Flow Racers is a reader-supported site. Purchases made through links may earn a commission.

What Is A Double Diffuser In F1?

In the 2009 F1 season, one of the biggest talking points was the double diffuser. It was an engineering marvel that took the F1 world by storm, and helped some teams shock others at the first races of the season. But many fans may wonder what the double diffuser was in F1. 

The double diffuser was a “double-decker” diffuser which arose from an unusual interpretation of the 2009 aerodynamic regulation changes. The double diffuser allowed a much larger diffuser than the regulated dimensions, which allowed significantly more downforce to be generated by it.

There is a lot more to the story of the double diffuser, and it involves finances and politics. It is also part of the reason that Brawn GP and their magical 2009 season is now permanently a part of F1’s lore. Let’s take a closer look at what the double diffuser was and why it was so special.

What Is A Diffuser For On An F1 Car?

A diffuser on an F1 car helps the air that has run underneath the car to exit in a way that produces a lot of downforce. It is the portion of under-body work at the car’s rear. In order to do its job, the diffuser normally has a number of vanes and fences to direct the air flow.

By accelerating the air under the car and expanding it out from behind, a low-pressure region is created underneath the car. This low-pressure region effectively means that the car is sucked down onto the track. The diffuser helps the body of the car itself to act like an upside-down airplane wing, forcing the car down onto the track. This creates downforce but without adding much drag.

A diffuser aims to channel as large a volume of air as possible, as this can be translated into a more significant low pressure and greater downforce. Therefore, the larger the diffuser area, the more air it can pull from underneath the car, and the larger the downforce.

When Was The Double Diffuser Used In F1?

The double diffuser was used in F1 during the 2009 and 2010 seasons. It was born of a unique interpretation of a new set of regulations that came into force in 2009. But the double diffuser was banned from the 2011 season, because it was not part of the original intention of the regulation change.

Revised regulations implemented in 2009 were made to allow closer racing between cars. The intention of the revision was to make the aerodynamic performance of the cars less sensitive to the ‘dirty air’ of the car ahead of them. The regulations aimed to remove complicated external aerodynamic elements that created additional turbulence in the car’s wake (the airflow behind it). 

Further, they aimed to reduce the total aerodynamic downforce and make them less ‘peaky. Aerodynamics are considered peaky if they work well in a very narrow band of conditions, but significantly worse outside of those ideal conditions. This results in a significant portion of the cars’ downforce being lost if the car is in turbulent air from the car ahead of it.

Diffuser Overhaul For 2009

One area that was substantially rewritten in this change was the diffuser of the car. This allowed some unusual interpretation which then led to the double diffuser appearing in 2009. Teams were still allowed to run their double diffusers in 2010, because F1 acknowledged the resources that had been spent in their development. However, they were written out of the rules for 2011 and beyond.

The same approach of avoiding peaky aerodynamics to allow closer racing has been followed for the 2022 regulation change. The change was more significant there, bringing ground- effect aerodynamics back into the sport for the first time since it was prohibited in 1983. In fact, the driving force behind the 2022 regulations was Ross Brawn himself, previously the team principal of Brawn GP.

How Did The Double Diffuser Work?

The double diffuser worked with aerodynamics like a regular diffuser. But the double diffuser came about from an interpretation of new regulations, which allowed for a “double-deck” diffuser to be designed. Here, the upper deck of the diffuser was fed by openings in the floor underneath the car. 

In an F1 car, aerodynamic downforce needs to be balanced between the front and rear. If there is too much downforce at the rear, the car understeers, and the front of the car will struggle to turn enough into the corners. If there is too much downforce at the front, the car will oversteer. The front will turn in to the corner, but the rear will slide outwards due to low grip at the rear tires. 

The 2009 change aimed to reduce the diffuser’s size from 2008, but this also cut the downforce it could create. However, the official wording said, “No bodywork that is visible from beneath the car … may be more than 175mm above the reference plane.” This meant there could be some diffuser bodywork above 175mm from the bottom of the car so long as it was not visible from directly underneath it.

The new upper deck essentially did not have a regulated limit to its height, and so it could be a fairly substantial size. The overall greater size of the double-deck diffuser was then able to accelerate a greater volume of air out from underneath the car, and therefore create more downforce. The crash structure of the rear of the car was actually shaped to provide the upper deck of the diffuser.

Aerodynamic Balance

The combination of the front and rear downforce can be summarized by the position of the center of pressure, similar to a center of gravity but for downforce. This center of pressure should ideally be very close to the car’s center of gravity.

The intention of the 2009 regulations was to limit the size of the diffuser, thus limiting the rear downforce that could be generated. This would in turn decrease the amount of downforce that the teams could apply with the front wing if they hoped to keep an aerodynamically balanced car. Even though it was possible to gain more downforce in front, it would have been counterproductive.

The double diffuser interpretation allowed the area of the diffuser to be significantly increased, and therefore the rear downforce generated. More than this, it also gave the freedom to design more downforce into the front wing. The overall downforce gain was therefore quite pronounced within these specific regulations.

Which Teams Used The Double Diffuser?

Teams that used the double diffuser included Williams, Toyota, and the Brawn GP team. While the Brawn GP team didn’t discover the double diffuser, they were the team that extracted the most from the concept. As such, it was a major contributor to their one and only world championship win in 2009. 

