The Bridgestones, as you've noted, are very temperature dependent. Having done about 6 track days on my original set, I've now felt the progression of not only decreasing tread depth but also aging rubber. The heat cycling has taken a toll on the tires, and I now understand why one would replace them even with tread remaining...they're just rubber bricks. Anything below 70F and the tires are like rocks with diminished grip. I notice it on a spirited corner and the back end comes loose. Then after some aggressive driving, the rubber gets much tackier and the grip improves dramatically. During my last track day, I could literally put my nail into the tire. One way to judge your tire temp is how the tires sound when you spin them. A cold tire gives a silent spin while a hot tire squeals like a stuck pig. At 18,000 mi, your rubber may have heat cycled enough that its lost its grip unless you really get them warm.
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2013 Cadillac V-Wagon, RIP Z
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