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How to bed in new rotors

On your car after a brake service or when going racing. 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.

A Typical cast iron material used in aftermarket brake discs or rotors is grade GG25 or GG 20 and almost all discs from Asia will use these grades and contain these elements.

Carbon	Silicon	Manganese	Phosphorus	Sulphur	Nickel	Moly	Chromium	Copper
2.2	1.00	0.65	0.07	0.07	0.05	0.02	0.15	0.20

EBC new High Carbon GG15HC material is an even higher specification and contains these elements

Carbon	Silicon	Manganese	Phosphorus	Sulphur	Nickel	Moly	Chromium	Copper
3.72	1.74	0.05	0.05	0.108	0.40	0.45	0.16	0.80

More Carbon and Silicon in the GG15HC produces a more thermally stable disc that is quieter, resists cracks , dampens vibrations better and handles heat better.

This link is useful for further information: EBC Premium Replacement Rotors

Assuming that a brake disc has been made properly, with a good pearlitic structure to begin with, the onus then rests on the user to not create the conditions of heating and rapid cooling that will cause structural changes and therefore cracks to be formed in the braking surfaces.

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 thebed in new rotorsis 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).

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.

HOW HOT CAN A BRAKE ROTOR BECOME AND WHAT HAPPEN WHEN IT OVERHEATS ???

Once a disc goes above 727°C the pearlite in the matrix will start to decompose (degenerate) into ferrite. This change cannot be reversed, so once its starts to happen that is it. Ferrite is very weak so lots of ferrite can cause the strength of a disc to drop. The change from pearlite to ferrite can also cause the formation of craze cracks on the friction surface of the disc. Small cracks can then emerge to form larger cracks and in extreme cases the disc itself may crack completely.

Heating above 700°C and stopping quickly i.e. parking, can also help induce stress into a disc – some thing you do not want – this can cause distortion and judder.



	“Crazing Marks”…..these will progressively develop into “Full line cracks” after a few thousand miles and must be monitored and rotor changed. On track use any “Craze” line more than 10mm in length means the rotor must be replaced immediately as the crack progression will naturally be much faster on the track.

This means two things. FOR THE NORMAL URBAN DRIVER, these stress or “Craze” cracks should never appear and if they do you have either over heated the disc with aggressive driving or have other issues such as caliper drag or over thickness on the brake pads etc that has caused the system to overheat.

FOR THE TRACKDAY AND RACE DRIVER, you have to accept that the life of your discs will be shorter in track use and be on the lookout for these small cracks and their developments, REPLACING the disc/rotor before they get too large. Generally if a crack appears to be longer than 6-7mm or deeper than 0.5 mm the disc needs to be replaced. If you bed in new rotors slowly you will extend useful race life but in track use life of a cast iron rotor is MUCH less and that needs to be considered and the disc monitored carefully. The use of EBC new HIGH CARBON discs will greatly improve this problem on the track. Please remember our advice on caliper drag here: view caliper drag article which includes advice on brake fluids (click here to see how to check if your brake fluid needs changing) and venting or ducting of air. No street based car can be transformed into a race car by a few changes such as fitting new sport grade brake pads etc. Things are very different when temperatures push up into the 500-1000 degree zones.

ARE THERE GOOD AND BAD DISCS????.

If made from a quality pearlitic cast iron, most standard discs are similar but of course the higher specification High Carbon discs are a far better product for performance driving, that is why Premium car builders like Porsche and BMW use High Carbon for their OEM products.

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