Introduction to Brake Noise & Vibration

Weiming Liu & Jerome L. Pfeifer Honeywell Friction Materials

Presentation Outline
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Basic Noise Terminology Squeal

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Definition Contributing factors Analysis tools Definition Contributing factors Analysis tools

Roughness and Groan

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Summary

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Basic Noise Terminology

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Sound - transmitted acoustic pressure waves (particle oscillation within an elastic media). Frequency - Rate of the acoustic wave repetition (cycles per second = Hz). Sound Pressure - Related to radiating surface area, surface vibration velocity, and distance from the source. Broad range in sound pressure necessitates the decibel (dB) notation.
Pascal 0.000021 0.00021 0.021 0.21 0.21 2.1 21 2100 Decibel 0 20 60 80 80 100 120 160

Threshold of hearing W hisper (3 ft) Normal speech (3 ft) Shouting (3 ft) Auto horn (5 ft) Chipping hammer (3 ft) 75 piece orchestra Medium jet engine (10 ft)

Receiver Human Dog Cat Bat Cricket Robin Porpoise

Frequency (Hz) 20 ~ 20000 15 ~ 50000 60 ~ 65000 1000 ~ 120000 100 ~ 15000 250 ~ 21000 150 ~ 150000

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Frequency Range for Brake Noise and Vibration Issues

Roughness (5 - 60 Hz) DTV

Howl Howl Groan Moan

500

High Frequency Squeal Wire brush

L. F. Squeal
1K 10K

20K

Frequency (HZ)

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Brake Squeal
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Brake squeal is a phenomenon of dynamic instability that occurs at one or more of the natural frequencies of the brake system. The excitation comes from the friction couple. The disc rotor is acting like a speaker (sound wave radiated from the rotor surfaces). Pads and rotor coupling has major impact on mid to high frequency (4 ~ 16 kHz) squeal. Low frequency (1 ~ 3 kHz) squeal typically involves caliper, anchor bracket, knuckle and suspension, in addition to pads and rotor.

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Factors Influencing Low Frequency Brake Noise

Low frequency brake noise Rotor Thickness Vane pattern Hat section Material Caliper Bridge Anchor bracket Clips Rail slippers Knuckle Stiffness/mass Suspension Stiffness/mass Damping Pad Mu level/variation Material Geometry Insulator

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Factors Influencing High Frequency Brake Noise

High frequency squeal

Rotor

Pad

Thickness Vane pattern Hat section Material

Mu level/variation Material Geometry Insulator

Caliper Anchor bracket

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Tool Box for Brake Squeal

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Investigation, analysis, and validation tools:

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Vehicle test Dynamometer test Modal test Finite element simulation

Common solutions:
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Reduce excitation (chamfer design) Increase damping (insulator selection) Shift component natural frequencies (rotor, shoe plate, caliper and anchor bracket modification)

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Vehicle Test
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The ultimate judge for successful noise solutions. Test procedures (North America):
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Los Angeles City Traffic (LACT) - Standard brake noise and wear validation test; Different routes for each OE Detroit Suburban Traffic (DST) - Mainly for DTV Other tests from different OEMs and suppliers - For noise investigation and quick validation

Drivers noise ratings, noise counts, sound pressure and/or acceleration data are collected throughout the test.

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Typical LACT Noise Results

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Squeal events are identified from FFT spectrum. The number of noise occurrences and amplitudes are summarized. Additional calculations may be requested by customers.

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Dynamometer Test
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Provides noise validation within a controlled environment. Lower cost and quicker to run than vehicle test. Test procedures:
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SAE J2521 - Developed after AK with the addition of inertia stops AK - European originated procedure (mainly drag stops) Simulated LACT - A series of stops similar to LACT driving conditions The combination of the above

Sound pressure data are typically collected throughout the test.

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Typical Dynamometer Test Results

Noise occurrence over frequency, temperature, pressure, and speed range can be identified.

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Identification of Squeal Events

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Brake squeal can be identified and classified subjectively (by drivers) or objectively (processing microphone and accelerometer data). Not every up and down on the FFT spectrum curve can be considered as a squeal event. Brake squeal has a distinctive peak with possible harmonics displayed in its FFT spectrum.

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Repeatability and Reproducibility of Brake NVH Testing

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The underlining mechanisms of brake squeal involve multiple aspects in the brake system. It is not uncommon to face the following problems:
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Correlation between vehicle and dynamometer tests Repeatability and reproducibility between vehicles or dynamometers

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Correlation is more difficult to achieve for low frequency squeal since more components come into play. Environmental factors such as weather, ambient temperature, humidity and road dust may also influence test results. Best practices to improve repeatability and reproducibility in brake NVH testing:
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Use the correct level of hardware Set the correct test parameters Ensure consistency of test parts Document all variations and significant events during the test Properly collect and analyze data

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Modal Test
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Provides understanding of structural dynamic characteristics of the components and brake system. Component modal test measures natural frequencies, modal damping ratios, and/or mode shapes of pad, rotor, insulator, caliper, anchor bracket, knuckle, etc. System modal test is conducted on brake system on vehicle or dynamometer. Impact Hammer Test - quick and inexpensive. Laser Vibrometer - more elaborate with expensive equipment.
y y

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Mode shape determination Operating deflection shape (ODS) measurement

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Typical Modal Test Results

Natural frequencies and modal damping ratios may be estimated from measured frequency response (=acceleration/force) curves. The actual squeal frequency may differ from individual component frequency because of system coupling effect.

