Brake: Brake Rotor Thickness Variation and “Ghosting”

What it Does

Brake Rotor “ghosting” occurs when a brake pad deposits an excessive amount of friction material onto the rotor surface. This will often leave a ghosted imprint of the pads shape on the rotor surface.

Why This Matters

Uneven deposits of friction material on the rotor surface cause rotor thickness variation. Rotor thickness variation then causes vibration under braking. This vibration has an impact on ride quality, and it may also reduce the lifespan of steering tie rod joints. (These joints are exposed to the vibration from the brake assemblies and transmit it up to the steering wheel.)

Possible Problems

Poor pad quality or wrong friction formulation:

Brake pads on domestic and Asian vehicles are typically “adherent” type. This refers to brake pads that are designed to give up a small amount of friction material that coats the rotor friction surfaces. This material transfer is encouraged by heat. Low quality pads or pads that run in the wrong heat range can run too hot against the rotor, and that heat will encourage excessive transfer of material. This material transfer often occurs after the vehicle has come to a stop, when the brakes are hot, and pressure is being held on the brakes to keep the vehicle from moving forward.

Driving habits or conditions:

The more that driving habits or conditions encourage brake heat, the more likely it is that rotor ghosting or thickness variation are going to occur. For example, if two identical vehicles are stopped from 50mph, but one coasts much of the way, and the other comes to a rapid, aggressive stop, the second vehicle will have more heat in the brakes. Now, as both vehicles set still with pressure held on the brakes (which means the pads are still in contact with the rotors), the first vehicle is much less prone to material “baking” onto the rotor surface because there is less heat to encourage such behavior.

Mechanical Faults:

Concerns such as seized caliper slider pins, sticking caliper pistons,and binding brake pads can also cause brake assemblies to run too hot. These conditions cause the pads to rub against the rotors even when brake application is not desired, and the resulting friction generates heat, which can encourage excessive transfer of pad material.

Notes:

  1. We commonly observe rotor ghosting on vehicles that do not have perceptible vibration under braking. This is not surprising because for the vibration to be obvious while driving, it has to grow to the point that it is felt (this could be in the brake pedal, the steering wheel, or through the body of the vehicle.) When the vibration is in an early stage, much of it may be “absorbed” as the caliper moves back and forth on its slider pins.
  2. Rotor thickness variation can cause vibration without visible ghosting being present.

Recommended Parts Brands

Brake Pads:

  • OE grade or comparable
  • Akebono

Brake Rotors

  • OE grade or comparable
  • Brembo
  • Centric
  • (Installed runout should be kept as close to zero as possible.)

Repair Notes

Runout measurements should be taken during rotor installation. What commonly happens is that the rotor can be installed in a position that minimizes install runout, which lowers the risk of vibration developing due to thickness variation.