Tires: broken belt, inflation pressure, carrying capacity

Question
what causes belts to bereak in tires?

Answer
Rick,

The term "broken belt" is widely misused.  I'd estimate that 95% of the time the term is used, the belts aren't broken.  

So of the 5% of time the term is used correctly, the cause is an impact with an object or a cut through both the tread and the belt.

The other times the term is used to describe a separation - usually a "belt leaving belt" separation (the common term is tread separation and I'll use  that term for the rest of this response.)

There are a lot of different causes for tread separations, but the vast majority are related to overloading, underflation, and/or speed.

The load carrying capacity of a tire is related to the volume of contained air in the tire (and this is defined by the tire size with more volume = more load), the inflation pressure of the tire (with more pressure = more load), and the operational speed (with more speed = less load).  This relationship is complex so it is defined by tire standardizing organizations.

This link will help you better understand that relationship

http://www.geocities.com/barrystiretech/loadtables.html

There are other things that cause a tread separation, such as tire age, but each of those are topics in themselves - and this is just not the format to discuss those.