In the early '70s, drum brakes gave way to more powerful and efficient disc brakes. Disc braking system greatly relies on the performance of rotors and brake pads to deliver superior braking.
The function of rotors and brake pads can be explained through this simple experiment. Take a paper plate and spin it on the tip of your finger. Once, the plate assumes high speed, press it with another finger on top of it. What happens? The plate suddenly comes to rest. This is exactly what happens when you put your foot on the car brake pedal. The paper plate in this case is the rotor, and your fingers are the two brake pads. When you apply brakes, a lot of heat is generated. Naturally, both components undergo a lot of wear and tear, and need replacement in course of time.
Why to Replace?
Before we discuss when to replace them, let us see why do we need to replace them in the first place. To understand this, we will have to continue with our previous experiment. Now imagine, you are spinning the plate at the speed of a car. Now, when you stop it, a large amount of heat is generated due to friction. If you compare this with the heat generated due to friction between rotors and brake pads, you will realize that every time you hit the brakes, the rotors and brake pads have to endure tremendous amount of heat. Considering the number of times you hit the brakes while driving, your brake pads start eroding the metal surface of the rotors. The high friction and heat also cause warping of the rotors.
Warping is nothing but the dis-figuration of rotors with uneven thickness. Warped rotors and worn out brake pads can cause a complete loss of braking system over time. They are also responsible for most of the car brake problems encountered. Hence, it is a must that you replace rotors and brake pads in due course of time.
When to Change Rotors?
More often than not, your car itself will tell you when do you need to replace the brake rotors. If they are warped, you will experience wobbling when you apply brakes. Besides, squealing and screeching noises from the car on application of brakes is also an indication that something is wrong with the braking system. Mostly, these noises warrant replacement of rotors. Moreover, a closer look at the rotors will also enable you to decide when to change rotors.
Car mechanics often measure the thickness of rotors to check their performance. If the rotors are thinner than the minimum required thickness or 'discard thickness', then replacing them is the only option. Otherwise, a corrective procedure called resurfacing can be done. However, there is a limit to the number of times you can do this procedure. Warped rotors warrant immediate replacement. You may not need to replace all the rotors at one time. The condition of rotors and brake pads will determine when to replace front rotors and rear rotors. Similarly, you are rarely required to replace pads and rotors together, as rotors mostly outlive the brake pads.
How Often You Need to Change Rotors?
The frequency of replacement of rotors is greatly influenced by environmental factors. People living in severe winter regions may need to replace them more often than those living in moderate climate regions. The salts and dryness cause corrosion of rotors, thereby reducing their life. Also, the condition of roads and your driving skill also contribute to the life of rotors. You can get them changed by a professional mechanic, or you may choose to do it yourself.
If you frequently take your car for servicing, the mechanic should be able to tell you if it's time to replace rotors. Otherwise, you may inspect them regularly for damages. If not, your car will anyway tell you, if it's time to get the new ones.