Parts of a Brake Pad: What Each Layer and Component Does

The short answer

Backing plate: the steel base that gives the pad its shape and transfers force from the caliper. Friction material: the replaceable layer that presses against the brake rotor and wears away over time. Underlayer: an optional intermediate layer between the friction material and the backing plate. Attachment method: adhesive bonding, rivets or mechanical retention features that hold the friction material to the plate. Shim: a thin plate or coating on the back of the pad that helps reduce noise and vibration. Slots and chamfers: cuts and angled edges in the friction material that help manage noise, heat, gas and debris. Wear indicator: a mechanical tab or electronic sensor that warns when the pad is getting thin.

Backing plate

The backing plate is the structural foundation of the pad. It is usually made of steel and is shaped to fit a specific caliper. The caliper piston or pistons push against it, and its edges or tabs locate the pad in the caliper bracket so it can slide and stay aligned with the rotor. Many backing plates are painted or coated to resist corrosion, because rust building up between the plate and the friction material can contribute to the friction layer separating. Some plates also have holes, raised features or a textured surface that the friction material grips during manufacture.

Friction material

The friction material, sometimes called the lining or compound, is the part that actually does the braking. When the pad is clamped against the spinning rotor, friction converts the vehicle's motion into heat. This layer is designed to wear gradually, which is why pads are replaced periodically.

Friction materials are usually grouped into three broad families. Organic or non-asbestos organic compounds use fibers, fillers and resins and are generally associated with quiet operation and softer feel. Semi-metallic compounds contain a significant proportion of metal fibers and are generally associated with heat tolerance, at the cost of more noise and dust. Ceramic compounds use ceramic fibers and fillers and are generally associated with low dust and quiet operation. These are general category traits rather than guarantees: actual formulations are proprietary and vary widely between brands and product lines, and no specific compound data or test results were supplied for this guide.

Underlayer

Many pads include an underlayer, also called a thermal layer or bonding layer, between the friction material and the backing plate. Its typical roles are to improve adhesion to the plate, reduce heat transfer into the caliper and brake fluid, and help damp vibration. The underlayer is not meant to be a braking surface, so a pad worn down to this layer is due for replacement.

How the friction material is attached

The friction material is held to the backing plate in one or more ways. Bonded pads use a high-temperature adhesive. Riveted pads use rivets through the plate and lining, an approach more common on some heavier-duty and older designs. Many current pads also use mechanical retention, in which the friction material is molded into holes or a textured surface on the plate so it is physically locked in place in addition to being bonded.

Shims and insulators

A shim is a thin layer fitted to the back of the backing plate, where it sits against the caliper piston or caliper housing. Shims can be single-layer steel, multi-layer laminates with rubber or other damping material, or a coating applied directly to the plate. Their main purpose is to reduce brake noise, especially squeal, by damping vibration and isolating the pad from the caliper. Some shims clip or stick onto the plate, and some pads come without shims, in which case the vehicle may reuse separate shims from the old pads.

Slots and chamfers

Many pads have one or more slots cut across the friction surface and angled edges, called chamfers, at the leading and trailing ends. Slots can help release gases and dust, give the friction material room to flex, and change vibration characteristics to reduce noise. Chamfers ease the leading edge into contact with the rotor, which also helps limit noise. The number, position and angle of these features are specific to the pad design.

Wear indicators

A wear indicator tells the driver when the friction material is getting thin. A mechanical indicator is a small spring-steel tab attached to the backing plate; once the lining wears down far enough, the tab touches the rotor and makes a high-pitched squeal while driving. An electronic indicator is a sensor embedded in or clipped to the pad that triggers a dashboard warning when it is worn through or contacts the rotor. Whether a vehicle uses a mechanical tab, an electronic sensor or neither depends on the make, model and model year, so the owner's manual or service information for the specific vehicle is the place to confirm this.

Related parts that are not part of the pad

Several components are often sold alongside pads but are not part of the pad itself. Abutment clips, also called anti-rattle clips or hardware, sit in the caliper bracket and give the pad a smooth surface to slide on. Pins, bushings and springs belong to the caliper assembly. Some pad sets include this hardware and some do not, so it is worth checking the package contents for the specific part number.

What this guide does not cover

This guide describes the general construction of disc brake pads. It does not give minimum thickness limits, compound specifications or performance figures, because those vary by vehicle and manufacturer and no source data was supplied for them. Drum brakes use brake shoes rather than pads, and their construction differs. For replacement intervals, wear limits or compatibility, follow the vehicle maker's service specifications or the pad manufacturer's published data.