The flat surface on the Nissan disc brake rotor is the contact area they make with the brake pads. It is this area that the friction is created that is the stopping force of the brake system. There are at least two rotors on every Nissan made today, and on some models there are four. If there are only two, then they are located on the front wheel assemblies. The brake rotors not only have the flat disc-like surface the brake pads come in contact with, but some will also come with the wheel hub and bearing for your spindles and axles. The disc brake rotors can last the life time of your Nissan just as long as the brake pads are changed on time and before they totally wear out. When the friction material of the brake pads wear out, the metal backing plate is what comes in direct contact with the brake rotors and discs. This metal to metal contact can be easily heard when a grinding noise is coming from the wheel assemblies when the braking system has pressure being applied to it. When this is happening, grooves or divots are being cut into the flat surface of the Nissan disc brake rotors. If the surface of the brake rotors and discs are not resurfaced during the brake job, the new pads will then be exposed to an uneven surface and will wear out prematurely. This resurfacing of the rotors is called turning the rotors. This is when a lathe is used to remove portion of the disc until they are smooth again. If the grooves are too deep and the discs are too thin when they are removed, then new rotors will be required for the brake system on your Nissan to be properly functioning again.