subject: How To Bed In New Rotors [print this page] On your car after a brake serviceOn your car after a brake service. The important thing to remember is that when replacing rotors you need to bed them in even more gently than pads. It is a regular misconception that a new rotor can be installed on a car and there is no need to bed them in.
The conclusion for those of you who don't want the explanation is that to bed in new rotors you drive them gently for the first 1000 miles.
To bed in new rotors you need to first understand that all rotors are made from cast iron which is a natural material, very common in fact and it contains certain elements that can change or combine in their particle structure during both their formation into a cast object and in early use as a brake component which dramatically affect the performance, stability and strength of the iron material.
Because iron is known as Eutectic, the different elements solidify at different temperatures and therefore the cooling phase of a cast iron rotor must be controlled to avoid the elements from combining into brittle structures that cause the disc to fracture.
Assuming that the brake disc has been made properly to begin with the onus then rests on the user to not recreate these conditions of heating and rapid cooling that will cause brittle structures to be generated in the rotor material.
In other words getting a rotor very hot and cooling it very quickly is not good news especially in its early life.
The best way the bed in new rotors is to use them gently over the first few hundred miles, not getting them too hot and letting them cool gently, for certain avoiding hard braking if at all possible (safety considered).
How Hot Can A Brake Rotor Become?
It normal driving, cast iron brake rotors can see temperatures in Centigrade of 400-500 but pushing this temp up above 727 degrees which bring about a structural change where the material enters something called the "Austenite" phase . Cool it quickly and the Austenite forms into another brittle structure called martensite . So going out there, slamming on the brakes and getting the rotors glowing early in their life just keeps them in the brittle phase and after a short time cracks may appear.
By driving gently and not pushing the brake disc temperatures above the 250-400 C range (called the bainite temperature) and letting the disc air cool (by not parking the vehicle right after a heavy brake application, coasting lets the discs air cool nice and slowly) you can to a degree achieve what the industry calls austempering. This retains the austenitic (highly desirable) structure of the iron and the disc will stabilise. We are not saying you need to do 1000 miles on a racetrack to bed rotors in but more suggesting gentle street driving conditions a disc better before it sees race use.
The addition of copper in the mix is also a great advantage as it encourages the quasi austempering process, thats why copper is in most better brake discs or rotors.
If you can do this by moderate street driving for up to 1000 miles you will condition your iron disc or rotors and that is the best way to bed in new rotors.
