What are brake rotors and where are they located?

Brake rotors, referred to as brake discs, are components in a disc brake system.  A disc brake system consists of brake calipers, pads, and rotors.  The rotors bolt onto the wheel hubs or axles, and spin at the same rate as the wheels.  When you press the brake pedal, hydraulic pressure is transmitted to the caliper, which acts as a clamping device pressing the brake pads against each side of the rotor.  The friction this creates allows the vehicle to slow to a stop.

Most cars and trucks have either 4 brake discs, one for each wheel or two front brake rotors and 2 rear drums instead of brake discs in the rear.

Types of brake rotors

Most automotive brake rotors are made from cast iron with different variations in the design and appearance.  The most common types of brake discs used in cars and trucks:

Solid and Vented Brake Rotors

“Solid Rotor” refers to a disc that has no vents.  This brake rotor was standard for years, and is still found on vehicles today (more often as a rear brake rotor).  The downside to a solid rotor is its inability to efficiently dissipate heat created from the braking process.  Excessive heat causes brake pads to lose their friction capability, resulting in brake fade. 

Vented Brake Discs

Vented brake discs were developed to combat the heat dissipate issue of "Solid Rotor"; these rotors have cooling fins placed in between the two sides of the disc.  The fins allow air to flow through the disc aiding the cooling process, resulting in reduced brake fade and better heat dissipation.  Vented discs are widely used type of brake rotor found in the front of modern vehicles.

Drilled and Slotted Brake Rotors

Inspired by racing and high performance, drilled rotors and slotted rotors come from the factory on some vehicles.  Both versions have the benefit of increased heat dissipation, and allow for water and gasses to escape from between the brake pads and the rotor surface.

Drilled brake rotors have a series of holes drilled through the rotor surface creating less surface material for the brake pad to grab.  Drilled rotors are prone to cracking since they are inherently weaker than a brake rotor with a solid face.  Slotted brake rotors are known to wear brake pads down quicker than other rotor types as the slots tend to shave the brake pad surface.  Another downside is that mud and dirt can lodge inside the slots or holes.  For these reasons, they are not recommended for use in off-road applications.

Hub and Brake Rotor Assemblies

At one time, the majority of cars and trucks had the wheel hub and brake disc incorporated together.  This type of rotor can still be found today, specifically on 2 wheel drive pickup trucks and vans.  When replacing this style of brake rotor, the wheel bearings must be removed from the original brake rotors and pressed into the new ones.

Brake Rotors with Parking Drum Brakes

Many vehicles with rear disc brakes use emergency or parking brake shoes.  The brake rotor in these applications is a rotor / drum combination.

How do I know if my brake rotors need to be replaced?

Pedal pulsation is caused by thickness variations of the disc; often the result of brake pad friction material adhering to the surface of the rotor.  At one time it was common to resurface brake rotors, however today it is often more cost effective to replace the rotors rather than machining them.

How do I replace a brake rotor?

Raise and secure the vehicle, then remove the wheel and tire and unbolt the caliper.
Unbolt the caliper bracket and pry off the brake pads. The rotor should simply slide off.  If you have issues pulling the rotor off, threat a lug nut and strike the hub area of the rotor with a ball end of a ball peen hammer.

Clean the new rotor with brake parts cleaner before installing it.
Slide the rotor on and thread a lug nut to hold it in place.
Insert the caliper bracket into place.
Start the bolt by hand and torque the bolts to your vehicle's specifications. 
Place an old pad into the caliper and use a large C-clamp to reset the pistons. 
Insert the pads into the bracket and place the caliper on.  Thread the caliper bolts by hand and torque.

Once the wheel and tire are fitted securely to the vehicle, and the vehicle is back on the ground, pump the brake pedal repeatedly until it feels firm - that helps reset the pistons. Test your brakes at low speed before road testing the vehicle.