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Judgment Of Brake Phenomenon
- Jun 01, 2017 -

Brake Squeal

Sometimes a loud noise or high pitched 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. Techniques include adding chamfer pads to the contact points between caliper pistons and the pads, the bonding insulators (damping material) to pad backplate, the brake shims between the brake pad and pistons, etc. All should be coated with an extremely high temperature, high solids lubricant to help reduce squeal. This allows the metal to metal parts to move independently of each other and thereby eliminate the buildup of energy that can create a frequency that is heard as brake squeal, groan, or growl. It is inherit that some pads are going to squeal more given the type of pad and its usage case. Brake Pads typically rated to withstand very high temperatures for extended periods tend to produce high amounts of friction leading to more noise during brake application.


Cold weather combined with high early-morning humidity (dew) often worsens brake squeal, although the squeal generally stops when the lining reaches regular operating temperatures. This more strongly affects pads meant to be used at higher temperatures. Dust on the brakes may also cause squeal and commercial brake cleaning products are designed to remove dirt and other contaminants. Pads without a proper amount of transfer material could also squeal, this can be remedied by bedding or re-bedding the brake pads to brake discs. Some lining wear indicators, located either as a semi-metallic layer within the brake pad material or with an external "sensor", are also designed to squeal when the lining is due for replacement. The typical external sensor is fundamentally different from the noises described above (when the brakes are applied) because the wear sensor noise typically occurs when the brakes are not used. The wear sensor may only create squeal under braking when it first begins to indicate wear but is still a fundamentally different sound and pitch.

Brake Judder or Shimmy

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 120 km/h (74.6 mph) to about 60 km/h (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 disc thickness variation (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 and roughness of disc surface,misalignment of axis (runout), elastic deflection, wear and friction material transfers.Either type could potentially fixed by ensuring a clean mounting surface on either side of the brake disc between the wheel hub and brake disc hub before usage and paying attention to imprinting after extended usage by leaving the brake pedal heavily depressed at the end of heavy usage. Sometimes a bed in procedure can clean and minimize DTV and lay a new even transfer layer between the pad and brake disc. However it will not eliminate hot spots or excessive run out.

Brake Dust

When braking force is applied, the act of abrasive friction between the brake pad and the disc wears both the disc and pad away. The brake dust that is seen deposited on wheels, calipers and other braking system components consists mostly of disc material.Brake dust can damage the finish of most wheels if not washed off. Generally, a brake pad that aggressively abrades more disc material away, such as metallic pads, will create more brake dust. Some higher performing pads for track use or towing use may wear away much quicker than a typical pad causing additional dust from heightened brake disc wear and brake pad wear.