The Incredible Brawn GP Story

Honda announced quite suddenly at the end of 2008 that they had decided to immediately withdraw from Formula 1. The original intention was to simply close the team down, but Ross Brawn, the team principal, managed to negotiate a buyout of the team, together with an operating budget from Honda equal to what it would have cost them to close down the team.

The car for the 2009 season had of course already been largely developed and was in the process of being built at the time. Ross Brawn’s faith in the design of the car and the people in the team was what had motivated him and the team to take on the risk of trying to keep the team going after Honda pulled out.

The team had managed to gain back enough downforce in the design of their car to be roughly equal to the downforce of the 2008 cars. The main innovation that had made that possible was the double diffuser. As explained by the principal aerodynamicist John Owen 10 years later, the double diffuser itself didn’t give all of that downforce and lap time.

The key for Brawn GP was that it allowed far more stable air flow over the car, regardless of other tweaks elsewhere on the bodywork. That allowed the development of other aerodynamic elements to be far easier and more straightforward. As a result, numerous other gains were relatively simple to find in various areas, such as deflectors on the rear brake drums, and a new front wing. 

The car ran in a mainly white livery, as they had no major sponsors. They had very little money and had only two working chassis and very few replacement parts. But they were by far the quickest car at the start of the season and won six of the first seven races of the season. That built the foundation that carried them through to winning both the Drivers’ and Constructors’ World Championships.

Protest Against The Double Diffuser

Only three of the 10 teams had arrived at the interpretation of the rules that allowed the double diffuser. The remaining seven teams felt that the interpretation went against the spirit of the regulations (even though it did technically comply to the letter).

Although all the teams were aware of the concept during pre-season testing, it was only when Brawn GP galloped away to a 1-2 win of the first Grand Prix of the year, the Australian GP, that some teams launched a protest questioning the legality of the interpretation. Not only did they want the double diffuser banned, but they also wanted the result of the Australian GP overturned.

The stewards did not uphold the protest but ruled in favor of the Brawn GP car being legal. A protest was raised again at the next Grand Prix in Malaysia (Brawn GP also won that race), but their car was ruled legal once again. An appeal was then lodged with the FIA Court of Appeal. 

If You Can’t Beat Them, Join Them

Even at Australia, the seven remaining teams knew that the outcome of the protest could very well go against them. As a result, even though there were teams vehemently opposing the approach, they were all quietly developing their own versions in case the double diffuser was classified as legal. 

That is in fact what happened. The FIA Court of Appeal delivered their verdict confirming the legality of the double diffuser only days before the third race, the Chinese Grand Prix. The remaining teams were forced to design and fit them in order to be competitive. 

McLaren, Renault and Force India were the first of the remaining teams to apply a double diffuser to their cars. McLaren’s car lent itself quite easily to incorporating the upper deck of the diffuser, and they already had a solution in place for the Chinese Grand Prix. Renault also managed to bring a single prototype to that race. Force India had a solution by the next Grand Prix.

Red Bull’s car was probably the most difficult to incorporate a double diffuser into. Adrian Newey had introduced the unusual decision (at the time) to use pull rod suspension for the rear axle. The suspension joined the crash structure and the gearbox very close to the regulated diffuser area.

Red Bull therefore needed to raise their gearbox (and adjust their rear suspension) in order to accommodate the upper deck of the double diffuser. The upper deck was incorporated within the crash structure of the car, so that needed to be overhauled as well. Work also had to be done at the front to generate additional downforce there, in order to maintain an aerodynamic balance on the car.

Red Bull finally managed to get the double diffuser onto their car by Monaco, the sixth race of the season, two months after the season opener. By the eighth race at Silverstone, Brawn’s initial advantage was all but gone, with the fastest few teams all having competitive pace. Red Bull went on to challenge for both the Drivers’ and Constructors’ Championships but were the runner-up in both.

Why Did F1 Ban The Double Diffuser?

F1 banned the double diffuser because it was clear that enough downforce had been added back to the cars to be at similar cornering speeds and lap times to the 2008 cars. Due to the investment that had been made in the double diffusers by teams in 2009, it was left as legal for the 2010 season. 

F1 still wanted to slow the cars down by reducing the downforce and reducing turbulence in the air out of the back of the cars (the wake) to allow closer following and hopefully better racing. The goals of the 2009 regulation change had not been achieved

Those regulations had been formulated by an Overtaking Working Group, which had been tasked with studying how to allow closer following and therefore improve overtaking. This group was sure that the double diffuser was not helping these efforts

The double diffuser was considered a major factor of the reason that the 2009 aerodynamic regulations had not delivered. As a result, the regulations around the diffuser were clarified and tightened up for the 2011 season to ensure that the double diffuser was outlawed.

What Happened To Brawn GP?

Brawn GP was bought by Mercedes, which means that it only existed only for their one magical 2009 F1 season. After their incredible success, Mercedes ran as a ‘works’ team for the 2010 season onwards. It was the first time that Mercedes was a constructor in F1 since 1955.

Previously, Ross Brawn had years of success with Michael Schumacher at Ferrari in the early 2000s. With Mercedes buying the team, Brawn was able to convince Schumacher to come out of retirement to race for the new Mercedes team, which has gone on to win seven consecutive Drivers’ titles and eight consecutive Constructors’ titles. They dominated the hybrid era from 2014 to 2021.

Final Thoughts

The double diffuser will always be connected to Brawn GP winning both World Championships in their first and only season. Their interpretation of 2009’s regulations allowed them to be miles ahead of their rivals. The technology was short-lived, but the story is now part of legend.