Categories: Replacement Parts

Why Do Brake Rotors Warp?

Brake Rotors Warp – Introduction

Brake rotors are the large metal discs that can be seen inside the wheels of cars and trucks. They rotate along with the wheels inside of a stationary brake caliper that contains brake pads.  The caliper contains hydraulic pistons that cause brake pads to clamp down on the rotors which causes friction slowing down the car.   This friction generates tremendous amounts of heat which the Brake discs and pads have to withstand.  They also have to dissipate that heat into the air and cool down as quickly as possible to prevent overheating with subsequent brake presses.  The surface of the disc can wear unevenly over time, meaning braking will become jittery and less effective. This is referred to as warping.

How Brake Rotors Warp

A common misconception when rotors are referred to as “warped” is that they are no longer straight when rotating.  This rarely happens in cars.  Due to the design of the rotors, it is almost impossible for them to actually bend to a noticeable degree.  They would have to be defective as the temperature required to make metal soft enough to simply bend would be very high.

Warping actually refers to the surface of the rotor wearing unevenly. Heat is the number one cause of this, and can cause warping in multiple ways:

  • Glazing the brake rotor: This happens because brake pads, like tires, are made with different materials depending on the intended purpose. When standard brake pads get very hot from high-speed driving and braking, or from pressing on brakes for a prolonged period of time, the material can actually begin to get too soft and basically “paint” the brake rotors. This causes severely decreased brake performance due to the brake pads inability to generate as much friction against the rotors.
  • Overheating the Rotor: Brakes normally don’t wear down very much due to the fact that the metal of the rotor is harder than the brake pad applying friction to it.  The pad wears down while the rotor remains largely unaffected. With excessive heat however, the metal can become soft enough for the pad to wear down the surface of the rotor. This creates slightly less dense spots in the metal wear that down faster and while some more dense spots wear less, causing warping.

Preventing Warped Brake Rotors

To prevent the brake rotors from becoming glazed over, be careful not to do much heavy braking with the vehicle relative to what is done during normal operation. When going down a steep grade for prolonged amounts of time, try to control the speed of the vehicle by shifting the transmission into a lower gear or doing periodic heavy braking with periods of relief to allow the rotors to cool. For automatic cars, try shifting into “L” gear which will put your vehicle in a lower gear.  Vehicles with a manual or other shiftable transmission can decide on the best gear to use based on the grade, speed, and RPM’s. When the brakes are hot, be wary of continuing driving with the brake pedal hard down on one spot.

When the brake pads are first installed, some manufacturers recommend that they be broken in to ensure they don’t leave too much material on the brake rotor. Methods vary by manufacturer, but usually this involves getting the car up to road speed and then heavily braking until it is traveling almost stopped. Repeat a few times working your way up to braking to a complete stop.  Always exercise caution with new brake pads and break-in procedures especially right afterwards. These procedures allow the brake pad to perform better during hard braking further down the road.

The steps that can be taken to prevent excess wear on the surface of the brake rotor are similar to the steps for avoiding glazed rotors. Be sure to avoid hard braking when the brake rotors have experienced prolonged use and may be close to overheating.

What Do Warped Rotors Feel Like?

There are a few common symptoms to look for when diagnosing warped rotors:

  • If the steering wheel vibrates only when coming to a stop, the brake rotor is likely warped.  Other times, vibrations may be due to improperly balanced wheels or alignment issues.
  • If the brake rotors are glazed over, you may hear some squeaking or notice that the brakes require excessive pressure to achieve the same stoppage.
  • If braking becomes inconsistent, jittery, or squishy, the brake rotors should be inspected along with the rest of the brake system.
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