Rick's Free Auto Repair Advice

Brake rotor finish

Brake rotor finish

How smooth does the brake rotor finish have to be?

Brake Rotor Roughness Average

Brake rotor surface “smoothness” is rated by its Roughness Average (RA). The RA should be below 80. The proper RA is critical to achieving the proper transfer layer for adherent style brake organic and ceramic pads as well as abrasive semi-metallic brake pads.

If the surface is too smooth, it’s impossible to form a transfer layer onto the rotor face. If it’s too rough, the brakes will make noise, wear faster and even lose stopping power.

rotor surface smoothness
Non-directional finish

A non-directional finish is also critical to proper bedding and noise-free operation. When a brake rotor is machined on a lathe, the cutting tool can leave spiral “record grooves” if the machining is done too quickly. If left that way, the grooves wear off too much friction material.

A properly machined rotor should have a non-directional finish applied after using the cutting tool.

©, 2022 Rick Muscoplat

Posted on by Rick Muscoplat


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