Brake Rotor Inspection
Friction from the pads contacting the rotors surface causes heat. This heat combined with normal wear result in warped and distorted rotors. This is where the ventilation fins come into play. These fins between the inboard and outboard surfaces aid in cooling and actually circulate air flow as the vehicle travels down the road. A solid rotor usually smaller in diameter is often found on the rear of front wheel drive compacts.
There are two types of rotors found on today's vehicles. The most common type, the floating caliper is found on FWD vehicles. It has the hub and rotor cast as two separate parts. This type of rotor is easy to remove and cost less to replace. The other type is found on trucks, some luxury models and older rear wheel drive vehicles. It has the hub and rotor cast into one integral unit.
Rotor thickness and cracks in the surface must be taken into consideration when deciding whether to replace or machine a rotor. If the rotor shows signs of cracking or is thinner than specifications it must be replaced. Check the rotor for run out using a dial gauge and then compare with the manufacturer's specifications. Use an outside micrometer to measure rotor thickness in several places around the rotor. Rotors will become coned or dished from normal wear. This is the ridge felt on the outside edge or outside diameter of the rotors surface.
They're made from cast iron and the brake pads a much softer material. Check for pad deposits collecting around the surface of the rotor. A distortion in the rotor will collect pad material in that spot. When a rotor checks within specifications it may be cut on a brake lathe. Make sure not to remove too much surface material and bring the thickness measurements below threshold. Remove only as much from the surface as needed and always perform a final slow cut.
The smoother the surface the better. If you can feel ridges as you move your fingernail or a pen tip across its surface, it needs to be machined smoother. This roughness will wear the pads sooner and may result in a vibrating or humming sound while braking. After machining a rotor, wash its surface with soap and water to remove any excess metal shavings.