The class requires to run spec CRC tires CLN2230 and CLN2232. We aren't allowed to true the tires down at all. Everyone glues the sidewalls of their tires though and this helps prevent traction roll. One of the guys at our track measured my car the other night while I was working on some setup and my front was around 7mm and the class has a minimum height requirement of 3mm. I agree though 4mm seems like a lot to shim..