1 Copyright © 2011 by ASME
ASME 2011 International Mechanical Engineering Congress & Exposition
ASME 2011
November 11-17, 2011, Denver, Colorado, USA
IMECE2011-62084
A STUDY OF SQUEAL NOISE IN VEHICLE BRAKE SYSTEM
Xu Wang
RMIT University
Bundoora, Victoria, Australia
Sabu John
RMIT University
Bundoora, Victoria, Australia
He Ren
RMIT University
Bundoora, Victoria, Australia
ABSTRACT
Disc brake squeal can be classified as a form of friction-
induced vibration. Eliminating brake noise is a classic challenge
in the automotive industry. This paper presents methods for
analyzing the unstable vibration of a car disc brake. The
numerical simulation has been conducted, and its results are
compared with those from the experimental tests. The root
causes of brake squeal noise will be identified. Potential
solutions for elimination of the brake squeal noise will be
proposed. Firstly, new materials and technologies for the disc
brake application will be explored, secondly, it will be
illustrated how to avoid the brake squeal noise problem from
the brake system design. Brake disc design changes for
improving cooling performance, and service solutions for brake
squeal noise will be presented.
INTRODUCTION
Brake noise can be classified into brake judder occurring at
0 ~ 100 Hz, brake groan/moan at 100 ~ 500 Hz, brake howl at
500 ~ 1000 Hz, low frequency squeal at 1000 ~ 3000 Hz and
high frequency squeal at 3000 ~ 16,000 Hz. All the brake noises
are responses of self-excited vibration except for the brake
judder which is response of forced vibration. Brake squeal
occurs when the frequency falls in the range of 1,000-20,000
Hz and it results in a high pitched squeaky noise [Himanshu, et
al, 2005].
When a vehicle is braked and decelerated, the kinematic
energy of the rotating brake rotor is converted to thermal energy
through friction at the interface of brake pad and rotor. The
brake pads have high-friction surfaces and serve to slow the
rotor down or even bring it to a complete halt. Frictional heat is
generated on the rubbing surfaces due to the interaction
between the pads and rotor disc. This action allows the rotor
disc to absorb 90% of generated heat energy by means of
conduction from the friction interface. The surface temperature
of the brake rotor disc as well as the pads will rise. Disc brake
squeal was generated due to both the thermal effects and the
structural compliance of brake components. In addition to
executing the instability study of a typical passenger car brake
system using the complex eigenvalue analysis method, non-
linear contact pressure analysis and a fully coupled transient
thermal analysis are required. Many studies have recently been
conducted to reduce the squeal noise using finite element
numerical analysis [Hassan, et al, 2009].
Thermal imaging camera was applied to analyze the heat
distribution built up within the brake disc in an attempt to
further understand what causes the disc to deform while the disc
is in operation. Thermal imaging has shown that the vane
pattern of the disc can cause a corresponding temperature
profile on the surface of the brake disc. This relates to uneven
heat transfer from the disc surface which should be avoided in
order to minimize thermal distortion [Bryant, et al, 2008].
A polymer structure was considered for brake pads in
which the partial stress does not increase in accordance with the
dramatic changes in the real contact area when the polymer
undergoes shear deformation, as the molecular structure of a
polymer is capable of maintaining a high friction coefficient
while reducing variation [Kenji, et al, 2009]. Friction
experiments using parts formed from phenol resin and
Polyamide-imide (PAI) were used to verify the improvements.
The occurrence of brake squeal was halved by adopting a brake
pad using PAI instead of phenol resin. A brake pad produced
using the resin binder was found to be capable of improving
brake squeal without reducing the effectiveness of the brakes.
The squeal noise was reduced by approximately 20 dB with the
brake friction coefficient at the same level [Kenji, et al, 2009].
Automotive brake squeal continues to challenge the
industry engineers despite the efforts have been made to reduce
its occurrence during the past years. The squeal noise has been
a frequent source of complaint to many customers although it
does not affect the performance of a vehicle. Brake squeal is
high frequency noise (1 - 20 kHz) of a brake system, which is
complex and influenced by many factors. Research into
predicting and eliminating brake squeal has been conducted
since 1930s. However, the root causes of the brake squeal noise
are still not clear, effective design and service solutions need to
be explored. In this paper, experimental tests and numerical
modal analyses will be conducted to identify the root causes of
the brake squeal noise. A study on materials and design of brake
components will be carried out to define suitable design
guidelines to reduce or to eliminate squeal problems.
Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition
IMECE2011
November 11-17, 2011, Denver, Colorado, USA
IMECE2011-62084
1
Copyright © 2011 by ASME
2 Copyright © 2011 by ASME
EXPERIMENTAL MEASUREMENT
AND DATA ANALYSIS OF BRAKE SQUEAL NOISE
The brake squeal noise was measured by microphones and
analyzed using the software Head Acoustics Artemis. The
experimental setting is shown in Figure 1.
The experiment was carried out in a basement parking place in
order to record clear braking sound without other background
disturbance such as other cars or road noise. One microphone
was installed inside the testing vehicle at the driver’s left ear.
Two microphones were installed outside the vehicle at 7.5 meter
away from either side of the testing vehicle and at a height of
1.2 meter from the ground. Both the two microphones (Model
330 Series) point toward the centre line of the vehicle. The
testing vehicle had an odometer reading of 60000 km and was
driven along the path with a length of 20 meter. As the testing
vehicle was decelerated by braking with a deceleration of 0.5 g
(4.9 m/s2) passing through the two external microphones, the
sound pressure signals were recorded through sound level meter
preamplifiers (Model 330 Series) and by laptop computers with
sound card and the software Cooledit97. The sound pressure
data was also recorded by the Head Measurement System
(HMS) III in the front passenger seat and analyzed using the
software Artemis. The noise spectra measured inside and
outside the vehicle are shown in Figure 2 and Figure 3.
Figure 1 Microphone positions in the measurement.
It is clearly seen from Figures 2 and 3 that the noise peak
frequency bands are around 375, 765, 1138, 1559, 1941 and
6479 Hz. The noise spectrum contour patterns measured inside
and outside the vehicle are shown to be similar except that the
spectrum amplitude level inside the vehicle is much lower than
that outside the vehicle. This has established the correlation of
the brake noise heard inside and outside. The low-frequency
noise at 375 Hz is called “brake “groaning” while the low
frequency noise at 765 Hz is called “brake howl”. Any noise
having a frequency above 1000 Hz is considered a squeal
[Hassan, et al, 2009]. The low frequency brake squeal noise
frequencies are identified at 1138 Hz, 1559 Hz and 1941 Hz.
The high frequency brake squeal is identified at 6479 Hz. The
main brake squeal noise problem for the test vehicle is seen to
be a low frequency brake squeal. SPL amplitudes of the squeal
noise at 1941 Hz inside and outside the vehicle are listed in
Table 1. It is seen in Table 1 that the outside noise at the
frequency had 10 dB(L) higher sound pressure amplitude level
than the inside noise.
Table 1 Sound pressure amplitude level comparison for the noise
inside and outside the vehicle at 1941 Hz.
Inside Outside
SPL (dB) 62 72
Figure 2: Sound pressure spectrum contours for squealing noise in
different frequency ranges, recorded outside the vehicle (Max 95
dB(L)).
FINITE ELEMENT ANALYSIS
A numerical model of a disc brake system was formed
using the software ANSYS. Meshing of the matching surfaces
of the brake pad and the brake rotor, the pistons and the caliper,
the locating stud of the caliper support and the locating hole of
the brake caliper was conducted with special treatment.
Meshing element of the matching surfaces is one-to-one
corresponding. There are 20,000 elements in the model as
shown in Figure 4.
After the material properties of the components were
imported into the model, an applied force, constraints/boundary
conditions were defined for the model, normal modal analysis
was conducted by the unsymmetrical ANSYS solver over the
frequency range of 1-20 kHz, where all the natural frequencies
and mode shapes of the brake system were solved. The modal
shape results at the natural frequency of 6474 Hz are shown in
Figure 5.
3 Copyright © 2011 by ASME
It is seen from Figure 5 that the bending modes of the
pads and disc have similar characteristics. These bending modes
couple due to friction, which forms unstable modes and produce
a squealing noise. Therefore, the geometry parameters and
material properties of the braking system should be modified to
eliminate the brake noise.
