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Rotors

Learn and Shop: Brake Rotors

Brake rotors are best described as the "heat sink" in the braking system. Linings forced against the rotors create the friction force needed to stop the vehicle. As the brakes are applied and friction is generated to dissipate kinetic energy, thermal energy (heat) is produced. Brake pad linings are only efficient within a specific heat range, depending on the pad compound, and so the rotor must be able to absorb and dissipate this heat so as not to put the pads into thermal break-down.*

Material

Typical rotors are made of an iron alloy or steel, as this material copes with heat very well and is very strong, resisting damage from impacts. More advanced alloys will have higher carbon content, and high end rotors will feature heat- or cryogenic- treatments to increase hardness (to reduce wear or wear-out). The most advanced rotors have full-carbon construction, or may be made of two pieces where a separate rotor "hat" and "disc" are fastened together. Such rotors are lighter weight and the most resistant to overheating.

Ventilation

Rotors may be vented or solid, where solid rotors are usually used low-heat situations such as in rear-applications. Vented rotors have opposing friction surfaced separated by vanes which create an air channel for better cooling. More advanced rotors have curved vanes for even better air-flow.

Finishes

Rotor friction surface finishes can be plain, cross-drilled, slotted, or having any combination of drills and slots. Drills are commonly used to promote further cooling and reduce weight of the rotors. Slots (be they waves, dimples, curved, or straight) are commonly used to keep rotor faces clear of brake dust. The performance benefits of drills and slots are considered minor on street applications, but many manufacturers use drills and slots to give their rotors a trademark "look." Caution should be used when purchasing low-quality rotors with these features, as cracking can be promoted if they are over-heated.

Coatings

Being mostly iron, rotors will develop unsightly rust, even just being exposed to air (although the integrity of the part itself is not compromised by the rust). Rotors may come with a variety of coatings on them to prevent surface corrosion. Many low cost rotors come as bare metal with a light oil applied to prevent surface corrosion while in distribution. Rotors with black finished areas are more resistant to surface corrosion, although many sorts of paints can burn off with extreme heat. Some manufacturers may nickel (silver) or zinc (gold-ish) plate their rotors to further inhibit rust from forming. Others get even more creative offering a rainbow of colors to choose from. All such coatings do not significantly impact the braking performance, either positively or negatively, and are mostly for looks and brand identification.

Treatments

Due to rotor wear being the most significant contributor to rotor replacement, some manufacturers will apply advanced treatments to rotors after they are cast. Heat treating is a process of heating the material and cooling it progressively to refine the grain structure of the material. This makes the rotors harder and more stable and extends their lifespan. Cryogenic treatments cool the rotors to very low temperatures in an effort to achieve the same goal. Both treatments have been very effective and increasing rotor consistency and life.

Sizing

Rotors are commonly referred to by their large overall diameter and rotor disc thickness in either inches or millimeters (mm). Other important dimensions may be rotor hat offset, hub bore diameter, and hub bolt-pattern. These dimensions will all be interchangeable with a vehicle's OEM specification, and will not usually be stated when purchased as a replacement rotor for a stock one.

* This is the main cause of excessive shaking, often referred to as "warping." In situations of "warping," the rotors are not actually deforming: Rather, the brake pad linings have coated the rotors with an uneven transfer layer because of over-heating.

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