Rear diffusers have been extensively tested in the new 2017 WRC cars, in order to increase grip at the car’s rear. New regulations for 2017 allow free shape diffusers, which can stick out 50 mm more than the rear bumper. Before reviewing the new diffuser proposals, we’ll take a look at the basics of a diffuser.
Due to space limitations, air flowing under the car has higher velocity than air flowing above the car, resulting in a pressure gradient responsible for downforce. The role of the diffuser is to manage the air exiting from car underbody, as it returns to higher-pressure airflow behind the car. It acts as an expansion chamber, allowing a smooth transition, which reduces turbulence and drag in the car wake and improves airflow under the car. As a result, higher downforce is obtained, resulting in higher grip levels.
Main features of a diffuser are length and angle. The angle must ensure there is no flow separation, in order to ensure optimal transition.
Strakes may be added in order to increase diffuser efficiency, as they ensure only underbody air is drawn.
Pictures from last tests carried out by WRC teams show they are all very similar, except that tested by Ford M-Sport team. It is significantly wider and with the lowest angle.
Ford Fiesta WRC, tests in France, November 2016
On the contrary, the diffuser tested by Citroën is the one with a higher angle.
Citroën C3 WRC, test in Wales, November 2016
Toyota’s design is the simplest, with just four strakes.
Hyundai i20 Coupé WRC diffuser recently tested is the shortest from all diffusers evaluated, while there are two different angles throughout the diffuser, what might result in lower stability.
Hyundai i20 Coupé WRC, tests in Spain, September 2016
Even though VW has decided to quit the WRC, it is still worthy to evaluate their rear diffuser proposal, which was also very simple.
VW Polo WRC 2017, tests in Catalonia, September 2016
All diffusers tested contain only four internal vertical fences, except Ford Fiesta WRC, which contains seven. Distribution is regular in Ford, Citroën and Toyota, while Hyundai and VW show uneven distribution.
All teams have integrated the exhaust outlet on the top center of the diffuser, except Hyundai. See why here.
Pictures from car rear also allow comparing new rear wing proposals. All pictures show the latest design except that of Hyundai, which can be seen in the picture below (by Zagatoï).
Wings from Polo and i20 show high similarity, while C3 is also similar with the lower angle of elevation. Both Yaris and Fiesta use long side wings, while Toyota is the only car with vertical fins over the top blade.
Final diffuser configuration (updated on July 28th, 2017)
Pictures below show final diffusers design used by each manufacturer during 2017 championship:
- Citroën and Toyota use only four intermediate vertical fences, while Hyundai uses six and Ford up to seven.
- All manufacturers have located exhaust gas exit on the top center of diffuser except Hyundai, where it is located inside diffuser on the right side.
- All diffusers have similar length except Ford’s, which is significantly longer. Also height is similar for all of them, except Ford’s, which is lower.
K.Meeke/P.Nagle, Citroën C3 WRC, Rally Catalunya 2017, 1st
J.Hanninen/K.Lindström, Toyota Auris WRC, Rally Catalunya 2017, 4th
D.Sordo/M.Martí, Hyundai i20 Coupé WRC, Rally Catalunya 2017, 15th
E.Evans/D.Barritt, Ford Fiesta WRC, Rally Catalunya 2017, 7th
The picture below shows the difference in diffuser height and diffuser gap to the road between the Ford Fiesta WRC (M.Østberg/T.Eriksen) and the Hyundai i20 Coupé WRC (T.Neuville/N.Gilsoul) form Rally Catalunya 2017.
Length of Citroën diffuser strakes is greater than that of Hyundai, as shown in the picture below, both taken at the same location.