Disc rotor material is a hot topic in the brake market today. While various grey iron formulations have been used in brakes for more than 20 years, marketing has put their own spin on disc rotor materials to create a point of difference from competitors. A small amount of information and the clever use of numbers is often enough to plant the seed of doubt into the consumers mind so they hang off every power word and phrase of the marketeers.
We hope to shed some clarity on the topic in this brief article to enable the consumer or technician to make an informed decision when choosing a brake rotor that’s fit for purpose.
First some basics;
The first law of thermodynamics - “Energy can be transformed from one form to another, but can neither be created or destroyed”
The typical road vehicle engine converts the energy from a chemical reaction (combustion) into forward motion or Kinetic energy. In order to slow or stop a vehicle you must convert this kinetic energy into another form of energy, heat via friction through the brakes. Some energy is also lost through wind resistance, rolling resistance (tyre and road surface) and drivetrain resistance (engine braking) but this is much less significant than the work done by the brakes. The process of transferring heat energy from one body to another is call heat flux.
What do the numbers mean?
Referring to the globally recognised ISO standard 185 for grey cast iron, the grade number refers to the tensile strength of the iron in MPa (Mega Pascals). e.g. G250 = 250 MPa
The USA also has an SAE standard that has an extra zero on the end. E.G – G3000 which refers to the tensile strength in PSI (I.E 30,000 PSI). So G3000 is 30,000 PSI = 207MPa. A lower strength than ISO 185 - G250. G250 is commonly used in small engine cylinder blocks and disc rotors.
So, what are high carbon (HC) disc brakes?
When most people think of high carbon discs, they think carbon brakes, expensive, high temperatures and Formula 1. Not so!
Carbon is a primary element added to grey iron disc rotors to enhance the thermal properties, friction and noise damping ability. The typical carbon content of G3000 iron is approximately 3.25%.
High carbon is not a standard. It is a term used in the foundry industry to categorise an iron with a carbon content between 3.6% and 3.9%.
“It is generally accepted that in order to be classified as a high carbon iron, the carbon content of the material has to fall within the range 3.6 - 3.9%.”
Dr. John Krosnar, Chief Executive of European Automotive Components.
So, lets do the math, 3.65% (HC) – 3.25% (G3000) = 0.4% of the total mass in grey iron. Or 12.5% more carbon if you want to make it sound more exciting. At a stretch we could say 20% more carbon (exciting) or 0.65% of the total composition (not so exciting).
The process;
When grey iron is produced, the scrap metal and pig iron is melted. The chemistry is corrected by adding carbon, silicon and various other trace elements to produce the correct grade. The metal matrix can only absorb only so much carbon and the remainder is precipitated out into flakes. These flakes serve two critical purposes, thermal transport and noise damping.
As carbon is an excellent conductor of energy, the flakes act as small highways for the heat to travel away from the friction surface quickly. The size, shape and orientation (microstructure) of the carbon flakes is more important than the amount of carbon as you want the heat to travel as efficiently as possible away from the source. The difference in thermal conductivity between G3000 and HC can be as little as 5% or with some clever chemistry 10% or more.
The second and more significant purpose of high carbon brakes is noise damping. Carbon has a very low density whereas iron is very dense. Therefore, the carbon flake acts like a void or cushion within the disc rotor. Brake squeal or groan is basically a harmonic wave. If you want to disperse or cancel a wave (noise) you need to change the speed at which is travels. As noise passes through the dense iron and low-density carbon the speed is interrupted, causing the noise to disperse or dampen.
An example of carbon flakes in disc rotor material.
Noise damping;
G3000 (&G250) - Decrease in amplitude per cycle = 60
High Carbon – Decrease in amplitude per cycle = 126
100% more effective in noise suppression.
So, which is better?
Your common G3000 disc rotor material is robust, a little higher strength than HC with good wear resistance. A simple but effective general-purpose brake rotor.
Your high carbon disc rotor material sacrifices a little strength for more efficient thermal and noise damping properties. How effective these properties are depending not so much on the amount of carbon but the size, shape and orientation. There is an art to doing this well. Some vehicles with light weight press metal or cast aluminium suspension arms may be more prone to noise issues. A small compromise for weight reduction on the axles. Often these vehicles come standard with HC brake rotors. High performance vehicles also come standard with HC disc rotors due to the potential kinetic energy generated at high speeds. Regardless of whether you drive at 220km/h, the brakes must be designed to cope with the vehicle’s maximum speed. An effective noise damping and high-performance solution. Many European vehicles are supplied with HC rotors as standard.
Lastly, the less common G250 grade is the higher strength material with excellent low wear properties but it is prone to noise issues. You may see this used in commercial applications.
At DBA we work with different material chemistries and microstructures to produce the best braking solution to suit your needs. Every batch of castings is inspected and approved by our engineers before machining to ensure our strict material requirements are met every time. We know what’s inside the box!
Steve Gavin
Innovation and R&D Manager at Disc Brakes Australia
