EB2013-TE-005

EFFECTS OF DIFFERENT FRICTION MATERIALS ON HOT JUDDER

- AN EXPERIMENTAL INVESTIGATION 1Fischer, Sebastian

*;

2 Sardá, Angelo; 1Winner, Hermann

1Fachgebiet Fahrzeugtechnik der Technischen Universität Darmstadt, Germany;

2Continental Teves AG & Co.

oHG, Germany

KEYWORDS – – hot judder, friction material, hotspots, order analysis, dynamometer

ABSTRACT

Hot judder is a forced, brake-induced, speed-dependent vibration, typically occurring during

light to moderate brake applications at high speeds. The heat generated between the friction

couple induces waving of the disk followed by an inhomogeneous circumferential

temperature distribution on the brake disk causing thickness variations which lead to brake

torque variation (BTV) and brake pressure variation (BPV). The influence of brake pads

within this cause-and-effect-chain leading to hot judder is not yet thoroughly identified and

understood. Therefore, using different friction materials, hot judder is compared during stop

and drag brake applications. The methodology applied to analyze the effects of different

friction materials is of a primarily experimental character. Three geometrically equal brake-

pad prototypes were manufactured with different friction materials. These are made of non-

asbestos organic, low-metallic and copper-free friction composites. To detect differences

regarding their hot judder characteristics, a set of experiments was designed, executed on a

flywheel dynamometer and evaluated.

INTRODUCTION

In order to meet today’s high requirements regarding comfort during brake applications of

passenger cars the phenomenon of thermal judder, also called hot judder, needs to be avoided.

Hot judder is a forced, brake-induced, speed-dependent vibration, typically occurring during

light to moderate brake applications at high speeds. (1)

When hot judder occurs in an already developed brake system, a commonly used

countermeasure consists of changing the brake pads. It is well known, that different brake pad

materials influence the hot judder characteristics of a brake system. However, a methodology

to implement this variation process is not known. Mostly a trial and error approach is chosen

often leading to excessive testing on flywheel dynamometers or test drives which can

consequently be cost and time consuming. Hence, the influence of the brake pad on thermal

judder has been the focus of numerous research projects (e.g. 1, 2, 4) in the past and is still a

field of interest for brake system producers. The influence of brake pads within the cause-and-

effect chain leading to hot judder has not yet been thoroughly identified and understood and is

therefore the main objective within this research project.

KNOWN CAUSE-AND-EFFECT CHAIN OF HOT JUDDER

The effects of judder that the driver might recognize include brake pedal pulsations,

vibrations of the wheels, seats or the human body, rotating oscillations within the steering

wheel or drumming noise. The excitation sources of these effects are mainly brake torque and

brake pressure variations (BTV/BPV) generated within the brake system. Depending on the

design of different parts of the transmission path, for example the wheel suspension or the

steering, the driver will notice varying strong effects. (3)

Figure 1: Cause-and-effect chain of hot judder (1)

The cause-and-effect chain leading to BPV and BTV according to Sardá (1) is shown

schematically in Figure 1. The starting point is typically a light to moderate brake application

at high speed. The heat generated between the friction couple brake pad and disk causes a

specific heat flow through the surface of the disk towards the cooling channels inside it. A

temperature gradient in axial direction is generated and the disk cannot uniformly expand.

Thereby thermal stresses within the disk arise that lead to an undulated deformation of the

brake disk. This waviness of the brake disk represents a state of equilibrium at an

energetically favorable state (1). The wave-like form of the disk topology leads to an

inhomogeneous surface pressure distribution between the brake pad and the disk, with higher

values on top of the waves than in their valleys. Furthermore, the uneven surface pressure

distribution is followed by an uneven circumferential temperature distribution resulting from

the variation of the friction power. This inhomogeneous, circumferential temperature

distribution stabilizes the waviness, because hot spots form on top of the waves and

strengthen the current thermal stress state. Due to their high local temperature they expand the

disk material locally and cause thickness variations. These dynamic disk thickness variations

combined with the waviness lead to brake torque variations (BTV) and brake pressure

variations (BPV) and therefore to hot judder.

