USED TIRE GRADING
A ¾ tread tire means the tire still has 75% of its original tread remaining. For example: if a tire comes new with 4/32 tread, a ¾ tread tire will come with approximately 3/32nds tread.
A 50% tread tire will have ½ of its original tread remaining. For example: a 4/32nds tread tire will have approximately 2/32nds tread remaining.
A Heat Cycle is when a tire goes from ambient temperature to full or operating temperature.
Heat cycling race tires breaks the uneven bonds of a new tire to allow them to reform in a more uniform manner. As a result the tire compound is stronger and more consistent throughout.
Vulcanizing is when additional cross-links are formed between molecules.
The reason tire grip or performance tends to go away after a certain number of heat cycles is the rubber gradually hardens and loses flexibility reducing the tire's grip with each cycle.
The number heat cycles that your tires will get will depend on whether or not they were properly scuffed, ambient and track temperature, track surface, length of each track session, and most importantly, driving style.
RADIAL VS. BIAS TIRES
Radial Tires: Sidewall and Tread function as two independent components of the tire.
Sidewall flex is not transmitted to the tread allowing the vehicle to transfer more power to the ground.
Bias Tires: Sidewall and Tread are interdependent and form a thick layer that is less flexible and more sensitive to overheating. Sidewall flex is transmitted to the tread allowing the contact patch to deform for less grip.
Shaving removes tread rubber for shallower tread depths. The lower tread block results in increased responsiveness, reduced tire slip angle, wider contact patch, and better cornering
Tires manufactured with only 4/32" to 4.5/32" of molded tread depth do not require tire shaving for autocross or track use.
275/35/18 = Section Width / Sidewall Aspect Ratio / Rim Diameter
Section Width (in millimeters) is the measurement from the widest point of a tires outer sidewall to the widest point of the inner sidewall when mounted and measured on a specified width wheel.
Sidewall Aspect Ratio is the tire's profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall.
Rim Diameter is the overall height or diameter of the wheel/rim.