Jenson Button on his underdog F1 victory: "For a start, we had no engine"

Naturally, the Woof Index drops appreciably over a full season, so the chance of an underdog victory in the F1 Drivers' Championship is vanishingly small.

But it did happen. In 2009, a fairy tale took over the world's most single-mindedly commercial sport – and the story is now richly retold in a Disney Plus documentary series, Brawn: the Impossible Formula 1 Story, fronted by petrolhead film star Keanu Reeves.

It began with a team in chaos. "For a start, we had no engine," says Jenson Button, who was the designated number one driver in the Brawn GP team.

Even I, no motorsport expert, can see that this was a critical shortage. Honda had pulled out of F1 racing before the season began, and it looked as if the team they'd left behind would fall apart.

Then, they were taken over by motor racing maestro Ross Brawn, who had masterminded the dominance of F1 by Michael Schumacher and the Ferrari team in the early 2000s. Brawn had been working for the Honda team, but now he controlled it.

Button had to take a pay cut. "I had one other option [to move to another team], but I decided to stay. I wasn't going to get results elsewhere. The whole team chose to take a pay cut so we could go racing – I wasn't the only one, and I was still getting more than anyone else, so don't feel sorry for me. We did it so we could win races. That's why we were in Formula 1."

The car itself looked better once it had a Mercedes engine in it. All the other teams had been testing for a while when Brawn finally got a car onto the track, but Button was cautiously optimistic. "It felt smooth. It felt reliable. Ross told me, 'You're eight-tenths of a second quicker than anyone else.' And I thought, 'OK!'"

Yes, Cinderella, you shall go to the ball – and get there damn quick. "That was the moment," Button says, savouring it all over again.

"I knew then that we had a car that was seriously good, but we still weren't ready. Back in those days, we had mid-race refuelling, and our fuel guy had gone off to be a plumber and hadn't been replaced.

"In the first race, we lost seven or eight seconds at every pit-stop. So we asked him to come out of retirement every weekend for the rest of the season - and he agreed."

Button won that first race, and went on to win six of the first seven. "It's amazing how quickly the goalposts move," he says. "The first win was amazing. The second was great. So by the next race I was telling myself, I have to win. You start to put real pressure on yourself." That pressure was to become almost too much.

The first part of that season was a heady time. Button had always been admired for his smoothness as a driver. But behind that widespread admiration was the hint that maybe he couldn’t really do it under extreme pressure, that he would always be just a hair’s breadth away from excellence.

Yet he had become the driver that all others were trying to catch, in the car that everyone envied. "Many things gave us the edge," Button says. "There had been a change in the regulations, and when that happens there’s always room to find something."

What the design team found was something called a double diffuser. Formula 1 is as much about aerodynamics as engine power. The innovations that Brawn introduced allowed the airflow underneath the car to pull it down onto the track. It stuck better, so it got better grip and that brought better control at high speed. In short, you could go round corners quicker.

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"The best bit was that nobody looked at the rest of the car," says Button. "We weren't the only ones to have developed a double diffuser; two other teams had a similar thing. Everyone was talking about whether or not it was legal. Well, it was, but the point was that the whole car was fantastic, from the nose cone to the rear wing. Everything worked in unison.

"And the team was so united. We didn't think we had a job, and then we were given a chance to go racing. It was such a journey. When I see anyone from that year, we're always smiling before we speak – it was a special year in all our lives."

But it nearly wasn't. The seventh race was the last race Button won in the 17-race season. The other teams had caught up technically and it had gone from being a breeze to a struggle. Button's competence brought him a string of minor placings, which won him useful points, but were a long way from decisive.

The penultimate race was in Brazil and Button had qualified way back in 14th place on the grid. "It wasn't that we didn't have the pace; we made a bad decision about tyres," he says. "I was really struggling under pressure – the thought that this might be my only chance to win the championship. There was silence in the car as we drove away from the track.

"Then my dad said, 'Come and have a beer.' And he reminded me what I had achieved and where I had come from, as a lad from Frome in Somerset. We talked for an hour, I let it all out and suddenly I could breathe again. I slept really well. And the next morning, I knew I was ready."

Not half. Button started near the back and began overtaking. "Races like that are great. A long series of important overtaking manoeuvres. You get your fists out. I was driving through the field, ticking them off."

He finished fifth and won the World Championship with a race to spare. "It was the best way to win it. It was the story of our season in a single race. We had to fight just to be on the grid."

No team owner, no engine, a pay cut and no one to even put the fuel in the car… but Button was champion of the world. He had a great car, a beautiful driving style, and at the end he showed his racing mettle.

He left the team the following season and joined McLaren alongside the young Lewis Hamilton. Believe it or not, he says 2009 wasn’t his best season – that was 2011, when he finished second to Sebastian Vettel. "If I'd been the driver I was in 2011 back in 2009 – it wouldn't have been nearly so exciting," he says with a smile.

As it was, 2009 was a year of beauty – and the last time the underdog barked in Formula 1.

All four episodes of Brawn: the Impossible Formula 1 Story are on Disney Plus from Wednesday 15th November.

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