How Disc Brakes Work
Think for a moment about the principle of disc brakes. The wheels are bolted to smooth steel discs (or rotors), with the caliper enclosing the rotor like a clamp. When you press on the brake pedal, hydraulic pressure forces the brake pads against the rotor, slowing the vehicle by friction (not unlike a bicycle's handbrakes).
The brake pads are a hard ceramic or semi-metallic material, and it typically takes 40,000-60,000 miles for them to wear down to a minimum thickness where they would need to be replaced.
The pads' friction material is bonded to a steel backing plate, with a spring-steel tab that protrudes at a right angle. When your pads are down to a minimum thickness, this tab should drag along the rotors' surface and make a loud squeal as a wear indicator. If you let your pads go too long before replacement, the steel backing plate and rivets will start to dig into the smooth steel surface of the rotors. At that point, your rotors will have to be replaced.
It is still a best practice to replace brake rotors every time your brake pads are replaced. Brake friction material will transfer to the surface of the rotor during your everyday driving. If you replace your brake pads without replacing your brake rotors, you will often get braking noise and diminished performance.
Here's something else to consider when you're in need of a brake job: When you hit the brakes, physics comes into play and the weight of your vehicle transfers toward the front. As a result, front brakes are often going to wear out about twice as fast as rear brakes, and you will often just need the front brake pads replaced rather than a four-wheel brake job.
Your bottom-line cost for a brake job will depend on a few factors:
- The make and model of vehicle, as European and Japanese parts are just about always more expensive
- Whether you need front brakes or a full four-wheel brake job
- Whether you have any additional expenses, like replacing brake wear sensors
- What grade, brand, and quality of aftermarket brake pads you use
A brake job doesn't have to be expensive, especially if you get to it quick enough that other braking components aren't damaged. Of course, the most important reason to not put it off is your safety, and that of everyone else on the road!