The final laps of the Italian Grand Prix left everybody - including race winner Lewis Hamilton - very confused. Adam Cooper brings you the full story behind the tyre pressure saga.
The Italian Grand Prix turned on its head at 3.04pm, although at that stage neither race winner Lewis Hamilton, his Mercedes team nor the millions of people watching at home had any idea what was about to happen.
Up until that point, the Monza race had been pretty routine for the reigning world champion and his pit wall. Having made a good start, and eased away from Sebastian Vettel in the first stint, he made an untroubled pitstop and was cruising to a dominant victory.
He even had time to ask the pit wall if there was anything he could do to help save the engine – the upgraded unit remember – for future races.
Then everything changed as we saw pictures of a worried Toto Wolff at the centre of a debate in the garage. There were also some unusual radio messages from Hamilton's engineer.
"Strat Mode 3. We need to pull a gap, don't ask questions, just execute," was one.
Then later: "We need some good lap times, we'll explain at the end."
Hamilton responded: "What do I need to do? I can't go much quicker."
"What you are doing is great, Lewis," was the reply.
The obvious conclusion was that Mercedes had spotted a technical problem or perhaps a tyre issue that might require him to make an urgent late pitstop, hence the need to create the gap.
Clearly as confused as the rest of us by what had unfolded, and having to up his pace, after the chequered flag, he said: "those last few laps were not cool."
It was only in those minutes after the race that driver, the media and public learned of the tyre pressure issue, and it became clear what Mercedes was worried about.
The team had received notification from the FIA, in a document timestamped at 3.04pm, of an investigation into a tyre pressure anomaly on the grid.
It had prompted fears that there could be a time penalty at the end of the race, possibly of the order of 25s – the post-race equivalent of a stop and go.
Incredibly, Lewis actually crossed the line 25.042s ahead of Vettel, more by luck than judgement, one suspects...
Initially the team had no real answers as to what had happened.
"All I know is we set our pressures fully supervised by the Pirelli engineer, he was perfectly happy with them as they were set," said Paddy Lowe immediately after the flag.
"So other than that I don't understand it and we'll go and investigate. I think with what I'd call an abundance of caution, because we haven't done anything wrong, we thought let's make a gap."
The tyre pressure saga had kicked off earlier in the weekend when Pirelli told the teams that, as a result of the Spa dramas, it wanted to impose higher minimum pressure limits.
After further discussions, figures of 21psi front and 19.5psi rear – representing an increase of 1psi over more normal baselines – were agreed on for Friday.
Tyre pressure is significant because it's such a key parameter of car performance, and teams don't like to be forced to go higher than they would like.
As one senior race engineer explained: "For a given track temperature for a given car and given brake temperature and rim heating, there is a pressure where it's optimum for grip, wear and everything else.
"Here, because everything was high, everyone was looking to come down. It helps your pace, wear and everything. If you do a long run 0.5psi represents a difference in wear and performance."
Things moved up a notch on Saturday morning when Pirelli wrote to the teams in effect warning that they had to respect the tyre pressures minimums.
In the letter, first revealed by Motorsport.com, Pirelli's top engineer Mario Isola told the teams: "Our prescriptions about minimum starting pressures are based on the assumption that running pressures are higher than starting ones. These are the historical values we've seen, and we therefore need you to respect this in order to operate the tyres safely.
"If we find, during any session, that your stabilised pressures are equal to or lower than the starting pressures, we will give higher starting pressures limit to your team, as agreed with FIA."
Dodging the guidelines
This was a response to Pirelli and the FIA's frustration that teams were apparently trying various clever methods to get around the pressure requirement.
In other words, they were passing the test when the tyres went on the cars, but then running below the limit, in some cases with the rears at 18.5psi. Given that this was a question of safety, the FIA was clearly concerned, as were Pirelli.
"We need to be sure that our tyres are operated in the range of camber and pressure we want," Isola told Motorsport.com on Saturday. "We need to be sure that the running pressure is in-line with the starting pressure.
"We are checking and we are enforcing the usual data that are the starting pressure, the maximum camber and the temperature in the blankets. We just reminded everybody that this is for us very important. We need to be sure that all this data is respected. And we work with the FIA about that.
"I fully understand that the teams need to find performance, that is clear. On the other side we have to be sure that the tyres are working in the right way. It's always a balance between the two. It's clear that nobody's happy when you control something."
And as ever, the teams look for any advantage, and sometimes Pirelli has to rein them in.
Isola added: "It's not an easy job! For me it is not correct to say that people are cheating. They are trying to find the room in the regulations to do something that is allowed because it is not forbidden.
"If we realise that this new idea has a negative impact on the tyre, we need to react, and to police it. They've found some grey areas where they can work at the limit, that's all.
"It's more operatonal than set-up, but it's a combination of both, because also the set-up had an influence on how the tyre works."
Saturday's final practice session and qualifying passed without incident, but it was interesting to note that there were penalties in GP2 as a result of tyre pressure anomalies.
