SPA-FRANCORCHAMPS, Belgium -- When small margins decide Formula One races, it's usually measured by tenths of a second at the finish line.
That appeared to be the case at Sunday's Belgian Grand Prix, when a jubilant George Russell led Mercedes teammate Lewis Hamilton across the line by 0.526 seconds. Two hours later, however, a similarly small margin, albeit measured in kilograms, had cost him the very same victory.
Once fuel had been drained from Russell's car, it tipped two separate sets of FIA scales at 796.5kg -- 1.5kg short of the required weight to be in compliance with F1's technical regulations. As devastating as it was for F1's latest race winner, there was no arguing with the facts and no grey areas to exploit, leaving Mercedes and Russell with no choice but to accept the disqualification.
"We have to take it on the chin," team principal Toto Wolff said after the race. "We have clearly made a mistake and need to ensure we learn from it.
"We will go away, evaluate what happened and understand what went wrong. To lose a one-two is frustrating and we can only apologise to George, who drove such a strong race."
In a social media post, Russell described the disqualification as "heartbreaking."
In a sport where every last detail is accounted for, double checked and checked once more, it's rare for a car to be found underweight after the chequered flag. It's not unusual for teams to design a car to be as light as possible, but ballast is used to ensure it tips the scales at (or over) 798kg come what may after a race.
The last time it happened was in 2006, when Robert Kubica's BMW was disqualified from seventh place at the Hungarian Grand Prix for being 2kg underweight.
Crash damage (defined as "accidental loss of a component of the car" in the regulations) can be a valid excuse for a car tipping the scales short of the minimum weight, but Russell's Mercedes crossed the line intact.
Mercedes will spend the coming days fully investigating the reason for the oversight, but initially it seemed linked to the remarkable one-stop strategy Russell used to take the victory.
By making a single set of hard compound tyres last 34 laps, Russell's Mercedes vaulted from fifth place to the lead as his rivals made a second pit stop. The strategy was a gamble, not least because most teams believed a two-stop would be the fastest way to the flag ahead of the race.
When Russell suggested the idea of a one-stop over the team radio, Mercedes' strategy modelling said both the planned two-stop strategy and the eventual one-stop would leave the car fifth at the finish.
"We have these race planners, which tell us where the cars are going to come out at the end, and at some stage we saw we had to cover [Oscar] Piastri and [Charles] Leclerc with Lewis, so [putting Hamilton on a two-stop strategy] was absolutely clear cut," Wolff said. "Nobody expected the hard to last, and we went longer and longer and then the planners said P5 for George in either case, one stop or two stops.
"So we said, 'Let's take the gamble.' Both drivers at that stage felt happy with the tyres, but with Lewis we had to do it to cover [the other cars]. So we extended the stint and the tyre remained absolutely fine. It was pretty flat in terms of performance until the end."
Russell had nothing to lose, and as his stint continued over 20 laps, he took the lead of the race as Piastri made his second stop. Remarkably, Russell's tyres were still in good shape at this stage, and as the laps counted down, the performance did not drop off as the team feared it might.
But while the performance of the tyres was still good, the physical rubber on the tyre was still wearing away. A set of F1 tyres (minus rims) weigh 42kg when new, but can lose as much as 3kg in wear during a long stint.
Russell's second stint from Lap 10 to Lap 44 was longer than any other driver in Sunday's race and longer than Mercedes had ever expected to go ahead of the start. In the aftermath of the disqualification, the team believed the loss of rubber through tyre wear was a likely reason for the car being underweight at the finish.
"We don't yet understand why the car was underweight following the race but will investigate thoroughly to find the explanation," Mercedes trackside engineering director Andrew Shovlin 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."
Mercedes had not run the hard compound tyre at any point during the weekend prior to it being put on Russell's car in the race. The team had suspected it would be the stronger race tyre, but with just two sets available per car for all three days of the event, it had saved both sets for both cars for use in Sunday's race.
Adding to the team's trip into the unknown was a series of major setup changes after Friday practice. The car had looked worryingly uncompetitive when it was fitted with an upgraded floor during first and second practice, meaning it reverted to a previous spec of floor and made significant changes to the setup.
With rain throughout Saturday's final practice, there was no way of knowing how the changes on the car would impact dry running, including the likely wear rate on the tyres. What's more, the team's historical data was difficult to rely on because the tyres were behaving differently to previous years after large sections of the track in Sectors 1 and 3 were resurfaced.
Another factor, which is unique to Spa-Francorchamps, is that because of the length of the circuit (4.3 miles), drivers do not complete a cooldown lap before returning to the pits after the chequered flag. Instead, they take an immediate right after Turn 1 and drive the wrong way down the pit lane to parc ferme and celebrations with their teams.
On a normal cooldown lap at any other circuit, drivers stray into the run-off areas to pick up discarded rubber and debris on their hot and sticky tyres. This practice can add as much as 2kg in weight to the tyres (although that figure is dependent on how much the driver goes looking for it) and race engineers routinely remind their drivers to get plenty of "pickup" after the chequered flag. However, with no cooldown lap at Spa, Russell was unable to pick up any rubber.
Combine all of the above, and Mercedes' mistake is more understandable, even if, as Shovlin said, it remains inexcusable.