warped rotor

Why Do Brake Rotors Warp?

Disc brake rotors are solid and sturdy parts. So if you think you have warped rotors, it can be pretty confusing. How on earth does that happen?

Warped rotor

The thing is, rotors don’t actually permanently warp. It may feel that way when braking, but what really happens is that they become irregular due to excessive lateral run out of the rotor face. You can feel through the brake pedal as little as .002” of run out. This amount of run out is far smaller than you can see. In other words, rotors always remain true, but they can wear out irregularly for several reasons: 

  1. In many shops, wheels are installed using an air-wrench/gun and not hand torqued in a star patterned sequence where all lug nuts are torqued uniformly to spec. This creates stress in the rotor that shows up as the rotor heats up. The stresses prevent uniform expansion of the rotor, which can be felt in the brake pedal.
  2. Shops sometimes install rotors without cleaning rust from the hub. This causes a small amount of lateral run out. This may not be felt initially, but it will result in hot spots on the rotor. Over time, this will cause irregular wear that will eventually be felt. 
  3. Before installing rotors, the hub should be checked for run out. Most OEM’s spec hub run out at less than .000”. Any run out in the hub will be magnified by the rotor due to the larger diameter. Possible causes of run out in the hub are bearing wear, or improperly pressed in bearings. 

The Most Common Causes Of Irregular Rotor Wear

There are a few causes of irregular rotor wear. Let’s talk about the 4 most common ones:

1. Extreme Heat

You may be saying, “Hold up. Didn’t you just say that it’s impossible for rotors to get hot enough to permanently warp?”

While it can’t get hot enough for the entire rotor to permanently warp, it can get hot enough for the brake pads to wear down the rotors. Under normal circumstances, the rotor metal is harder than the brake pad material. So rotors shouldn’t wear down from contact with the brake pads. There are times when the rotor can get very hot, though. When that happens, the metal becomes soft enough for the brake pads to wear it down.

When a rotor is manufactured, it naturally has areas of varying density. This is due to variations in base metal that occur during the casting process. So when the brake pads wear it down, spots that are a bit less dense will wear down faster. That also means that the harder parts stick out. As a result, the rotor feels warped when braking. In most cases you won’t be able to see irregular wear. You have to measure the rotor to detect it. However, in this case, you may be able to see the problem. The high spots will get hotter than the rest of the rotor. A bluish tint may be left around the high spots. We say “may” because you can’t always see this.  

2. Glaze

The friction material of brake pads are designed to withstand a lot of heat. Yet, there are situations (such as high speed driving or riding the brakes) where the friction material can get too hot. When that happens, the friction material loosens up and coats the rotors. This results in a soft mirror-like glaze on the surface of the brake rotors. This reduces the stopping power of the brake system.

This can happen from hard driving, and then coming to a stop at light with your foot on the brake. In this case, glaze is deposited unevenly across the rotor face. This makes the rotors feel warped.

3. Pad/Caliper Misalignment

When the brake pads and/or calipers aren’t aligned right, you’ll see uneven wear on the rotors. For example, if one side of the caliper is sticking, you’ll see more wear on one side of the rotor. This usually causes an overheating situation that can result in glaze on one side of the rotor.  

4. Physical Damage To The Rotor

Rotors can get gouged. Most commonly, it’s because the brake pads are too worn down. When you don’t replace the pads on time, the metal backing will damage the rotors. If it gets bad enough, you’ll see excessive wear that can look like warping.

Two Ways To Fix The Issue

Resurface rotor
Image Credit: Auto Fanatic

It’s dangerous to drive with “warped” rotors. This is because they compromise your vehicle’s ability to stop. If you have out-of-spec rotors, fix the issue as soon as possible. There are two ways to fix an out-of-spec rotor:

  1. Replace the rotor with a new one.
  2. Resurface the rotor (aka machine turning). Resurfacing a rotor makes it smooth enough to operate within proper tolerances.

The exception is with high performance cars. In that case, don’t refurbish your rotors. A complete replacement is the best course.

The Drawbacks Of Resurfacing Rotors

If you ask us, replacing the rotors is almost always the better solution. Resurfacing rotors may be sometimes cheaper than purchasing new rotors. But is it really saving you money? Not always. Resurfacing your rotors has several issues. For example:

  • The rotors may not be thick enough. Resurfacing can only be done if the rotor has enough material remaining to meet its minimum thickness spec after turning.
  • Resurfacing removes material from the rotor, which shortens its life. Some rotors may not last very long after being resurfaced. If that’s the case, you’ll get more value out of brand new rotors.
  • Resurfacing the rotor removes material, which reduces the rotor’s ability to absorb heat. This may be ok for a gently driven passenger car, but compromises the brakes of a performance car. 

If in doubt, replace the rotors. As long as you get a set of quality rotors, you’ll get good value out of them.

High Performance Driving

High performance driving can cause a unique issue that feels like warped rotors. As brake rotors absorb heat and temperatures go beyond 900°F, cast-iron 1-piece brake rotors actually begin to try and grow up to 1mm in diameter, but they can’t. They’re not allowed to since they are of a “fixed 1-piece rotor” design. A one piece rotor cannot expand in diameter, and they instead temporarily distort laterally (left and right).  

That’s when and why you feel your brake pedal pushing back at your feet. The brake rotors are absorbing more heat than handle. A two-piece brake rotor upgrade fixes that problem. The design of a two-piece rotor allows for the disk to expand in diameter.

Finding Quality Rotors For Your Vehicle

New rotor

At BuyBrakes.com, we carry a large selection of high quality rotors. You would find many different types of rotors for all kinds of makes and models. For example, you’ll find the following rotors on our site:

  • Smooth-plain
  • Slotted
  • Cross-drilled & slotted
  • Cross-drilled
  • Dimpled & slotted
  • 2-piece rotors
  • High carbon
  • Cryogenic frozen

We carry rotors from many different brands, as well. If you have warped rotors, look no further than BuyBrakes.com! Here, you’ll get brand new rotors that will serve your vehicle well for a long time!