You know that your brake pads do not last forever, but did you know that the same is true of your rotors? Like the pads, brake rotors wear every time you step on the brake pedal. How quickly they wear depends on several factors, such as the type of brake pads on your car, environmental conditions, driving style, and the quality of the rotor itself. They probably do not need to be changed every time you get new brake pads – they might be resurfaced instead – but no matter what, your rotors will need to be replaced eventually.
What Is a Brake Rotor?
A brake rotor is a component of a disc brake system found on the majority of passenger vehicles today. The rotor is the “disc” in the system. To understand why a rotor might need to be resurfaced or replaced, it is helpful to know a bit about how the system works.
A brake rotor is a heavy metal disc (not a rock and roll album from the eighties, an actual metal disc) attached to each wheel. When you step on the brake pedal, a pump (the master cylinder) exerts hydraulic pressure through the brake fluid in a series of tubes. These tubes (brake lines) lead to clamping mechanisms called brake calipers that suspend a pair of brake pads along the sides of each rotor.
Force applied through the calipers causes the brake pads to squeeze the rotors. When the pads and rotors make contact, friction increases and causes heat that is used to slow and stop your vehicle (on the nerdy side, the kinetic energy of your vehicle-in-motion is converted to thermal energy that eventually reduces the motion to zero).
Picture this: You hold in your hand a slice of bologna (a disc) sandwiched between two slices of bread (pads). If that bologna disc was spinning and you squeezed the bread with your caliper (hand), the friction would cause the bologna to stop spinning. If there was a wheel attached to that bologna… oh, never mind! Forget that analogy.
You can probably get a better picture from observing the way a bicycle hand brake works, or how a person in a wheelchair might squeeze the wheels to slow down. That’s the idea.
Why Does a Brake Rotor Need To Be Serviced?
When you think of routine maintenance for your car, you might envision changing the oil and oil filter, maybe the engine air filter, possibly a tire rotation, even brake pad replacement. After all, brake pads are meant to wear down. But the rotors? Why do those need attention? Well, for a number of reasons.
Brake pads contain a sacrificial friction material that wears away with use. Rotors do not. But they still wear down over time. Back in the day (when heavy metal hair bands were a thing), most original equipment rotors were designed with enough thickness to last quite a long time. But today, in order to save weight for fuel economy, OEMs make rotors that are thinner and lighter. Consequently, those thinner rotors wear down to their minimum thickness specifications (in many cases) by the first time the brake pads need to be replaced.
Minimum brake rotor thickness is an important factor for safe braking. As your rotors wear and get thinner, they also lose mass – mass needed to absorb and dissipate heat. The strength of the rotor is also affected when it gets too thin and it could fail.
Rotors get too thin, not only because they are thinner, to begin with, but due to other factors. If you tend to “ride the brakes”, your brake system will receive undue abuse – excessive heat, premature pad wear, and rotor wear. The same goes if you brake aggressively with frequent sudden stops. Not only that but the type of brake pads you use, whether organic, ceramic or semi-metallic, also makes a difference in rotor wear and thickness.
So, a technician should measure the thickness of your rotors every time your car has brake service or whenever the brake system is inspected.
Besides wearing thinner, your brake rotors can also wear unevenly. You might be able to sense when one does. Your brake pedal will tend to pulse or vibrate when a rotor “warps”. Actually, a brake rotor does not warp, but it does develop areas of uneven wear that make it feel like it is warped.
In addition to measuring for minimum thickness, a technician will check for runout, a measurement of how true the rotor spins. Uneven wear and excessive runout can be caused by poor manufacturing or quality control, overtightening or uneven tightening of the lug nuts, and dirt or rust between the rotor and the wheel hub. Uneven wear can also be produced from hard spots in a rotor casting that wear differently than the surrounding metal.
Also, if your brake system is not working properly, your rotors can wear unevenly. For example, if the guide pins in a brake caliper are stuck and prevent equal pressure from being applied to both sides of a rotor, the rotor (and the pads) can wear unevenly.
