Did a 5mm inter tyre tread difference decide the Brazilian GP?

The rain-affected Brazilian Grand Prix delivered what was perhaps the biggest shock podium of the Formula 1 season so far.

Max Verstappen’s charge from 17th on the grid to the front had been anticipated by very few people, and it marked his first triumph since the Spanish Grand Prix back in June.

Right behind the Dutchman were the two Alpine cars of Esteban Ocon and Pierre Gasly, who scored more points for the squad in a single afternoon than they have all season so far.

The joy of the top three was in contrast to the struggles that other teams faced in the wet conditions, with recent benchmark squads McLaren and Ferrari struggling with a lack of pace.

World championship contender Lando Norris was fighting front-locking problems, while Ferrari’s Charles Leclerc described his SF-24 as pretty horrendous to drive.

“We were just not fast enough,” declared the Monegasque after the race. “The car was extremely difficult to drive and very, very pointy, very digital, very oversteery.”

While Verstappen and the two Alpines were certainly given a helping hand to their result by the red flag that handed them a free tyre change, it would be wrong to say that this was a fluke result won by a roll of the dice.

Even after the red flag resumption, the trio were the fastest cars on track, showing that the end result was certainly more down to how the relative cars performed in the wet.

It is a well-known phenomenon that some cars are more suitable for wet conditions than dry, as multiple elements come together to help drivers overcome deficiencies that are exposed in the dry.

Pierre Gasly, Alpine

Photo by: Alpine

One factor that almost certainly helped Alpine was the fact that the wet masked one of its main weaknesses: engine performance. With the tricky conditions more about managing throttle input than simply having the most power, the squad was on a much more level playing field than it is in the dry.

But one other interesting element popped up as a factor that could explain the shuffling of the order in the wet – and that is the aero impact of wet-weather tyres.

The current generation of ground-effect cars are very sensitive to ride height, and just a couple of millimetres of difference in ground clearance can have a pretty big impact in terms of downforce levels, with all the juicy performance coming as close to the track as possible.

So with the diameter of the inter tyre that most teams use being 5mm greater with its tread pattern than the slick (725mm compared to 720mm), there is a direct impact on where the car platform is running compared to where it sits on a slick.

And it must be remembered that teams were already finding that they could not run as close to the ground as they would have liked in Brazil because of the Interlagos bumps, so those cars already falling out of the ideal window will have been further pushed away by running on inters.

But it is not just the minuscule ride-height impact that can make a difference when it comes to the aero impact of the tyres, because sidewall stiffness is perhaps an even more important element.

How the tyre deforms under cornering load, and when downforce is applied, has a big impact on the car’s aerodynamics, which is why teams put a lot of effort into ensuring that their cars are optimised to take the changing shape of tyres into account.

That is why wind tunnel tyres are designed to replicate in perfect scale the sidewall deformation that real-life tyres have.

A change of sidewall stiffness and a subtle impact on ride height is certainly more than enough to alter the aero map of a car, potentially shifting the balance and making what is a benign car in the dry quite pointy in the wet.

Haas team principal Ayao Komatsu, whose own team seemed to be worse off on the inter than the slick, said it was not a new phenomenon for his squad – as Spa earlier this year had exposed problems of his car losing rear downforce when put on to rain-weather tyres.

Pirelli tyres on the car of Oscar Piastri, McLaren MCL38

Photo by: Steven Tee / Motorsport Images

“The amount of aero balance we need to take out just shows the rear of the car is weak on the intermediate tyres, which is a new problem this year,” he said.

“You design the car with your wind tunnel tyres for dry conditions, obviously. Then, I can’t remember when we first ran the inters or wet, but straight away we lost so much stability.”

On the flip side, the Red Bulls and Alpine certainly seemed to be a step ahead of the opposition in the wet.

What the data says

While teams do not yet have answers as to whether the aero influence of the tyres was decisive in Brazil, analysis of lap times definitely points to some shift in trends.

Most interesting is that the Brazilian GP weekend, with its dry sprint event and wet rain race, offered us a snapshot of performance differences across the two conditions.

And while qualifying comparisons are not totally indicative, because some cars did not show their full potential in the same conditions as others, they do at least show how some teams moved around in the pecking order – with Alpine and RB certainly looking relatively better in the wet and Ferrari dropping back.

The below results show the fastest car from each team in Q3.

Sprint qualifying result – Dry

Qualifying – Wet

But a more accurate gauge of the pace of the cars, and how things moved around from the dry to the wet, comes from race pace.

Looking at the fastest car from each team, based on clean racing laps – so not including pitstops nor restarts – we get the following data set.

Sprint – Dry

Race – Wet

Red Bull and Mercedes’ pace was certainly much improved in the wet relative to its rivals, while McLaren and Ferrari fell back.

And Ferrari’s was perhaps the biggest drop-off, as Alpine and RB proved to be quicker over the stints.