UNDER THE HOOD
Pat Symonds on Pirelli’s eternal struggle
The high-speed accidents experienced by Max Verstappen and Lance Stroll in Baku raised once again the subject of tyre durability. The somewhat obscure nature of the press release issued by Pirelli after its analysis of the failed tyres led to some rather ill-informed comments in various publications, so it’s worthwhile trying to understand both the nature of the failures and why the case was effectively closed as ‘unproven’.
To fully comprehend the problem, we need to understand a few different facts about tyres and basic mechanical engineering. First and foremost is to know how a tyre is constructed and how it carries load. Of course, a tyre contains a lot of rubber and other polymers but the real load-carrying capacity of a tyre is governed by the materials used to reinforce the rubber.
These materials and how they are arranged are known as the ‘construction’ of the tyre. The particular mix of the rubber is known as the ‘compound’. Here we can lay one of the first myths to bed. When, in a racing tyre, we refer to the compound we are actually referring to the tread compound. This is varied from circuit to circuit and between the hard, medium and soft compounds that we have all become familiar with. In current Pirelli speak, they are termed C1 to C5 – with C5 being the softest compound.
What is generally not appreciated is that these various tread compounds are wrapped onto the tread portion of a tyre, the main part of which is built from the same compound of rubber irrespective of what is used for the tread. The notion, therefore, (and often written) that a soft compound is more prone to failure than a hard compound is incorrect. The tread compound of the tyre plays a very small part in the load capacity of the tyre.
Examining the construction in more detail, the rubber is reinforced in the sidewall, shoulder and belt areas with a number of different materials. These can be nylon, steel or – more usually in a racing tyre – an aramid fibre known as Kevlar. Kevlar is a very strong, lightweight fibre used extensively in crash structures, military and aerospace applications, and even bullet-proof vests.
The next thing we need to understand is how materials behave when they are subjected to load. I’m sure everyone knows that if you apply a load to a material it stretches as the load increases and then, when it reaches what is known as its ultimate tensile strength, it breaks. What is less commonly known is that if you repeatedly stress something at a lower load it can also break. This is known as fatigue failure, and different materials behave in different ways.
With steel for example, providing you keep the load to a low level of around 50% of the ultimate tensile strength, it will survive any number of cycles. Aluminium is different in that if it is subjected to a cyclic load, it will eventually fail almost irrespective of the magnitude of that load. In both cases the behaviour is non-linear. In other words, while a high load will only be survived for a few cycles and a low load for many cycles, you can’t simply say that a load half way between will lead to a failure in half the time – it will in fact be much less.
Kevlar is generally used in applications where its strength is the more important property. In a tyre the fatigue properties can be just as important. The material, when subjected to cyclic load, behaves slightly differently to either steel or aluminium in that, like aluminium, it has a finite fatigue life but the life is also a function of exactly how the fibre interacts with its neighbours in a multi-stranded rope or tow.
Most important of all the aspects of tyre durability is the loading condition. Every time a tyre rotates the construction is exercised as it enters and leaves contact with the ground in a process known as deradialisation. The tyre flattens in the contact patch area, leading to complex loading in the sidewalls and shoulder of the tyre. The tyre designer, knowing the vertical load on
the tyre and the speed the car will experience, designs his tyre to cope with this at a given inflation pressure.
If the tyre operates outside the expected parameters, for example at a lower pressure, a phenomenon known as standing waves can occur. These are resonant deformations of the sidewall causing extreme loads in the tyre carcass. The knowledge of these is nothing new: an article in Motor Sport magazine in September 1958 mentions the difficulty of avoiding this destructive condition.
What is essential to understand about standing waves is that for a given construction there is a critical speed at which they occur. Below that speed the tyre behaves normally and above that speed huge deformations are seen in the sidewall and shoulder which appear stationary but are in fact moving rapidly. The critical speed is a function of load, camber angle and, most importantly, inflation pressure.
An increased inflation pressure effectively increases the stiffness of the belt, which is the dominant parameter in avoiding standing waves. While the exact values for a Pirelli F1 tyre are not known, typically a 1 psi increase in pressure will raise the critical speed by around 3 to 5km/h on a wide racing tyre. Significant also is the tread depth, as a worn tyre has a lower critical speed due to the lower geometric section properties of the tread decreasing the effective belt stiffness.
Understanding how critical tyre pressures are leads one to ask why the running pressures are not mandated, rather than the starting pressure. With the advent of 18” wheels next year, calibrated tyre pressure monitoring becomes mandatory which should put paid to problems of relating cold and running tyre pressures, something which should be easy but, due to imperfections such as moisture in the tyre, can actually be extremely difficult.
“UNDERSTANDING HOW CRITICAL TYRE PRESSURES ARE LEADS ONE TO ASK WHY THE RUNNING PRESSURES ARE NOT MANDATED, RATHER THAN THE STARTING PRESSURE”