2oldman,
If you are still around, please look for a puncture in the hoop of tread. Look from the inside as punctures are much, much easier to find on the relatively smooth surface of the innerliner, rather than from the outside with that tread pattern obscuring things. 90% probability you will find one.
What you have is called a "Run Flat". It is caused by a loss of inflation pressure - usually a puncture. This is a very common thing. The mode of failure has been documented in several books on analyzing tire failures.
The failure mode is the loss of inflation pressure, followed by excessive flexing of the sidewall in the area where the sidewall transitions from sidewall to tread as the sidewall gets thicker. (I call that the buttress area, but others have different terms for this area.) The excessive flexing breaks the sidewall cords, and the result is 3 pieces - 2 sidewalls and a hoop of tread - very distinctive.
One may ask: "What about the other 10% where you can't find the puncture?"
That's because the puncture occurred in the area that is now damaged. In Rex Grogan's book about analyzing tire failures, he has a chart that confirms this.
But I also don't want to discount the possibility that the snags in the sidewall are what let out the pressure, but those are a low probaility.
No, this is not a *tread separation*. A classic tread separation - the kind that many attribute uniquely to ST tires, has the separation between the 2 belts, where the top belt and tread detaches from the bottom belt, leaving the bottom belt on the rest of the casing. For those of us who analyze failed tires for a living, that type of failure is called a *Belt Leaving Belt Separation* - as documented in the TIA's Tire Conditions Manual.
A side note: Belt leaving belt separations are not unique to ST tires. Any radial tire can have that type of failure.
