Formula One Double Deck Diffuser explained
April 7, 2009 Only two rounds into the 2009 FIA Formula One World Championship and the largest number of rule changes in the history of the sport have well and truly reshuffled the deck. We took a close look at the Kinetic Energy Recovery System (KERS) before the opening round got underway in Melbourne, Australia, but it turns out the biggest news in Formula One at the start of the season is the rear diffusers being used by the Brawn, Toyota and Williams teams to improve aerodynamics. The diffusers in question were cleared by the FIA as long ago as January but the matter will again be considered by the FIA's International Court of Appeal on April 14.
The controversy surrounding these devices stems from a clever interpretation of the rules by the three teams in question and the fact that they obviously have a clear on track advantage, Brawn having claimed both poles and won both races so far this year. The 2009 regulations were designed to reduce downforce and increase overtaking and this included a smaller diffuser in a more rearward position. With an estimated 50% reduction in downforce because of these changes, teams have been working hard to regain the lost downforce.
McLaren, Ferrari, Renault and Sauber have all made very literal interpretations of the revised 2009 rules regarding the rear diffuser. The new regulations now limit the main part of the diffuser to a width of 1000mm, a length of 350mm and a height of 175mm so all of the channels on these cars are the same height and length, with no difference in height between the main central section and the side channels.
The Brawn, Williams and Toyota diffusers have exploited a loophole in the rules as did most teams last year. The rear diffuser regulations are simply a section of the wider bodywork regulations, which also include sections which allow bodywork in areas not intended for the diffuser. More important is the fact that all three designs use a 'window' or hole to feed the top side of the diffuser. That hole is horizontal in the case of the Williams, vertical for the other two teams, and is located where the flat floor meets the diffuser. The Brawn and Williams cars have 'double deck' diffusers while the Toyota has a 'triple deck' version.
The area of contention surrounds the central crash structure at the rear of the car. Where the literal interpretation suggests a single 1 meter wide channel the same height all the way across, the three diffuser teams have split that into 3 channels using the crash structure as a central channel.
Toyota’s diffuser exploits regulations that allow extra bodywork within a 150mm zone in the center of the car, the team has cleverly shaped the TF109s rear crash structure so that it effectively lengthens and heightens the diffuser’s central section, which also features a very low splitter at its base. The same area has been exploited in recent years with small winglets mounted atop and below the rear crash structure.
Additionally there is an ambiguous rule which appears to allow more than one surface to exist in this area. All three teams have been able to create a double decker diffuser, their main diffuser is as long, wide and tall as the rules allow, but they have made the middle section stop short of meeting the flat floor - instead the floor extends into the upper diffuser creating an opening to allow airflow above the main diffuser which creates more diffuser exit area, and the higher expansion of the flow through the diffuser creates more downforce.
The cars from McLaren, Ferrari, Renault and BMW Sauber don't have these holes in the diffuser and the main point of debate is whether the diffuser's three channels can be considered as separate entities, or whether they must be considered as one whole. Given the wording of the regulations, one can argue a case either way, though the majority of teams apparently took the spirit of the rules to mean that all three channels must have the same height and length, with no holes to feed them.