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Brake Squeal

Binder - Advanced Technical Information
This bulletin replaces bulletin Group 4,
#1605, dated 03/24/2016.

Attention: Service Managers/Service Technicians
Vehicle Type:

All

Model Year:

All

Concern:

Customer reports that vehicle brakes squeal.

Information: Remember that brake squeal is a normal result of different factors acting upon the brake system
and does not constitute a defect.

• Normal brake noises, such as squeal, occur under a narrow set of operating condition.

• An example would be low speed, light brake pedal application.

• Abnormal brake noises occur under wider operating conditions.
• An example would be during light or heavy braking at any speed or temperature
In the interest of customer satisfaction, we will perform a brake inspection to assure the customer
that there are no safety or performance issues associated with brake noise. Porsche will cover this
inspection under warranty.
The customer may also refer to the owner’s manual section on Driving and Driving Safety regarding
brake squeal.
Disclaimer
Porsche vehicles have high performance brake systems. Due to the stresses to which Porsche
vehicles are oftentimes subjected, Porsche vehicles presented for brake service may exhibit exceptional wear at earlier than expected times compared to wear found typically on other vehicles. All
the conditions described below are the result of normal brake system wear and are therefore not
warrantable.

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(cont’d): “However, disc brake squeal remains an elusive problem, and there is not yet a method to completely suppress it.”

Reference: This is a book published by the Society of Automotive Engineers.

Disc brake squeal: Mechanism, Analysis, Evaluation, and Reduction/Prevention /[edited by] Frank

Chen, Chin An Tan, and Ronald L. Quaglia. SAE)
www.SAE.org
Brake Friction Materials are Different
There is a common misconception that all brakes should exhibit the same characteristics. In the
North American Market (U.S. & Canada), one brake friction material is used more than any other is.
It is called Non Asbestos Organic (NAO). The dominance of this friction material in the market has
aided this misconception.
The characteristics of this material are:

• Excellent Noise, Vibration, & Harshness (NVH) characteristics
• Excellent lining wear
• Produces low amounts of wheel dust
• Friction level fades at high temperature

This list matches customer expectations pretty well. However, the last characteristic about friction
level fading at high temperature makes it unsuitable for Porsche.
Porsche uses a Low Met friction material to meet our high performance standards. The characteristics of this material are:

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• High coefficient of friction
• Excellent fade characteristics (friction level does not fade at high temperature)
• Very effective at stopping the vehicle
• Noise can be more than other friction materials
• Wheel dust is a normal by product
• Lining life is low (compared to NAO for example)

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Information
(cont’d): The graph below illustrates the difference in fade characteristics between NAO & Low Met friction
materials:

Low met performance characteristics are necessary to maintain the balance of performance in the
Porsche vehicle as a whole.

What is Brake Squeal?
Brake squeal is a noise that is the result of minute vibrations. The minute vibrations occur as the rotor moves in relation to the pad. A combination of high friction levels and low pedal pressure create
ideal conditions for noise.

The condition that is creating the noise is called “stick-slip”.

This stick-slip condition is most prevalent during light braking. Light braking smooths the rotor and
the pad surfaces. When this is coupled with imperfect pressure distribution on the pad (due to low
pedal pressure), the conditions for stick-slip exist. The noise occurs as the transition is made from
stick to slip.

Slip

Frequency

Velocity

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(cont’d): Some clear indications of a normal brake squeal resulting from the conditions described are:

• When the squeal occurs, applying more brake pressure makes it go away
• The rotor has a polished mirror like finish on it
• The pads have a smooth surface finish, like a glaze

Other Contributing Factors
Other factors also contribute to brake squeal. All components (even the bolts) have what are called
“bending and twisting modes.” As force is applied to a component, it will bend and twist in different
ways. The bending and twisting modes change as brake pressure and temperature changes. As the
components are mounted together, coupling effects also become a factor.
The amount of bending and twisting is so small as not to be visible to the naked eye. It is a normal
characteristic of all components. It is determined through Frequency Response Testing.
This is why proper torque on fasteners is critical. Other factors include component temperature,
mass, geometry, insulation, and lubrication. Weather and driving conditions are also major factors.
,

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(cont’d): Countermeasure Limitations

Brake squeal can fall into a very wide frequency range. The range is divided into three groups:

1. Low Range 1 - 3 KHz

2. Mid-Range 3 – 6 KHz

3. High Range 6 – 20 KHz (above 14KHz can be very difficult to hear)
The frequency of the noise will determine what countermeasure to use. Some common countermeasures are listed below:
• Pad Chamfers

• Various slot configurations in the pad
• Pad slippers

• Shims of various types
• Tuned mass dampers
• Fixed calipers
Countermeasures are limited on a given brake system due to limitations with space and performance. Testing must be performed to determine if a countermeasure is effective without deteriorating performance.
Measuring the Noise
Since the frequency of the noise determines the countermeasure, it is not possible to address the
issue without knowing the frequency.
This is why it is necessary to capture the information using an FFT (Fast Fourier Transform). There
are applications for smartphones and tablets that will do this.

Two real life examples from Porsche vehicles are illustrated:

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(cont’d): The base frequency shown (1550 Hz) is the noise where the problem is coming from. The other five
peaks are all harmonics. This means that if the base frequency is eliminated, the others will go away
as well.
In a second example, we see a squeal frequency without harmonics. This frequency is over five
times higher than the last example. The countermeasure required for this noise will be completely
different from the last example.

As mentioned above, the apps are available for smartphones and tablets. Two apps are listed below:

iOS
FFT Plot
Android
Speedy Spectrum Analyzer
Note: FFT plots are required for brake noise warranty cases; please refer to the procedure below.
Procedure

1. Test drive with the customer to verify the noise. The customer should be able to reproduce the
noise on the test drive.

• This will help ensure you are addressing the same concern.

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Information
(cont’d): FFT Plot (Required for Warranty)

2. Take an FFT plot of the noise during the test drive with the customer.

• This is important because the frequency of the noise can vary widely

• The frequency helps point to the source of the noise

• The intensity (measured in decibels dB) helps us know how bad it is and how much we have
improved it after our work

• The
plot needs to be recorded from inside the vehicle where the noise is heard by the customer.

• An omnidirectional microphone with a greater sensitivity can be attached to a tablet or smartphone to improve recording ability

3. The third step is to perform a thorough visual inspection.

4. Perform repair work as necessary.

5. A final FFT confirms that the noise has been improved or eliminated.
Component Inspection Guidelines
Fluid

Fluid replacement is required every two years by the maintenance schedule.

• WM 470855 Changing the brake fluid

• There is also a section in the Owner Maintenance booklet dedicated to recording the brake
flushes every 2 years

Moisture in the fluid is heavier than the fluid, and accumulates in the calipers.

• Moisture affects the viscosity of the fluid and its ability to lubricate

• Corrosion and damage to the caliper can also result
The right rear caliper is the farthest from the reservoir and may be affected more than the other
calipers.

• This is due to the larger volume of fluid in that circuit and therefore a larger amount of moisture
can settle in that caliper

If squeal is occurring on one wheel in particular, fluid condition is likely to be a part of the problem.
Countermeasure

Flushing the fluid is necessary to get rid of the moisture accumulation.
Tip: Warming the fluid by driving and braking before flushing it, can help get a more thorough flush
of the fluid.

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