Technical analysis: Diffuser debacle

This week a row has erupted over the design of two teams' diffusers, after the new Williams and Toyota emerged sporting radically different diffuser designs to the other cars launched so far

Williams came up with a 'double decker' diffuser design, while Toyota initially tested an extension to the middle of their diffuser, and then later added a double decker section of their own. Both these designs raised eyebrows up and down the pitlane, as they appear to stretch the wording of the new rules.

Having spoken to both teams' technical directors at the Portimao test, neither feels their designs are that different to their rivals' and clearly both are confident that they are not contravening the regulations.

As part of the 2009 package of aerodynamic rule changes designed to reduce downforce and increase overtaking, the FIA mandated a smaller diffuser in a more rearward position.

With the shock of losing 50 per cent of their downforce because of these changes, teams have been working hard to get the bodywork shaped to the new rules to regain the lost downforce.

One of the critical elements in making a diffuser work is internal volume, so 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 - as seen on the McLaren in the picture.

However the diffuser rules are simply a section of the wider bodywork regulations, which also include sections which allow bodywork in areas not intended for the diffuser. Williams and Toyota have exploited these areas with their new cars. No doubt other teams know of these loopholes, but have yet to run their interpretations.

There are two interpretations of the regulations being exploited with these diffusers.

1) Both cars appear to use the same loophole that allowed the teams to run an extra channel above their diffuser under the outgoing rules.

Although the rules now demand a diffuser height of 175mm above the reference plane, this is measured from below - using the 'bodywork facing the ground' articles in the rules. Therefore the actual diffuser can be taller, if they can get around the second paragraph of the article 3.12.7 that demands a continuous line where it meets the flat floor at the axle line.

This is an ambiguous rule which appears to allow more than one surface to exist in this area. Thus both teams have been able to create a double decker diffuser, their main diffuser (highlighted in yellow) is as long, wide and tall as the rules allow (the red line), but they have made the middle section stop short of meeting the flat floor - instead the floor extends into the upper diffuser (shown in green).

This is a creative way of interpreting the F1's Technical Regulations as outlined below:

Article 3.5.2: The width of bodywork behind the rear wheel centre line and more than 200mm above the reference plane must not exceed 750mm.

Article 3.12.7: No bodywork which is visible from beneath the car and which lies between the rear wheel centre line and a point 350mm rearward of it may be more than 175mm above the reference plane. Any intersection of the surfaces in this area with a lateral or longitudinal vertical plane should form one continuous line which is visible from beneath the car.

Williams and Toyota's interpretation has a precedent, as it was exploited by most teams last year. The area above the lower diffuser is covered in article 3.5.2, which allows bodywork to be up to 200mm above the reference plane (25mm higher than the lower diffuser) and as wide as 750mm.

This can create about 10 per cent more diffuser exit area, and the higher expansion of the flow through the diffuser creates more downforce. A couple of teams have questioned whether this interpretation is allowed under the current rules, although it is not thought any has lodged a formal question with the FIA.

2) Toyota have an additional diffuser aft of the main diffuser (shown in blue). This sits in a 150mm wide area that is intended for the rear crash structure and rear wing mounts. The same area has been exploited in recent years with small winglets mounted atop the rear crash structure.

Although the rules demand no bodywork above 175mm, this only applies to the area between the rear axle line and point 350mm behind it: article 3.10.4 creates a void between 350mm and 500mm behind the axle.

This extra 150mm x 150mm area can be up to 400mm high, some 225mm taller than intended for the diffuser, and it is this area that Toyota have taken advantage of. Again the taller exit creates more potential for downforce.

In addition, the rule also allows this part to extend beyond 500mm (behind the axle line) as long as it sits alongside the rear impact structure (200mm and 400mm above the reference plane). This is an area yet to be exploited by any team's diffuser:

Article 3.10.5 states: Any parts of the car less than 75mm from the car centre line and more than 500mm behind the rear wheel centre line must be situated between 200mm and 400mm above the reference plane.

Teams often approach the FIA during the design process to clarify whether their interpretations of grey areas are within the regulations, although in this instance sources have informed that neither Williams nor Toyota submitted their design to the governing body - although Toyota are believed to have exchanged correspondence with the governing body regarding diffusers.

Interestingly, FIA sources have revealed that a diffuser design related to the current intrigue has been approved - although it has not been confirmed whether this is one used by Williams or Toyota.

Testing is not bound by the technical regulations, so the issue could continue to be debated until the opening race. Melbourne is the first time that the cars are formally scrutineered, and the first opportunity for any protest to be lodged - although it is possible that the FIA could clarify its view of the rules in question before Melbourne.

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