Energy is some weird stuff that can neither be created nor destroyed. It can only be changed into different forms, but during these transformations, the total amount of energy in any given system-or in the universe for that matter-never changes. Being aware of this key fact is critical when designing a performance brake system. A car transforms the thermal energy released from the burning air/fuel mixture into rotating energy to accelerate it down the road. Short of wrecking, the only way a car can stop is if a brake system converts kinetic energy back into thermal energy as the calipers clamp down on the rotors. In other words, any time you add horsepower but neglect to upgrade the brakes, your hot rod is mired in a state of imbalance.
In essence, a properly designed brake system is one that heeds to the basic laws of physics. However, packaging constraints, airflow limitations, and all-around driveability are just some of the challenges brake engineers must deal with. Consequently, brake design is a balance of rotor size, diverse alloys, various pad compounds, and caliper design. To help us understand how it all comes together, we had lengthy discussions with some of the biggest names in the aftermarket brake industry. They include Todd Gartshore of Baer Brakes, Carl Bush of Wilwood Brakes, Michael Jonas of Stainless Steel Brakes, and Brad Burleson of MBM Brakes. Here's the scoop.
Drilling & Slotting
"Cross-drilling provides the most effective path for eliminating gasses trapped between the surface of the pad and rotor. These gasses can create a boundary layer and reduce friction. Such gasses are formed when bonding agents used to manufacture pads are released at temperatures normally only witnessed in racing conditions. Ironically, virtually all current race pad compounds are pre-burnished to eliminate this outgassing. On race-only vehicles, we generally use a slotted surface without any cross-drilling to allow for any outgassing to escape.
"For road cars we take the same approach used in Porsches, Ferraris, and Corvettes by offering drilled rotors largely for the visual excitement. When the holes are located properly-directly behind, but not in, a vane-and the hole is properly chamfered, there is no detrimental impact on rotor life."
"There is a lot of misconception about what holes and slots really do. The drill and slot look is definitely part of contemporary sports car styling, but people that choose drilling for performance reasons must truly understand what they're getting into. The biggest reason to drill a rotor, from a performance point of view, is to get rid of weight. However, the more weight you take out of the rotor, the less heat it can effectively manage. So racers are always looking to reach the threshold of just how light they can go while still maintaining an acceptable life cycle from the rotors. Some people would have you believe that taking a stock dimension, stock weight rotor and lightening it with holes will make your brakes to run cooler. That's just not the case. A lighter part will spike to a higher temperature for any given amount of heat put into it, as it has less mass to dissipate the heat. Aftermarket two-piece drilled rotor assemblies, such as the SRP rotors used in all Wilwood brake upgrade kits, will usually begin with thicker wall faces, more vanes, and more mass in the rotor contact face area to better handle high temperature spikes."
Brad Burleson: "Without a doubt, cross-drilled and slotted rotors offer performance benefits. Slotting helps channel brake dust and water vapor off the surface of the pads, which helps to increase braking power. With older-style brake pads, the cross-drilling also helps to dissipate heat, which also increases braking power. One drawback to cross-drilled rotors is that they can crack around the holes if overheated, so it's a good idea to inspect them any time you are servicing your brakes."
"A rotor is a heat sink, and you need weight in the rotor to absorb heat. Simple physics says that heat gets absorbed by the largest mass. If there isn't enough mass in the rotor, it will get transferred to the caliper where you don't want it. The problem with cross-drilling is that it reduces rotor mass, and therefore reduces heat capacity, which leads to a hotter rotor. As rotor temperature increases, the pad compound must be changed accordingly. Cross-drills can even act like a cheese grater and heat a pad up even more. The part of the rotor that actually cools it down are the vanes, and they can be either curved or straight. Curved vanes promote better airflow, as they act like a fan with greater surface area. As air comes through the car to cool a rotor, it has a hard time flowing past a spinning wheel. Air naturally travels along the path of least resistance, which is through the vanes. Since air can't make a 90-degree turn at the face of the wheel, it can't even get to the cross-drills to assist with cooling. Cross-drills may look cool, but their original purpose was to reduce unsprung weight in open-wheel race cars. Since these cars don't have fenders and offer much better airflow to rotor vanes, the reduction in mass doesn't hurt braking performance. On the other hand, slotting acts like windshield wipers for the pads. Brake pads are like a sponge, and dirt and dust gets trapped on the pad surface. Clean pads run cooler, which makes them last longer. An ideal slot wipes the entire pad from top to bottom, and left to right is ideal."