The shape of a Formula One tyre

Tyres are round. That’s the answer you will get if you ask the man on the street what shape a car’s tyres are, but as with everything in Formula One, the answer’s not quite that simple.

"The shape of a tyre is a very important part of how a tyre performs," confirms Hirohide Hamashima, Bridgestone Director of Motorsport Tyre Development, “and it’s an area that is not as simple as you might otherwise think."

Certain dimensions for a tyre are laid out in the FIA’s regulations, but there is far more to a tyre than maximum contact patch and the width of the wheel and tyre combination. "The regulations give us a regulatory framework in which we must work, but within these limits there are areas where we can make a difference," says Hamashima.

The size of a dry front Formula One tyre is 270/55 R13, whilst a rear is 325/45 R13, but what do these measurements actually mean? The first measurement (for example, 270) relates to the tread width in millimetres, the second measurement (for example, 55) relates to the aspect ratio of the height to the width, whilst the final measurement (for example, 13) relates to the rim diameter in inches.

There are four main jobs that a tyre has to do. It has to enable a car to go forwards (enable acceleration), to stop a car from going forwards (to allow the car to brake), to support the car’s weight (both stationary and when moving when an F1 car’s effective weight increases because of downforce generated by the wings) and also enable the car to change direction.

The front tyres play a crucial part in starting the direction change of a car, whilst the rear tyres of a Formula One car are the only ones to transfer the engine’s power to the road. In contrast to this, on many road cars the front tyres have to both steer and deliver the engine’s power to the road.

"Both the front and rear tyres have cornering as part of their job, whilst the fronts also have the additional aspect of changing the car’s direction as a result of a driver’s steering inputs, the rears also have to provide traction to enable forward motion in response to a driver’s throttle inputs," says Hamashima.

"The tyre sizes are determined by the regulations, and as with most single seater racing cars the front tyre is smaller than the rear.

"The traction demands of a Formula One car with their high power engines mean a wider rear tyre is desirable and in Formula One’s past we have even seen far wider rear tyres because of traction and cornering requirements."

So, for more grip you need a wider tyre? To an extent yes, but you can go too wide. "Obviously we are constrained by the regulations, but even if we had free reign on the size of a tyre, we would not see cars with massively wide tyres because the current generation of Formula One cars are not designed for them," says Hamashima.

"A wider tyre means a bigger contact patch and more aerodynamic drag and rolling resistance, which can slow the car in a straight line, although a slightly wider tyre could enable higher speeds in the corners."

A tyre works best when it is in its correct operating temperature. Heat generation in the tyres relies on a certain amount of work going through the tyres, making a tyre wider, and its contact patch greater, means that not so much heat is generated for any given compound.

"The requirements for the rubber compound we make are closely related to the tyre size for the application," explains Hamashima, "and this is the same for many applications, not just motorsport.

"For instance, when we move to slick tyres next season we will have a larger contact patch with the road from the same size of tyre as there are no grooves and this means we will use softer compounds as the greater contact patch distributes the various demands on the tyre and we don’t need the same amount of compound strength to maintain the groove integrity."

The tread area is an interesting aspect of a tyre’s shape. A largely flat surface means a slick tyre, whilst the current regulations stipulate grooves.

"Currently we have circumferential grooves square to the wheel axis around the entire circumference of the tread – grooved tyres. Next season we will have slick tyres and no grooves so there will be a larger contact area and more grip available. This is a significant difference, but as the regulations stipulate fewer means to create downforce we have to be aware of the potential for less load affecting the tyre shape."

A tyre’s shape is not the same when it is at standstill as it is when moving. “When considering the contact patch for a Formula One tyre we need to look at the downforce created by the cars,” explains Hamashima. "When you have a car that is stationary, the weight of the car compresses the tyre, but when a formula one car is in motion, especially with high levels of wing, much compression comes from the loads created by the aerodynamics."

The shape of a tyre from the outside is one thing, but the shape of the inside of the tyre and its construction is another topic entirely...