Other experimental investigations have shown that based on the non-uniform thermo-

mechanical load in circumferential direction, a variation in the friction coefficient can emerge

between the brake pad and the brake disk. The friction coefficient changes because of the

uneven compression of friction film material and the generation of load-bearing plateaus on

the top of the waves. This is an additional cause for BTV and BPV. (2)

For the analysis of the influence of different brake pad materials regarding their hot judder

characteristics, this cause-and-effect chain according to Sardá (1) clearly identifies five

important characteristic measures:

brake disk waviness

brake disk temperature distribution

disk thickness variation

brake torque variation

brake pressure variation

METHODOLOGY OF THE APPROACH

The applied methodology to analyze the effects of different friction materials on hot judder

has a primarily experimental character. Three geometrically equal brake pad prototypes with

different friction materials were manufactured. These are composed of low-metallic, non-

asbestos organic, or copper-free friction composites in order to have a wide range of

materials. The low-metallic brake pad is used as reference brake pad. In order to detect

differences in their hot judder characteristics a set of experiments was designed, executed on a

flywheel dynamometer and evaluated.

Specification of brake applications

As described above, hot judder occurs during specific light to moderate brake applications at

high speed. This results in a combination of two important characteristics namely the brake

power should be high enough while combined with a low to moderate surface pressure

between the brake pad and the disk. For the specification of a brake application we examine

the calculation of the brake power and the brake torque on a flywheel dynamometer: (5)

re

eff cl eff Pis

( ) 2 ( ) ( )

( ) 2 2 ( )

B B

B B

P t n t M t

M t r µ F r µ A p t

(1)

With clF N Clamping force

effr m Effective friction radius

PisA m2 Piston area

BM Nm Brake torque

Bp bar Brake pressure

ren 1/s Revolution speed

- Friction coefficient

Assuming for simplification that the friction coefficient and the effective friction radius are

constant during a brake application, the only factors that define a brake situation are the brake

torque, the brake pressure and the revolution speed. The revolution speed can be constant or

can change during the brake application, differentiating between drag and stop braking

applications. Either the brake torque or the brake pressure can be controlled on the flywheel

dynamometer and leaving, therefore, four possible combinations of these factors. The chosen

combinations and the rationale are briefly discussed in the following sections.

DOE for critical brake power - drag brake application

The first step is to identify for the reference pad brake a critical brake power that results in hot

judder. This is done in discrete steps with a full factorial matrix of combinations between

revolution speed and brake torque. To generate a constant brake power per revolution both

torque and velocity values will be held constant and hence only contain drag brake

applications. Having two factors and choosing five steps per factor, the resulting matrix will

include 25 different brake situations with the analyzed brake power ranging from ca. 10 kW

to 110 kW. The torque will be varied from 100 Nm to 500 Nm and the velocity from 0.5vmax

to 0.9 vmax of the reference passenger car (vmax= 250 km/h).

Comparison of three friction materials - drag brake application

Once a critical brake situation is identified for the reference brake pad, this brake situation

will be chosen to compare the three different brake pads. It is not possible to decide which

brake pad in general is most critical for the generation of hot judder, because the situation is

only critical for the reference brake pad. But it is possible to answer the question: Does the

exchange of a brake pad influence hot judder characteristics in general and what is changing?

Furthermore, every brake application will be repeated five times to observe the repeatability

of the characteristic hot judder measures.

DOE for critical brake pressure - drag and stop brake application

The second basic condition for the appearance of hot judder is a low to moderate surface

pressure between the brake pad and the disk during the brake application. This influence

factor will be observed by a constant brake pressure both for drag and stop braking. In both

instances, the objective is to identify a critical brake situation for the three brake pad

materials. Hence it will be possible to arrange the friction materials according to their hot

judder generation characteristics based on the following evaluation parameter.

Evaluation parameter for critical braking applications

Based on the described cause-and-effect chain, BTV and BPV are the cause of the effects of

hot judder the driver recognizes. Therefore, an evaluation should be based on their behavior.

Industry also uses an evaluation parameter based on BTV in their static and dynamic test

fields. It is defined as:

rel max mean/BTV M M (2)

With relBTV - Relative brake torque variation (BTV)

maxM Nm Maximum brake torque variation (BTV)

meanM Nm Mean brake torque

The relative brake torque variation equals the maximum occurring BTV during a brake

application in relation to the mean brake torque. The higher the value the more critical the

brake system needs to be observed regarding hot judder. The relation to the mean brake

torque additionally suggests that the higher the mean brake torque is, the more BTV is

accepted. In general, this evaluation parameter is used in this paper, but a few changes were

made that will be explained later. Additionally the other characteristic measures described

earlier in the cause-and-effect chain were observed thoroughly.

TEST SETUP

The equipment on the flywheel dynamometer must be able to measure all separate steps

within the described cause-and-effect chain. Therefore the key additional equipment is:

Capacitive displacement sensors (3 for each side of the disk brake),

measures the topology of the disk (Disk waviness, DTV)

Infrared camera,

measures the brake disk temperature distribution

Synchronic data acquisition connecting the dynamic topology data of the brake disk and the

dynamic brake disk temperature distribution is essential to retrace steps within the cause-and-

effect chain and evaluate differences with varying friction materials. Figure 2 shows the

schematic data acquisition at the test bench.

