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“When new brake rotors are installed it is absolutely essential that they run true. All EBC rotors are manufactured and inspected to have less than 0.002 inches (.05mm) of run out. If after your new rotors are installed they have more than this amount of run out, then there is a run out problem on your car. This can be quite easy to resolve and is usually due to one of two things. First the mounting faces where the disc locates on your vehicle must be ABSOLUTELY CLEAN and free from rust or scale which develops during the lifetime of the old rotor. Even the tiniest amount of dirt or scale can throw these run out figures to five times the factory limit. After installing the rotors and tightening them using correct procedures by tightening wheel nuts diagonally with a torque wrench (not an airgun) it is vital to take a few minutes to check rotor run out with a dial gauge and if one of these is not available by holding a screw driver firmly against a part of the caliper body and rotating the disc / rotor to listen or look for distortion. If you do not correct distortion above 0.004 inches (0.1mm) at this point you will DEFINITELY experience brake judder within a few thousand miles. The actual cause of brake judder is not this run out figure (it will be almost impossible for you to detect small run out whilst driving) but over a period of time a “thin spot” would develop on an area of the rotor caused by intermittent pad contact which is known technically as DTV (disc thickness variation). As you apply pedal pressure these thin spots will cause pulsation. If the vibration or shimmying is noticed on the steering wheel it is usually a front rotor problem. The problem is usually only ONE ROTOR not necessarily the pair. If the pulsation is noticed through the bodywork of the car, such as the seat or brake pedal, it is usually a rear rotor that is at fault. “

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Yeah, see all the black/dark spots on the rotors before? thats the build up of pad material.

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“The second cause of brake vibration is black spotting of the rotor which is caused by the rotor over heating and a hard spot occurring intermittently around the rotor surface. This in technical terms is the formation of cementite which is a very hard by product of cast iron (rotor material) caused by over heat and sudden cooling. If you witness black spotting the only solution is to replace the rotor or have it turned. Black spotting occurs when the rotor has either worn too thin (having been turned more than once), or by brake pads that are ineffective in balancing rotor and pad temperatures.”

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ROTOR RUNOUT
The wobbly movement or a rotor running out of true (see Fig. 1) is referred to as lateral runout. Rotor runout should not exceed .002 to .005 inches, depending on the vehicle m a n u f a c t u r e r ’ s specifications. Rotor runout should be checked on the car and not on a brake lathe. With the downsizing in mass and front wheel drive braking ratios come some very tight machining s p e c i fi c a t i o n s . Runout exceeding this range can create a vibration, pulsation or brake noise. Some runout is forgivinas the calipers will track the wiggling rotor. When the rotor makes intermittent contact with the pads, a force or pressure is applied to the caliper piston. Due to the solidity of the hydraulic column acting on the hydraulic cylinders, a pressure is applied to the brake pedal and a pulsation is experienced by the driver. In addition to a pulsation, a chatter, vibration or squeal condition may be encountered. Excessive runout can knock the pads too far from the rotor and cause increased pedal travel. Prior to turning the rotor, make certain you examine the wheel bearing adjustment, brake hardware and suspension components for wear. Turning the rotors will do little for worn or loose parts. Rotor runout is measured with a dial indicator positioned on a stable surface such as a suspension component and should not exceed the manufacturer’s specification (usually .002 – .005 inches).

PARALLELISM
Lack of parallelism is a variation of the rotor thickness (see Fig. 2). Many of the same factors that contribute to runout affect parallelism. Checks should include loose wheel bearings, excessive lateral runout of the rotor, a lazy or dragging caliper resulting in a contact of the friction against the rotor, a hard or abrasive friction, improper rotor machining procedures, uneven torque or incorrect wheel lug nut tightening sequence, stop lamp or cruise control switches keeping residual pressure applied on the system. With a micrometer, check for parallelism by measuring the rotor thickness at four or more equally spaced points about one inch from the edge or at the middle of the pad contact area. A thickness variation of as little as .0005 (one half thousandth
of an inch) can contribute to pedal pulsation, vibrations and brake grabbing. While the caliper will follow some rotor runout, the pads must track an absolute parallel surface. This is why a thickness variation of as little as .0005 inch will cause pedal pulsation.

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Cast iron is an alloy of iron and silicon in solution interspersed with particles of carbon. At elevated temperatures, inclusions of carbides begin to form in the matrix. In the case of the brake disk, any uneven deposits - standing proud of the disc surface - become hotter than the surrounding metal. Every time that the leading edge of one of the deposits rotates into contact with the pad, the local temperature increases. When this local temperature reaches around 1200 or 1300 degrees F. the cast iron under the deposit begins to transform into cementite (an iron carbide in which three atoms of iron combine with one atom of carbon). Cementite is very hard, very abrasive and is a poor heat sink. If severe use continues the system will enter a self-defeating spiral - the amount and depth of the cementite increases with increasing temperature and so does the brake roughness. Drat!

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