Figure 3 Sound pressure spectrum contours of the squealing noise
in different frequency ranges, recorded inside the vehicle (max 85
dB(L)).
Figure 4 A FEA model of brake system.
It is important to determine the modal behavior of
individual components (disc and pads) when predicting the
brake squeal noise. A modal analysis performed on the free pad
and free disc model will give insights into potential coupling
modes. The natural frequencies and mode shapes of brake pads
and disc can also be used to define the type of squeal noise that
may occur in a braking system. As shown in Figure 6 and Figure
7, it is seen that the second bending mode of the pad and ninth
bending mode of the disc may couple to generate dynamic
instability in the system.
Figure 5 Brake disc mode shape at 6474 Hz.
Figure 6 Free brake disc mode shape at 6343 Hz.
The bending modes of pads and disc are more significant than
twisting modes, they eventually couple to produce squeal noise.
The second bending mode of the pad has a frequency of 6640
Hz and the ninth bending mode of the disc has a frequency of
6343 Hz. These pad and disc bending modes may couple to
produce an intermediate lock, resulting in a squeal noise at a
frequency close to 6474 Hz, which is very close to the measured
squeal frequency of 6479 Hz.
The finite element modal analyses for other natural
frequencies near 1138, 1559, 1941 Hz also show similar results
which illustrate that the brake squeal was caused by mode
coupling occurring between the out-of-plane bending modes of
the rotor and the brake pad. For higher modal frequencies, the
finite element modal analyses shows in plane mode coupling
occurring between the rotor and brake pad, although the higher
frequency squeal noise was not noticed from the vehicle brake
noise measurement data.
ROOT CAUSES OF BRAKE SQUEAL NOISE
4 Copyright © 2011 by ASME
Coupled vibration of the brake rotor and pad generates an
uncomfortable noise. Brake squeal noise was caused by large
amplitude nonlinear vibration [Himanshu, et al, 2005]. The
brake squeal at 6474 Hz is associated with frictional excitation
couple occurring between the out-of-plane bending modes of
the rotor and the brake pad, with a phenomenon known as
modal “locking”. Modal locking of two or more modes of
various structures provides appropriate conditions for brake
squeal as the brake disc rotor typically vibrates with 2 to 4
nodal diameters. Higher frequency squeal is produced by
friction induced excitation imparted on coupled resonance
occurring between the in plane modes of the brake disc rotor
and brake pad.
Figure 7 Free pad mode shape at 6640 Hz.
There are several factors that influence the brake squeal
noise, they are:
• rough finish on resurfaced rotor discs
• loose fitting brake pads inside the brake calipers
• lack of silicone compound on the back of brake pad
• missing springs or anti-rattle clips that should be on
the caliper or pad
• improper tightening sequence of lug nuts or caliper
hardware
• contamination of the brake pad such as that caused by
leaked brake fluid
• humidity weather
• temperature variations of brake disc and pads
Brake disc surface conditions contribute to the brake
squeal noise generation. Main brake disc surface damage
patterns which would induce the brake squeal noise are given in
Figure 8.
SOLUTIONS FOR BRAKE SQUEAL NOISE
Correct selection of brake disc material is one of most
important factors in elimination of brake squeal noise where the
brake disc material and its surface treatment should have stable
mechanical and frictional properties, high wear resistance
through the range of expected service temperatures, high heat
absorption capability, high thermal conductivity, high vibration
damping capacity, minimal thermal expansion and high degree
of corrosion resistance.
Cast iron is a popular material for brake discs. Due to its
properties and low cost in manufacturing, it has been widely
used in disc brake system. Furthermore, some new technologies
such as coatings and surface treatments can be applied to cast
iron discs to eliminate the brake squeal noise to prevent or
minimize brake disc surface corrosion. These include alloying
to improve thermal conductivity and/or wear resistance,
alloying or heat treatments to modify the microstructure for
improved vibration damping, composites of gray iron and other
metals or ceramics.
Figure 8 Main brake disc surface damage patterns causing brake
squeal noise.
