When a brake pedal pulses beneath your foot, no doubt your first thought is "warped brake rotors." Your next thought might
be about how it happened, because if you're going to get paid for the repair, you have to make sure it doesn't happen again.
Fortunately, most of the things that cause pedal pulsation happen in the service bay. With close attention to detail, it's
easy to avoid those particular comebacks.
The stain doesn't wash off, indicating it's a heavier concentration of brake pad friction material imbedded in the metal.
This is common and rarely enough to feel in the pedal.
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The term 'warped brake rotor' is used pretty loosely to define any brake rotor that isn't absolutely flat and parallel with
the plane of rotation. But there are other things that can cause pedal pulsation. There are also ways rotors can warp that
don't produce pedal pulsation but do affect braking. Understanding these things is the only way to guard against them or deal
with them successfully if they do come back.
Rotor run-out
The bronze tint is friction material imbedded in the rotor.
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If you know how a disc brake works, it's not hard to understand why you can sometimes feel warped rotors in the brake pedal.
If the rotor is shaped like a potato chip, the high spots will push back against the caliper pistons as they rotate between
the brake pads. That push is transmitted directly to the pedal, and if steering or suspension bushings are worn, it might
even cause a slight steering wheel shimmy.
Several things can cause a rotor to warp that way. Most of us are aware that uneven heating or cooling is one culprit. A frequently
cited example is driving through a deep puddle on a dry day with hot brake rotors. Another example is parking the vehicle
without a "cool-down lap" after very heavy brake use, such as road racing or towing in hilly terrain. In that scenario, so
much heat has soaked into the pads and calipers that it keeps one section of the now stationary rotor hot while the rest of
it cools. Both of these scenarios will warp a brake rotor, but in most parts of the country, they don't happen very often.
With fixed calipers, a seized piston can impart bending forces on the rotor. Eventually, the rotor might distort and cause
uneven pad wear.
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By far, the single most common cause of warped brake rotors is improper installation of the wheel.
There was a time that this was not so, but modern brake rotors are made thinner to reduce a vehicle's unsprung weight. They're
not just thinner at the friction surface, there's less material everywhere, particularly in the hat section. This makes the
rotor more susceptible to warping due to a combination of heat and improper lug nut torque.
Manufacturers specify lug nut torque for the same reason they specify cylinder head bolt torque: to control distortion of
the parts being bolted together. Most front-wheel-drive cars have one-piece cast iron brake rotors shaped like a top hat.
The hat section of the rotor fits over the wheel studs and is clamped between the hub flange and road wheel. When the rotor
is mounted onto the hub, the hat section does not fit flat against the hub flange; there's a slight gap either toward the
center of the hub or toward the outer diameter of the hub flange. That gap is less than two-thousandths of an inch, and on
most cars, it's supposed to be there.
