On almost all modern cars, whenever you replace your brake pads, you should also replace your brake rotors. This is because brake rotors, as they are used, develop small grooves where the pads run along them and develop microcracks from heat and environmental conditions. The grooves make it so that a new pad will not seat correctly on a worn rotor. Because of heat cycling, older rotors are more prone to expand inconsistently, causing warping, and a pulsating brake pedal. Below we highlight a few key characteristics of brake rotors and show you what to look for when buying.

MATERIALS

In general, most brake rotors are made of some form of cast iron. Exact metal content varies based on several factors—where the rotor is made, manufacturing process, company—and information about what exactly your rotor is made of isn’t readily available. You’d have to take each rotor to a lab to get analyzed for that. Other materials (like ceramic) are available for extremely high-performing automobiles, such as higher-end race cars, but these are not available for most mainstream, daily driven vehicles. In general, cast iron rotors are durable, cheap, and perform well—i.e., they have a reasonably high friction coefficient and dissipate heat at an effective rate. As most diesel owners are interested in fuel economy more than high performance, cast iron rotors work nicely. Many quality rotors are advertised as having a high carbon content. Their highlighted feature is that they produce less noise than regular rotors. All sorts of other advantages have been claimed, such as better heat dissipation, higher friction, improved damping, etc., but the main purpose of adding more carbon was to reduce the noise, and it’s unclear whether adding carbon actually does all these other things.

COATINGS

Many rotors nowadays are coated with a rust-resistant material. Most manufacturers that offer coated rotors state the reasons for doing so as such:

• Resistance to the formation of rust where the rotor meets the hub.
• Coatings that do not have to be cleaned off before installation, that have immediate functional capability, unlike older oil based coatings that need to be cleaned off before bedding of pads.
• Constantly widened production range.
• Resistance of rust build-up on the outer edge of the rotor.
• Nicer appearance when installed.

NANO COATING

A new offering from rotor supplier Fremax is Maxcoating: rotors coated with paint that contains exclusive, patented nanotechnology. These rotors receive a protective coating containing nanoparticles that claim to increase corrosion resistance. These characteristics, associated with a water-based formulation, enable the application of thinner layers, increasing the life of the coating. Its anti-corrosion characteristic is good for brake discs and drums used in extreme weather conditions, such as regions with heavy snow where the application of salt on the roads accelerates the oxidation of car components. The company says that laboratory tests show the technology confirms approximately 1,000 hours of protection, which can double the life of the brake part.

VENTS

Many modern cars have vented rotors, generally in the front, but some even have all four rotors this way. This means that the rotors have an air void, or space between the two halves of the rotors. The vents you can see in the picture are between the two braking surfaces and allow heat to escape through the middle of the rotor. This helps maintain reasonable brake temperatures, even when pushed to the limits. 

SLOTTED & DRILLED

Some manufacturers offer “high-performance” drilled or slotted rotors, and some even offer rotors with both features. It is debatable all across the internet whether these actually provide any real benefit in daily driving. Some people seem to love them, and others don’t think that slotted or drilled rotors are necessary or effective. Adding to this point, it even says on the boxes of most slotted or drilled rotors that they’re NOT to be used for racing (rotor might not be durable enough), so caveat emptor. They do look nice, though.

SLOTTED

Slotted rotors have slots or grooves cut into their faces. This can help keep the rotors cool by giving hot gases a path to escape when under high pressure from a brake pad. The slots can also help to clear debris off the face of the rotor—a moderate performance upgrade, although they do have some drawbacks. Slotted rotors will wear pads faster due to the slicing nature of the slots against the pads. They can also be noisier, some refer to the sound being similar to a dull swarm of bees. You also might notice some pedal flutter when braking from high speeds.

