Residual stress is a common structural weakness that occurs during the casting process of iron, the most prevalent material in brake rotors. This same weakness is inherent in all casted iron, but makes a more dramatic impact in applications where the iron is heated and cooled repeatedly such as automotive braking systems causing expansion and contraction of the material.
During typical driving, residual stress is unlikely to cause any problems at all. However in racing or aggressive driving situations where the brakes are at high temperature for a extended periods of time, these residual stresses can lead to thermal cracking and deformation of the rotor which can lead to vibrations or brake ‘judder.’ In an effort to work around this inherent issue with casted iron, often times heat treating is employed to add additional protection from heat related issues.
Each application, product and manufacturer has their own specific steps for heat treating, but we’ll discuss the process in general for explanation purposes. Typically the product (in this case brake rotors) are heated in a strictly controlled environment, increasing the temperature very slowly, and at predetermined intervals until they are at the maximum defined temperature. At that point the rotors will remain heated at the precise maximum temperature for a period of around 8 hours. When that heating interval ends, the rotors are very slowly cooled again using strict controls gradually reducing their temperature until they reach ambient temperature. Often times, this process can take over 24 hours. By strictly controlling the temperature and humidity, this process helps strengthen the bonds between molecules within the rotor.
Commonly known as “profiling,” DBA uses our own variation of the process that we refer to as Thermal Stability Profiling, or TSP. This TSP treatment re-aligns the microstructure of the iron resulting in added resistance to deformation, and increased heat resistance.
The two most common materials used in brake rotors are cast iron and carbon fiber. (also known as Carbon-Carbon) Carbon fiber offers high thermal resistance with a low weight, but can be costly and is currently only used in the upper echelon of racing or as a high dollar option on a few ultra-premium vehicles. Cast iron is what typical rotors are comprised of based on a benefit-to-cost ratio.
There are three main types of cast iron with varying levels of graphite in the mix, Grey cast iron, CV cast iron, and ductile cast iron. Grey cast iron contains carbon flakes which produces the most effective noise dampening properties of all irons and steel. It also enables high thermal conductivity properties making it the first choice for braking.
Grey cast iron is the most common material choice for mass-produced rotors as the raw materials are easily sourced and cost effective to process.
The potential downside is that it can be cracked or deformed when exposed to frequent drastic temperature changes due to the accumulation of hoop stress.
Ductile cast iron is number two on the list, and is an excellent material with similar tensile strength as steel. The formation of carbon in ductile iron is in spheres. Whilst these small balls of carbon enable excellent strength properties it does not dampen noise or carry thermal loads as effectively as grey iron.
Our last main type of cast iron is CV, or Compact Vermicular cast iron. CV cast iron falls almost in between Grey and Ductile, depending on the manufacturing process. Quality control with CV is very difficult resulting in an inconsistent material, occasionally bordering on Grey while other times being more like Ductile.
After extensive testing, DBA has derived a version of Grey cast iron with a unique blend of additives including higher levels of carbon (C) that achieve a higher level of durability and better resistance to sharp temperature changes than the typical Grey cast iron. This proprietary DBA mixture offers a higher FC200 rating versus only an FC150 rating of the standard Grey cast iron OE-type material. (FC ratings are used as a measurement of the strength of cast iron) This material leads to a superior rotor with more tolerance for sharp temperature changes, while still having a high surface hardness, as well as a high resistance to cracking and other distortions.
Brake rotors are most commonly offered in solid and ventilated designs. In many modern vehicles the front rotors are ventilated and the rears are solid, however, there are also many vehicles that offer ventilated rotors front and rear. Ventilated discs have a space between the brake surfaces that allow for airflow to pass through the rotor for cooling purposes. Solid discs don’t offer this feature, which is why they are frequently used in the rear of vehicles where the brake system is often under less stress.
Vent designs come in 3 common styles, straight fin, curved vane fin, and DBA’s Kangaroo Paw pillar fin design. Straight fins allow for airflow, but when a rotor is spinning, the straight fin design doesn’t do much to help in extracting the heat from the rotor due to the perpendicular alignment of the straight fin compared to the center of the rotor’s rotation. A better option is the curved vane, which acts much like a fan blade, swirling the air and forcing it outward from the hub center by taking advantage of the spinning motion of the rotor during driving.
DBA rotors employ specifically designed pillars with a layout and post shape that works much like the curved vane when it comes to heat extraction, but offers even less obstruction to the airflow which reduces turbulence and increases the flow of hot air as it exits the rotor.
While slotted rotors may look cool, the real advantage of slotted rotors is an increase in braking power, in many cases up to 15-20 percent. Often slotted rotors have a higher resistance to brake judder, an increased cooling affect and a higher operating friction level. Sounds great right?
Unfortunately, all this magic comes with a price. Slotted rotors are more expensive to build due to the precision machining required which translates to a higher price tag. Also, slotted rotors have a tendency to be noisier during braking and will often wear through brake pads at a faster pace than solid faced rotors.
In an effort to eliminate the negatives and accentuate the positives of slotted rotors, DBA offers our new T3 slot design that addresses a few of these problems. The T3 slot design has a gradual overlapping curved design that is significantly quieter than straight slots virtually eliminating their inherent whirring noise during braking. The T3 slot shape also reduces the excessive brake wear by keeping the pad in constant contact with the rotor face, rather than allowing the slot to ‘cut’ material from the pad with each passing slot. While we’ve managed to eliminate some of the downside of slotted rotors, with the DBA T3 slotted rotors, you still get the good stuff like increased braking power, added cooling and a reasonable price.