What Happens if You Put New Pads on Bad Rotors?by Chris Weis
Brake rotors need to be machined to correct distortion or damage to the braking surfaces. Restricted access to this service or field expediency may force the installation of new pads on bad rotors. Short-lived mobility may allow the avoidance of tow charges and permit the vehicle to reach a suitable repair location. This temporary repair method is not recommended. No reputable repair facility would engage in such activity, and many municipalities enforce legislation that prohibits ineffective or dangerous workmanship on brake systems.
Brake friction material is held to the pad backing plate in one of two ways. One method bonds the two substances with a strong adhesive, while the other style employs rivets to join the pieces. Either pad construction can severely damage the rotor if the friction material is worn away. The backing plates of bonded pads might contact the rotor somewhat evenly as material wears away. Riveted pads begin scoring grooves into the rotor as soon as friction surfaces wear down to the rivets.
In the Groove
The deep grooves ground out of a rotor by pad rivets almost always ruin the part beyond repair. Similar marring from bonded pads render the same result. New pads only contact the highest surfaces of the rotors, greatly reducing stopping power. As new pads attempt to mate to the irregular surface, rough edges fracture the friction material. Material is quickly removed by the spinning rotor, much like the way a cheese grater cuts through a block of cheddar.
Whatever damage occurs to a rotor, a side effect arises. Brakes generate and dissipate huge heat loads in normal operation. Malfunctioning parts or abusive driving habits can exaggerate the heat load beyond tolerances. The excess heat created by too much friction changes the metal of the rotor. Surfaces take on a blue color and do not respond to normal machining methods. Rotors exhibiting "hot spots" must be discarded.
The braking surfaces of rotors are made in specific thicknesses. The needed value is determined by vehicle weight and horsepower as well as composition of materials used and construction design. Minimum thickness for any particular brake rotor usually is exceeded by all but the slightest marring. Metal is removed from the surfaces in the machining process, and no responsible technician will attempt to true a rotor too near the thickness limits for safe usage.
- Automotive Suspension Steering Alignment and Brakes; Walter E. Billiet and Walter Alley
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