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Damiano Capra, an experienced tire engineer working in the field of motorsportWhen finding ways to improve vehicle performance, particularly in motorsport, there are two tire operating parameters that are considered important – pressure and temperature.

This is because it is universally recognized that pressure and temperature affect grip. The two parameters are not independent of each other. For example, changing the tire’s pressure will affect the temperature. However, if we look just at the first order effects, we could say the temperature mainly has an effect on the size of the contact patch (which is also related to the tire carcass), while the temperature has a greater impact on the elastic and shear modulus of the compound, as well as an effect on the chemicals in the tread.

If we restrict the analysis to the relationship between temperature and grip, the first thing we need to ask is “Which temperature reading do we need to focus on?”. It is generally considered that the so-called ‘bulk’ temperature is more closely correlated with the grip level. It therefore becomes clear that the bulk temperature needs to be controlled.

There is however one problem: the bulk cannot be measured on the vehicle. Due to this fact, as a compromise, we usually use the surface and innerliner temperature as a reference, accepting that this is not exactly what is needed but that they must be in some way correlated.

This assumption is correct, however the level of correlation is debatable: if we take two tires that are the same, running at a certain innerliner and surface temperature on two different tarmacs, each with a considerably different level of roughness.

Here the indentation of the tarmac asperities on the tire varies between the two, placing different amounts of stress on the tire itself where the so-called bulk lies. These varying stress levels will cause different levels of heat generation inside the tire. As such, although the two tires are running in the same conditions, supposedly with the same surface and innerliner temperature, in reality they will have a different bulk temperature, which is the main factor affecting the grip.

If the above statement is clear, it is evident that whatever relationship between surface and innerliner has been defined a priority, this cannot be true for all cases. To achieve a more accurate reading of the bulk temperature, a thermal model is required.

There are several thermal models on the market based on various theories. Models however, have their limitations, that is, a model may be better suited to a certain application, and the selection process requires a thorough evaluation of the strengths and weaknesses of each model.

Based on my experience, I believe another key parameter needs to be taken into account when evaluating a thermal model – the background of the model developer. Since there are different ways to develop a thermal model and research is always ongoing, it is vital to select a supplier that is used to working with other companies in the same field.

A supplier should also be dedicated to continually improving the model. More importantly, the model must have a proven scientific background, such as, for example, being based on university research, which is also crucial to ensure it incorporates the most modern technologies and ideologies.

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