When it's time to change from slick to wet tyres in Formula 1

It's funny to think that an entire Formula 1 car ¬is entrusted to just a few square centimetres of rubber gripping the asphalt. Looking at the whole field of 20 cars, that roughly adds up to around eight square metres and 700 kilograms of advanced technology fighting the laws of physics, courtesy of Pirelli's tyres. Here's how it all works, in theory and in practice. Centrifugal force tries to throw the cars to the outside of every corner. The tyres try to do the exact opposite. In Formula 1, these tyres are 305 millimetres wide at the front and 405 millimetres wide at the back, generating as much grip as possible. The slick tyres, as the name suggests, have no tread pattern on them, in order to provide optimal contact between the rubber and the road.

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At the Mugello circuit, which Formula 1 visited for the first time in mid-October, the cars exit the second Arrabbiata corner at more than 270kph. Those two corners, as well as the ones that immediately precede them (Casanova and Savelli) are taken flat-out: in other words, with the throttle pressed to the floor. It's then that the battle with physics commences: grip versus centrifugal force. Keeping the car on the road are only the tyres, which become hotter and stickier as both ambient and tyre temperatures rise. But the tyres don't keep the same shape throughout the battle: they deform and get squashed by the immense aerodynamic loads, planting them even more firmly onto the track.

A Formula 1 grand prix has one thing in common with an everyday drive: it could rain at any moment, making life instantly more complicated. At this point, slick tyres are no longer suitable. Sure, they can squash the water closer to the surface, but they can't make it disappear. Instead, the slick tyre can become like a water ski – and that's when aquaplaning begins. So when rain starts falling, it makes sense to switch to either intermediate tyres – for a damp track – or full wet tyres, when the surface is properly waterlogged.

Intermediate Formula 1 tyres are a good choice for when the track is changing. When it starts to rain, it's important to understand exactly when is the right moment to change from slick to intermediate tyres. The same process applies in reverse, when the track is instead going from wet to dry. These are known as the ‘crossover points'. Intermediate tyres are softer and have tread blocks that are less resistant than a slick tyres when it comes to dry conditions, so it's important to know when to make that all-important change.

The history of Grand Prix racing is filled with episodes that show how drivers have been able to seemingly walk on water or predict changes in the weather. Timing is all important, as a lap on the intermediate tyres when you should be on slicks can cost five or six seconds, for example.

Being on the slicks when you should be on a wet-weather tyre can be even more costly in terms of time – and there's the added risk of losing grip completely and aquaplaning into an accident. As a general rule, if a normal slick lap time can be measured as 100%, once the lap time goes up to 110% or 112%, it's time to move onto intermediates. A similar percentage increase from the normal intermediate lap time then means that it's time to go onto full wets. To give a concrete example: if a normal dry lap time is 1m30s, it's time to think about intermediates once that lap time increases to around 1m40s. So if we're talking about an average speed of around 200kph for the 1m30s dry lap on slicks, the average speed only decreases to around 180kph with the intermediates fitted. It's a massive challenge.

What are the technical characteristics that allow an intermediate tyre to work so well even at such high average speeds in the wet? The thermal capacity of the compound is important: in other words its ability to generate heat even at low temperatures, which is usually the case when it's raining. The other important element is the tread pattern, which is made up of blocks and channels. The blocks need to find the grip and also deliver a certain degree of flexibility by moving around. If they don't flex enough, the tyre is too hard and won't generate enough heat, but if they flex too much, the tyre can overheat. The channels instead have the important job of dispersing water. At 300kph, a single intermediate tyre can disperse around 35 to 40 litres of water per second. That means that a Formula 1 car at full speed on the straight can shift around 150 litres of water per second running on the intermediate. If it's on the full wet, that figure can be doubled. An astonishing amount of water.

Managing this unique set of challenges is the job of the driver. The team can help out on the pit wall, providing theoretical information about how much water there is on the track and where. But it's then down to the driver to understand which tyre will be the best choice to cope with all the different conditions they might come across. The 2019 German Grand Prix – a race that was characterised by both wet and dry weather – was a perfect example. Many drivers made a mistake, with spins and off-road excursions seen from even the top stars like Lewis Hamilton and Charles Leclerc. The man who found the ideal balance, somehow sensing the grip available perfectly, was Max Verstappen, who triumphed with Red Bull after an hour and three-quarters of racing at an average of nearly 169kph. That was purely down to his remarkable abilities. But also down to the capabilities of the Cinturato intermediate and full wet tyres, whose tread patterns resemble the tyres we use on the road every day.