Jenson Button maintained a narrow lead over Fernando Alonso for the first 36 laps of the Italian Grand Prix at Monza, but lost the race when he pitted first. It was a game changing decision by McLaren.

After the race Button said it was the wrong decision, while McLaren boss Martin Whitmarsh said it was the right one. So what is the truth? Did the call to pit Button before Alonso cost him what would have been a historic and memorable victory?

Button wins the start (Darren Heath)

Button got the lead at the start, but it was clear from the performance of the Ferrari that it was a slightly faster car on the day.

While Button had opted to run with a F Duct rear wing giving reasonable downforce, Alonso was running with a compromise aerodynamic set-up, involving less downforce and more straight line speed.

Button’s belief after qualifying was that this would mean that the Ferrari would slide around more on its tyres, leading to them losing performance sooner. In practice the soft Bridgestone tyres were so durable that in Sebastian Vettel’s hands they were able to do 52 laps, still at a competitive pace to the end. Vettel’s fastest lap was on lap 50!

As the race went on Button’s pit crew, like Ferrari’s were monitoring the performance of the cars which had switched to hard tyres. Early examples were the Virgin and Lotus cars, but it was Robert Kubica’s stop on lap 33 which caught their attention.

Prior to his stop Kubica was lapping in the 1m 26.4 range, very consistently. On lap 35 he did 1m 25.7 and it was this improved performance which made McLaren make the decision to bring Button in on lap 36. That and the fact that Rosberg had just pitted from fourth place on lap 35 and there was now a gap that the team could bring Button out into, ahead of Hulkenberg.

McLaren were convinced that the tyres had reached a crossover point, where the new hards were faster than the used softs. If they delayed bringing Button and Ferrari pitted first, then they believed he would be at risk not only from Alonso but also Massa on new tyres.

Martin Whitmarsh basically admitted after the race that they had convinced themselves that whatever they decided to do, they were going to lose the lead to Alonso anyway. I asked Stefano Domenicali if he agreed with that and he said yes, the Ferrari was faster on the day. But it was only by a tenth or two. So it really was down to the timing and execution of the stop.

Ferrari were also monitoring Kubica and also considered bringing in Alonso on lap 36, the lap Button came in on. They didn’t know when Button would pit but their concern was about backmarkers; they have painful memories of Montreal this year where Alonso could have won the race, but for losing time in traffic on his in lap to the pits. They were only three seconds on the road behind Adrian Sutil, who was lapping two seconds a lap slower. Luckily for Alonso, he pitted on lap 36, thus allowing the Spaniard enough space to do a fast lap and then and in-lap before catching Pedro de la Rosa.

So Kubica was 0.7s faster on the hard tyre on his first flying lap but what about the out lap?

Button believes that the reason he lost the race was the immediate performance of the hard tyre on the first lap out of the pits. Here the advantage would be with the car that pitted second as his task would simply be to emerge from the pit lane ahead. The performance of his new tyre over the out lap would be academic because he would already have track position.

So it’s clearly all about the fine detail here – should Kubica’s performance in those first sectors out of the pits on new tyres have given McLaren pause for thought? For this we need sector timing data, which is only available to the teams. Luckily we have been able to get hold of it.

Breaking down Button’s out-lap, it’s clear that the new tyre took time to getup to speed. It was not faster than the old one until the third sector. The first sector time involves the pit stop and the drive out of the pits, where Button lost 2/10ths to Alonso.

The middle sector was 29.1s, which was 2/10ths slower than on his previous lap. It’s only when Button got to sector 3 that he did a 28.3, which was half a second faster than before.

So this bears out Button’s theory that the hard tyres took most of a lap to get up to speed. Was this obvious from Kubica’s first lap on them? It was.

Kubica’s experience exactly foreshadowed Button’s – he was 2/10ths slower on the middle sector and then half a second faster in the third.

When he saw Button peel off into the pits Alonso knew what he had to do. He had been 9/10ths behind Button on that lap before the McLaren stop. He did a personal best for the race in Sector 1, which gave him two tenths, then in Sectors 2 and 3 he matched his recent times. The following lap, his in lap, he repeated that.

So the real difference between the two cars was in the final sector of their in lap, the pit stop itself and the first sector of the outlap. Alonso gained 7/10ths of a second here, which when combined with the two tenths he gained when Button was in the pits, was enough to give him the lead, by the slenderest of margins.

Another factor is that the Ferrari mechanics were faster than the McLaren ones in the actual stop, but it’s not as simple as many pundits are saying that the mechanics were 8/10ths faster on Ferrari’s stop.

The calculation the timing computer does is based on a formula which deducts a standard amount of in and out time from the total time the car is in the pits to give the stationary time. But that is not an actual measure of how long the car is stationary for. Button was in the pits for 8/10ths longer than Alonso, but some of that lost time was on the way in and, more importantly, on the way out.

“When I exited the pits I had very little grip on the prime tyre,” said Button. “Lots of shuddering which means the tyre isn’t working, so little bit disappointing to find myself in that position and Fernando coming out of the pits in front.”

The pit stop, combined with the initial sluggish performance of the hard tyres in the first two sectors of the lap, is what lost Button the lead.

Re-printed from James Allen on F1 - The official website