DRILLED

Drilled rotors have holes drilled into the braking surface, through the rotor. The holes reduce the weight of the rotor and help dissipate heat. Originally, drilled rotors were also intended to solve another specific design problem in race cars. During intense braking, certain materials in the pad caused a gas to be created that acted as a lubricant on the rotor, essentially creating an invisible layer between the pad and rotor. The holes allowed the gas to escape down into the rotor, and out the vents at the edge of the rotors. Nowadays, however, pad designs have improved and off gassing is minimized with most current pad compounds. Drilled rotors are not without their disadvantages though. As one would expect, a rotor with holes drilled into it isn’t as durable as one without. Because of the holes that are drilled into the rotors, stress cracks can start around the holes, leading to uneven surfaces. Also, water can get into the cracks, and expand with heat, causing warping and shortened life. Sometimes these rotors crack and last a shorter time period than normal rotors, and like slotted rotors, they wear out pads more quickly.

SLOTTED & DRILLED

You guessed it: these are rotors that are both slotted and drilled. Someone investing in these rotors is likely going for a specific look, or would be looking for a different performance improvement in their braking. Just note though, that these carry all of the benefits, and downfalls of the slotted or drilled rotors on their own. These are honestly usually overkill for most drivers, but are an option.

BREAKING IN YOUR BRAKES

Replacing your brakes can be a relatively easy job, done by most DIY mechanics using simple tools. One thing that is often missed though is the break-in of the pads and rotors. Once you’ve replaced your brake rotors with a new pair, it’s important to properly break them in. Breaking in your pads/rotors helps clean off any coatings that may have been applied to the rotors, and helps to transfer a thin film of resin from the pads to the rotor. If you drive erratically or brake heavily immediately after putting a new set of brakes on your car, it can cause the friction material to transfer unevenly, and this in turn will cause uneven braking and judder. Correctly braking in your brakes will also help to perfectly match any imperfections in the pads and rotors, essentially mating them together for the best available contact. With most common brake pads, all you need to do is drive the car normally, without any panic stops or excessive braking for a couple hundred miles and the pads will bed-in correctly. For high-performance pads and rotors, the process usually involves slowing down from gradually increasing speeds and letting the brakes cool in between. In the pads you purchase come with instructions for bedding the pads, follow those procedures. If not instructions come with the pads or rotors, just break in normally as posted above. 

ROTOR ISSUES

WARPING

Rotors are machined to be almost perfectly flat, to give smooth and predictable performance. Rotor warping is when the surface of the rotor has changed shape and begun to affect the braking efficiency and smoothness. This will often result in a pulsating sensation when applying the brakes. Under normal conditions, warping of a rotor is not common. Rotors will not typically warp on their own and need some sort of outside influence to do so. Warping can happen for a couple reasons. The most common reasons for rotors to warp are heat, pad deposits, incorrect installation, or previously damaged parts. Drilled rotors and older rotors are more prone to warping as microcracks appear in the rotor. Water can get into these and cause the face of the rotor to swell in certain areas. Sometimes, the rotor has been installed onto the wheel incorrectly, and that can cause it to warp. Also, if a car has a warped hub or accident damage, it can cause a rotor to warp in a short time. More often, a process occurs where under intense heat friction material from the pad deposits itself onto the rotor and makes its surface uneven. To say that the rotor is warped, in this case, is a bit of a misnomer, because the rotor itself hasn’t warped due to strain or anything like that, but rather material has been added to the rotor. Aside from extremely intense braking situations, such as those experienced while racing, you shouldn’t experience warping of a rotor in day-to-day driving.

RUST

One of the biggest killers of rotors in cars, especially those in coastal areas or the salt belt, is rust. The surface that the pad runs against on the rotor will always be exposed to the elements. In daily driving, the pads will consistently sweep this area, keeping it pretty clean and rust-free, yet cars that are not driven often enough will likely see rust start to build up on the rotor surface. Coated rotors can help in the areas where the pad does not touch, such as the outer edges and the hub face, but the business part of the rotor will always be raw and exposed to the environment. If rotors are allowed to sit with rust on them, pitting can develop, which will likely lead to excessive noise and warping. Usually though, if a rotor only has light rust build-up, simply driving the car will allow the brake pads to “clean” the surface of the rotor.