Brake Rotors - Metallurgy and Composition

When it comes to vehicle safety and performance, your braking system is one of the most critical components. It can be a little tricky to find cost-effective, high-quality performance rotor options on the market, especially with big name brand products that overuse buzz words like “performance” without giving any quantifiable insight into the actual specs of their products. Safety related items shouldn’t be a secret.

That’s where FactionFab comes in—offering a high-performance, affordable solution that stands out, with the data to back it up. We feel a strong obligation to educate and inform our potential customers about the products you’re buying so that you can feel confident and safe when driving your car. Let’s break down the types of brake rotors available and show you why FactionFab should be your go-to choice.

The Basics of Brake Rotor Metallurgy

Brake rotors convert your car's motion into heat through friction when the brake pads clamp down on them. However, not all rotors are created equal—rotor material plays a significant role in heat dissipation, braking consistency, and overall braking feel.

Most passenger vehicles come with what the industry calls a “gray cast iron” rotor, which has long been the industry standard for their balance of cost, durability, and performance. Cast iron rotors are affordable and effective, with good heat management capabilities, making them suitable for most everyday drivers. But that term still leaves a lot of vagueness in actual composition, specs, performance, and cost.

There are two main grading scales for rotors that give us some indication of the rotor performance, as determined by SAE J431, Iron Grade (G) and Hardness Grade (H). Together, these make the overall Casting Grade of a rotor, an example being G11H18 (the casting grade of our FactionFab V2 Rotors). Let's dive into what those two grades and numbers mean.

Iron Grade is the first half of the total grade, which is the G11 in our example. This designates the tensile strength to hardness ratio of the rotor, both measured in MPa (megapascals, the metric version of PSI). G7 is the lowest grade in the SAE J431 grading scale (0.07 t/h ratio, or 7% tensile strength over hardness), with G13 being the highest (0.13 Ratio, or 13%). Since this is a ratio and not an actual measurement, it is unitless.

The Hardness Grade measures how resistant the rotor is to surface indentation and is the same hardness calculation used in the above Iron Grade (G) listed above, but listed separately to give greater definition to the characteristics of the rotor. This is the H18 in our example. This number is the actual force per area, in MPa. A typical rotor will range from H10 (1000 MPa) to H24 (2400 MPa). It should also be noted that with cast iron, anything above H21 (very hard) usually requires heat treating / tempering, while anything below H17 (softer) typically requires special annealing processes. H18-H20 are more naturally occurring hardness specs during the casting process, with differences depending on compositional makeup.

The casting grade system for brake rotors changed more than 20 years ago, but many companies are still using the old system based on the PSI rating. You might see some rotors advertised as G2500 or G3000, which were common. With the requirements of heavier cars and improvements in modern brake technology, the new system uses both the Iron Grade and Hardness Grade together to help us see more detail about how a rotor will perform.

Economy passenger vehicles typically use a G9H17 grade rotor, which is the successor of the old G2500 rating. More modern cars will use G10H18 with performance cars using G11H18, both of which would have been classified as G3000 under the old standard. The new standard gives us an even better breakdown of some differences between popular rotor specs that you might come across when shopping for your car.

This difference between the G10H18 and G11H18 is shown when we use the 2 grades to solve for the actual tensile strength difference between the rotors.

Comparison: G10H18 vs. G11H18

• G10 designates a t/h ratio of 0.100 (or 10%).
• G11 designates a t/h ratio of 0.110 (or 11%).

Since hardness (H18 = 1800 MPa) is the same for both grades:

• G10H18: Tensile strength = 0.100 × 1800 MPa = 180 MPa.
• G11H18: Tensile strength = 0.110 × 1800 MPa = 198 MPa.

So the G11H18 rotor is going to have greater tensile strength compared to the G10H18.

So what do I need for my car?

Now the unanswerable part of this conversation is “Which spec is best?” To even discuss a potential answer, you must consider what the rotor will be used for. Demand varies widely among vehicles when you consider that both a 2000lb lightweight sports car is not anywhere close to 80,000lbs in a fully loaded semi, but both would use some version of a cast iron brake rotor, and the SAE J431 grading scale gives us common options from G9H12 up to G13H19. Too soft and you see high wear and potential for warping. Too strong and you get great performance – until the rotor cracks because it doesn’t have the ability to expand and shrink with the heat cycles during heavy load times. Higher strength rotors are also more difficult to produce, requiring more expensive additives, stricter heat / casting conditions, and higher end lathe/milling machines for final processing. These are best reserved for track cars not used on public roads and require more frequent inspection and rotor replacement. The photo below is a common result of a high strength rotor that has developed micro cracks after heat cycles.

For most passenger vehicles, the G10H18 and G11H18 specs above have a good balance of performance and cost. You may find outliers still available at a lower spec for economy vehicles, but they are becoming rarer every year and we would recommend skipping them.

If you have a sport-focused car, we think the G11H18 range is going to be the best choice. Higher tensile strength is a benefit when braking at high speeds to provide consistent pedal feedback and performance compared to the lower tensile strength option, but it still has some flexibility to be driven on a daily driver where your canyon run may see rain or snow that would create micro cracks a high-strength rotor.

