Brake rotors are big circular discs with hubs that are sandwiched between the wheel and the wheel hub. The outer part of the disk is machined on both sides so that it spins between the disc pads that are mounted in the caliper.
When brake fluid pressure is applied to the caliper, the brake pads pinch the machined surfaces. Since lug studs that pass from the hub through the wheel also pass through the center of the rotor, pinching the rotor slows or stops that wheel.
Rotors are in a hostile environment and do a lot of hard work, and over time, they get thinner. That’s just the nature of the rotor.
Continuing to use a worn brake rotor can affect your safety. That begs the question, what are the signs of bad brake rotors? How to tell if your vehicle’s brake rotors are bad?
You can’t always tell the condition of the rotors by simply looking at the rotors. Rotor thickness is measured using a micrometer before and after machining. A rotor can look just fine and still be too thin to machine or reuse, so don’t expect to be able to just look at a rotor and determine whether it’s good or not. Even if a rotor has grooves, it may still be thick enough to machine and reuse.
Having said that, here are indicators of brake rotor problems.
It’s normal for brake rotors to have visible grooves or scratches on the machined surface of the rotor where the pads make contact. However, these scratches will gradually become deeper after prolonged use. Faint scratches are probably not going to be concerning. However, if the scratches are deep or the rotor has cracks, it’s time for measurement, machining, and usually a replacement.
First of all, note that rotors cracking is very rare. But in case it happens, you’ll likely feel or hear the cracked rotors before you see them. Cracked rotors typically make loud banging noises or pulsate on the brake pedal. If your brake rotor has a crack, then it’s considered destroyed and you should replace it immediately. Continuing to drive with a cracked rotor can compromise your safety because the rotor can disintegrate and cause brake failure.
If you frequently peek at your brake rotors, you might notice that they can rust. Brake rotors are typically made from cast iron or steel, which can rust quite easily when exposed to moisture or salt. This shouldn’t be concerning because the friction from the brake pads should clean them off during use.
In some cases, a vehicle may be parked right after driving through water so that some water is trapped between the pad and the rotor and it can make a rusty patch in that one spot, which will cause a shuddering feeling when applying the brake.
You’ll see the rusty pad-shaped patch on the rotor in these cases. Sometimes, you can remove the wheel and use some 80 grit sandpaper to smooth this patch. Usually the rotor needs machining or replacing if this happens.
Brake rotors can turn blue when they overheat. This can happen if the brakes were excessively used during a mountain descent.
However, some high-performance brake rotors are perfectly usable even if they turn blue. Some rotors are designed to handle high temperatures. Hence, high heat exposure won’t cause these rotors to warp.
The point is that you don’t need to replace the rotors just because they’ve changed colors unless you notice brake pulsation while stopping.
However, most vehicle rotors will warp when they turn blue. Warped rotors are considered worn and can be a safety issue since they’ll cause issues such as excess vibrations while braking, which is unsafe. Sometimes, blue rotors can even be caused by overheating due to improper installation.
You don’t need to replace the brake rotors just because they’ve changed colors unless you notice brake pulsation while stopping.
– Richard McCuistian, ASE Certified Master Automobile Technician
Just like the brake pads, the brake rotors will also lose material over time. Eventually, the friction will cause the brake rotors to become too thin.
This happens normally over time even if the pads are replaced before the lining is gone. But some shops will machine rotors without measuring rotor thickness and if the rotors become too thin and the pad lining wears, in some cases, the pad backing may be ejected during a normal stop.
You’ll need to measure your brake rotor’s thinnest spot using a micrometer. Brake rotor micrometers have a pointed contact so you can measure the rotor where it’s thinnest at the bottom of a groove. Most rotors have a minimum operating thickness. If the brake rotor has worn past this thickness, then you need new rotors.
Pulsating or vibrating brakes are the most common warped brake rotor symptoms. Pulsating brakes mean the brake rotors don’t have a flat contact surface with the brake pads. Instead, the rotor surface can have small curves and indents which can cause the brake pad and caliper to vibrate. Sometimes the uneven surfaces will vibrate the pads and caliper so much that you’ll feel the vibrations on the brake pedals and even your steering wheel.
Squealing or screeching noises happen because the brake pads rub against an uneven brake rotor. This means some portions of the rotor have less metal, while some parts have more. When there are a lot of uneven surfaces on a brake rotor, the contact patch between the rotor and the pad becomes smaller, causing a screeching noise.
This is the most obvious symptom of an old and worn-out brake rotor. You might notice this when stopping at an intersection and you notice that the brakes might require more input before slowing down your vehicle. Worn-out brake rotors cause reduced performance because they can become uneven, resulting in less contact and therefore less friction with the brake pad.
If you think your rotors are bad, ask a mechanic to inspect your brakes immediately. Brake rotors play a significant role in your vehicle’s braking system and driving with a faulty rotor is unsafe. Driving at higher speeds is going to be risky because you’ll have reduced stopping power. Plus, there’s a chance your vehicle might have brake failure.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.