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rmay635703,
Some insight into sidewall cracking and why you may see some variation between tires:
The purpose of the sidewall rubber is to protect the casing from abrasion by curbs. This is more or less a "thickness" kind of thing. Sidewall compound doesn't have much effect. As a result, the sidewall compund tends to be the dumping ground for otherwise unuseable rubber within the tire factory.
There are different types of rubber and there are pretty extreme differences in ability to resist cracking. Unfortunately, some of these rubbers are unsuitable for use within the casing itself, not to mention use in the tread. That means that some tire manufacturers will use crack resistant rubbers in the sidewall and others won't.
Antioxidants (AO's) are used within the tire to slow the deterioration of rubber due to oxygen. AO's are very expensive and their use is carefully monitored within the tire factory. Use of AO's slows down the onset of cracking.
The net effect is: While cracking is commonly used as a tool to judge the state of the tire casing's rubber, it is somewhat unreliable. Clearly tires that have severe sidewall cracking likely means the casing rubber is also deteriorated, the reverse in not necessarily true. In other words, tires with little or no sidewall cracking aren't necessarily OK. Cracks in the tread rubber are a little better guage, but that is also a little unreliable, especially between tire manufacturers.
There have been group discussions between tire manufacturers about tire aging. Some tire manufacturers want their tires to look old when they ARE old, so the tire will be removed before the casing rubber reaches a critical point. In this case, "Old" means the tire's casing is deteriorated, which would be a combination of stress, time, and heat.
However, this is not a universally held position, so some manufacturers are inclined to view sidewall cracking as an item requiring a "fix" of some sort (as opposed to those manufacturers who view this as a tool to judge the state of the casing.)
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