Figure 2: Data acquisition and signal flow at the flywheel dynamometer

Figure 3: Sensor setup and working principle of the infrared camera at the flywheel dynamometer

Parts of the sensors and the test setup as well as the schematic working principle of the

thermal camera are shown in Figure 3. All tests were conducted with a brake pressure throttle

valve which regulates the maximal braking pressure variation by the dynamometer control.

Otherwise the test bench control would try to compensate the BTV or the BPV depending on

the currently controlled parameter.

RESULTS

Critical brake power for reference pad causing hot judder

Figure 4 shows an exemplary part of the drag brake applications to identify a critical brake

power causing hot judder. It illustrates the amplitude spectrum of the torque time signal for

five brakes. The amplitude spectrum is chosen to analyze the data because of an identified

eigenfrequency within the measuring chain of the brake torque. The measuring chain of brake

torque was therefore analyzed regarding its frequency response with the application of an

impact hammer. The 1st eigenfrequency is identified at 188 Hz, which corresponds to the 6

th

harmonic order of the revolution speed of the brake disk at the current speed of 225 km/h

(Figure 4). As a result we have a strong BTV signal within the eigenfrequency, which is not

caused by hot judder or any brake system related phenomenon.

Figure 4: BTV amplitude spectrum for five drag brake application with a velocity of 225 km/h and rising brake

torques of 100 Nm to 500 Nm

Within the time domain it would not be possible to clearly separate the different signal parts

and consequently, regarding the analyses of the BTV signals only frequency domain is used.

This causes the already described change of the introduced evaluation parameter. For the

required data regarding the critical brake power, the maximal brake torque variation is not

identified out of the time domain, but from the signal part that shows the maximal BTV and is

not consistent with the eigenfrequency, as can also be seen in Figure 4.

The result is shown in Figure 5 where the relative brake torque variations for every 25 brake

applications (combinations of brake torque and velocity) are the sampling points for the

shown interpolated curve. The critical brake application is identified as a combination of 200

km/h and 210 Nm. This equals a brake power of ca. 36 kW.

Figure 5: Relative BTV for the full factorial drag brake application DOE to identify a brake power leading to hot

judder (torque variation in five steps: 100-500 Nm, velocity variation in five steps: 125-225 km/h)

Comparison of the brake pads regarding a drag brake application

The identified critical brake application for brake pad A is applied to brake pad B and C and

repeated five times. This makes it possible to compare the three brake pads in terms of their

characteristic measures for hot judder and identify the differences. Figure 6 shows the

amplitude spectrums of the disk thickness variations, the brake torque variations and the

waviness of the disk for the three brake pads. Table 1 contains the mean values for the critical

amplitudes in the dominant order for the brake application repeated five times. Brake pads A

and B each show one distinct dominant order for BTV, DTV and the waviness (15th

/8th

).

Regarding the copper-free brake pad C, the dominant order differs for these measures. The

DTV shows a 14th

dominant order and the waviness an 11th

dominant order and both can be

found in the BTV amplitude spectrum. The circumferential temperature distribution in Figure

7 could provide a possible explanation for these diskrepancies. The circumferential

temperature distributions for each brake pad for rotation 1000 of the brake application are

shown below (obtained with the infrared camera).

Figure 6: Amplitude spectrums of BTV, Wav and DTV for the brake application (Mmean = 210 Nm, v = 200 km/h

= 0.8vmax) for brake pads A, B and C – seven equidistant rotations throughout the brake are analyzed - red

arrows mark the dominant orders for the separate measures

For brake pads A and B, the number of hot spots on the brake disk is equivalent to the

dominant orders of the other characteristic measures. Brake pad C causes a more inordinate

appearance of hot spots. Their appearance is not as equidistant in circumferential direction

compared to brake pads A and B and both the 11th

or 14th

order could be found, depending on

the analyzed radius. Nonetheless, the BTV amplitude spectrum shows both DTV and the

waviness excitation as torque variations in the 11th

and 14th

harmonic order.

