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Brake Rotor Inspection

There are two types of rotors found on vehicles today. The most common type 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 typically found on trucks and older rear wheel drive cars has the hub and rotor cast into one integral unit.

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. Another type, the solid rotor usually smaller in diameter is often found on the rear of front wheel drive compacts.

Road test the vehicle and feel for a vibration while braking. The pulsation caused by distorted rotors is usually felt more towards the end of the braking cycle and when hard braking occurs. Front brakes do most of the braking and usually warp before the rears. The pulsation caused by excessive front rotor run out will be felt in the steering wheel while the rears felt in the seat.

Rotor thickness and cracks in the surface should 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 is should be replaced. Check the rotor for run out using a dial gauge and check against manufacturers specs. Measure the thickness in several places around the rotor as rotors will become coned or dished from normal wear.

Rotors are usually made from cast iron and the pads a much softer material, so check for pads deposits around the surface of the rotor. A distortion in the rotor will collect pad material in that spot. When a rotor checks within specs and is machinable 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 finger nail or a pen tip across the surface it needs to be machined smoother. This roughness will wear the pads sooner and may result in a comeback.