Brake rotors need to be serviced because of environmental issues as well. Corrosion – rust – can destroy your rotors. It will weaken a rotor over time. It will eat away your brake pads in short order. It decreases your ability to stop. And rust will develop without you doing a thing when your car is parked for a long period of time. Therefore, it is important to keep maintaining your car even when it is sitting still.
If you live on a dirt road (especially one that is treated with calcium chloride) or you live in an area of the country where salt on the roads is common (in the snow or near the ocean, or both), the likelihood of corrosion developing on and eating up your rotors is higher.
While heat is essential to the braking process, excessive heat will cause your rotors to wear faster and unevenly. And if your rotors are unable to deal with excessive amounts of heat, they will transfer it to other brake system components. Excessive heat can even cause the brake fluid to boil and add unwanted vapor to the system, causing your brakes to become less effective.
When you brake, friction builds up between the brake pads and rotors. The heat is absorbed by the rotor and dissipated by slotted vents, or cooling fins, between the faces of the disc. Some rotors have holes in them to vent the heat, whereas others have no vents at all.
Excessive heat can be caused by your driving style. If you ride your brakes or brake aggressively, not only will you wear your rotors faster, but you will also cause them to overheat. Pulling a trailer or hauling a heavy load is also harder on your brakes and causes more heat to develop, as does driving in stop-and-go traffic or through hill country. Overheating leads to hot spots and glazing, the former causing your pedal to pulsate, the latter making it difficult to stop quickly.
Inspecting Your Rotors
Any time your brake pads are replaced, your rotors should be inspected for overall condition, minimum thickness, runout, and signs of glazing or cracking. An inspection will reveal whether your rotors are in need of resurfacing or replacement – or (in some cases) nothing at all.
Some repair shops offer a courtesy inspection when you schedule an oil change service. Unfortunately, with the emergence of extended oil changes on late-model vehicles, inspection of other systems may be delayed, including the brake system. And things can go wrong with your brakes between oil changes.
When you are having your brakes inspected or repaired, make sure to enlist a qualified technician at a trusted repair shop. A complete brake job should consist of more than simply slapping on a set of pads, but a lot of cheap brake services do exactly that. Most likely, your brake rotors will require resurfacing or replacing roughly every other brake job.
Resurfacing Your Rotors
When a brake rotor wears unevenly or grooves are worn into it, or corrosion develops on its surface, one remedy is to have it resurfaced. Resurfacing is the process of machining away a small amount off of the faces of a rotor. This is done on a special machine and creates a new face that is smooth and spins true.
Resurfacing can only be done if there is sufficient material left on the rotor to allow it to maintain the manufacturer’s specification for minimum thickness. For that reason, resurfacing is less common today than it once was. Late-model vehicles often feature thinner, lighter rotors that simply do not have enough metal to machine, at least not very many times. Some manufacturers do not recommend resurfacing at all.
If a rotor has enough material present – and if the manufacturer recommends that it be done – the rotor can be resurfaced.
If a rotor is worn down to the minimum thickness specification, or it cannot be resurfaced without doing so, or if the manufacturer says it needs to be replaced, then it must be replaced.
Fortunately, the cost of rotors has come down in recent years, making replacement a less painful process. In fact, replacing some rotors with aftermarket alternatives costs only a little more than resurfacing them. For cost-conscious drivers, this is a plus. Others might prefer to err on the side of safety and longevity and choose original equipment rotors or higher-end aftermarket ones.
Another advantage of new rotors is that they are essentially ready to install right out of the box. No need for the added time of machining each one.
Most industry professionals will recommend replacing your rotors somewhere between 30-70K miles in any case. So, have a conversation with the folks at a trusted repair shop about the advantages of resurfacing or replacing your rotors – and deciding what is right for your car.
Columbia Auto Care & Car Wash | Author: Mike Ales | Copyright
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