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Fundamentals of Brakes

December 1, 2015

I often here about the debate between cross-drilled rotors and slotted rotors. To understand the benefits and short comings of these two types of rotors I feel it is first necessary to understand the basics of how your brakes work in the first place. We all know what are vehicle's brake systems are there for, but few understand what they are actually doing. Not from a mechanical level, but from an energy level. The brakes on your car, whether disc brakes or drums accomplish the task of stoping your vehicle by converting the kinetic energy of your moving vehicle into thermal energy or heat. That is the key, that your brakes convert kinetic energy into heat. It is your rotors and surrounding components that are there to absorb this heat and then transfer it to the air so they can convert more kinetic energy into heat. 

When does a brake system become inadequate? Answer: When the system cannot effectively transfer enough heat away to operate at "suitable" temperatures. If a brake system is in a situation in which it is "forced" to convert too much kinetic energy to heat, the brake system becomes saturated with heat energy because it is unable to transfer it away fast enough. At these elevated temperatures your brake pads, rotors, or other components can begin to fail from the excessive heat. 

So our question now is how do we improve our brake system with just a rotor change. Well the bigger (larger diameter, thicker, etc.) your rotor is the more heat is can absorb without being "over-heated". Larger rotors also inheritly have more surface area for heat to be transfered to the air. This is why you always see high power sports cars with larger disc brakes, or why you find larger vehicle with large brake systems, because the faster or the heavier the vehicle is the more kinetic energy it will have at speed. Without some other major modifications we cannot easily use a larger rotor though. Another option is to improve the rotor's ability to transfer heat to the air, thus improving your braking capacity. This is often found in the form of cooling vanes which are sandwiched between the two rotor surfaces. These cooling vanes add surface area to which heat can be transferred to the air, and promote air flow through the rotor core. So the more cooling vanes you have the potential heat transfer that can occur. Finally there is the infamous slotted and cross-drilled rotors. Both of which sacrifice rotor mass to provide more efficient transfer of heat to the air. I don't think there is any doubt that cross-drilled rotors are more effective at transferring heat, making them the optimum choice for braking performance, but they do have their disadvantages. Because of the rotor's loss of mass and the addition of the holes you now have a rotor which will have localized high-heat concentrations which cause thermally induced stresses to develop in the rotor which can potentially lead to warped or even cracked rotors. Slotted rotors are similar, but they effectively sacrifice the effeciency of transfering heating by improving the duribility of the rotor. One other phenomenon that both slotted and drilled rotors help eliminate is "gasing" of the pad/rotor interface. Under the extreme tempertures of heavy breaking gases are spent in this interface, which effectively can cause your pads to "float" on the gas layer. Slotted and drilled rotors avoid this by giving the gases a route to escape, much like thread on a tire does for wet driving. So if you are looking for performance my recommendation would be to go with the cross-drilled, but if you are looking for an excellent performing rotor that will give you long life, I would recommend the slotted rotors. 

 

 

 

 

 

 

 

 

 

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