I see the term "Dry Rot" used by many in the RV community when describing old tires that have visible external cracks.
Some have attributed this to the dry air in the Southwest part of the US. Some with specific references to Arizona.
Sidewall cracking occurs for a number of different reasons but these reasons all end up at the same place. The elastic properties of the rubber have been degraded over time and when the rubber is flexed it cracks rather than stretches.
Some reasons for the loss of flexibility or "stretchyness" can include exposure to UV or Ozone or simply old age. Each of these items is different but they each attack the bonds that exist between the various materials such as rubber, sulfur, carbon black, oils waxes and numerous trace materials used in the process of manufacturing rubber. When the chemical bonds break or "crack" the loads in the rubber get transferred to the nearby material which then has to resist the forces trying to stretch it.
These cracks may start out at the molecular level but they do not repair themselves so they can only continue to grow. Eventually they are large enough to be seen on the outside of the tire and if the tire is exposed to the damaging elements long enough the cracks can grow large enough to allow air to escape or for tire components separate.
I have previously discussed the way increased heat can actually accelerate the aging process of rubber in posts about direct exposure to sunlight and that the use of white tire covers can reduce the accelerated aging process.
For those interested HERE is a report issued by NHTSA on tire aging. You will note that on page 3 of the report titled "Background" they identify that "degradation is accelerated with higher temperatures", You may also note that there is no mention of UV as a significant contributor to the aging process.
IMO Sidewall cracking is in itself seldom more than a cosmetic issue. However it can be an indicator of possible "old age" and degradation of the internal structure of a tire. Maybe a good analogy is when you run a temperature.
I do not recall ever hearing of someone having a temperature for no reason. Your elevated temperature is almost always an indication or symptom of some other medical problem that needs attention.
Since the consumer has no good, low cost way to learn the condition of the tire structure you are confined to looking at various symptoms. Spotty tread wear is one symptom. Tread and/or sidewall snaking is another and of course sidewall cracking can be another.
Tires do not actually suffer from "rot" as one might see in a piece of wood or some old food. They can have signs of surface cracking but as long as the cracks are shallow and a tire dealer has completed a full inspection of a tire and said it was okay to run I would go with the dealer finding.
You might want to review my post on How do I inspect my tires and note that signs other than just cracking can be much more telling than just sidewall cracking.
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