What Were the First Disc Brakes?
The disc brake, as we know it, was first patented in 1902, but it failed to gain acceptance in the U.S. until the early 1960s when vacuum assist made the pedal effort acceptable for the driving public. European makers adopted disc brakes in the '50s, rapidly following Jaguar's dominance in the 1953 24 Hours of Le Mans race using four-wheel disc brakes. While exotically sculpted and finished calipers get all the glory, the disc brake rotor is where the rubber meets the road, or more literally, the pad meets the rotor and kinetic energy is converted into heat. The rotor has two critical functions: provide a consistent, high coefficient of friction surface for the pads to engage, and act as a heatsink and at the same time shed that heat into the surrounding air. When talking rotors, mass is your friend since mass helps regulate heat. With aftermarket and high-performance brakes, a lot of engineering goes into rotors from the materials used to the design of the cooling vanes. There's also a lot of science in keeping that heat from migrating into the caliper and subsequently the brake fluid, but caliper design is another story for another day. For now, let's focus on disc brake rotors.
The first disc brake rotors were one-piece castings somewhat similar to the one-piece rotors specified on most cars and trucks, and many aftermarket applications today. They perform well but carry a burden of excess weight, and they promote greater heat transfer to the hub/axle and bearings. In smaller sizes like the Wilwood setup below, which is designed to be used inside smaller classic OE style wheels, compact disc brakes can provide long-lasting service with far better braking compared to drums.
However, as rotor diameter and thickness increase to improve brake leverage and heat absorption and dissipation, a solid rotor will distort in a bell curve shape under heavy use, potentially causing uneven pad wear and excess drag. Those concerns plus the ability to optimize for heat related material expansion led to the development of two-piece rotors like the Wilwood 14-inch rotor 4 piston setup shown on the below.
What Is a Two-Piece Rotor?
Two-piece rotors were first used on motorcycles to save weight and allow for the fixed calipers required on the bikes. The use of an aluminum "hat" or "bell" bolted to an iron rotor ring was introduced to the high-end racing classes in the '60s. As with many track-bred innovations, the technology trickled down over the next few decades to more mainstream racing and high-performance street driven cars, and now trucks. Here you can see a pair two-piece rotors below.
How Is the Hat Connected to the Rotor?
There are three common ways of attaching rotors to hats—or bells, as they say in Europe: fixed bolt and nut; T-bolt style floating rotor; and stanchion style floating rotor with anti-rattle clips.
Back in the '60s and '70s, the typical way to attach the hat to the rotor ring was with bolts that clamped the hat into threaded holes in the rotor. But a simple set of bolts isn't adequate to the critical task of keeping the hat firmly attached to the rotor. Everyone has seen nighttime pictures or video of brake rotors glowing red under racing cars caused by the enormous heat generated by heavy braking at speed. Those cycles of heat, expansion, and contraction, as well as thermal tempering of the bolts themselves means that the bolts will come loose or fail completely over a relatively short period of time. Using the right type of bolts, along with safety wire, can solve these issues.
What About Heat Expansion in Two-Piece Rotors?
The negative effects of heat cycling can be worse on two-piece rotors because of the vastly different rates of expansion between cast iron, steel, and aluminum. Aluminum expands at roughly twice the rate of cast iron and grade-8 steel fasteners. Early users of two-piece rotors learned that the bolts had to be safety wired to avoid catastrophic separation of the rotor and hat. Safety wiring was the standard for keeping bolts in place in the '60s, and these complex and time-consuming safety wire schemes were all that stood between finishing a race or having the equivalent of a 150-mph flywheel come loose under the car. The modern fasteners used by companies such as PFC, Brembo, AP, Stoptech, Alcon and Wilwood lock in place without the need for safety wire, but people still like the warm-fuzzy feeling of security it offers. Plus it looks cool.
Which Bolts to Use with Two-Piece Rotors?
