Brake Pads

The brake pads are designed for high friction with brake pad material embedded in the disc in the process of bedding while wearing evenly. Although it is commonly thought that the pad material contacts the metal of the disc to stop the car, the pads work with a very thin layer of their own material and generate a semi-liquid friction boundary that creates the actual braking force. Of course, depending on the properties of the material, disc wear rates may vary. The properties that determine material wear involve trade-offs between performance and longevity.

The brake pads must usually be replaced regularly (depending on pad material), and most are equipped with a method of alerting the driver when this needs to take place. Some have a thin piece of soft metal that causes the brakes to squeal when the pads are too thin, while others have a soft metal tab embedded in the pad material that closes an electric circuit and lights a warning light when the brake pad gets thin. More expensive cars may use an electronic sensor.

Although almost all road-going vehicles have only two brake pads per caliper, racing calipers utilize up to six pads, with varying frictional properties in a staggered pattern for optimum performance.

Early brake pads (and linings) contained asbestos. When working on an older car's brakes, care must be taken not to inhale any dust present on the caliper (or drum). Although newer pads can be made of exotic materials like ceramics, kevlar and other plastics, care to avoid inhalation of brake dust produced during operation should still be practiced no matter what materials are employed.

Brake squeal

Sometimes a loud noise or high pitch squeal occurs when the brakes are applied. Most brake squeal is produced by vibration (resonance instability) of the brake components, especially the pads and discs (known as “force-coupled excitation”.) This type of squeal should not negatively affect brake stopping performance. Simple techniques like adding chamfers to linings, greasing or gluing the contact between caliper and the pads (finger to backplate, piston to backplate), bonding insulators (damping material) to pad backplate, inclusion of a brake shim between the brake pad and back plate etc, may help to reduce squeal. Cold weather combined with high early morning humidity (dew) often makes brake-squeal worse, although the squeal stops when the lining reaches regular operating temperatures. However, some lining wear indicators are also designed to squeal when the lining is due for replacement. Overall brake squeal can be annoying to the vehicle passengers, passers-by, pedestrians, etc especially as vehicle designs become quieter. Noise, vibration and harshness (NVH) is one of the important priorities for today's vehicle manufacturers.

One solution is to put a small amount of copper grease onto the back of the pads where they contact the brake caliper piston and on the pad shims, if present. While this will normally stop the squeal, getting grease on the pads or disks will affect braking performance.

Dust on the brakes may also cause squeal; there are many commercial brake cleaning products that can be used to remove dust and contaminants from the brakes.

Apart from noise generated from squeal, brakes may also develop a phenomenon called brake judder or shudder.

Brake squeal on some pads is caused by a layer put in by the manufacturer that makes them squeal when they need replacing.

Brake judder

Brake judder is usually perceived by the driver as minor to severe vibrations transferred through the chassis during braking.

The judder phenomenon can be classified into two distinct subgroups; they are Hot (Thermal) or Cold Judder.

Hot judder is usually produced as a result of longer more moderate braking from high speed where the vehicle does not come to a complete stop. It commonly occurs when a motorist decelerates from speeds of around 120 km/h to about 60 km/h, which results in severe vibrations being transmitted to the driver. These vibrations are the result of uneven thermal distributions believed to be the result of phenomena called Hot Spots. Hot Spots are classified as concentrated thermal regions that alternate between both sides of a disc that distort it in such a way that produces a sinusoidal waviness around its edges. Once the brake pads (friction material / brake lining) comes in contact with the sinusoidal surface during braking severe vibrations are induced as a result and can produce hazardous conditions for the person driving the vehicle.

Cold judder on the other hand is the result of uneven disc wear patterns or DTV. These variations in the disc surface are usually the result of extensive vehicle road usage. DTV is usually attributed to the following causes: waviness of rotor surface, misalignment of axis (Runout), elastic deflection, thermal distortion, wear and friction material transfers.

Brake dust

When braking force is applied, small amounts of material are gradually ground off the brake pads. This material is known as "brake dust" and a fair amount of it usually deposits itself on the braking system and the surrounding wheel. Brake dust can badly damage the finish of most wheels if not washed off. Different brake pad formulations create different amounts of dust, and some formulations are much more damaging than others. This applies to the use of metallic brake pads. Ceramic brake pads contain significantly fewer metal particles in them, and therefore produce less corrosion of surrounding metal parts.