Amid the suggestion that Formula 1 teams could be running below the tyre pressures specified by Pirelli, it seems the perfect time to answer a question I’ve been asked a lot: ‘What advantage may teams be deriving by running their tyres below Pirelli’s minimum specified pressure?’
Well, the minimum pressure is imposed to limit the grip of the tyre and the stresses imposed upon its structure. Running below that will increase the contact patch of the tyre – ie there will be more rubber on the road and therefore more available grip.
The weakest point of the construction of the Pirelli tyre seems to be around the inner sidewall where the belt (the cords running around the circumference of the tyre) and the tread meet. This is where most failures seen in the last few years seem to have occurred, including those suffered by Max Verstappen and Lance Stroll at the Azerbaijan Grand Prix.
When the tyre is running at a lower pressure, the rubber is more flexible and bends more under both lateral (cornering) and longitudinal (braking and accelerating) loads. A repeat cycle of these stresses can cause a stress failure in the sidewall – a circumferential break, in this case.
The bending of the tyre’s structure is part of the mechanism of grip. The more the tyre can bend around the tread while that tread remains planted to the surface, the greater its grip.
The optimum pressure for grip can result in more grip than the tyre’s structure is able to sustain, especially on F1 cars which place a more combined load on the tyres than any in history due to their combination of torque, weight and downforce.
The tread of the tyre will distort in the contact patch (the part in contact with the track surface) and then attempt to return to its normal shape as it leaves the contact patch. The strength with which it does this imposes loads on the sidewalls. These loads are amplified by centrifugal forces (forces directed outwards from the centre) which are immense on a car travelling at 200mph or more.
Any ‘give’ in the structure will multiply those forces. Imposing a minimum pressure allows the forces to be contained to within the structure’s operating limits.
