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3 Brake Issues That Cause Disc Thickness Variation

Your vehicle’s brake system has operated flawlessly for tens of thousands of miles. However, you’re now noticing that whenever you apply your brakes, you experience brake judder—a steady vibration throughout your steering wheel and seat. Brake judder is caused by disc thickness variation, which means the surface of one or more of your rotors is uneven. Inspect your brake system for these three issues to determine the cause of your vehicle’s disc thickness variation:

Cementite Spots

Cementite is a compound that forms when cast iron overheats and experiences a molecular change. During normal brake usage, your rotors typically won’t reach temperatures that are capable of causing cementite formations along your rotors. However, slamming on your brake pedal when you get cut off on the freeway, riding your brake pedal while descending a hill, and other extreme braking habits can cause your rotors to overheat and form cementite spots.

Cementite spots are problematic because they’re significantly harder at a molecular level than cast iron. For this reason, cementite spots on your rotors won’t wear at the same rate as the rest of your rotors’ surfaces. As the cast iron surface of your rotor wears away during normal brake usage, the cementite patches will remain in place and create ridges on your rotors’ surfaces.

Luckily, it’s very easy to identify the presence of cementite on your rotors since it’s darker than cast iron. With your vehicle raised on jack stands, remove each of your tires and inspect the interior and exterior surfaces of your rotors for dark patches of cementite. If you see any cementite, then you’ll need to either resurface or replace your rotors.

Improper Pad Bedding

Every time you install new rotors or brake pads, your brakes need to be bedded. Bedding your brakes requires you to repeatedly accelerate to freeway speeds and lightly press on your brake pedal until your vehicle slows to about half of your original speed. This process allows your brake pads to transfer a thin layers of film onto your rotors that protect your rotor surfaces from unnecessary and uneven wear.

If you changed your brake pads or rotors and failed to properly bed your brakes, then both your rotors and pads have sustained uneven wear each time you’ve pressed your brake pedal. Unfortunately, if your disc thickness variation was caused by improper bedding, then you’ll need to resurface your rotors using abrasive brake pads or a brake lathe. Once the surfaces of your rotors are ready for use, you’ll need to replace your pads.

Excessive Run-Out

Every time you apply your brakes, your pads and rotors grind together and thin layers of pad material transfer to the surfaces of your rotors—a process known as adhesion. Additionally, some of the material from both your pad and rotor grind away and turn into brake dust—a process known as abrasion.

Not all brake pads rely on an equal amount of adhesion and abrasion. Some brake pads are designed to stop your vehicle primarily through adhesion, and other pads stop your vehicle primarily through abrasion.

Excessive run-out can occur when the surface of your rotors weren’t completely clean when your brake pads were installed and bedded. For this reason, run-out is more likely to occur with resurfaced rotors than brand new rotors.

When excessive run-out occurs as a result of adhesion, brake dust will collect on the dirty or elevated surfaces of your rotors. When excessive run-out occurs as a result of abrasion, the dirty or elevated surfaces of your rotors will wear away and turn into grooves. Both of these issues result in disc thickness variation by either elevating or removing a section of your rotors’ surfaces.

If you’re experiencing brake judder as a result of any of these issues, then hire a professional mechanic to perform the necessary brake repairs. In most cases, your pads and rotors will need to be replaced. However, by replacing your brake components, you’ll gain the opportunity to properly bed and maintain them from the very first mile.

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