If you need even higher performing brakes, you may even look into carbon ceramic composite brake rotors, which have much higher heat thresholds and longer lifespan, but extremely high costs (more than 10x the cost of a similar sized cast iron rotor in most cases). These typically are reserved for extreme performance versions of supercars with a focus on track use, and the carbon ceramic composition doesn’t have many of the drawbacks (brittleness) that come with the G12H21 and higher grades.

So is High Carbon a hybrid supercar rotor or something?

Actually, no. This simply refers to the carbon content in the iron that is used in the casting. Gray Cast Iron isn’t actually 100% pure Fe iron, and like everything else in the universe (until you start talking quantum particles), it's made up of other primary elements found on the periodic table. These other elements are added to tailor the material for added strength, corrosion resistance, heat transfer attributes, cost, etc. We can determine a pretty exact composition of most metals by performing an Optical Emissions Spectroscopy (OES) test, which analyzes the light emitted when a sample is subjected to an electrical discharge. 

A typical brake rotor composition will have some of the following compounds:

• Carbon (C): Increases graphite content, enhancing noise damping, heat transfer, machinability, and wear resistance, but too much can reduce strength.
• Manganese (Mn): Improves strength and hardness but may reduce vibration damping if used excessively.
• Phosphorus (P): Improves castability and wear resistance but can lead to brittleness if levels are too high.
• Sulfur (S): Enhances machinability but can cause brittleness and reduced impact strength.
• Silicon (Si): Increases strength and oxidation resistance but may reduce ductility.
• Copper (Cu): Improves strength, thermal conductivity, and oxidation resistance without harming machinability.
• Nickel (Ni): Enhances toughness and corrosion resistance but can be expensive and may reduce machinability.
• Chromium (Cr): Increases wear resistance and hardness but can reduce machinability if used in excess.
• Molybdenum (Mo): Enhances high-temperature strength, wear resistance, and thermal stability, especially under heavy loads, but can lead to brittleness.

There is a small range of acceptable percentages of each of these elements for the material to qualify as the gray cast iron that we want to use for brake rotors, and each batch needs to be carefully measured and adjusted to make sure it’s right (we spec and test on our rotor castings). You can see that the amount of each element is really about finding a sweet spot between strength, heat transfer, and brittleness.

High Carbon simply refers to a higher-than-normal amount of carbon in the iron, which in cast iron is still relatively small. Typical iron rotors have a carbon content between 3.0-3.5%, with High Carbon iron rotors coming in between 3.6-3.9%. This extra carbon content enhances heat conductivity, making them less prone to cracking in high-stress situations like prolonged braking or high-speed stops. The added graphite structure (carbon compounds) also reduces vibrations and noise, resulting in a quieter, smoother driving experience.

This table shows the composition analysis of our first batch of FactionFab V2 rotors.

The photo below is a micrograph image showing the carbon/graphite structure of these rotors, an important part of the inspection process to verify consistent and even distribution through the grain. A bad example would have heavier blotches of black with wider spread between the carbon.

What about everyone else?

We were disappointed at how difficult it was to find actual data and specs from some of the biggest brand name rotors (we can’t list names). We had to do some pretty in-depth research to get anything, and our actual testing showed that they didn’t match what we expected. Lab testing results of 2 popular Subaru favorites are listed below and compared to our V1 rotors. Let's walk through some outlier specs to highlight what it all means on each of the rotors.

For strength testing, our V1 is typical for a G10H18 grade, with Competitor 1 being closer to a G11H18, but the Competitor 2 tested in the G13H19 range. While competitor 2 would likely be a great performing rotor, it should absolutely live its life on a racetrack and be replaced frequently due to its higher potential to crack.

And for compositional analysis on these 3 rotors, we have the following make-up next to our V2 target (which is our chosen spec listed as a ballpark, not law).

We were a little surprised to see that our V1 rotor tested a little low on the carbon content to our old target of 3.5% (we bumped up our requirement for the V2 to 3.6%), but it was still the best in this batch of testing. Competitor 2 also claims to be a high-carbon rotor, which should indicate a C value of 3.6 or greater (shame shame). 

The high manganese (Mn) content in competitor 2 is likely what contributes to its hardness, and typically a value less than 1% is acceptable for rotors to reduce brittleness.

The high copper (Cu) and Molybdenum (Mo) of the FactionFab V1 rotor is a positive that aids in heat dissipation and rust resistance but comes at the cost of overall strength.

FactionFab Rotors: The Ideal Balance Between Performance and Cost

With enhanced heat dissipation and increased friction, FactionFab rotors are highly efficient at keeping your brakes cool during prolonged use—whether you're tackling city traffic, curvy mountain roads, or high-speed highways. The high carbon content also contributes to noise reduction, ensuring a quieter, smoother driving experience without sacrificing strength.

And for a quick price comparison, the FactionFab V1 rotor had a retail price of $115.00, while the same fitment Competitor 2 rotor comes in between $150-200, and the Competitor 3 came in between $100-150 (keeping it vague for professionalism).

At the end of the day, the challenge for drivers is finding a rotor that offers premium, safe performance without the premium price. That’s exactly what FactionFab rotors deliver. Made from tested and confirmed high carbon G11H18 iron, FactionFab rotors strike the perfect balance between heat management, durability, and affordability.