White Papers

It is hard to imagine the energy required to stop a vehicle traveling at 110 mph. Kinetic energy is ½ mass times the velocity squared KE = ½ mv2. Stopping a 2500 lb car requires 352,800 ft-lb of energy, the equivalent of dropping the car from an 11 story building. When you stop, almost all of this energy is converted into heat through the rotor in a very short time. Brake Rotors are designed to get hot, often in excess of 1400 degrees F. Contrary to popular belief, the temperature does not cause the rotors to warp, and this is not the reason why you get pulsating brakes. So what causes this brake judder?

The answer lies in understanding how friction works, and how high spots form on the rotor to cause the pulsating brakes. There are two primary mechanisms of friction at work during the stop: 1) adherent friction and 2) abrasive friction. For adherent friction, a thin layer of material is continuously transferred between the brake pad and the rotor. The breaking of the chemical bonds creates a resistant force to stop the vehicle. Abrasive friction is the mechanical wearing of the rotor and friction surface, like sand paper on wood. Brake pads use both mechanisms, but at higher temperature, adherent friction is what stops the car. The brake pad deposits material on the rotor, and as the pad gets hot, more material gets deposited. These deposits are caused by the resin (glue) in the pad. All brake pads use a binding resin that holds the friction component materials together. This resin turns to liquid as the pads get hot, and the resin material along with some other material in the pad gets deposited on the rotor. On overheated rotors, sometimes you can see the glazed resin material. It can look like someone brushed syrup on the rotor. The resin glazing forms a high spot on the rotor surface.

Another way to form a high spot on the rotor is if you clamp down on the brake after a hot stop. When the pad gets hot, it wants to transfer friction material to the rotor and it does not care if the rotor is stationary or rotating. If you are at a stand-still, the pad will imprint material on the rotor in the shape of the pad. The next time you brake, as the pad passes over the high spot, the temperature increases relative to the rest of the rotor. This causes more material to get deposited on this spot until it gets high enough to cause pulsating brakes. It does not take a lot of material to cause this brake judder, only .0007 inch is enough to start to feel the pulsation. Power Stop friction is engineered with lower resin content to maintain a uniform boundary layer of friction on the rotor. Another way to prevent glazing or non-uniform friction deposits, is to install drilled rotors. Drilled rotors help keep the pad cooler to prevent pad glazing.

Key Steps To Prevent Brake Pedal Pulsation:

• It is important to bed new friction on a new rotor. I recommend 5 moderate to aggressive stops from 40 mph down to 10 mph in rapid succession without letting the brakes cool and do not come to a complete stop. Then do 5 moderate stops from 35 mph to 5 mph in rapid succession without letting the brakes cool. You should expect to smell some resin as the brakes get hot. After this is complete, drive around for as long as possible without excessively heating the brakes and without coming to a complete stop (Try for about 5 minutes at moderate speed). This is the cooling stage. It allows the heated resin in the brake pads to cool and cure. After the brakes have cooled to standard operating temperature, you may use the brakes normally.
• Don't clamp down on the brakes after a hot stop, try to keep the car rolling slowly to avoid depositing friction material on just one part of the rotor.
• Use our drilled rotors combined with Power Stop pads. When it comes to preventing pulsation, brake pad selection is just as important as the rotor selection.
• Since more vehicle weight means more energy, and since this energy is converted into heat, towing or truck applications are more susceptible to pulsation problems. I recommend cryogenically treated rotors combined with Power Stop Z36 Truck and Tow Pads to improve durability.

Engineers measure the friction force of the pad (the resistance level) by using the coefficient of friction called mu. The higher the mu, the more aggressive the pad is. Most OE pads have mu of about 0.3 up to 1500 degrees. However, on some aftermarket pads, mu can drop to half its original level at higher temperatures! That means you can lose half of your stopping power. Engineers call this "brake fade". All brake pads use a binding resin that holds the friction component materials together. This resin is liquid during the hot press manufacturing process so that it coats and binds the fibers, minerals, etc. When it solidifies, it locks the materials into a homogeneous matrix. However, during a brake application, the resin gets hot and turns from solid, to liquid, and then to gas. This gas is trapped between the pad and the rotor (along with some friction dust), and this inhibits brake performance. At higher temperature, the driver must apply more brake pedal pressure to stop the car. The following graph shows the coefficient of friction for a popular aftermarket pad. The test has 15 consecutive stops (labeled on the bottom) to measure the stopping power shown with green lines with black dots. Each one of the 15 stops is at a constant deceleration rate similar to what you would experience in stop and go city traffic. The red lines shows temperature and the blue lines shows pedal pressure. In this fade test, the pad bite (mu) drops from about 0.28 to 0.14 after 6 stops. This means that you have to press the pedal twice as hard to stop the car. I would not want this pad on my car.

The following graph shows the same test on Power Stop Evolution ceramic pads. The pad bite holds at 0.3 mu. This has the same pad bite as the OEM brakes. The pedal pressure( blue lines) is constant, despite the increase in temperature (red lines). Note that the other aftermarket pad temperature peaks at 700 degrees Centigrade or 1300 degrees Fahrenheit. The Power Stop pad remains about 90 degrees cooler. This helps prevent pad glazing.

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.