176 Auto Brakes Copyright by Goodheart-Willcox Co., Inc. Th is chapter is designed to provide a clear understanding of front and rear disc brake operation. Caliper hydraulic components were covered in Chapter 5. Th is chapter covers all major types of disc brake rotor and caliper designs and related parts. Studying this chapter will prepare you for the service and repair information in Chapter 13. Disc Brake Rotor Th e job of the disc brake rotor is to provide a smooth brak- ing surface for the pads to contact. When the stationary pads contact the spinning rotor, the resulting friction slows the rotor and stops wheel rotation. Much of the result- ing frictional heat is absorbed and dissipated by the rotor. Figure 12-1 shows a typical rotor and pad assembly. Note: In this and other chapters, we will be referring to inboard and outboard components. Inboard components are those installed on the inside of the wheel assembly. Outboard components are installed on the outside of the wheel assembly. Rotor Construction All stock disc brake rotors are made of cast iron and are sim- ilar in appearance. High performance aluminum rotors are available, but not widely used. Rotors are manufactured in many diff erent diameters and thicknesses. Since the linings clamp against the rotor to stop it, both sides are machined to provide a smooth braking surface. Th e rotor must also be the same thickness throughout its diameter. Variations in thickness will cause pulsation when the brakes are applied. As well as its diameter and thickness, rotors can be either ventilated or solid. Rotor Diameter Th e rotor diameter directly aff ects the system’s braking abil- ity. A larger diameter rotor provides more braking area and exposes more area to the air, which aids in heat dissipation. Rotor diameter is chosen depending on the brake system and the vehicle’s intended use. Most disc brake rotors are 9″–11″ (22.86–27.94 cm) in diameter. Larger rotors are used on sports cars, trucks, and vans. Rotor Thickness To absorb heat, rotors must have suffi cient metal. Th e rotor is built with enough metal so the surface fi nish can be restored by removing a layer of metal. Th is removal process is called machining or turning. Th e minimum thickness of the rotor is usually stamped on the hub or the inner part of the rotor body, Figure 12-1. If the rotor is below the minimum thick- ness before or aft er machining, it must be replaced. Solid Rotors Solid rotors have no openings between the machined sur- faces. Th e rotor is cooled by air passing over the outside surfaces of the rotor. Solid rotors are smaller than ventilated rotors and are used on lighter vehicles. Ventilated Rotors Ventilated rotors have internal fi ns between the two friction surfaces. See Figure 12-2. Th e fi ns are arranged to create a centrifugal air pump inside the rotor. As the rotor spins, the fi ns draw air into the center of the rotor and discharge it from the edges. Th is extra air circulation causes the rotor to give up frictional heat much more rapidly than would oth- erwise be possible. Th is type of rotor is only used on larger cars and light trucks and on many smaller vehicles. A variation of the ventilated rotor has both internal fi ns and holes drilled into the braking surfaces from one side to the other. Th is type of rotor is shown in Figure 12-3. Th ese are used on high-performance and competition vehi- cles. Th ey are not installed on most production automobiles because they quickly wear out the brake pads. Rotor Attachment Rotors are attached to a wheel hub. Th e hub is attached directly to the wheel spindle, knuckle, or axle through wheel bearings, discussed in Chapter 16. For proper brak- ing ability, rotors must run true without any scoring, lateral runout, or wobble. For this reason, the rotor must be accu- rately attached to the hub. Most rotors are separate from the hub, and are held in place by the lug nuts and/or wheel studs when the tire and rim are installed, Figure 12-4. Some rotors, especially those used in the front of rear-wheel-drive vehicles, are integral with the hub (the hub and rotor are formed as one piece of cast iron). An integral rotor is shown in Figure 12-5. Splash Shield Most disc brake assemblies have a splash shield installed over the inner surface of the rotor to keep as much water and debris as possible away from the rotor. Th e rotor’s outside sur- face is protected by the wheel itself. Th e splash shield is a sheet metal stamping or a molded plastic assembly that cov- ers part or all of the rotor’s inner surface. Th e splash shield defl ects water and debris from the inside rotor surface. Th e