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Many of us have been there before - we apply the brakes to slow down our car and suddenly, the steering wheel shakes, the car lurches and vibrates, and the car comes to a shuddering stop while you wonder what was wrong with your brakes.

Traditionally, this was blamed on warped rotors, and the prescription was to either get the rotors machined or to replace the rotors. Technically this wasn't wrong, but it's not completely right either. To understand what is going on, lets examine how the brake pads and rotors interact. It's not as simple as it seems on the surface, and relies on complex interaction between friction materials.

When you step on the brakes , there are two types of friction that are working on concert to stop the car - abrasive friction and adherent friction.

Abrasive and adherent friction

Abrasive friction is when when two materials are rubbing against each other - in this case it would be the brake pads and the brake rotors. This one is simple to understand - when you step on the brake pedal, it pushes hydraulic fluid into the pistons inside of the caliper. The pistons press onto the brake pads, which press onto the rotor. This creates friction, which converts the kinetic energy of the moving car into heat. This usually is the mode of friction that we're dealing with when the brakes are cold. Abrasive friction wears down the rotor and the brake pads as the molecular bonds in both materials are broken down. As the temperature increases with braking, the effectiveness of abrasive friction decreases.

Adherent friction is the mode of friction when a microscopically thin layer of friction material, a few hundredths of a nano-meter thick from the brake pad is transferred to the rotor. As the pad is pressed onto the rotor, this thin layer of friction material is continuously breaking and reforming molecular bonds between the rotor's surface and the brake pad. This mode of friction is the primary mode of friction when the brakes heat up to a higher temperature.

Uneven pad deposits on a brake rotor

When you feel a vibration as you step on the brakes, this is caused by uneven deposits of friction material on the brake rotor. The transition from abrasive friction in areas of relatively low material deposit to adherent friction in areas of higher pad deposits causes the coefficient of friction to vary, which is what is causing the car to pulse as it comes to a stop.

The deposits of friction material can be deposited unevenly if the brakes have gotten hot and then improperly cooled down, such as by coming to a complete stop and holding the brake pedal down. The brake pad is still hot, and when it is hot the pad material will transfer to the rotor - whether it is rotating or not. Contaminants on the surface of the brake rotor (such as grease) can also cause uneven deposits of friction material.

Brake rotor being machined on a brake lathe Brake rotor being machined on a brake lathe

Technically yes, the rotor is warped because the buildup of pad deposits is a few thousandths of an inch thick. To restore proper smooth braking performance, the uneven deposits need to be removed, and a fresh uniform layer of friction material needs to be laid down. The friction material can be removed by having the rotors machined, either on a brake lathe or with a Blanchard grinder. When this this friction material is removed the brakes need to be bedded in to again establish another uniform layer of friction material on the brake rotor. This friction material will help prolong the life of the brake rotors and also keep the brakes quiet, and the nice uniform transfer ensure smooth and predictable braking performance.

Bedding in the brakes

To bed in the brakes, the brakes need to be hot enough to transfer friction material. The usual procedure is brake moderately from high speed a few times. The important factor is to not come to a complete stop - always keep the car moving so the rotors are ventilated and so the pad material does not imprint on the rotor. Refer to the brake pad's manufacturer for specific instructions on pad bed-in.

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Hopefully this quick summary sheds some light on the mystery of shuddering and "warped" brakes.

Shop Volvo Brake Kits at FCP Euro


Sources:
GoMoG - Bedding-In Your Brakes
PFC Brakes - Pad and Disc Bedding
Essex Parts - Know Brakes 1: How to Bed-in Brake Pads and Rotors
Essex Parts - Know Brakes 2: Swapping Between Street and Race Brake Pads
Stop Tech - Pad and Rotor Bed-In Theory, Definitions and Procedures Removing the Mystery from Brake Pad Bed-In
Moss Motors - Warped Brake Discs
Metal 2014 - Characterization of the Friction layer Formed on the Cast Iron and Stainless Steel Brake Discs During the Friction Test
Safe Braking - Myth: Brake Judder and Vibration is Caused by Discs That Have Been Warped from Excessive Heat
Disc Brakes Australia - Friction Material Deposits
Enduro Mag - Pad Bed-In Theory: An Education
Powerstop - What Causes Brake Pulsation?
NASIOC - Uneven pad deposits - what's the best fix?
Art's Automotive - In-House Machine Shop


About the Author: Andrew Peng

4bc258bba12eb53f892c34317c49b78eAndrew is an aerospace engineer and car fanatic that enjoys working on his garage of Volvos and Subarus. When he's not busy attending car meets and shows or taking things apart, he enjoys driving his cars and finding interesting new ways to break them. He can be reached via his personal website at http://andrewpeng.netFacebook, Google+Instagram, or Twitter.


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Written by :
Andrew Peng

Andrew is an aerospace engineer and car fanatic that enjoys working on his garage of Volvos and Subarus. When he’s not busy attending car meets and shows or taking things apart, he enjoys driving his cars and finding interesting new ways to break them. He can be reached via his personal website at http://andrewpeng.net, Facebook, Google+, Instagram, or Twitter.


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