Some new materials, such as aluminum metal matrix
composites, are a class of metal matrix composites in which an
aluminum matrix is reinforced with ceramic particles, whiskers,
or short fibers. These materials have the potential for redefining
the property limits of aluminum materials because of their
unique combinations of properties. For example, they would
provide the stiffness of titanium, better wear resistance than
5 Copyright © 2011 by ASME
steel, and tailor-able coefficient of thermal expansion, all while
maintaining the light weight characteristics of aluminum.
Another brake disc material silicon carbide (SiC) is a
compound of silicon and carbon with chemical formula SiC.
Grains of silicon carbide can be bonded together by sintering to
form very hard ceramics which are widely used in brake disc
applications requiring high endurance, because its operational
temperatures are not limited by brake disc rotor material, and its
frictional properties are better at higher temperatures. It is light
weight despite of a high cost.
Brake disc shape should also be fine-tuned for fin
configurations and inlet outlet fin geometries to maximize
airflow for effective heat removal during braking to attain high
cooling performance. Cooling performance can be achieved
through optimization using CFD.
Matching of brake pads and brake disc materials plays an
important role in the brake squeal noise generation and control,
which needs a lot of fundamental material property studies and
tests. Anti-squeal shim is one of effective service solutions to
reduce the brake squeal noise. The shim is installed on the
backside of the pads, between pads and caliper pistons. The
shim has a sandwich structure of constrained layer damping
with two steel plates separated by a viscous-elastic core shown
in Figure 9. This shim is a very thin and can be attached onto
the back-plate of brake pad, which attenuates the vibration
energy from the brake pad. The shims provide a permanent
vibration damper and reduce the vibration transmission from the
brake pad to vehicle chassis.
Alternatively, anti-squeal grease can be applied onto the
back of the pads when the brake pads are removed as shown in
Figure 10. Anti-squeal grease is a kind of high-temperature
silicon grease. This may be one of the low cost solutions for
elimination of the brake squeal noise.
CONCLUSIONS
Vehicle brake squeal noise has been studied. Vehicle
measurement and finite element analysis simulation have been
conducted to identify the root causes of the brake squeal noise.
Potential solutions for elimination of the brake noise have been
recommended.
It is concluded that the disc surface finish, installation
quality, weather conditions, and contamination on the brake
pads all contribute to the brake squeal noise. The brake squeal
noise which was measured from the vehicle tests was caused by
the frictional excitation couple occurring between the out-of-
plane bending modes of the rotor and the brake pad. Higher
frequency squeal is produced by friction induced excitation
imparted on coupled resonance occurring between the in plane
modes of the brake disc rotor and brake pad. Matching of brake
pads and brake disc materials plays an important role in the
brake squeal noise generation and control, which needs further
studies in future.
New brake disc materials, better surface treatments and
cooling designs, will help to reduce the possibility of the brake
squeal noise generation. Service solutions such as adding anti-
squeal shims and greases are recommended for elimination of
the brake squeal noise.
REFERENCES
1. Hassan, Muhammad Z., Brooks, Peter C. and Barton, David
C. (2009). Thermo-Mechanical Contact Analysis of Car
Disc Brake Squeal, SAE Int. J. of Passeng. Cars – Mech.
Syst., Volume 1, No.1, pp 1230-1239.
2. Himanshu, M., Wayne, N., Tom, K., Louis, K., and Erwin, J.
(2005), Brake Analysis and NVH Optimization Using
MSC.NASTRAN,
www.mscsoftware.com/support/library/conf/auto99/p01699.
pdf
3. Bryant, D., Fieldhouse, J., Crampton, A. and Talbot, C.
(2008). Thermal Brake Judder Investigations Using a High
Speed Dynamometer, SAE Paper 2008-01-0818.
4. Kenji A., Masaaki N., Yukihiro S., Yasuo F., Hiromichi Y.,
and Igor S. (2009). A Study on Friction Materials for
Reducing Brake Squeal By Nanotechnology, TOYOTA
Technical Review, Volume 56, No.2, August, 2009, pp85 –
89.
6 Copyright © 2011 by ASME
Figure 9 Anti-squeal Shim
Figure 10 Anti-squeal Grease
Adhesive
VE core
Friction material -pad
Back plate
Shim
Steel
Steel
Shim Back plate
Brake disc
Pad
Brake disc
Pad
