I have been mixing tire brands for many years as have many other Coaches and track junkies... In fact, my favorite set up has a different manufacturer rear and front.
Even after just 1 session, the profile of every tire has changed - just look at the side of the tire that runs the side where the majority of hard turns runs based on the design of the track....
And that's also why many people who do a lot of track days flip tires - it evens out the wear so they can get a few more sessions out of them.
1st off, the profile of the tire (steepness of crown, shape of tire curvature) does not change "after 1 session." Generally the tire profile will keep uniform and look similar to where it started, even after the tires are smoked.
2nd, people who flip tires are flipping slicks. When you flip a dot track tire, you are upping the risk of a critical tire failure, that's first hand experience...
I have ridden with a lot of really fast guys both locally and internationally, and I have never heard of anyone using different tire brands. The bike simply won't handle the way it would on a matched set...
The simple fact of the matter is tire manufacturers develop their products with performance in mind. When you start messing with the tire combination, you start messing with the handling.
It's about having fun, if you can have fun with you're handling thrown off, go for it. I just know that most people who spend the money on a trackday want to get their money's worth, not go home frustrated thanks to handling issues.
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