When Should You Replace Brake Rotors?

Hawk Performance

08/28/2020

"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.

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How Do Brake Rotors Wear Out?

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:

  • Quality standards: The materials and quality control standards implemented by the manufacturer during the brake rotor construction, treating, and distribution processes.
  • Physical characteristics of the brake rotors: Solid, drilled, slotted, or vented affects heat dissipation capacities differently.
  • Quality of the brake pads: Interacting with cheap, hard pads or pads not properly installed will cause damage.
  • Driving style and environment: City, mountain, or aggressive driving, and extreme environments (think desert heat or frigid winter cold) all cause quicker wear rates for rotors.
  • Car weight: The heavier the car or the more upgrades or modifications added to a vehicle, the more quickly rotors and pads wear down.
  • Axle position: The front axle usually bears more weight than the rear, as well as (typically) forward bias proportioning. Because of this, the front carries more of a braking “load," resulting in the front usually wearing rotors and pads more quickly than the rear.

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How Do Brake Rotors Operate?

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.

  • The driver wants to stop the car, and depresses the brake pedal.
  • A plunger in the master cylinder forces brake fluid out of the reservoir.
  • Through rigid brake lines, the fluid flows to the wheels.
  • Flexible brake lines then carry the fluid into calipers.
  • The fluid pressure pushes out the brake pistons in the calipers.
  • The pistons press on the backing plate of the brake pads.
  • The brake rotors and pads start rubbing against each other, with the pad linings squeezing the surface of the rotors from the outside and inside.
  • The friction causes the vehicle to decelerate or come to a standstill.
  • As a large amount of heat is generated, the brake rotors and pads heat up to a high temperature.

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.

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Brake Rotors 101

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.


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When Should They Be Replaced?

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:

  • After pressing the brake pedal, the driver feels a vibration in the steering wheel and/or the brake pedal.
    • Cause: Pad Deposits. Brake pad material can collect on the face of the rotor and create high spots. Typically, this is the result of an overheated - or underheated - brake system. These deposits typically show themselves more in pedal feedback, as the caliper piston is pushed in and out of the caliper.
    • Choosing the right brake pad compound formulation for your vehicle application can help prevent pad deposits. That said, repeating the bed-in process can often remove the deposits; if not, turning or replacing the rotors may be necessary.
    • Note: While steering wheel shudder and/or feedback is possible, severe steering wheel vibration is typically a result of a failing suspension component, and not related to a brake component.
  • The brakes produce very loud noises when braking.
    • Cause: Corrosion, or worn out components. Heavily corroded brake components can affect the tight tolerances between them, creating dragging or grinding effects. Many brake pads are offered with a "mechanical wear sensor," which is a metal clip designed to rub against the rotor when the brake pad thickness becomes too low and create audible noise to alert the driver that their brakes need to be replaced. Though these systems are aimed at bringing attention to worn brake pads, they are also a useful reminder to check your rotors, too!
  • The brake rotor has developed surface cracks.
    • Cause: Excessive heat. It's important to understand that there are two types of rotor "cracks," both of which relate to the expansion and compression of the rotor as it cools and heats, but that represent different issues.
    • Heat Checking: As shown below, this condition produces small hairline fractures that develop on the friction surface of the rotor. When operated at high temperatures, such as in a racing scenario, this heat checking is completely normal and expected, and rotors displaying this are not necessarily deemed to need replacing.

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  • Crack to Edge: On the other hand, if a crack extends and reaches the outer or inner edge of the rotor - as in the picture below - the rotor has reached the end of its lifecycle and must be replaced immediately.
  • Note: The better the quality of the rotor, the longer the rotor will last before either types of cracks occur, but it is vital to know that heat checking is not a result of poor materials and, when operated at very high temperatures, neither are cracks.

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  • The brake rotor's working surface has become damaged or grooved.
    • Cause: Worn component or debris. Heavily worn brake pads can cause damage to the rotor if the metal backing plate comes in contact with the rotor. Additionally, rust, corrosion, and road debris such as stones can work in between the brake pad and the rotor, causing grooving in the rotor's friction surface.
  • The rotor has "warped": Contrary to popular belief, brake rotors very rarely warp. The physical and thermal energy required to bend - or "warp" - a rotor is astronomical, making this an incredibly rare occurrence. Instead, any unevenness is purely a result of pad deposits.

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.

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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.

Trust Your Instincts!

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!