The Technical World of Formula 1 Explained : F1 Tires...Part 1

Introduction

The tire is the interface with which the car is connected to the road. Acceleration, deceleration, downforce and changes in direction are all transmitted through the tires.

Slick Tire

Grooved Tire

Since 2009 Formula 1 has gone back to slick tires from grooved tires. Slick tires provide more grip due to the higher surface area of rubber in contact with the road, what is referred to as ‘the contact patch’. Slick tires were also used pre 1998, but from 1998 to 2008 the sport decided to change to grooved tires because they provide less grip, to try and simulate the grip levels of a wet track because it produced a better show.

The size of the tires is limited by regulations; the size of the rear tire must be 325/45R13 while the front tire must be 270/55R13. Taking for example the rear tire, what these measurements mean is that the width of the tire must be 325 mm wide and the diameter of the wheel rim must be 13’’ or 330 mm. The 45R figure gives the aspect ratio of tire sidewall height to tire width. The tire sidewall is 45% the height of the tires width, 45% of 325mm is 146.25 mm. So the overall height of the tire is 2(146.25) +330 which is equal to 622.5 mm for dry tires. Wet weather tires are slightly taller to increase the car’s ride height and prevent the floor or the front wing from hitting any standing water on the track. If that would happen downforce would be lost at that instant and the driver would probably lose control.

Coefficient of Friction

The pneumatic tire had not yet been invented when Sir Isaac Newton defined the laws of motion in which he included the laws of friction. So when it was invented in 1988 by John Dunlop, everyone expected it to obey Newton’s laws which suggested that no tire would ever be able to develop a force in any direction that would exceed the load applied to it. For example no car would ever be able to achieve acceleration higher than 1g. But no one told Dragsters, which soon enough sailed through the ‘barrier’ of 1g of acceleration. But there wasn’t a barrier and the tires do not obey Newton’s laws of friction which were written for friction between two smooth surfaces. The tire can and of course does develop forces higher than the load acting on it. Today’s racing tires under ideal conditions with a load of 2000N can generate a force of approximately 3200N.

The ratio between the load on the tire and the force generated by the tire is the Coefficient of Friction.

The force of friction is equal to the coefficient of friction multiplied by the normal reaction of the weight of the car against the road. F_F=μN

F_F => Force of Friction

μ => Coefficient of Friction

N => Normal Reaction

So in the above case the 3600N force divided by the 2000N load gives us a μ of 1.6. This means that under ideal and constant condition the tire can accelerate, decelerate or corner with an acceleration of 1.6g which is more than enough to make your neck hurt. At this point though it is important to state that the coefficient of friction does not have units, it is an indication of how much force a tire can develop with a certain load applied to it.

Both of the factors though, Force of Friction and Normal Reaction, are constantly varying with speed, road temperature, tire temperature, ambient temperature, road condition, aero loading, suspension setting and the pressure of the gas (in this case Nitrogen) inside it.

Heating Up & Thermal Degradation

Tire with blanket around it

Taking one of those variables, temperature, which has been a very important factor with the 2012 Pirellis as tires have an operating window in which they give the most grip, this is important for qualifying as you want to heat up the tires quickly enough to give you maximum grip during your flying lap and this is also the reason tire blankets are used in Formula 1, as they pre-heat the tire before the driver goes on track. Thermal degradation has been another big factor with the Pirellis. It is important to state that the Pirellis were deliberately designed to degrade quickly to improve racing, something they have achieved.

To describe a racing tire I will use honey as an example, I know it sounds weird but bare with me. Imagine a tire made up of frozen honey. When frozen it is hard and doesn’t stick on anything. When it is brought up to the ideal temperature, a temperature at which only the outside layer will start to melt slightly and become tacky and stick to whatever it is in contact with while leaving a small amount of residue behind (the dark lines you can see on the racing line of the track).

If you continue to heat it up though the energy will work further into the structure of the honey tire and it will no longer have a sticky boundary layer but instead it will develop a thicker molten surface which will behave like a lubricant between the harder inner honey and the outer molten honey. This is what happens when the tire overheats. If you continue to heat it up it will no longer be able to hold its structural integrity and it will withstand neither the frictional nor the centripetal forces applied to it.

Elastic Hysteresis

How tires heat up and degrade is explained by the effect of Elastic Hysteresis.
The graph below illustrates this effect. The blue curve is the curve produced when the material is loaded and stretched, which is shown on the graph as the force applied (load) vs the extension of the material. The red curve is the curve produced when the material is unloaded. Since Force (N) x Distance (m) = Work Done (J) the area of the graph between the two curves is the energy dissipated as heat and sometimes sound (tire screeching) when the material returns to its original shape. As heat energy is dissipated it heats up both the tire itself and the surroundings (tarmac, air). Te tire is loaded and unloaded a lot of times during the course of a single lap and every time this happens the tire loses energy and subsequently grip. This is how thermal degradation occurs.