"Safe Driving" means different things to different people, but no matter their age or experience, every driver can agree a car's brake system is at the very heart of that safety. Presuming every piece of that system is functioning properly is what gives us equal confidence to gradually compress the brake pedal at the end of a slow roll up to an intersection, or stomp on it to avoid a collision. Like brake pads, brake rotors are a critical component of that system; the heaviest and largest single pieces on each axle, they, too, are therefore subject to replacement as they wear.
Every driver should know the indicators that their brakes are nearing the end of their duty cycle, and while it's relatively easy to determine when brake pads are starting to wear out, knowing what the warning signs of a worn rotor are can be a little trickier.
Knowing what the indicators are that rotors have become worn out or damaged is affected by a slew of circumstances and causes, and the way those elements are combined over the life of the vehicle.
Factors that contribute to rotor wear and tear include, but are not limited to:
In the simplest form, brake rotors are squeezed by brake pads to slow and stop a vehicle, but it's really not that elementary. Let's take a look at how the full system works together, step by step.
In modern cars, the pressure of the brake liquid is modulated by the ABS (Anti-Lock Braking System), ASR (Anti-Slip Regulation), and ESP (Electronic Stability Program) systems.
Brake rotors are one of the most visible parts of the full brake system. They are those metal discs shaped like a flying saucer mounted on the wheel hubs, visible behind the hub cap. Typically, they are formed by casting solid "blanks" in molds from special metal alloys which must be resistant to wear and high temperature; in most cases, these are cast iron and cast steel. In addition to the holes for the bolts that hold them to the axle, they are also molded with internal cooling vanes, and, depending on the vehicle and end use they are intended for, they may then be custom machined to add slots and/or drilled holes.
As brake pads are compressed against a rotor, the resulting friction creates heat and gasses that can get trapped within the system. Solid rotors are the least expensive option, but have a tendency to overheat. Higher quality rotors, by comparison, are vented in a variety of ways to decrease that heat. Drilling holes and machining slots into the surface that interacts with the brake pads not only allows for heat to be more quickly dissipated, it also keeps gasses and debris from being trapped, extending the wear life of the rotor and keeping the brake system stronger longer. Slotted-only rotors are fitted with internal ventilation channels that direct heat away and toward the back of the vehicle.
In premium-class and racing cars, custom brake rotors made of ceramic or carbon materials are used; these are extremely expensive, but rarely overheat.
Brake rotors tend to wear longer than brake pads, commonly at a rate of two-to-one, but should be checked at every maintenance and service your car receives. The most surefire way to determine if they are at the end of their life is to measures the physical thickness to determine if they are too thin. The minimum thickness is defined in the vehicle’s service book, and some brake rotor manufacturers even engrave it on the rotor's surface.
Additionally, brake rotors may also need replacement when any of the following signs appear:
Regardless of the cause of the wear, if even a single rotor is worn out, it’s better to change the entire assembly at one go, including front and rear brake rotors AND front and rear brake pads.
Brake rotors are extremely influential on driving safety, which is why they need to be replaced with new ones as soon as they are found to have been damaged. Brake rotors must be selected in accordance with the car manufacturer's instructions, and it is best to choose branded products, ideally cross-drilled and vented styles, manufactured from good materials and to high quality standards. Researching known issues with your make and model of car may offer insight into better-than-OEM solutions, as well, as some aftermarket manufacturers have identified rotor weight and engineering weaknesses and addressed these in their product offerings.
You know better than anyone what safe braking feels like in your own vehicle. Some of the damage a rotor can suffer will cause an immediate change in braking abilities, and that change requires prompt and urgent attention. Because brake rotor wear and tear can be gradual over time, however, it's equally important to check them at every service and note their thickness, corrosion level, and surface condition. Additionally, if you've made any upgrades to your car or truck that affect the wheels or the overall weight of the vehicle, the way the vehicle is being used, or added towing or increased payload, it's time to change the brake system components to match.
Replacing your brake pads and rotors with new, high performance products is the best way to guarantee your safety as you head out on the road!
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