Rotors. A. Standard solid rotors. B. Ventilated rotors.
Rotors can be solid or ventilated FIGURE 32-27. Solid rotors are less expensive and usually found on smaller vehicles. Ventilated rotors are used to improve heat transfer to the atmosphere. These passage ways are designed to use centrifugal force to cause air to flow through the center of the rotor when it is rotating. Ventilated rotors are used on heavier vehicles or high-performance vehicles. Some ventilated rotors are directional, meaning they are designed to force air through the rotor in one direction only FIGURE 32-28. If the rotor is rotated in the wrong direction, it will not pump air properly and will overheat easier.
Disc brake rotors with holes or slots machined into their surface dissipate heat quicker FIGURE 32-29. They also help to remove water quickly from the surface of the pad in wet driving conditions. Since the pads wipe across the holes or slots, the surface of the pad is prevented from becoming hard and glassy smooth from the friction and heat of use. However, this scraping action reduces the overall life of the brake pad, so these types of rotors are generally only used in high-performance or race vehicles.
Directional ventilated rotor.
Slotted and drilled rotor.
Rotor with minimum thickness stamped on it.
Rotor thickness and heat capacity.
Most disc brake rotors are stamped with the manufacturer’s minimum thickness specification FIGURE 32-30. This minimum thickness ensures an adequate amount of thermal mass for stopping power FIGURE 32-31. When material is removed, there is not as much material to absorb heat and the rotor heats up faster. The excess heat can lead to brake fade sooner. Also, when the brake pads wear, if the thickness of the rotor were below this minimum, the piston could be pushed out beyond the edge of the sealing ring, which would cause the brakes to lose hydraulic pressure and fail. Make sure the rotors are always above the manufacturer’s minimum thickness before putting them back in service.
TECHNICIAN TIP
If you hear a loud scraping or grinding noise when you test-drive a vehicle after servicing the brakes, check to see if the dust shield is contacting one of the rotors. If so, it can make a lot of loud grinding noises. Since it is thin sheet metal, chances are good that it is bent. It can usually be bent back into shape easily.
