Introduction to brake pads
Coefficient of friction: The scientific definition for coefficient of friction is the ratio of force required to move or stop one surface, with a given perpendicular force applied to it, as it drags across another surface. The layman can think of coefficient as the slippage of the pad as it clamps down on the rotor. The coefficient measurement ranges from zero to infinity, but most racing pad's range from .15 to .65. The mathematical formula for coefficient of friction is:
Mue = F/W
* Mue = coefficient of friction * F = force required to move one surface over the other * W = perpendicular force
Transfer layer: his is what determines the stability and effectiveness of the pad's coefficient of friction. It is the simultaneous transfer of pad material between the rotor and the pad. Pad material is constantly moving between these two surfaces during brake engagement. Every pad leaves a transfer layer. This is one reason why a new pad will not work optimally against a rotor that had previously run a different type of pad.
Temperature continuum: This is a brake pads' comfort zone and signifies the range in which the pads are more effective. Some pads are designed to perform optimally at cooler temperatures while others do not start performing well until they are hot. The expanse of the temperature continuum is dependent upon the materials (powder, fibers, and resins) used to manufacture the pads. The thermal stability of these materials determines their combined effectiveness. Different materials have different properties at different temperatures. Very few brake pads are designed to retain performance while transitioning along the temperature continuum. The key to a great brake pad is to have one material transition in while another material is fading away; all without the driver feeling any difference in the brake pedal.
Thremal stress This happens when the rotor is heated up and cools down rapidly causing an uneven heat distribution throughout the rotor. The result is the expansion of the rotor material in one area, while not expanding in another area. Heat checks in the rotor are a symptom of thermal stress. Be careful not to confuse small cracks in the transfer layer laid on the rotor by the pads as thermal shock. Bedding of a new rotor will help reduce the possibility of thermal stress/shock.
Resins: Resins are organic materials used to bind and reinforce the different components of a brake pad. The resins act like a thermoset plastic, which holds the components together like glue. This glue forms the matrix of the brake pad. When heated, resins tend to shrink and crack, and at very high temperatures they may turn to carbon. Carbonized resins cause weakening of the matrix and accelerated pad wear.
Brake fade There are several types of brake fade. The first is the conventional burning off of resins as the pads are being broken (bedded) in. This is known as green fade and occurs when gases from burnt resins are trapped between the pads and rotors. When this happens the pedal feels firm but the car will not stop. The second type of fade is when the pads are forced to work outside their temperature continuum. This is the point at which the resins burn off rapidly, and the pad has accelerated wear. The third type of fade is glazing. This is when the resins, which soften during active braking, then cool and solidify on the face of the pads. This glazed surface is hard and slick, and will not give the same coefficient of friction as an unglazed or new pad.
Pad taper: This is a rather disputed point among drivers, mechanics and manufactures of calipers and brake pads. The following are a few theories as to why pads taper. One type of pad taper is leading edge to trailing edge taper. One theory as to why this occurs is that the leading edge (front of pad) is hotter than the trailing edge (rear of pad) and thus the front part will wear faster. Another theory for leading to trailing edge taper is as the pad material smears against the rotor it flows from the front to the rear of the pad and accumulates there, giving the pad a tapered face. A third theory is that pads themselves are not made to the proper density, and thus have uneven wear. The second type of pad taper occurs in the form of top to bottom taper. In this case the top radius of the pad wears at a faster rate than the bottom radius of the pad. The caliper flexing under hard braking conditions may cause this. This problem is very common in racing where original equipment calipers are used. In many cases the original equipment calipers were not designed to withstand the high heat and high line pressure of a racing environment. Under these strenuous conditions the original equipment caliper bridge bends and flexes.
Pad-bedding and rotor-bedding: rst, slowly build heat into the pads by making slow to medium speed stops. Then make a series of very hard high-speed stops. Some fade may be experienced when doing the high-speed stops. The brakes should not be dragged during this procedure. Let the brakes cool down for twenty minutes or more and they are then ready to race. Never use new pads to bed new rotors. It is best to bed new pads on old rotors that had run the same type of pads. It is best to bed new rotors with old pads that are the type that will be used in the race. A bedded rotor should have a uniform, polished appearance with no cracks or grooves.
Brake Pads for HPDEs
An extensive discussion on brake pads for HPDE is going on in the forums http://www.trackpedia.com/forums/showthread.php?t=136
Hawk HP Plus on Porsche 996 stock
I've ran these for several events now and am very happy with them. They stop well on the street and stop very well with no fade on the track also. I did Mid America Motorplex, Brainerd International Raceway and Road America with no issues. You'll get about 6 days per set of pads. Rotor wear looks acceptable.
EBC Redstuff Ceramic on Porsche 996 stock
Horrible. They do stop well on the track but when cold there is no stopping power at all to the point where it's dangerous driving on normal roads with them. Foot to the floor and the car doesn't even lock up when they are cold. They eat rotors. My rotors were destroyed after 6 track days. I switched to Hawk HP Plus and haven't looked back.