Figure 7: Circumferential temperature distributions on the brake disk surface (piston side) for one rotation in a

Cartesian grid (rotation no.=1000, Mmean = 210 Nm, v = 200 km/h = 0.8vmax)

Regarding the drag brake applications for the three different brake pads, it can be determined

that the dominant orders of the waviness; the temperature distribution, brake disk thickness

variation and the brake torque variation are congruent, the only exception being the constraint

described above for brake pad C. Because the chosen brake application is critical only for

brake pad A, a comparison of the absolute values has no significance to their hot judder

performance. A comparison of the dominant orders for the different friction materials shows a

clear discrepancy. The non-asbestos organic friction composite caused a distortion of the

brake disk in the 8th

harmonic order; the low-metallic friction composite caused a dominant

15th

harmonic order and the copper-free composite caused a dominant 11th

/14th

harmonic

order. Therefore, it can be stated that the change of a brake pad material has an influence on

the dominant order and consequently on the dynamic behavior of all characteristic measures

within the cause-and-effect chain of hot judder.

Table 1: Results of the drag braking applications within the critical brake situation of the reference brake pad

(Mmean = 210 Nm, v = 200 km/h = 0.8vmax)

Material BTV Temperature DTV Waviness

dom.

order

∆Mmax

/Nm

dom.

order

∆Tmax

/°C

dom.

order

DTVmax

/µm

dom.

order

Wavmax

/µm

A REF 15th

164 15th

51 15th

128 15th

184

B NAO 8th

(306)* 8th

132 8th

44 8th

159

C Cu-free 11th/

14th

124/86 11th/

14th

74/56 14th

47 11th

64

* Dominant order of the brake torque variation is too close to the eigenfrequency (ef) of the torque measuring

chain in the 7th

order for 200 km/h

Comparison of the brake pads regarding stop brake applications

The brake pressure within a deceleration brake application from 225 to 80 km/h, and also the

starting temperature of the brake disk were varied for each brake pad material as shown in

Table 2. This test design is based on a hot judder test commonly used in industry. Figure 8

shows an exemplary comparison of the BTV amplitude spectrum for two tested brake

materials. The red dotted ellipses show the areas of interest again not consistent with the

eigenfrequency where the maximal brake torque variations are obtained.

Figure 8: Exemplary BTV amplitude spectrums for the stop braking application (pB = 10bar, v = 225-80km/h)

Having identified the three critical brake pressures based on the relative BTV as shown in

Table 2, it is possible to arrange the friction materials according to their hot judder generation

characteristics. Comparing the plotted relative BTV’s in Figure 9, the copper-free brake pad C

shows the worst hot judder behavior regarding the stop braking applications with relative

BTV of 134%. The brake pad B shows the lowest values in relative BTV with 19% and

therefore the best behavior. The reference brake pad A reaches its maximum relative BTV of

59% at a higher brake pressure (15 bar) and within the higher stating temperature (200 °C)

compared to the other two.

Tabel 2: Relative BTV for the stop braking applications

T Start

/°C

pmean

/bar

Mmean

/Nm

dom.

order

∆Mmax

/Nm

vτ.max

/km/h

BTVrel

-

T Start

/°C

pmean

/bar

Mmean

/Nm

dom.

order

∆Mmax

/Nm

vτ.max

/km/h

BTVrel

-

Brake Pad A REF

100

10 148 16 25 160 0,17

200

10 128 15 72 181 0,56 15 195 15 61 150 0,32 15 172 15 101 197 0,59 20 262 15 76 147 0,29 20 241 16 42 192 0,17

30 372 14 104 146 0,28 30 348 15 78 152 0,22

Brake Pad B NAO

B NAO

100

10 162 14 30 157 0,19

200

10 139 15 27 182 0,19 15 246 16 16 171 0,07 15 234 18 14 172 0,06 20 304 17 25 160 0,08 20 258 15 44 182 0,17

30 428 17 20 182 0,05 30 409 15 19 216 0,05

Brake Pad C Cu-free

100

10 173 15 231 174 1,34

200

10 158 15 148 188 0,94 15 242 15 254 187 1,05 15 223 15 83 187 0,37 20 310 15 159 147 0,51 20 283 16 58 184 0,20

30 468 15 147 146 0,31 30 433 19 46 156 0,11

Figure 9: Relative BTV’s for stop braking applications (left side with a brake disk starting temperature of 100°C

and on the right side 200°C – critical brake pressures marked with white dot)

Comparison of the brake pads regarding a drag brake applications

Similar to the full factorial DOE for only the reference brake pad with constant torque and

velocity combinations, full factorial DOE’s for constant pressure and velocity combinations

were carried out to detect the critical brake situation for all three brake pads. Here the pressure

was varied in the steps 5, 10, 15, 20, 30 and 40 bar and the velocity between 0.6, 0.7, 0.8, 0.9

and 1.0 vmax 30 sampling points for each interpolated curve shown in Figure 10 were

conducted. The table included in Figure 10 shows the detected theoretical critical situations.

