Why peak tyre temperature is an elusive state for F1 cars to attain

At a macro level the change in competitiveness this season has been obvious. From early domination, Red Bull found itself regularly challenged by McLaren with Ferrari always there or thereabouts. Mercedes too built on its ever-increasing competitiveness, at times being the class of the field. 

However, taking a more microscopic view there are considerable swings from day to day and even from session to session. It appears that this is more prevalent this season than ever before – so what could be causing these apparent inconsistencies?  

Of course, the closeness of the field perhaps amplifies the effect. A few years ago a couple of tenths cost very few grid positions – today it can be catastrophic. Nevertheless, there are some factors that can play a part. For example, if a car has poor aerodynamic yaw sensitivity it might show well in a windless morning practice but lose relative performance during a gusty qualifying session. Set-up also seems to be more critical on the current cars but the most dominant factor, and the one you hear drivers mention more than any other, is tyre temperature. 

So, let’s examine why this appears to be such a problem. To do so we need to understand how a tyre generates grip and what factors affect that grip, but firstly let’s answer the obvious question: how important is grip? At an average circuit, a 1% loss of tyre grip loses around 0.3 seconds of lap time – roughly equivalent to carrying 10kg of extra fuel or losing around 10 points of downforce. This shows the significance. 

A tyre is a complex mechanical and chemical amalgam. The all-important rubber is a complicated material that behaves very differently at different temperatures and different excitation frequencies. It’s termed a visco-elastic material because it neither displays a simple stress/strain relationship like a suspension spring, nor is the relationship purely viscous like a suspension damper. A spring produces a restoring force which is proportional to its displacement, a damper one which is proportional to its velocity or how rapidly you compress it. Rubber lies in between in that it responds to both displacement and velocity and, just like a suspension damper, it loses energy by heating up. So as the tyre is deformed by cornering, braking or traction, as well as some deformation on the straights, it heats up. It’s also subjected to heating from the brakes but, conversely, the portion not in contact with the road will be cooled by the rush of air over it. 

There are two mechanisms of grip which go under some different names. The first and most important is generally known as hysteretic grip. This is the grip which occurs as the tyre deforms into the roughness of the small stones that form the aggregate of the track. The depth of deformation and therefore the mechanical grip is a function of temperature. If the tyre is too cold, it will not deform into the track surface; too hot and it will not retain enough mechanical strength to provide grip. 

For all the money and time spent on car development, the biggest challenge for teams is often getting the perfect performance from the tyres

Photo by: Mark Sutton / Motorsport Images

The second mechanism is adhesive grip. This is the molecular attraction of the rubber to the smoother surface on the top of the aggregate. It’s a much smaller contributor to the grip but can still be significant on smooth tracks. As a generalisation, the ideal temperature for hysteretic grip is slightly lower than that for adhesive grip. 

Determining the ideal temperature for the tyre is complex and depends on the roughness of the track and the slip velocity of the tyre over the surface. These factors have to be computed and put together for both types of grip in what’s known as a master curve. This is a plot of grip against temperature for a particular condition and rubber compound. Typically, we may see from this that peak grip occurs at around 105°C with our 1% loss occurring below 100°C and above 115°C. This is a narrow temperature window and explains why performance can be lost so easily. 

Unfortunately it’s not as simple as that. In order to determine the master curve accurately one needs access to the mechanical properties of the rubber, something no tyre manufacturer will share. The engineers therefore have to make some estimates. Further to this we need to decide what we mean by temperature. Rubber is a pretty good insulator. Its conductivity is about 1,500 times less than that of aluminium and only five times more than a Styrofoam coffee cup. The heat is generated in the contact patch, and it’s the temperature of the bulk of the rubber that determines its properties.  

This is a very short account of an incredibly complex system – probably one of the least well-understood in race engineering but one critical to performance

Unfortunately, we can only measure the outer surface temperature through remote infra-red devices and the temperature of the inner liner through similar devices mounted on the tyre pressure sensors. We therefore have to estimate the bulk temperature, which lies somewhere between our two measurements. We would like the tyre to be at a relatively uniform temperature. A particular killer for performance is high surface temperatures with low bulk temperatures. This occurs through excessive sliding, wheelspin, and, of course, locally through brake lock-ups.  

A word should also be said about graining and blistering. Graining occurs when the tyre is loaded at too low a temperature, in other words when it is relatively stiff, and is a shear failure of the surface leading to rolling of the tread surface and very low grip. Blistering occurs when the bulk temperature is too high, and the embedded gases expand and escape by rupturing the tyre surface. 

This is a very short account of an incredibly complex system – probably one of the least well-understood in race engineering but one critical to performance. This explains the attention drivers pay to getting their tyres in the right window to start their qualifying lap and possibly why the results aren’t always as expected. 

Graining and blistering (pictured) are also pitfalls of finding the ideal tyre performance

Photo by: Manuel Goria / Motorsport Images