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
Todd Gartshore: "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."
Carl Bush: "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."
Michael Jonas: "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."
Todd Gartshore: "Larger rotors do in fact improve braking performance due to the extra leverage they offer. This concept can be illustrated with your own toolbox. Try loosening the head bolts on your next project with a short-handled ratchet, and after turning yourself red from exertion, place the socket on your breaker bar. That's not too different with what happens with a larger diameter rotor. As the distance from the center to the perimeter of the rotor is increased, leverage is increased as well. The torque improvement of a larger rotor diameter can be individually calculated based on the before and after rotor sizes. Without changing the friction compound, increasing a rotor's diameter is the only way to improve brake torque and shorten stopping distances unless other areas of the brake system are changed."
Carl Bush: "Rotor diameter has a measurable influence on stopping power due to the leverage it provides to the caliper in order to stop the wheels. The leverage that a caliper has is based on the distance from the axle or spindle centerline to the center radius of the brake pad contact face. The amount that radial distance to the pad centerline is changed, by moving the caliper outward on a larger diameter rotor, is directly proportional to the increase in stopping power. For example, if you increase that distance by 10 percent, you increase leverage by 10 percent as well."
Michael Jonas: "Maximum rotor size is ultimately determined by wheel diameter and wheel spoke design. That said, a larger diameter rotor generates more stopping leverage than a smaller rotor. The gains in performance won't be that great when stepping up from a 13- to a 14-inch rotor because it's only 1/2-inch bigger on each side. However, there will be a substantial increase in stopping power when stepping up from an 11- to a 13-inch rotor. Cars with larger diameter rotors will require less pedal effort. The nice thing is that you don't always need a bigger caliper when upgrading to a bigger rotor. At SSBC, we now offer big rotor kits that relocate the stock calipers farther outward."
Todd Gartshore: "Two-piece rotors use a separate hat assembly that bolts to the rotor itself. By using a rotor with a separate hat, the overall rotor weight can be dramatically reduced. For example, a Baer EradiSpeed two-piece rotor for a C5 Corvette is 3.8 pounds lighter than the stock rotor. This particular example is not a direct comparison since the EradiSpeed design has more mass around the outer plates than the OE rotor, thus adding heat absorption improvements directly in the fire path of the rotor. Another benefit of a two-piece rotor design is longer hub life resulting from less thermal transfer from the rotor into the hat. In the hot rod market, the most compelling reason for two-piece rotors beyond weight is that they give you the flexibility to vary offset very easily for easy fitment behind different wheel designs."
Carl Bush: "In most two-piece rotor assemblies, the bolt-on iron rotors are attached to an aluminum hat, rotor mount plate, or sometimes directly to the aluminum hub. Two-piece assemblies offer several advantages, but one of the biggest reasons from a performance point of view is that more iron can be concentrated in the pad wear faces and cooling vanes while at the same time reducing unnecessary rotating and unsprung weight by mounting the rotor to an aluminum hat instead of using a single piece of iron or steel. Once again, the goal is to maximize cooling and durability, while eliminating unnecessary weight that does not contribute to cooling."
Brad Burleson: "Two-piece rotors allow you to reduce weight by making the hub out of a lightweight material like aluminum, but still leave plenty of mass in the rotor for effective cooling. Aluminum also dissipates heat faster, keeping the system cooler. They do cost more initially, but later on it is cheaper to replace just the rotor instead of the whole assembly. One of the biggest benefits is that they resist warping due to overheating much better than one-piece rotors."
Todd Gartshore: "All Baer calipers are built from aluminum. Although most are made from a 6061 alloy, a 2618 alloy is used in Baer's Forged MonoBlock units. This is the same alloy often used for pistons in race engines. The benefit of 2618 is that it's a very stable material at sustained high temperatures. Baer employs this alloy along with MonoBlock technology in both its 6S and 6R calipers. Due to cost versus performance concerns, we believe iron is still the best available choice of material for rotors."
Carl Bush: "Most brake rotors are made from one type of cast iron or another. OE brake calipers are generally cast iron, aluminum, or some combination of both. Aftermarket street performance and race calipers are usually aluminum, but there are also other high-tech exotic alloys that can be used for extreme special-purpose parts. In racing, you can find rotors made from a number of materials including iron, steel, stainless steel, titanium, aluminum, and a whole range of carbons and ceramics. The application of rotor materials isn't a subject to approach without a full understanding of the material and purpose. An aluminum rotor can perform flawlessly on a lightweight open-wheeled sprint car racing on a high-momentum dirt track. However, that same rotor would melt down in a heartbeat on a street car and set the stage for a potential catastrophic failure."
