Resurface existing rotors when installing new pads
In order to ensure a uniform mating surface between the new brake pad and existing rotor, it is optimal to resurface the rotor prior to installing and bedding-in new brake pads. Resurfacing the rotor will also help ensure maximum pad life and braking torque (i.e.stopping power). Further, when changing from one brake compound to another, especially when switching from semi-metallic brake pad to a ceramic based brake compound, it is necessary to turn your rotors to remove any existing friction material deposits on the rotor. These friction material deposits (called ¡°film transfer¡±) can often interfere with the proper¡±bedding¡± of new brake pads and, additionally, can cause a notable shudder dring the initial stages of new pad use.
Keep in mind, however, that when resurfacing the rotor, make sure you are within 20% of the factory specified rotor thickness after resurfacing, or else your rotor will be more prone to warping and cracking, or otherwise present a condition where catastrophic failure may occur (e.g. a rotor may crack severely and then shatter under hard braking).
Brake shudder
There are three main causes of ¡°brake shudder¡±, which is generally felt as a brake pedal or steering wheel pulsation or vibration. Having verified that your rotors are not warped (i.e. warped rotors are another, more obvious, cause of the brake shudder phenomenon), the following are possible, commonly overlooked, explanations for an experienced brake shudder:
• An improperly seated or mounted rotor (i.e. the rotor does not turn perfectly parallel to the brake pad surfaces). This can be due to:
• Debris trapped between the backside mating surface of the rotor and the wheel hub
• Improperly tightened lug nuts/blots
• Pads that are not properly realignling themselves after releasing the brake pedal, due to a very rough or dry (i.e. non-lubricated) brake pad to mounting clip interface.
• Heavy friction material film transfers on the rotors. Very heavy use of the brake pads, especially use that exceeds the manufacturer's recommended heat range, can result in uneven friction material deposits on the rotors. This results in a ¡°grab-release¡± effect when the brakes are applied. While this will generally even itself out over time, in extreme cases, the rotors may need to be resurfaced/turned/cut.
• Worn suspension parts (i.e. springs, shocks/dampers, bushing/ etc.). ¡°old timers¡± call this type of suspension shift under torque load'tramp¡±. In this situation, the car's alignment is slightly changing due to brake torque upon brake pedal application. Severe cases of this result in a very notable pull in the steering, and can become evident as a shudder. If this is the cause of your brake shudder, the suspect suspension components should be replaced.
Brake fade and why
Pad fade occurs for several resons. All friction material has a coeffcient of friction curve over temperature. Friction materials have an optimal working temperature where the coefficient of friction is the highest. Sometimes you can use the brakes so hard that you get the temperature over the point of maximum friction to where the coefficient of friction curve starts to decline.
The mechanics of this decline in the coefficient of friction are varied. At a certain temperature, certain elements of the pad can melt or smear causing a lubrication effect, this is the classic glazed pad. Usually the organic binder resin starts to go fire, then even the metallic eelements of friction material can start to melt. At reall high temperatures the friction material starts to vaporize and the pad can sort of hydroplane on a boundary layer of vaporized metal and friction material which acts like a lubricant. Pad fade is felt as a car that still has a decent, non-mushy feeling brake pedal that won't stop even if you are pushing as hard as you can. Usually it builds somewhat slowly giving you time to compensate for it, but some friction materials have a sudden drop off of friction when the heat is put on them resulting in sudden dangerous fade.
Green fade is a type of fade that manifests itself on brand new brake pads. Brake pads are usually made of different types of heat resistant materials bound together with a phenolic resin binder. These are thermosetting plastic resins with a high heat resistance. On a new brake pad, these resins will out-gas or cure when used hard on their first few heat cycles. The new pad can hydroplane on this layer of excreted gas. Green fade is dangerous because many people assume that new brakes are perfect and can be used hard right off the bat. Green fade typically will occur much earlier than normal fade so it can catch a driver that is used to a certain car's characteristics unaware. Typically the onset of green fade is rather sudden, further increasing the danger factor. Green fade can occur if you change the pads and drive on the street for a few hundred or even thousand miles, never braking hard, then suddenly start using the brakes hard.
Edge codes
An edge code is a means of identification that may be used to decribe the initial frictional characteristic of any brake lining.
Typically, a two-character code (e.g. EE, FF, GG, HH, etc) is used on specific friction formulation. These characters represent the coefficient of friction when a 1¡± square piece of friction material is subjected to varying conditions of load, temperature, pressure and rubbing speed on a test apparatus known as the chase machine.
The coefficient of friction measured by the Chase test describes the relationship between the two forces acting on the friction material. A clamping force is exerted on the friction material, resulting in a frictional or resistance force. A low coefficient of friction means that very little of the clamping force is transferred into resistance force. On the other hand, a high coefficient of friction means that given the same level of clamping force, a higher resistance force is generated by the brake pad.
For example, a pad that carries an HH code has a normal coefficient of friction of 0.55 or higher, and a hot coefficient of friction of 0.55 or higher.
The first letter of the code represents the normal friction coefficient. This is defined as the average of four test data points measured at 200, 250, 300 and 400 degrees farenheit.
The second letter of the code represents the hot friction coefficient based on a fade and recovery test. We all should know what brake fade is. If you've ever had to use the front brake extensively and found that its effectiveness quickly diminished, that's fade. Recovery is basically the period where the brake are gradually cooling off.
The hot friction coefficient is defined as the average of 10 data points located at 400 and 300F . on the first recovery cycle of the pad; 450, 500, 550, 600 and 650F . on the second fade cycle; and 500, 400 and 300F on the second recovery cycle.
The range of friction coefficients assigned to each code letter are as follows: C = less than 0.15. D=0.15 to 0.25. E=0.25 to 0.35. F=0.35 to 0.45. G0.45 to 0.55 and H=over 0.55
