If using a low treadwear street tire (between 140 and 200) on a track/road course what would be the highest spring rate you would be able to use before it would become detrimental? The way I understood it there is a point in spring rate when it is meant for slicks or race DOTs.

 

Your choice of spring rates doesn't depend solely on the tires. To say that X lbs/in springs works best when running between Y and Z treadwear would be neglecting vehicle weight, chassis stiffness and damping.

 

There isn't a magic number. Part of the problem is that best compliance over bumps will be a one rate and desired roll rate will be at another. You could slap on a really big roll bar but that has other drawbacks. Any good setup is going to compromise on something.

Looking at the SCCA STU field, 140 tire, 500-800 lbs/in have been used successfully.

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If 140-200 is your idea of a low wear street tire what would you consider sticky? Your getting into rather serious rubber in that territory.

With that said I do think there will be more to it than just spring rate. The quality of the damper often seems to have more effect on the ability to control the spring than the springs weight itself. You would also need to consider the use as if you are going to be on glass smooth surfaces you will be able to use more spring than someone who is driving on rough backroads or over a lot of potholes.