What You Should Know about Disc Brake Rotors

All modern road bikes, gravel bikes, and mountain bikes rely on disk brakes. What happens if your disc brake rotors start to wear? The buyer's manual will help you choose the best rotors for your vehicle.

Most people think about components to make their bikes look better when they upgrade. There is nothing wrong with a nice set of wheels, or fancy stems. However, investing the same money in small parts will often result in a better ride.

The disc brake rotor is an area often ignored, particularly by road cyclists who are used to using rims. You probably already have them, as they came with your bike. This guide can help you choose the correct rotor when it's time to upgrade or replace them.

Heat is the main factor that limits rotor braking, just like disc braking pads. The rotors also convert speed to heat. And the quicker they can shed this heat, the more you will be able to scrub. The heat can be shed faster with the use of advanced materials, large rotors and surface treatments, but these come at a price. When buying a set of new rotors, you should consider the following factors: size, material and shape.

When choosing a rotor, it's important to consider the cost, durability, weight, performance, and longevity. If you're planning to use metallic brake pads, make sure your rotors are compatible with them.

Standard mounting for disc brakes

The hubs you have will determine the type of brake rotors that you require. The centerlock is the standard used by most hubs today. This standard makes it much easier to mount rotors than the 6-bolt. Centerlock rotors can be used with six-bolt hubs by purchasing an adaptor. However, this adds weight and complexity. Once your wheels have been set up, either standard will work. However, Centerlock makes it easier for those who are frequent travelers and want to remove their rotors on the fly.

To change the Centerlock rotors you will need to loosen and tighten the lockring with either or to change the Centerlock rotors you'll need a Bottom bracket tool or Cassette tool. You'll notice that lockrings for a bottom-bracket tool are large and have external splines. Those with internal splines will fit the cassette tool. Torx T-25 is required for the installation of six-bolt rotors.

Size of brake rotor

Nick Murdick explains that riders looking to increase their stopping power may consider larger brake rotors. Bigger rotors are a good option for heavier riders, heavier bikes or those who need extra stopping power on the descents.

A larger rotor comes with an increased weight and may require you to purchase new adapters that mount your brake calipers. Some frames do not allow you to change the rotor diameter. However, many road or gravel frames with flat-mount brakes have a reversible mounting system that lets riders switch between 160mm and 140mm rotors. Ask your mechanic or frame maker if you are unsure if your frame will accommodate bigger rotors.

Most riders stick to their standard rotor size and will look for improved materials in rotors and pads if they need a bit more power and heat management. Your rotors may not all be of the same size. As standard, many road bikes have a rear rotor of 140mm and a front rotor of 160mm. For greater stopping power, mountain bikes tend to have larger front brake rotors.

Material of brake rotors

You will likely encounter a lot of marketing-speak. Steel, aluminum or multiple metals can be used to make rotors. Combinations like these allow for weight reduction, increased performance and reduced warping. It's more costly to use multiple metals rather than stamping the rotor out of steel.

You might upgrade your brake rotor from an OEM one-piece steel rotor (like SRAM Centerline X, SwissStop Catalyst and Shimano Ice Tech) to a floating design, where the brake track will be held by a central carrier made of aluminum that is connected to the hub, like the spokes on a wheel. The lighter carrier material allows manufacturers to reduce weight, and the rigid aluminum carrier prevents rotor warping. These floating rotors cost more because of their more complex manufacturing process.

Shimano's Ice Tech Rotors are made with two steel layers sandwiched in between aluminum. Shimano can reduce weight and increase heat management by using a finned layer of aluminum. This increases the surface area, which allows for faster cooling. Shimano adds a heat-dissipating layer to its most expensive XTR rotors and Dura-Ace rotors. SRAM's high-end rotors use titanium bolts to reduce weight.

Only Resin Brake Rotors

Murdick claims that some cheaper rotors have been stamped with “resin-only” to save money. They “skip heat treatment at the factory to cut costs.” This makes the rotors softer and, “If you choose metal pads, they will make a horrible noise, which can lead to rotors lasting incredibly short.”

These rotors should be avoided unless budget is a major concern, even if they are heavier and not as well-suited to dissipate heat.

Different brake rotor designs

Some rotors have larger cutouts along the brake surface to allow dirt and mud to escape. Ultralight rotors may have larger cuts along the brake surface. They also have a lower rotational mass which makes it easier to spin your wheel. However, they can also wear out pads more quickly. Big brands such as Shimano and SwissStop produce rotors that are very lightweight (an XTR 140mm rotor weighs only 95g) with a braking surface designed to allow mud to easily be removed from the rotor.

UCI demands that the edge of the rotors isn't 90 degrees. Most riders won't have any issues with this, as there have been very few injuries in the years that pro-road racers have used disc brakes. However, both Shimano & SRAM offer a slight bevel on their top-quality road rotors.

SRAM's brake rotors are a little bit thicker than Shimano's, according to my experience. However, both brands seem to perform well when used with each other's calipers. You can also use packaging as a guide to space out your pads if you're having trouble getting enough distance between them for a thicker rotor. Place a business card on top of the rotor and insert the caliper. Then align your pads.

When should you replace a rotor?

The minimum thickness of rotors is usually written along the side. The measurements range from 1.7mm up to 1.5mm. Murdick says that it can be difficult to measure since the thinnest part will be at the center of the brake track. A micrometer is a great tool to use, but Vernier Calipers have a depth gauge that works as well.

Contrary to what your friends may have told you, it is not necessary to replace rotors due to contamination. This is good news, as high-end brake rotors can cost more than $100 per pair. Murdick claims that they can usually be cleaned using rubbing alcohol, clean paper towel and a lot of it.

He said that if a rotor is glazed it's best to replace it, but you can wear down the glazing and restore performance. The glazed surfaces can be removed quickly by soaking the brakes and repeating break-in procedures in clean, fresh water. The dust from the brake pad material acts as a polishing agent.

Can you true a bent rotor?

Yes, in a word: yes. You might not need to buy the costly tools sold as rotor-truing devices. Murdick says, however, that “it does require some skill, and it is possible to make the problem worse.” Murdick uses only his hands and a rag. He says that the biggest mistake made by new mechanics is to focus on the track and not the arms radiating from the centre. It's only wise to bend the entire brake track if it was the one that bent on the trail. Instead, use your brake pads as an accurate guide and gently push on the arms of the rotor. Some rotors cannot be repaired. If a section of the rotor has been bent or bulged, it is time to replace that rotor.

Installing new rotors and bedding in the bedding

Remember to properly bed in your new rotors. Murdick suggests that “burnishing” is a more accurate term. When a pad is bedded in, the material of the pad is embedded into the surface of the rotor and it conforms to the rotor's shape. This results in better braking.

When mounting a 6-bolt rotor make sure you tighten them diagonally, not around the circle. Manuals will list torque specs, which can sometimes be low. It is worth double-checking. As a general rule, Centerlock lockrings should be tightened as you would with a cassette. After your rotors have been mounted, realign the calipers. If you don't do this, your brake pads may rub on the new rotor.

After your new brake rotors are installed, seated, and oiled, you can enjoy months of improved braking without having to think about them again.