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Technical Information

Technical Information

Noise reduction measures

With their growing quality awareness, drivers are ever more reluctant to accept brake noise. Vehicle manufacturers, conversely, are increasingly excluding this characteristic from their warranty conditions.

What causes this – often unpleasant – noise and how can it be prevented?
Because they work with dry friction, brakes invariably cause vibrations when they are applied. Depending on their frequencies, these vibrations can be perceived in various forms.

Low-frequency vibration is often referred to as creep groan, humming or judder, while medium and high frequencies cause, for example, squeal or so-called wirebrushing.

The various frequencies are affected by the friction coefficient of the pad/lining and the counter-material, friction speed, the temperature and contact pressure, the duration of excitation, and also by the design of all associated components and their natural-frequency and damping behaviour. Climatic conditions, such as air temperature and humidity also play an important role.

As a matter of fact, all components of the braking system and chassis, right down to the rubber car body mounts are involved in noise generation, with the brake assembly taking a leading role and the brake discs acting as “loudspeakers”. The size, design and condition of the whole wheel, i.e. the rim and tyre, also have a significant impact.

In the effort to combat brake noise, its causes have been the subject of extensive investigation for many years now. The results are applied at a very early stage in the development of new vehicles and contribute to improving driving comfort.

New friction materials for brake pads and linings, for example, are tested for strength, coefficient of friction, natural-frequency behaviour and damping properties already during their development. The backing plate’s shape, material and strength are optimised and the contour of the pad or lining is finely adjusted. Depending on the frequency, a variety of chamfers, grooves, additional inert masses or underlayers are used.

Basically, brake pads and linings can either be connected to the calliper with adhesive film or springs, or they are decoupled with the help of damping lacquers, damping shims or lubricants.

On critical brakes, the various measures are often used in combination, in which case it is important to observe the special installation regulations when fitting the brake.

When you change the pads, check that the contact areas of the caliper are free from dirt and grease.

Cut out in plate:

Banana slot and shouldered plate are possible

Chamfer:

Segment chamfer, anti-segment chamfer, tangential chamfer, axial chamfer, upper bow chamfer

Balancing weights:

Various forms; size and weight are critical, also on brake calliper or holder

Grooves:

Radial, tangential and diagonal grooves; X-groove

Underlayer:

Various materials, wedge-shaped

Damping shims and lacquers:

Various coatings, adhesives, shapes and materials

Noise classification by frequency

Noise-for-Profi-ENG

Low-frequency squeal: 1.5–4 kHz

noise-reduction-measures02-low-frequency-squeal
  • Temperature < 200 °C
  • Speed < 20 km/h
  • Pressure < 30 bar

 

High-frequency squeal: 8–15 kHz

noise-reduction-measures03-high-frequency-squeal

Graunch: 70–200 Hz

noise-reduction-measures04-graunch
  • Temperature < 200 °C
  • Speed < 30 km/h
  • Pressure > 15 bar

Wirebrush: 0–5 kHz

noise-reduction-measures06-wirebrush
  • Temperature < 200 °C
  • Speed < 30 km/h
  • Pressure > 15 bar

 

High-speed judder: 100–300 Hz

noise-reduction-measures07-squelch
  • Temperature > 100 °C
  • Speed > 160 km/h
  • Pressure > 10 bar

 

Moan: 200–600 Hz

noise-reduction-measures10-moan
  • Temperature < 150 °C
  • Speed < 1 km/h
  • Pressure < 10 bar

 

Squeal without brake: 1.5–15 kHz

noise-reduction-measures11-wqueal-without-brake
  • Temperature < 150 °C
  • Speed < 5 km/h
  • Pressure > 0 bar