is that there is interference between the upper wing and the lower plate, and less downforce is generated.

Moving the upper wing upwards, as allowed by 2022 rules, increases the separation between elements. The interference between the two wings is minimised and the system becomes more effective in generating downforce.

New (left) and old (right) Subaru WRX STI rear wing – images by Subaru Motorsport USA

A side view of the new and old wings confirms that the design of the wing is the same, with the exception of the separation between the upper wing and the lower plate, as well as a small modification in the endplates (as shown by the yellow arrow).

Old (left) and new (right) Subaru WRX STI rear wing – images by Subaru Motorsport USA

The modification in the rear wing proved to be very effective, and much more downforce was obtained at the rear of the car, which is good for the rear grip, but bad for the balance of the car. An unbalanced car becomes more difficult to drive, and this is critical in jumps, as the rear wing keeps working while flying, leading to nose-up landings that can compromise the integrity of the car.

T.Pastrana/R.Gelsomino, Subaru WRX STI, North England Forest Rally 2022, 3rd – image by Subaru Motorsport USA

To compensate for the extra downforce at the rear, the team searched again in the rules and found that there was room for extending the front splitter. The car was not reaching the maximum overhang allowed, which cannot exceed OEM plus 7%, measured from the centerline of the front wheels forward.

The air flowing over the front splitter causes some front downforce, due to the impact over the flat (upper) surface of the splitter. The bigger the surface, the more pressure is exerted, and more downforce is obtained.

T.Pastrana/R.Gelsomino, Subaru WRX STI, Ojibwe Forests Rally 2022, retired – image by David Cosseboom (@davidcosseboom)

The result is that both the Subaru WRX STI of Pastrana/Gelsomino and Semenuk/Williams incorporated an extended front splitter in the next event of the ARA Championship, the Ojibwe Forests Rally. The image below shows the increase in surface with respect to the previous version.

New (top) and old (bottom) Subaru WRX STI front splitter – images by David Cosseboom (top) and Subaru Motorsport USA (bottom)

In summary, through the two aero modifications incorporated into the Subaru WRX STI, the car generates now more front and rear downforce, and the balance seems to be not significantly altered. These are probably the last aero modifications added to the car in 2022, which will have to be completely modified to fulfill the 2023 Technical Rules. But this is food for another article, hopefully soon.

T.Pastrana/R.Gelsomino, Subaru WRX STI, Ojibwe Forests Rally 2022, retired – image by David Cosseboom (@davidcosseboom)

 

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