The only purpose of the BMW disc brake pads is to apply friction to the rotors of your car to slow down and stop the forward or backward motion of your vehicle. The pressure that is applied to the back side of the brake pads is supplied from the hydraulic fluid that is compressed and moves the piston in the brake caliper. The material the disc brake pads are made from for your BMW include asbestos, carbon fiber or a composite material. The reason they have to be periodically replaced is because when the brakes are applied, the friction being applied to the brake discs removes a small portion of the pads. The time period a set of brake pads will last is dependent on how aggressively the brakes are used by the driver and the smoothness of the surface of the discs. All BMW models are now equipped with disc brakes on all four corners of your vehicles suspension. One characteristic of this type of configuration is that the front BMW disc brake pads will wear out twice as fast as the ones mounted on the rear of your vehicle. This is created on purpose because the brake bias or amount of pressure applied to the discs is generally twice as much towards the front than what is being applied to the rear. This is done for a smoother and more controlled stopping action by your vehicle. On most of the quality replace brake pads made for BMW automobiles are a small warning plate. This is the portion of the pad that will come in contact with the disc or rotor when the pads are nearing the end of their lifespan. When the grinding sound of this plate makes contact with the disc indicates a brake job should be scheduled. If this warning is not heeded, then damage to the rotors will occur.