I've ridden many a modern sportbike tire to the end of their tread life and have never had major issues with the compound changing that I am aware of. A bigger issue to me is the profile of the tire changing from wear.
--xTalon
Probably discussing nits here.
Race tires are made and the compound chosen in order to cope with the high heat excursions encountered during racing. They can chose a compound that withstands high heat, but that may not give the best traction for the track surface du jour. So, it is a compromise, traction vs withstand/longevity. Softer tires may get you around the corner quicker, but will they last the entire race without becoming soft, slippery goo, or fail entirely as the heating abuse from cornering braking and acceleration increases? It's now nearly a science and they can rate such tires in heat cycles, temp limits, etc.
Street tires are not expected to endure the rapid, repeated, high heat cycles, that race tires do. They are also expected to last longer than a race day heat, too. Take a street tire to the track, and if you don't slide off, the tires will not likely last very long, because of the higher heating involved.
So, if you use your street tires as though they were race tires on the street, you can expect them to harden quicker than with "normal" use, what ever that is. The rubber for street tires likely has far more silica in it to lengthen mileage replacement. But, the rubber component will still age faster in high heat conditions than in cooler ones.
The base material property is the same. You don't have to use a tire at all for it to develop age cracks. This is caused by the rubber out gassing and losing it's plasicizers. Store your new tires in a hot ventilated warehouse for a few years and it will harden up, guaranteed.
The phenomenon is similar to a pan of water on a stove. The pan will hold water for quite a long time unheated. However, if you apply heat, even well below boiling, the water "out gases" much more rapidly.
The nit part is just how much grip is lost, over how fast a time; 6 months, a year, 2 years, 5 years? I would expect tires under heavy use in Arizona Summer would show hardening and grip lost earlier than tires driven "normally" in Canada. I don't know how to quantify that difference, though, especially since road surface traction qualities differ greatly within townships, not to mention states and countries.
I'm not likely to find out first hand, either, because doing so means pushing to the edge. Now that I have come to realize how little life I have left, I'd prefer my remaining years to have ambulatory qualities. So, I'll let you youngsters find where the grip is lost, and hope a speedy recovery for those that exceed it.
In summary, you can wear the tread off the tire long before the compound hardens. You can also have lots of tread on a tire whose rubber has hardened with time/temp.
But, I do agree that tread profile matters a great deal.
I don't know if tires played a part in ASCS's accident. But, If he was treating the tires like a racer would, then the definitive answer is ... maybe?
I wonder if a thumbnail test would lend clues?
Cheers,
