Originally Posted By: zeng
CapriRacer, could you kindly explain the mechanisms of how/why the hair line cracks appear on the tread rubber compound ?
Is it solely due to harder compound being in use for purposes of achieving longer tread life ?
Cracks in rubber are the result of the combination of the material properties of the rubber, the amount of stress applied to that rubber, and the length of time that stress is applied. Please be aware that material properties can change over time - particularly if attacked by oxygen. There are antioxidants (AO's) added to rubber to slow down the attack, but there is only so much that can be added.)
Harder rubber compounds are more prone to cracking, but those compounds tend to be used where cracking isn't much of an issue.
In this case, it appears the cracking is in the cap tread compound, but where the compound isn't worn away (as opposed to the part that contacts the road surface, which is wearing away faster than the rubber can crack.)
It's all a balancing act. Choosing the appropriate rubber compound to achieve the desired properties - and cracking is just one of those.
And be careful of the hard vs soft rubber idea. There is a three way technological triangle involving treadwear, traction, and rolling resistance. In order to improve one property, one or both of the others has to be sacrificed (sort of, as there are technological improvements made that can change this relationship, but those result in smallish improvements and the 3 way relationship still applies.)
So good wearing tread compounds tend to be harder, and both good traction or good RR tread compounds are soft. (and just to state the obvious, a hard compound isn't necessarily a good wearing one. It's that the things that achieve better wear, result in a harder compound.)