By Andrew Markel
Editor, Brake & Front End
is tough for a technician to know what they are getting when they buy a
rotor. If the technician or counter pro goes only on the price on the
screen, it is difficult to pick the best rotor.
Design and Engineering
rotor manufacturers care only about the external dimensions of a rotor.
The engineer wants it to fit and disregards the performance attributes.
In the industry, this is called the “round and heavy” philosophy. Some
aftermarket rotor manufacturers will take the OE design and make it
better in order to exceed customer expectations. Engineers will put in
years of research and development and racing in order to develop and
manufacture a rotor that can go beyond OE specifications.
manufacturers will research and formulate high-temperature and
high-strength alloys in order to have the best metallurgical
microstructures. Also, they will develop casting and machining
processes that maximize certain properties of the metallurgical
Casting and Machining
Many cheap castings
are poured with the least amount of iron possible. When these are
machined at the factory, they are essentially performing a “skin cut”
where they remove the least amount of scale and material possible. The
outer layer of iron is not as stable as the material underneath. This
creates problems with porosity and strength. Eventually, these problems
create stress in the rotor leading to distortion. This results in a
Some rotors are processed through a controlled
heat-treating cycle that relieves internal stress in the rotor and
improves the thermal stability and microstructures. According to some
manufacturers, this eliminates any need for cryogenically treating a
A rotor should be mass balanced or it will shake or vibrate.
Look at the method used to balance the rotor. Some manufacturers will
either place weights in the fins or remove material from the outside
diameter of the rotor’s plates.
If the rotor was mill balanced, you
will see notches or cuts in the rotor. But, if too much weight is added
or removed, it is a direct indication that the rotor has some severe
internal problems that are causing this out-of-balance condition.
Adding or removing mass can result in the rotor being thermally
As a rotor heats up, the area where the material was
removed is different than the surrounding area. The way it absorbs and
dissipates heat is changed. This creates stress in the rotor when it is
heat cycled on the vehicle. Over time this will cause runout and
thickness variation. Some rotors are balanced using a machining
technology that mills the rotor concentrically eliminating these
One standard that can be used to judge
some rotors is if the manufacturer is certified by the International
Standards Organization (ISO). The aim of ISO is the development of a
quality management system that provides for continual improvement,
emphasizing defect prevention and the reduction of variation and waste.
16949 applies to the design/development, production and, when relevant,
installation and servicing of automotive-related products.
If a manufacturer follows ISO/TS 16949, the manufacturer can provide you with the following information upon request:
1. Certified metallurgical chemical analysis for every batch poured.
2. Hardness of the brake face in eight places of that batch of rotors.
3. Concentricity of the mounting pilot to the outside diameter should be less than .002”.
4. Micro finish should be less than 35Ra or .7 micro inch.
5. Runout of the brake face to the mounting surface should be less than .001”.
6. Thickness variation of the brake plates should be less than .0003 inches
7. Who performed the machining.
not just how heavy a rotor is or if it will fit. The rotor needs to be
properly designed for thermal capacity and superior heat dissipation,
and it must be manufactured with precision to meet today’s vehicle