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Busting the myth of Brawn GP's legendary double diffuser

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In the second part of our untold stories of Brawn GP's championship year, we look at why the car was so quick. It's not unusual for a major regulation change, like the one in 2009, to reshuffle the order, but no-one was expecting a team on the brink of folding at the end of 2008 to emerge multiple seconds faster in pre-season testing.

The most obvious visual differentiator on the Brawn car was its double-decker diffuser. The new regulations had intended to strip the cars of downforce by reducing the size of the diffuser, but a loophole was left open that allowed for a second diffuser to be built into the crash structure and fed by two holes in the floor of the car.

Toyota and Williams also exploited the grey area alongside Brawn GP, leading to their rivals launching an unsuccessful protest at the first race. The matter then went to the International Court of Appeal after the fourth round in Bahrain -- by which point Brawn had won three of the first four races -- but again the decision found the double diffuser legal.

There was an assumption, that still exists today, that the novel design was the key to Brawn GP's success in 2009. But those behind the concept tell a different story and, in fact, by the time the team reached the opening round in Australia, removing the double diffuser - which was an option given the potential for protests -- would only have cost the car 0.3s in lap time.

The truth behind the double diffuser

John Owen, Principal aerodynamicist
"What's interesting about the double diffuser is that everybody thinks of it as being the big secret of the Brawn car, but it's not. Well, it is and it isn't, and I think this is the key thing to understand.

"What was apparent with the new rules was that we would be stalling a single diffuser really easily -- almost everything we did was stalling the diffuser. The problem with that would be that you would try a new front wing and the overall numbers would be down -- so the conclusion from that was that it was a worse front wing. But you would also realise that the diffuser wasn't happy, so you are left the question of do you do a lot of work just to make sure that the diffuser is not stalling or do you just blame the new wing? More often than not, people would just say it's a bad wing.

"The beauty of the double diffuser is that once you put that on the wind tunnel model, it was so much more robust that the diffuser just worked on the car like it used to in 2008 regardless of what you did elsewhere. So we finally had a diffuser that was stable and that meant when we had a new front wing and it was better, then the numbers would just be better as it didn't stall the diffuser and we could see the true gain from the wing.

"One of the key areas we found early in the development process were the deflectors that we put on the rear brake drums. All the teams have them on their cars now and know how massively important they are, but back then they were a much smaller benefit. We'd tried to develop them before and they didn't work because of the single diffuser, but once we had the double diffuser they worked. And quite quickly we were finding a second per lap in performance from the deflectors on the rear brake drums and then the front wing offered another second a lap.

"So suddenly the whole project was coming alive and we were finding massive gains and the car car developed really rapidly. And then, because we were not quite sure if the double diffuser would remain legal or not, we put a conventional diffuser back on, re-optimised it, and we only lost about 0.3s in lap time.

"So, yes we wanted the double diffuser on the car, but actually the true value of it was in the seconds of performance we found in developing other parts that we probably wouldn't have ever seen without it. Now we had those seconds and we could keep them even if we went back to a conventional diffuser. So in the end we weren't too worried about the other teams protesting!"

James Vowels, Chief strategist
"Red Bull thought the double diffuser was the reason why we were seconds faster at the beginning of the year and they tried to copy it. The truth was that it was worth a couple of tenths, but they had to completely redesign a new gearbox and rear crash structure around it before they fitted a version to their car and realised it was just only a few tenths!

"It just made us chuckle inside because it caused them to be distracted away from the primary goal of adding downforce to the car. Our development budget that year was tiny -- it was just hundreds of thousands of pounds; it was just a joke how small it was -- but equally the money wasn't there, so the double diffuser sending our rivals off the scent was valuable."

John Owen, Principal aerodynamicist
"The concept for the double diffuser actually came from a guy called Mina, he was a Japanese aerodynamicist, and he came up to me in the corridor and wanted to talk to me about this concept they had that they had been trying in Japan. Back in 2008, when we were originally discussing the rules, it was apparent you could probably do something with creating openings in the diffuser, but we hadn't spent any time looking at it as we thought we'd get round to it later when we had done some more fundamental understanding of the new rules.

"We actually tried to close that opportunity down in Technical Working Group meetings because it was apparent there was a bit of an oversight there and we were concerned the aerodynamic performance of the cars would be a lot more than people thought, but that pretty much fell on deaf ears. They thought we were either scaremongering or we were perhaps completely mistaken -- after all we were not a very good team at the time so what did we know?