There are other issues with using conventional bolts. Because of the expansion rate differential and the stress relaxation it causes, SAE J429 grade-8 bolts shouldn't be used at temperatures above 800 degrees, grade-5 not above 450, and plated bolts should not be used above 250 degrees. Iron and steel glow red at 900 degrees. Your rotors need to be fastened according to how much heat you'll be generating. So use the right fasteners and torque them to what the manufacturer stipulates. The bolts supplied by companies such as PFC, Brembo, AP, Stoptech, Alcon and Wilwood are designed to handle the heat and stay locked in place even under extreme heat.
How Critical Is Bolt Size and Design?
Bolts must be sized so that the sheer plane, the surface where the hat and rotor are attached, is on the shank of the bolt, not the threaded portion. Bolts are weakest at the root of the thread, and bolt strength is reduced almost 30 percent when the threaded portion is placed in sheer. Therefore, the length of the bolt shank is critical, as is the material it is made from in order to have an extended service life and significant margin of safety. Look at the bolts used in your two-piece rotors of choice. Are they sized correctly? Do they have the right shank length? And what material are they made of?
Two-Piece Rotor Bolts
Like so many other technologies, aircraft and aerospace requirements led to the development super high-quality bolts and nuts that are specifically recommended for use in circumstances like rotor-to-hat attachment. High-end brake rotors from PFC, Alcon, AP, Brembo, and Wilwood use NAS A286 stainless bolts and nuts. These fasteners retain their clamping properties at temperatures up to 1,300 degrees and are more than twice as strong as grade-8 fasteners.
Specialized Two-Piece Rotor Bolts for More Clearance
NAS A 286 bolts come in conventional hex head styles as well as unique counter-sunk tri-wing configurations for tight clearance situations. NAS fasteners typically can be reused once for a rotor replacement. The torque specification is double the grade-8 fastener, and while they cost 10 times as much as other bolts, what would you rather have holding your rotors together?
How Do Floating Rotors Work?
The next evolution in hat and rotor technology is referred to as floating rotors. As rotor size has increased to keep pace with today's faster and heavier cars, the increased rotor mass, greater diameter, and thicker hats has exacerbated the effects of differential rates of heat expansion. To allow for that, rotors, or more often, hats are now slotted to allow the materials to grow radially while being safely clamped to prevent or minimize side-to-side motion. The aluminum hat and iron rotor "float" in relation to each other so that problems don't occur as the metals expand at their own rates when heated.
What Fasteners Are Used for Floating Rotors?
For street use, with acceptable noise levels (the two parts can rattle under some circumstances), two approaches have been used: T-shaped bobbins that lock the hat to the rotor, and more complex CNC shaped rotor stanchions often combined with anti-rattle clips. The bobbin is more compact and simpler to produce, saving some cost and again allowing for tighter fitments.
The stanchion style is better for eliminating or reducing noise, and due to its increased size and flat (rather than round) sides, it can support 1.125-inch-thick rotors in 15-inch and even 16-inch diameter sizes. For these rotors, most aftermarket brake manufacturers also use aerospace-grade fasteners.
What Type of Iron Are Rotors Made From?
The quality of the iron used in rotors is also important. All of the higher-end brake manufacturers use proprietary formulations in the cast iron rotors. They also control for uniformity and precision with their casting and finishing processes. The typical modern rotor is finished to 0.0005-inch tolerance. If your high-performance car will see significant track time, look for rotors specified for road racing and extended on-course use with pads to match. Just because all rotors—from a stock replacement to a high-end aftermarket unit—are iron doesn't mean they are all made from the same quality of iron.
Although proprietary most high end rotor manufacturers agree on a high carbon content making up between 3% - 3.7% as well as varying amounts of silicon, sulphur, maganese and phosphorus. Components affecting it's brittleness, toughness and strength include ferrite, cementite, pearlite and graphite carbon. All in all a fair amount of chemistry is involved in producing a truly effective performance rotor.