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Drilled vs Slotted Rotors, What is Better?
Drill holes and slots in rotors can both improve braking, but under different braking scenarios. To understand how these rotor modifications can improve stopping power, it is first necessary to understand the three forms of heat transfer:
Conduction: When there exists a temperature gradient within a body, heat energy will flow from the region of high temperature to the region of low temperature. This is conduction. Conduction heat flows from the edge of the rotor through the hub bearing, and it also flows through the brake pad into the caliper.
Convection: Air flow disapates heat from a body. Normally, the higher the air flow over the rotor, the more heat is removed. The rotor vanes act like a fan blade to move air from the inside of the rotor to the outside edge. The rotor vanes helps remove heat through convection.
Radiation: heat is removed through infrared radiation (electromagnetic radiation that is not visible). After you drive your car, stop and pop the hood, you can feel the heat with your hands above the engine without touching it. You are feeling the infrared radiation coming off the engine.
All three methods of heat transfer occur when you apply the brakes. During a typical stop, the heat transfer is about 25% conductive, 35% convective, 40% radiation. For a high temperature, high speed stop, the heat transfer is about 15% conductive, 40% convective, 45% radiation. At high speed, convection heat transfer is very important. This is why drill holes can help reduce the brake temperaure. The drill holes help air flow through the vanes. The brake temperature can drop up to 180 degrees. Brake pads work better at lower temperatures, and you reduce the risk of pulsating brakes as well.
Slotted rotors do not improve any heat transfer. However, the slots can improve brake output by removing gas and dust that is trapped between the pad and rotor. This gas and dust reduces the friction force by preventing the pad from fully contacting the rotor.
Given the choice between drill holes and slots, the drill holes will give you better braking power over slots for normal city/highway driving. This is why high end BMW, Porsche, Corvette, and Mercedes rotors are drilled, not slotted. However, for track racing (high speed stops), slotted rotors are the better choice.