"So when Mina came along with this concept, he had already got a part, he had already got something he'd tried. When I looked at it I felt I didn't really like the way it was laid out. It had a forward-facing intake underneath the car for the double diffuser, which never really struck me as a nice way to create an inlet as it was going to create problems with different ride heights, but it was clear the concept could work and clear you could do it. So at that point we looked at it more and we tried his version and came up with our own version and our first version was slightly better straight away and that's what we pursued.

"We enquired with the FIA's Charlie Whiting straight away as to the legality of what we had in mind, he looked at the simple sketches and said yes he didn't see any reason why that wouldn't be legal and we pressed on with it."

A Super Aguri supercar

Another well-worn story behind Brawn GP's success is that the development of the 2009 car benefitted from the use of three wind tunnels and the help of former members of the Honda-backed Super Aguri team, which had folded in 2008. While it's true that Honda devoted significant resources to 2009 early on, the actual development of the final concept started to come together as late as June, 2008...

John Owen, Principal aerodynamicist
"Under Honda ownership, we had been making a massive effort for 2009 because we knew we had a deficit to make up to the others. Honda were looking at improving the engine but we knew we had to make a big step and start as far ahead as we could on the chassis side. So we had a lot of different projects going on.

"What we set up was three independent teams: one here in the small wind tunnel in Brackley, one in Teddington with the aerodynamic department of Super Aguri and one in Japan with Honda in Tochigi, but unless the wind tunnels and wind tunnel models are absolutely identical, you can't really parallel process aerodynamic development. Initially people were thinking we could use all these tunnels and get all this benefit, but it didn't really work like that. So we said let's give the three teams the rules, don't let them talk to each other and just let them go and have a look at what they find.

"Back here, myself and the head of aerodynamics, Loic Bigois, oversaw the three projects to see where they were going. It was quite interesting, everyone found different things and everyone was looking at different stuff and it was a nice experiment to see how they were developing differently. And then from about probably June 2008 we started developing in our big wind tunnel here and started bringing the three projects together.

"So there was talk that we had been working on it for 18 months, but we hadn't. We'd been looking individually at the rules for quite a while but we only really brought it together in June time and started the main aerodynamic work. Most of the concept that we decided to go with was Super Aguri's, we took bits and pieces from what we had been doing in Brackley and we took the double diffuser from HGT in Japan."

Andrew Shovlin, Jenson Button's race engineer
"Our target was to try and recover the downforce we had in 2008 whereas a number of other people had just accepted that the car would simply have less downforce under the new regulations. We had very much focused on hitting the same level of performance and so we set our target and then looked for solutions that would allow us to hit it.

"At BMW-Sauber they had increased their target a couple of times, because it was so low originally and they'd hit it and had to go up again, whereas ours was ambitious, maybe pie-in-the-sky thinking. But it's very difficult because when you get a new set of regulations, there is no way of working out what the potential of those rules is.

"If you start with the 2008 car and legalise it -- so you rip off everything that is not allowed and make the bodywork compliant -- then you will take a massive hit. But over the space of two weeks you will recover half of that and then you will get onto a steadier development trajectory. That's because when we legalised the 2008 car to 2009 rules, we destroyed all the flow structures that were in place creating downforce and you are then trying to reconstruct the flow structures to get the thing to work correctly again.

"Your focus is on that, your focus isn't on 'Can we find a few more points of downforce today?' It is about getting the fundamental flow worked out and from that secondary phase of development you can start to get some notion of where you are headed."

John Owen, Principal aerodynamicist
"I remember looking at Red Bull's car concept at the beginning of the year and saying, 'They will be fastest by the end'. Perhaps frustratingly, the guys at Super Aguri had come up with a car that looked very much like the Red Bull, but we had said it was a bit too ambitious on the design side -- structurally it was quite challenging -- and at that point we still had any number of issues with our design.

"We were worried about compliance of components and suspension and it just seemed like a leap too far. We actively went down a route that was a bit more conservative on the mechanical side of the car to make sure we had a car that was a good platform.

"But I think it's a credit to the guys at Super Aguri that they did end up with a concept that proved to be the best and up there with Red Bull and their layout. We've got wind tunnel pictures from back then that show a car that looks almost exactly the same, with a very high, very narrow nose -- almost exactly the same as the Red Bull's. So we did dumb down the extremes of the aero package to make it much more practical."