A recommendation on how often brake rotors should be replaced on a vehicle.
History
Rotors have come down in price significantly in the past decade. Supply and demand has been a major contributor, as well as many countries are now offering manufacturing both foreign and domestic auto parts at a competitive rate in the aftermarket auto parts industry. To stay competitive with the aftermarket auto part stores, even dealers have come down in price. The other factor in pricing is the redesigning of the rotors. Many vehicles now offer what is referred to as a "knock-off" rotor, which means once the caliper is unbolted from the bridge, the rotor is simply removed from the hub. Older vehicles had hub-bolted rotors where grease seals and bearings had to be repacked and pressed. Some older four-wheel-drive vehicles had to have the four-wheel-drive hubs disassembled in order to remove the rotors. Many automakers are now offering both front and rear disc brakes with a "knock-off" rotor, which saves both time and money for replacing.
Geography
The need to replace rotors more often stems from the geographical aspect of the United States. Many places that do not experience severe winter weather will get longer lifespan out of several components. Areas that have severe winter weather that use corrosive sand and salt on the roads will greatly reduce the lifespan of many components exposed to the environment. Rotors are no exception.
With that in mind, places like southern states have a better chance of turning the rotors on the vehicles opposed to replacing them because the corrosiveness has not seized the rotor to the hub like it would in the areas that experience severe winter weather. There are circumstances in the Northeast where rotors have to be literally hammered off the hub of the vehicle in order to remove.
Types
There are two types of common rotors. One is a "vented" rotor where two plates are melded together with cooling vents in between. These are generally always used for front disc brake applications and also in rear on larger and heavier vehicles that offer four-wheel disc brakes. The other type of rotor is a single plate or solid rotor. Solid rotors are generally used only for rear disc brake applications and are generally found on smaller, lighter vehicles.
Function
When applying the brake pedal in a vehicle, brake fluid builds up hydraulic pressure and extends the pistons of the calipers, which squeeze the pads together over the front and rear surface of the rotor. Pads are commonly made with a semi metallic material, and rotors are obviously metallic, so there is a great deal of heat created by that friction.
Depending on the braking habits and conditions the driver is experiencing, certain conditions such as pulsations can occur where the surface of the rotor exceeded its limit and warped slightly. This is called run-out in the rotor, and that would have to be measured against the amount of thickness the rotor has left in order to successfully machine or turn the rotor down on a lathe in order to true it and eliminate the run-out. The problem there is now the rotor is thinned down to a more compromised state and the chances of run-out reoccurring are greater than simply replacing the rotor.
Expert Insight
Many mechanics would rather replace knock-off rotors when completing a pad replacement for many reasons. The first and foremost is for minimal comebacks and to lower customer complaints. The cost to replace rotors on common vehicles is hardly more than the price of the part to replace it since the caliper needs to come off the vehicle in order to replace the pads. As a rule of thumb, when pads need to be replaced, rotors are recommended or required to be replaced.
