The disc brake or disk brake is a device for slowing or stopping the rotation of a wheel while it is in motion. A brake disc (or rotor in U.S. English) is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon-carbon or ceramic-matrix composites. This is connected to the wheel and/or the axle.
To stop the wheel, friction material in the form of brake pads (mounted on a device called a brake caliper) is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop. Brakes convert motion to heat, and if the brakes get too hot, they become less effective, a phenomenon known as brake fade.
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. Dust on the brakes may also cause squeal; there are many commercial brake cleaning products that can be used to remove dust and contaminants.
Finally, some lining wear indicators, located either as a semimetallic layer within the brake pad material or with an external squealer “sensor”, are also designed to squeal when the lining is due for replacement. The typical external sensor is fundamentally different because it occurs when the brakes are off, and goes away when the brakes are on.
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) are among the most important priorities for today’s vehicle manufacturers.
Apart from noise generated from squeal, brakes may also develop a phenomenon called brake judder or shudder.
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: hot (or 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 74.6 MPH to about 37.3 MPH, which results in severe vibrations being transmitted to the driver. These vibrations are the result of uneven thermal distributions, or 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, 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 (disc thickness variation). 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, wear and friction material transfers.
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. Airborne brake dust is known to be a health hazard, so most repair manuals recommend the use of a chemical ‘brake cleaner’ instead of compressed air to remove the dust.
Different brake pad formulations create different amounts of dust, and some formulations, particularly metallic brake pads, are much more damaging than others.
Ceramic brake pads contain significantly fewer metal particles, and therefore produce less corrosion of surrounding metal parts.