Again, brake pad C shows the highest value for the maximal relative BTV (180%). Brake pad

B NAO reaches the critical relative BTV values in a brake situation with higher brake

pressure compared to brake pads A and C at a slightly lower speed (as seen in Figure 10). The

interpolated curves in Figure 10 can be interpreted as a sensitivity grid regarding the

generation of relative BTV for the different brake situations.

Figure 10: Sensitivity grids - Relative BTV’s for drag braking applications with constant brake pressure and

velocity combinations and the identified critical situations

CONCLUSION

The different experimental test designs carried out for this research allow certain conclusions

to be drawn and continuative questions to be raised:

Based on the evaluation parameter relative brake torque variation, the critical brake

pressures for the three brake pads and the two different brake applications stop and

drag braking have been identified:

o Stop braking:

For brake pad B NAO the critical brake pressure was detected at 10 bar and it

caused the lowest relative BTV of 19% followed by reference brake pad A

with a caused maximal relative BTV of 59 % at a mean brake pressure of 15

bar. Brake pad C caused the biggest relative BTV of 134 % at a mean brake

pressure of 10 bar.

o Drag braking:

For the reference brake pad A the critical brake pressure and velocity was

detected at 9 bar pressure and the maximum velocity of 250 km/h. It caused

the lowest relative BTV with 120%. Brake pad B NAO the critical brake

situation was detected to be at 24.5 bar brake pressure and a velocity of 230

km/h. The relative BTV have been at 170%. Also here for drag braking the

copper free brake pad C caused the highest relative BTV with 180% at a brake

pressure of 7 bar and a velocity of 250 km/h.

Changing the friction material of a brake pad influences the dynamic behavior of the

characteristic measures within the cause-and-effect chain up to BTV including the

waviness of the brake disk, the generated hot spots and the disk thickness variation.

The general influence is a known fact, but the significant difference caused by the

friction material that was identified within this experimental research is the variation

of the dominant order of these characteristic values. Each material within the same

brake application showed a different dominant order ranging from 8th

to 15th

and

therewith the excitation frequency of all effects of hot judder the driver might

recognize is changing as well. Due to the conducted experiments the interesting

question arises:

o Why are the dominant orders of the characteristic values of hot judder

changing when different friction materials are used (comparing brake

applications with equal brake powers)?

The occurrence of hot judder can be described for each friction material in the shown

sensitivity grids dependent on the mean brake pressure and the velocity. Further work

must clarify whether this grid is characteristically for groups of friction materials. The

measuring data provides the bases to derive sensitivity grids for each characteristic

value of hot judder. Therewith a stepwise analysis of potential correlations between

the sensitivity grids and specific brake pad parameters like for example the damping

factor or the dynamic stiffness of the friction material can be carried out.

OUTLOOK

This preliminary experimental study is the basis for further work within the research project,

including the formulation of hypotheses regarding the correlations between brake pad

parameters and hot judder characteristics and their validation and verification within another

set of experiments. A measuring method regarding different dynamic brake pad properties

within the hot-judder-relevant constraints is currently being developed. On the one hand it

will provide some of the brake pad parameters for the correlation analysis and on the other

hand it will be used for the parameterization of an analytic simulation model of the dynamics

in the brake system regarding hot judder. Additionally, an alternative measuring method

based on strain gauges for the brake torque without the influence of the eigenfrequency of the

current measuring chain is being worked on and shows first promising results. The work

currently being done is expected to lead to a more precise model description of the brake pad

within the cause-and-effect-chain which might answer some of the open questions about

phenomenon of hot judder.

REFERENCES

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Symposium, 2008

(2) Cristol-Bulthe, A.-L..; Desplanques, Y.; et al., “Coupling between friction physical

mechanisms and transient thermal phenomena involved in pad–disc contact during

railway braking”, in Wear 263, 1230–1242, 2007

(3) Engel, Hans Georg; “Systemansatz zur Untersuchung von Wahrnehmungen,

Übertragung und Anregung bremserregter Lenkunruhe in PKW”; Dissertation, TU

Darmstadt, 1998

(4) Kolluri, D.K.; Boidin, X.; Desplanques, Y.; Degallaix, G.; “Effect of natural graphite

particle size in friction materials on thermal localization phenomenon during stop-

braking”, Wear 268, 1472–1482, 2010

(5) Heissing, Bernd; Ersoy, Metin; “Fahrwerkhandbuch”, Friedr. Vieweg & Sohn Verlag,

165, 2007