Michael Jonas: "Rotors are typically built from cast iron with various levels of carbon added to them for strength. Some race cars and high-end street cars use carbon rotors that are very light and generate little heat. The downside is that they're extremely expensive and very brittle. It's possible to break a carbon rotor by over-tightening the lug nuts. As far as calipers are concerned, older cars used cast iron with steel pistons. Aluminum calipers are 4 to 12 pounds lighter than a comparable iron caliper, and that reduces unsprung weight and improves ride quality. The pistons inside the calipers are made from aluminum or steel, but lighter aluminum pistons aren't necessarily better. We offer aluminum and iron pistons, but prefer using stainless pistons because they don't transfer as much heat, and also dissipate heat more effectively."
Todd Gartshore: "Traditionally, cars have had smaller rotors and calipers on the rear than on the front. These days, many production cars, road racers, and autocrossers are using rear brakes that are just as big as the front brakes. While this arrangement isn't necessarily better, in some cases it is more practical. As more efficient caliper designs have become necessary for both safety and performance, calipers with integral parking brakes have begun to lose favor. On a race car, parking brakes aren't necessary, but that's not the case with a street car. A popular alternative to integrating the parking brake mechanism into the caliper is to build a shoe inside the rotor hat, which acts as a drum brake. However, to accommodate the shoe a larger rotor diameter is mandatory. Even though your street machine may not warrant a caliper and pad the same physical size and shape as the front, once the pistons are appropriately sized it's easy to achieve proper brake balance. Other than weight, there is no particular downside to a large diameter rear rotor, and the aesthetics of a brake system is now an important element of design used by hot rod builders and OE manufacturers alike."
Carl Bush: "You should put your priorities in proper order when deciding which rotor may be best for a given application. Sometimes, rotor selection is merely a function of wheel-filling aesthetics, and optimized performance is not the priority. However, for road racing and autocross, we usually prefer to use the biggest front rotors possible within the wheel constraints or rules. Rear rotor size will vary depending on the need for rear cooling capacity and leverage to help maintain optimized front-to-rear bias balance. Another factor to consider is that a lighter rear rotor reduces driveline inertia, which helps with acceleration and deceleration as well."
Todd Gartshore: "Four-piston calipers used to be considered pretty exotic, but now six- and eight-piston calipers are becoming more common. Having lots of pistons looks nice, but there is a point of diminishing returns. It really depends on the job at hand, size limitations, weight, and cost. As the piston count goes up, pedal response time, pedal feel, and ease of modulation are improved. Multiple pistons also afford the opportunity to stagger the bore diameters. On longer pad shapes, staggered bore sizes allow the pressure to be evenly distributed to reduce taper wear across the pad. That said, if you're more concerned with bragging rights than having better control over the pads, you have likely reached the point of diminishing returns in terms of piston count."
Carl Bush: "The final determining factor in caliper selection is not necessarily piston count, but overall piston volume. Piston volume determines how much clamping force can be generated based on the amount of hydraulic pressure that is being applied to the pistons, and that volume must be in harmony with the rest of the brake system. Using multiple-piston arrangements is beneficial in terms of how the clamping force load is applied over the length of a pad. As calipers get bigger, and the pads get longer, the benefits of spreading the clamping force out over a longer length of the pad with multiple small pistons become more measurable and justifiable."
Michael Jonas: "The number of pistons inside a caliper doesn't make a lot of difference in performance. What's more important is the size of the pistons. It's like comparing a V-12 Ferrari to a big-block Chevy. The Ferrari motor has more cylinders, but because they're so much smaller than in a big-block Chevy, it won't make as much power. The same thing goes for calipers; you want big pistons. Our Tri-Power three-piston calipers work well because the pistons are very large. The pistons are so large in our Tri-Power calipers that they actually generate more clamping force than many six-piston calipers on the market. While we do offer eight-piston calipers, the reason for this type of arrangement is to maximize the surface area behind the pad for even clamping force distribution. A master cylinder can only handle so much, so there is a limit to the number and size of pistons you can use."
