Everything you ever wanted to know about tires
 

Everything You ever wanted to know about tires.

http://media.wiley.com/product_data/...0471354619.pdf

Having worked for sears tire america I can say they are temperature sensitive and "spoil" so to speak like food does. The brown color isnt a good color and is usually followed by cracking and separation. The black color is actually painted on the tire and protects it.

Lastly tires are made up of parts and you can break a band, belt, etc inside on a pot hole and get a vibration or see a pimple on the tread as well as sidewall area.

A couple of thoughts:

First - the brown color is the antioxidants (AO's), which were put in the tire to protect the rubber from attack by oxygen. I suspect the brown color is because they are iron based (rust). When first placed into service, many tires will quickly show this color, as during storage and shipment some of the AO's will be used up, but since the tire isn't being flexed, the AO's pretty much stay where they are - BUT - during the first bit of flexing, the used up AO's quickly migrate to the surface and make a strong color change. The waxes that are in the rubber do the same and capture the used up AO's. The waxes and captured AO's eventually flake off.

Another time the brown color shows up is when a tire experiences high heat. That will sometimes use up the last of the AO's. The next step in the process is cracking, a symptom of the rubber breaking down - which leads to a separation.

Broken belts is a fairly rare phenomenon - but irregular wear is commonly misdiagnosed as this. Irregular wear is usually caused by misalignment and aggravated by insufficient rotation and inflation practices.