that means, the loss of grip (linear or progressive) is strictly 'tyre wear'
and the (thermal) deterioration of a tyre, which in turn affects the loss of grip, is 'tyre degradation'
is that right?
Georg, I didn't say that. My objection was against the improper explanation of degradation as "the rate at which a tyre looses grip".
Performance falloff is significantly more complex than simply tread wear. Actually, data (lots of if) seems to indicate that tread wear is not as important an indicator of tire performance as other indicators.
Data suggests that the de-vulcanization of tires happens significantly at very high temperatures. Which points to overheating tires and successive heat cycles as the primary reasons for performance falloff (ie, loss of viscosity/rubber compound strength caused by heat). We could also mention the continued curing process (under successive heat cycles) due to excess of cross-linking agents left over in the rubber, which leads to harder rubber and wider areas of the tire with different hardness coeffs.
The process, from linear (tread wear) to thermal-based degradation, is largely not linear, and is surely hard to predict.
So tyre degradation is not a (fixed, variable) value that is influenced by compound or friction between surface and contact patch, but a term to describe the symptoms of (thermal) deterioration, right?
No. Though you cannot reduce degradation (from a scientific/engineering POV, that is) to a single variable, you shouldn't think of it as a list of "symptoms".
Degradation, as I explained earlier, happens due to many factors:
-exposure to heat,
-exposure to UV,
- exposure to rain
- and even due to microbial action
Under race conditions, stresses are obviously higher. We're talking about lateral and vertical deflections due to forces in the range of several thousands of newtons, and an inevitable gap between the tire surface temperature and its core.
There are 3 main degradation types:
- linear
- graining
- blistering
Everything in a tire is constantly changing when submitted to stress (or forces). The viscosity, flexibility and hysteresis of rubber vary significantly during a stint, and depend on temperature and stress frequency. If you increase temperature, rubber changes to a more rubbery state; on the other hand, increase stress frequency and rubber changes from a rubbery state into a more vitreous state. Getting closer to the glass transition temperature elicits higher hysteresis and flexibility (on the other hand, higher stress frequency is conducive to higher glass transition temperature).
With the right level of hysteresis and flexibility, friction coefficients (for a given compound over a given type of surface) are maximized.
So, with the onset of degradation, the above changes - and changes faster (though some anomalies may still occur as when drivers report that tires are beginning to "lose it" and suddenly seem to be "coming back"). How?
Lets consider this. Tyre grip is a function of the frequency excitation of rubber under the application of a certain type of stress - in particular, that resulting from slippage over a track surface. Degradation affects the two friction mechanisms - indentation and molecular adhesion:
- indentation: hysteresis changes over time, which means the ability to asymmetrically deform over road irregularities (thereby creating the inevitable force field) changes.
- molecular adhesion: rubber molecular chains, due to its viscosity properties, resist deformation, thereby creating a friction force which resists sliding. With degradation, the state of the rubber compounds change, which means molecular chains create less force to oppose skidding (they break and do not reattach).
Engineers do add special elements that retard the degradation of rubber and other tire compounds by increasing the tendency of these broken chains to reattach, but that's only retarding the progress of degradation, not preventing it or stopping it.
So, it's not a list of symptoms we're dealing with here, but very specific indicators (though hard to monitor) of both the performance of the tire and its state.
The interaction of all these factors and mechanisms is reflected on the level of grip (or rather, the friction coefficients).