Banned: The double diffuser that triggered an F1 development race

The 2009 Formula 1 season will long be remembered as the year that Brawn GP emerged from the ashes of the Honda team and went on to win the world championship.

As soon as the season started, however, the team had to endure a row over a contentious design feature of its car: the infamous double diffuser.

The wide sweeping rule changes introduced for 2009, with the intent of improving the ability of drivers to follow one another and thus improve overtaking, had sought to reduce the overall downforce that was achievable.

But F1's designers sought to overturn the downforce deficit from the previous season - and three different teams managed to stumble across the idea of a double-diffuser design.

According to former Honda and Brawn CEO Nick Fry in his book Survive. Drive. Win., the idea was first floated by Honda engineer Saneyuki Minegawa in an aerodynamics meeting who, reading the regulations in his second language, interpreted the rules differently and suggested that there was scope to expand the effective volume of the diffuser.

Both Toyota and Williams also began 2009 with double diffusers, rumoured to be a legacy of the closure of Super Aguri and Honda, and the subsequent cross-pollination of ideas within the three teams.

The double-deck diffuser that could be seen on the BGP001, the Toyota TF109 and the Williams FW31 which were immediately put under scrutiny by the rest of the grid, who had not spotted the opportunity that the rules presented and sought to remove any advantage the three teams had gained.

The rules introduced for 2009 had set about limiting the dimensions of the diffuser to 1000mm wide, 175mm tall and 350mm long.

The use of the double diffuser arrangement hinged on the wording on article 3.12.7 of F1's technical regulations.

It stated: "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."

The fact that this only required for the designers to be compliant in one dimension opened up the possibility for channels to be created, which allowed airflow to move into the upper deck of the diffuser.

The design did not go down well with the other teams, who knew it would be an issue to incorporate a double diffuser on their cars in the short term. As such, they opted to protest the design at the Australian GP and lobby the FIA not only to ban the design going forward but also reverse the result of the Melbourne race.

But although the teams protested, they all had their own double-diffuser concepts in development at their respective factories in the event that the FIA disagreed with them and gave the trio of Brawn, Williams and Toyota the benefit of the doubt.

Ultimately, they were unable to lobby for the double diffuser's removal, and were required to fast-track designs from the drawing board and get them on the car as quickly as possible.

The consequence of that was that the front-end also had to be improved to match the increased downforce output at the rear, in order to retain the same aero balance.

Red Bull arguably had the biggest challenge, as it had reintroduced pull rod suspension at the rear of the car. This was a decision that has had an effect on the design of F1 cars ever since thanks to the packaging and aerodynamic advantages on offer.

It would, however, make life difficult for Adrian Newey and his team, as the pull rod joined the gearbox and crash structure assembly right at the point at which the diffuser opens out.

This made it extremely difficult to get the best from a double diffuser with major modifications but Red Bull, knowing it needed the solution, set about making the necessary changes regardless.

With the pull rod mounted in such a way, the team set about raising its gearbox in order to clear space for the double diffuser, with the channel for its double diffuser nestled away under the gearbox.

In order to maximise the amount of flow into the upper deck of its double diffuser, Red Bull created a large throat either side of the lower floor section that carries the plank.

This then tapered into a keel shape underbody, which merged with the crash structure to keep flow at maximum through the normal diffuser.

Over the season, the double diffuser was developed further, and Toyota even brought a more advanced triple layer variant to the second round in Malaysia.

The double diffuser's new-found legality eventually played into the hands of those disadvantaged by the new regulations, as the likes of McLaren had slipped down the order.

The original McLaren diffuser for 2009 was more respectful of the original intent of the regulations but, with the changes needed to accommodate a double diffuser not as dramatic for McLaren as others, it had a solution on the car just a week after the FIA threw out the team's appeal (above, plus developments).

The team had clearly been working on a solution in tandem with its protest, introducing the solution at the Chinese GP.

The BGP001 might have been considered to have the most refined double diffuser on the grid but that didn't stop the team from making further improvements - making strides ahead of the Spanish GP in which it altered the shape of the central section.

The FIA continued to allow double diffusers to be used throughout the course of 2009 and 2010, given the resources expended by everyone to include them in their designs.

But, from the start of 2011, the loophole that had allowed them to be installed in the first place was closed off in the regulations for good.