Correct me if I'm wrong, but I've read ozone plays a role. Rubber goods in urban settings don't appear to last as well as they do up away from town i n the Sierra (based on my rubber irrigation boots, and camping fuel bottle gaskets, don't have tire experience). Plastic wrapping, no sun, no bends/c reases help.
Reid Fisher
cracks in my gumwalls in San Martin, California, USA
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