George Russell’s disqualification for his Mercedes being underweight caught his team by surprise after Formula 1’s Belgian Grand Prix.
Indeed, with no obvious smoking gun, such as a missing car part, it had no immediate answer as to how it had ended up in the situation it did.
Some factors that may stand out as immediate considerations – such as Russell having burned more fuel because the Belgian GP had no safety car nor VSCs – can be discounted because car weight is taken without any fuel weight on board.
Instead, in its own submission to the FIA stewards, Mercedes noted that there was an error with some other calculation.
The FIA document stated: “The team acknowledged that there were no mitigating circumstances and that it was a genuine error by the team.”
But as the team begins its analysis as to how it ended up with a car that was 1.5kg underweight at the end of the race, there is already a suspicion that the main contributing factor could be Russell’s tyres – and the consequences of him doing the one-stop strategy that helped him take victory on the road.
Russell’s charge to the chequered flag came as the result of him electing to skip a second pitstop that most of his rivals went for.
He had originally pitted as early as lap 10 for a fresh hard and, as the pace looked consistent and degradation not too bad, the door opened for him to run until the end.
Russell was able to pull it off and briefly looked like having executed a clinically brilliant performance of tyre management on a day when an extra stop would have ensured he managed no better than fifth.
That decision to run those tyres for 34 laps may, however, be the very thing that ultimately cost him the win because there were weight consequences from doing so – as tyres wear down (and therefore lose mass) the longer they are run.
This was something that Mercedes trackside engineering director Andrew Shovlin said had emerged as an early theory as to what went wrong.
“We don't yet understand why the car was underweight following the race but will investigate thoroughly to find the explanation,” he said.
“We expect that the loss of rubber from the one-stop was a contributing factor, and we'll work to understand how it happened.
“We won't be making any excuses though. It is clearly not good enough and we need to make sure it doesn't happen again.”
The missing weight of 1.5kg may sound like a lot, but we are only talking about 375 grammes from each corner.
And as Pirelli head of F1 Mario Isola has explained, that is well within the boundaries of how much a worn tyre loses.
Asked about how much weight a tyre sheds over the course of a stint, Isola said: “Usually, and we were talking about this a couple of days ago, it should be around one kilogramme.”
With such a figure being bandied around, it is not hard to imagine that Russell’s tyres which had done 34 laps would weigh a lot less than those of his rivals who had stopped a lot later. Hamilton’s second stop had come on lap 26, so his tyres would have had 16 more laps of rubber on them.
F1 teams normally take expected tyre wear profiles, and the likely reduction in weight, into account when they come to settle on their race weight.
And while a one-stop is not unusual, what was different about Belgium was that it was unexpected. Russell and Mercedes had not gone into the race anticipating it was a viable option so it may have made its minimum weight calculations based on tyres that were going to do a much shorter stint.
And, as Isola explains, it is hard to be totally accurate in predicting what the wear rate is going be like ahead of a race.
“Each track is different, each situation is different, and the wear is not linear,” explained Isola. “It depends how much you push, and it depends if your balance is perfect, because then you would wear all four tyres.
“The volumetric wear here [at Spa] was not big though, because sometimes you just wear the tyre with the area that is on the inside shoulder.”
Russell was definitely pushing at the end of the race in his bid to hold Hamilton back, so that is something that could have triggered higher-than-expected wear.
There is another unique factor to Spa-Francorchamps as well, and that is that there is no opportunity for drivers to pick up marbles on the slowing down lap to help cover their tyres in extra bulk for the post-race checks.
It is a well-known tactic in F1 to add extra weight before the car returns to the pits, just in case anyone has ended up sailing too close to the wind.
However, due to Spa being the longest lap on the calendar, there is no slow-down lap after the race to pick up rubber, as cars are sent back into the pitlane from the La Source exit straight after the race.
According to Isola, the lack of marbles could easily account for the difference in weight that meant Russell got disqualified.
“Considering that he is 1.5 kilos underweight, 1.5 kilos on four tyres is possible if you are just talking about the pickup,” he said. “If you have a lot of pickup, then for 1.5 kilos, it would be less than 400 grammes on each tyre. It’s a number that is possible.”
But while the post-race marbles situation may offer some explanation, equally teams are clever enough to know that Spa is different to other tracks in not offering that extra opportunity to add some weight – so it is likely that Mercedes would have had that factored in its calculations anyway.
As Mercedes returns to its factory on Monday, it will need to dig through all its data from the Belgian GP weekend to understand at what point it tripped up.
It may have missed something with the major car changes it made on Friday night, abandoning the new floor it had brought, that impacted it weight.
Saturday’s wet weather may have meant it did not get an extra data point about car weight and tyre mass fluctuations, because there was no slick running as it ran in the configuration it elected for.
It may simply have elected to sail much closer to the wind when it came to picking its final car weight than others – with Aston Martin’s Fernando Alonso (from lap 13) and RB’s Yuki Tsunoda (from lap 15) having pulled off one stops without ending up underweight.
Or it may have simply miscalculated the tyre wear profile, or the lack of post-race marbles, to put itself in the wrong window.
As Mercedes boss Toto Wolff said about the factors that could have played a part: “I think it's a one stop and you expect lots of rubber, maybe more. But there's no excuse. If the stewards deem it to be a breach of regulations, then it was what it is.
“We have to learn from that as a team. But there are more positives today. Obviously for George, that's a massive blow for a driver whose childhood dream is to be winning these races and it’s taken away, but he's going to win many more.”