Given the general climate, it was logical to assume that that the F1 teams went into Sunday fully aware that the FIA was keeping a close eye on things, and thus it was imperative to comply with the limits.
The usual practice is for the pressures of the tyres used to start the race to be recorded by the team's individual Pirelli engineers in the last few minutes before the start, and this information is relayed to the FIA.
Different tyres are used for the reconnaissance laps through the pitlane and to get to the grid, so the actual race tyres are waiting on trollies on the grid in heated blankets.
Exactly when they are checked appears to be something of a grey area, and apparently the FIA found out that different Pirelli engineers and teams adopt slightly different procedures and timings.
However, in essence, the numbers are taken when the tyres are still in fully heated blankets. The Pirelli engineer with each team, sets (or oversees) the pressure setting just before the blankets are disconnected from the power supply, ie when they are at their hottest. This is normally 110°C.
The tyres are then put on the car and the blankets loosened in order to lower the car onto the ground to tighten the wheels, so all the time they're cooling down and losing pressure.
On this occasion the FIA decided to impose its own extra surprise checks at a late stage, between 3-5 minutes before the start, having agreed such a strategy with Pirelli on Sunday morning.
The top four cars on the grid – the Ferraris and Mercedes – were duly singled out, and in each case just the left rear was checked by FIA technical delegate Jo Bauer.
It was at this stage that the anomalies were discovered. Hamilton's tyre was 0.3psi below the limit, and Rosberg's 1.1psi below the limit. The two Ferraris were found to be above the limit.
This information was then processed by Bauer and passed to the FIA Stewards. As they were to discover, the key is that when Bauer's measurements were taken, the tyre blankets had already been disconnected from their power source as part of the usual procedure the Mercedes crew go through on the grid.
Mercedes duly pointed out that even in that short time the drop in temperature was sufficient to prompt the drop in pressure that the FIA recorded.
The Stewards accepted this argument, and agreed that the earlier Pirelli supervised measurement, in other words the normal procedure of measuring while still in the heated blankets, showed that the tyres were above the legal minimum when "they went on the car."
In what appears to be a case of miscommunication, the FIA had not been informed that Pirelli usually accepts the earlier measurement – with the tyres fully heated in their still connected blankets – as its definitive official "starting pressure."
When Bauer did his extra checks, it was at a stage when Pirelli had already accepted the Mercedes tyres as "legal", while knowing that, with the blankets disconnected, the temperature and pressure could only drop.
So in effect it would seem that Bauer was wasting his time by measuring pressures in the last few minutes before the start, with the blankets disconnected.
Given that the FIA is trying to police the situation at the request of Pirelli – and has to take all the parameters that Pirelli sets at face value – it would seem that perhaps the tyre maker didn't quite get its story straight.
Indeed, Pirelli confirmed to the Stewards that Mercedes had complied with its own requirements, vis a vis its blankets plugged-in measurements.
Once this became apparent the Stewards decided that, while there was an anomaly in the pressures recorded by the FIA, there was also a grey area in the procedures and timing of the measurements.
The Stewards also accepted that once the cars started moving, the tyres heated up again, and the pressures went up. Indeed, data from the cars showed that throughout the race the Mercedes tyres stayed well above the minimum requirement, and the FIA was satisfied by that.
The punishment potential
Taking a win away from Hamilton would have been a Draconian punishment given that this was not a clear cut case.
That was especially true as the race data indicated that no advantage was gained, since he was running above the minimum during the race. After all, given this is a safety matter, what really counts is the stabilised pressure when the cars are running.
Consider too the ramifications of an exclusion for Hamilton, representing a 25-point bonus for Nico Rosberg, who could not lose any points as he didn't score in Monza, but whose tyre pressure anomaly was in fact more extreme than Hamilton's...
The counter argument is that teams should have been prepared for such a spot check and put sufficient pressure in the tyres to allow for that window after the blankets were disconnected.
It could be argued they should ensure that they stayed above the limit somewhat may, at least to the point where the FIA could make a physical check before the cars actually drove off.
Legal or not legal?
It would be safe to assume that Ferrari did leave such a margin, and thus the Italian team is not happy that Mercedes sailed so close to the wind. Others were not impressed either.
"You are either legal or you aren't legal," said one F1 engineer. "Especially at a circuit like this where the speeds are so high, you've got to have a margin. This is Monza, anywhere where you want to be safe, it's here. You try to be a bit sensible about it."
The key part of the Stewards decision was the final paragraph, in which they "recommend that the tyre manufacturer and the FIA hold further meetings to provide clear guidance to the teams on measurement protocols."
In other words, if Pirelli wants the FIA to properly police pressures on its behalf, it has to come up with a definitive procedure that everyone accepts and understands, which is not the case at the moment.
While sorting out such procedures should be straightforwards, F1 dodged a bullet at Monza without them in place.
Had the Stewards accepted that Hamilton had been in breach - whether innocent or not - the implications would have been massive.
F1 would have had F1's first race-winning disqualification since Spa 1994, and Hamilton losing 25 points? World championships have been won and lost on much, much less.