Carl Bush: "Radial mounting is a very secure and accurate way of attaching a caliper. In this type of setup, the mounting bolts run through the top of the caliper instead of the side. The majority of aftermarket radial mount calipers use a bolt-on mounting bracket. The brackets can then be shimmed for perfect lateral alignment over the rotor centerline, and perfect radial height for alignment of the outside radius of the brake pads and the outside radius of the rotor. This is in contrast to only having a lateral plane of adjustment on a conventional lug mount caliper."
Fixed vs. Floating Calipers
Michael Jonas: "Fixed brake calipers have pistons on both sides of the rotor, while floating calipers position the pistons on the inboard side of the rotor only. The biggest difference between the two styles of design is packaging efficiency, but there are performance differences as well. When you go from a floating caliper to a fixed caliper, you may need as much as two additional inches of clearance near the inside portion of a wheel. As a result, fixed calipers require changing the backspacing of a wheel, which might necessitate stepping up to expensive billet or forged wheels. One drawback of a floating caliper is that it has to activate the inboard side of the caliper before the outboard side. On the other hand, a fixed caliper actives both sides at the same time, so its response time is a little quicker. For most street cars, however, this advantage is negligible."
Carl Bush: "Floating-mount brake calipers are usually part of an OE compromise for overall caliper cost and wheel clearance. Since floating-mount calipers only have pistons on one side, they can be narrower overall than a fixed-mount caliper. Floating-mount calipers are also likely to move around more, with deflection at the mounting points. Most drivers can feel an improved sense of consistency in brake response and pedal feel with a firmly anchored, fixed-mount caliper. The overall rigidity, and improved pad load distribution of a fixed-mount caliper also has a direct effect on better and more efficient pad wear than a floating-mount caliper, especially those that only have one central piston."
Todd Gartshore: "The brake pad is the easiest thing to change in a brake system, and is the most influential on how the entire system works. Think of a pad as the last ingredient that brings everything together that can also be easily tuned. However, just like more cam isn't always better for a street car, a more aggressive brake pad isn't always ideal. The biggest mistake people make is trying to use a race pad on a street car, and even guys with loud exhausts can't handle the noise level of a real race pad. The best pad in a street environment is a pad that has good response but is still driveable. In a street car, having good onset friction is important. Ceramic pads fill that requirement very nicely, but still have enough flexibility to work above 1,000 degrees Fahrenheit. In contrast, most OE applications rely on high-metallic compounds for their low-noise and low-cost characteristics. The downside is they become marginal at 750 degrees. The next step up the ladder from ceramic is a carbon pad, which creates a good deal of friction even at low temperatures. Carbon is more of a race pad, and can survive at temperatures up to 1,350 degrees. However, they require more pedal pressure, wear the rotors out faster, and create lots of dust.
"The old wives tale is that race pads won't work at low temperatures. They may take more initial pedal effort and be harder to modulate until they reach operating temperature, but they still have a higher friction level. The bigger issue is rotor wear. Pads operate through either direct friction or on a transfer film. Most race pads are designed to work with transfer film technology. When a pad reaches 450 degrees and above for a sustained duration of time, a transfer film is created on the rotor surface because of the porosity of the rotor. This reduces the wear rate of a rotor dramatically. Unfortunately, if there isn't enough temperature to create a transfer film or boundary layer, the rotor will wear out very fast. In reality, there is no such thing as a dual-purpose pad. Street driving, road racing, drag racing, circle track racing, and superspeedway racing all require different types of pads. From our experience, ceramic pads are the best all-around compound for the typical hot rod."
Carl Bush: "Brake pad compounds can vary tremendously, and it's sometimes hard to find just one style of pad that covers all conditions a driver may put his vehicle through. Typically, race pads are formulated to generate higher friction values with more resistance to fade and wear as temperatures increase. Street pads, including high-performance, dual-purpose street/track pads, push the envelope of friction and heat resistance, but also attempt to keep engagement noise and dust to a minimum. Track-only race compounds are generally very aggressive and can provide less than favorable noise and response when used at lower temperatures, such as those observed during normal day-to-day driving. It's best to avoid race-designated compounds for street driving, and pad compounds can have an unbelievable effect on response from your brake system, much like a good tire compound can have a positive effect on traction. Your pads first need to be matched to your temperature operating range, which loosely stated could be street temperatures, intermediate track use, or extreme condition competition. Then, the pad's friction value and response must be matched to your vehicle requirements. Following the pad manufacturer's guidelines and baseline recommendations is usually your best bet, but then feel free to tune with compound changes as conditions may warrant."