Brake Rotor Basics
Brake rotors are the rotating part of a disc brake system. They are attached to the axle with taper roller bearings and the wheel with studs and lug nuts so that they spin freely. A stationary, hydraulically-actuated caliper squeezes a pair of brake pads against the rotor’s surface to slow its rotation or stop the vehicle by converting the vehicle’s kinetic energy into heat energy via friction. The friction used to slow the vehicle creates heat that must be dissipated. Disc brakes dissipate heat better than drum brakes, making them a popular brake technology on today’s vehicles. They are commonly applied to the front wheels, or on all four wheels where greater braking performance is desired.
A drum brake uses friction (brake shoes) that press outward against a rotating cylinder-shaped part called a brake drum. When the brakes are applied, brake fluid is forced under pressure from the master cylinder into the wheel cylinder, which in turn pushes the brake shoes into contact with the machined surface on the inside of the drum. This rubbing action reduces the rotation of the brake drum, which is coupled to the wheel. Hence the speed of the vehicle is reduced. When the pressure is released, return springs pull the shoes back to their rest position.
Compared to drum brakes, disc brakes are less prone to the brake fade caused by excessive heat, and they recover faster. They’re also less affected by water than drum brakes as centrifugal force “spins” water off the rotor’s surface and the pads “wipe” the surface dry.
Rotors are usually made of cast gray iron, but may be made from more exotic materials, such as reinforced carbon–carbon or ceramic matrix composites when higher brake performance is required, such as in high-performance sports cars and racing vehicles. The Society of Automotive Engineers (SAE) has set standards for gray iron used in casting rotors, with SAE J431 G3000 as the standard for car and light truck castings, and it specifies the chemical composition, hardness, and tensile strength.
Rotor construction
Solid and vented
Solid rotors are used in older applications or on small vehicles that don’t generate excessive heat during braking.
Vented discs as seen in this photo have two surfaces separated by vanes that provide a larger surface area for greater heat dissipation that result in less brake fade and faster recovery (cooling).
Vented rotors are almost always installed on a vehicle’s front wheels where up to 75% of braking force is applied and greater amounts of heat are generated. It’s not uncommon for some four-wheel disc brake vehicles to have vented rotors on the front and solid rotors on the rear. Generally speaking, four-wheel vented disc brakes will be found on vehicles where enhanced braking performance is desired or will experience greater demands on the braking system.
Whether solid or vented, some smooth rotors are available with a coating on the non-contact surfaces to prevent rust and help ensure rust is not a cause of reduced brake performance.
Rotor surface
The rotor’s friction surface is commonly a flat/smooth surface upon which the brake pads exert their braking force.
On rotors designed for higher performance, the friction surface may be slotted, drilled, or a slotted/drilled combination. The advantage of these surface geometries is the ability to resist pad surfaces from becoming glazed and reducing their friction coefficient, as well as faster dissipation of water should they become wet. The drawback for these rotor surfaces is that pad life is sacrificed for greater performance.
Drum brakes are used in most heavy-duty trucks, some medium and light duty trucks, and few cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels, since most of the stopping force is generated by the front brakes of the vehicle and therefore the heat generated in the rear is significantly less.
History
Successful application of automotive disc brake system goes back to 1902, but with material limitations and mechanical actuation at the time, wide adoption of the technology was impractical.
Credit for the first automotive production disc brakes is given to the 1949 and 1950 American Crosley Hot Shot, but then reverted to drum brakes because of reliability problems (sticking and corrosion), particularly where roads were salted in winter.
Widespread introduction of disc brakes were first made on sports cars as they demand greater brake performance. Since the early 1970s, disc brakes have become more popular and are now common form in most passenger vehicles. However, on lightweight vehicles, drum brakes are used on the rear wheels to save cost and weight, and to simplify the parking brake.