Brake pads for Racing
Brake pad bedding
Bedding is a "real conditions" heat cycle, and the final step that prepares brake pads and rotors for service. Nearly every friction pad material, even OEM passenger car compounds, improve in performance and durability after bedding. Some friction compounds will transfer a layer of friction material onto the rotor faces. This transfer layer is an essential element to the performance and service life of the pads and rotors. New cast iron rotors must be heat cycled and bedded to remove any remaining internal stresses and to condition the rotor faces for service. Pad and rotor bedding cannot be accomplished in a heat oven or cryogenic chamber. For parts to be properly bedded, they must be subjected to the heat, pressure and torque loads they will see in real operating conditions. This can be accomplished in the car or on a special dyno capable of operating at real torque level conditions.
First, slowly build heat into the pads by making slow to medium speed stops. Then make a series of very hard high-speed stops. Some fade may be experienced when doing the high-speed stops. The brakes should not be dragged during this procedure. Let the brakes cool down for twenty minutes or more and they are then ready to race. Never use new pads to bed new rotors. It is best to bed new pads on old rotors that had run the same type of pads. It is best to bed new rotors with old pads that are the type that will be used in the race. A bedded rotor should have a uniform, polished appearance with no cracks or grooves.
Here is the truth about bedding. Some pad compounds can be cured to eliminate the need for any real break in time. This is true with pads that will run in the lower temperature ranges. It is not the case with the higher temperature race compounds. Some pad companies claim that their race compounds are ready for competition right out of the box. They claim that you do not need to spend time with bedding. The fact is, no experienced racer would ever take the green flag and head off into turn one with parts that have not been heat cycled. The performance is just not there yet and the chances for failure are proportionately higher. You may be able to take the family wagon right out of the garage and onto the highway after installing new parts, but do not attempt to race your car on competition pads or cast iron rotors that have not been cycled and bedded. Your are compromising the performance and increasing your chances for early failure or premature wear. Our engineers strongly urge you to take the time for proper bedding. We want you to get the maximum performance, reliability and service life from your pads and rotors.
The principals behind rotor bedding are largely the same as the principals that apply to seasoning new cast iron engine blocks or cylinder heads. One of the leading causes of rotor cracking is thermal shock. Thermal shock is the result of heat, or the rate which the heat is applied or removed. The cracking heard when pouring your favorite beverage over ice is fracturing caused by thermal shock. The same thing can happen to unseasoned cast iron. It is therefore imperative to run the rotors through one or two moderate heat cycles before subjecting them to the extreme heat conditions in competition. In addition to stress relief, the bedding cycle also conditions the rotor faces. This is important with pad materials that work with a friction transfer layer. Even if you are using pads that are already bedded or cured low temperature pads that do not need break in, you must still heat cycle new cast iron rotors before subjecting them to real service conditions.
Rotor bedding is usually done at more moderate temperatures than most pad bedding. Rotor bedding works best with pads that have already been run and are ready for service throughout their entire temperature range. The new rotors should be cleaned prior to installation to remove any oily residues. On the car, the rotors should be gradually cycled to 500 - 700 degrees F., and then allowed to cool. Keep in mind that pyrometer readings taken in the pits do not reflect the actual peak temperatures of the rotors on the track. Temperature paints can be very helpful in identifying the peak temperature. It is also a good idea to bed new rotors with a lower temperature pad compound: this will prevent overheating the rotors in their first cycle. Once the rotors have been subjected to the first head cycle and cooled down, they are ready to run with the higher temperature pads. A second lap session, followed by another complete cool down cycle is the best method of assuring that the bedding process is complete.
For proper bedding, the pads should be gradually cycled up to normal operating temperature and then allowed to completely cool to ambient air temperature. Race compound pads intended for high temperature service will naturally require more heat in their bedding cycle than pads intended for passenger car or lower temperature competition. It is always best to bed new pads on a rotor that has already been bedded or run. Used rotors must not be scored, cracked or grooved from wear when installing and bedding new pads. If you are running new pads and new rotors together for the first time, the first laps should focus on bedding the rotors. Once the new rotors have been bedded and cooled, the next cycle can be used to heat the pads up to their operating range.
Whether you are bedding new pads or rotors, keep these points in mind. Pump the brake pedal with medium pressure at low speed to be sure everything is working properly. Do not drag the pedal to increase the heat up time. Apply the brakes in gradual cycles to make sure the parts are evenly and thoroughly heat saturated. Avoid quick overheating of just the pad and rotor surfaces without taking the time to get good heat build up through the entire body of the part. If your car is equipped with brake cooling ducts, reduce or block the air inlet area. This will minimize the cooling cycle time and maintain more even temperature during the bedding laps. If the pads and rotors are both new, make the first laps with rotor bedding in mind. After the rotors or pads have been heated, run a few laps without touching the brake pedal to begin the cooling process. Use the brakes as little as possible to bring the car back to the pits. Once parked, release the brakes immediately. It also helps to rotate the wheels periodically during the cool down cycle. This will promote even cooling in the rotors by not allowing any one section to retain heat from within the brake pads for any extended time. Avoid any chance for extreme temperature shocks. Avoid driving through any standing water on the way back. If time permits, you can use a low temperature pad to bed new rotors. This method will allow the pads to fade before the new rotors reach a critical temperature. If you take these steps to prepare your parts for competition, your time will be well rewarded with performance and longevity from your part