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Finite Element Simulation

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Enabling multiple design iterations to optimize noise performance without building prototype parts and tests. Two types of models:
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Brake system instability model (complex eigenvalue analysis)

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includes all major corner components suitable for low to mid frequency squeal includes pads and rotor with simulated caliper-anchor bracket supports suitable for mid to high frequency squeal

Pad-rotor-caliper support model (frequency response analysis)

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Pad Shape Optimization Through FE Simulation

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Different pad shape, shoe plate thickness, and rotor geometry can be quickly evaluated. An optimized configuration may be chosen without any vehicle or dyno testing.

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Brake Roughness Definition

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Brake Roughness is the vehicle vibration, jerkiness, or pulsation that is felt by the driver and or passenger:
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During a braking stop Sensed through

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steering wheel (shake or nibble) floor boards or dash panel car seat brake pedal typically in the 5-60hz range frequency varies with wheel speed typically 1 to 2 pulsations per revolution but can reach up to 10

Created by repetitive variation in torque output from the brake

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Brake Roughness Definition (Contd)

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Severity of roughness dependent on:

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Magnitude & Frequency of the brake torque variation Sensitivity of the vehicle
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transmission path through suspension & steering components resonance frequency & damping of vehicle components

Brake Roughness, also commonly called Judder or Shudder, is often tagged with a descriptor that helps define the conditions where the roughness is noticed:
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Hot Judder, Brake temperature > 200 C Cold Judder, Brake temperature < 100 C Green Roughness, New brake Wet Roughness, Water soaked High Speed Judder, > 130 kph

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Possible Contributors to Roughness

Friction Materials Friction Materials Caliper Caliper Rotor Rotor
Machining Seal Rollback Compressibility Metallurgy Piston Material Lining Transfer Runout - Dynamic Slide Force Abrasives Runout - Static Pin Clearance Lubricants Corrosion Bridge Stiffness Mu Level Clamp Loads DTV Resins Centering Vane Pattern Corrosion Thermal Distortions Piston Clearance Mu Variation Scorch Residual Drag Plate Thickness Taper Wear Swell Surface Finish Shoe Plate Insulator Attachment Clips Dimensional Tolerances Chamfer Length Contamination Wheel Hub Footprint Clamp Load Lug Nut Torque Tire Imbalance Hub Flange TIR Wobble Static/ Dynamic Runout Bearing Torque End Play

Torque Variation

Vehicle Vehicle Sensitivity Sensitivity

Judder

Knuckle Flex Bushing Stiffness

Resonance's

Pad Assy Pad Assy

Dust Shield Dust Shield

Wheel // Tire Wheel Tire

Hub // Bearing Hub Bearing

Suspension Suspension

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Brake Torque Variation

Torque = Ff * r = 2u*FN * r = 2u*P*A* r = c*u*P* r Lets look at what parameters could vary and contribute to torque variation dT = c*u*r*dP + c*r*P*du + c*u*P*dr Although all three have been observed to cause torque variation, the most serious is from brake pressure fluctuation dP mainly caused by Rotor Thickness Variation (RTV)
Ff = braking force r = effective radius of brake u = coefficient of friction FN = force exerted by caliper on pad P = brake line pressure A = caliper piston surface area c = 2A

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Contributors to Rotor Thickness Variation

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Lining Transfer Thermally Induced Distortions / Growth Corrosion Uneven Wear Patterns from Off-Braking / On-Braking Drag

Uneven rotor wear caused by varying pad contact while driving at highway speed is the factor that usually has the most significant influence on creating brake roughness. There can be selective wear on either one side of the rotor, or on both sides of the rotor. Rotor Thickness Variation in the range of 15-20 um (or about half the thickness of a human hair) can create torque variation in the range of 50 NM. This normally is sufficient to create vehicle roughness issues.

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Factors Influencing Uneven Wear During Off-braking and On-braking Driving

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Mounted Runout of the rotor (bearings, machining, lug nut forces) Caliper Clearance (seal rollback, slide forces, bridge / finger stiffness) Aggressiveness of the friction material and pad dimensions Load Deformation of knuckle and caliper bracket Have to consider other brake system requirements such as pedal feel, rotor rust removal, rattle, wheel dust generation, etc. A recommended approach to minimize roughness is to look at the entire brake corner as an interactive system and address all these interactions instead of concentrating on only one component.

IDEAL = 0 IDEAL = High IDEAL = Low IDEAL = 0

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Groan Definition
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Groan is a low frequency noise that is more distinctly heard inside the vehicle than outside the vehicle:
y y y y

Emanates from body structure in response to vibrations/pulsations created in brake Frequency typically in range of 30 - 600 hz Frequency content usually controlled by resonance modes in body and suspension system components Frequency sometimes driven by higher order of wheel rotational speed

Dynamic Groan:
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Created during a brake stop More pronounced from middle to end of stop Often requires prior thermal conditioning stops and or more aggressive stops to bring out the groan Introduction to Brake Noise & Vibration
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Groan Definition (contd)

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Creep Groan:
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Created when vehicle starts inching forward under light brake pressure Occurs within narrow brake pressure / speed window
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0.2 - 2 rpm 30 - 70 psi

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Brake restraining the torque from automatic transmission Noise signature usually made up of a string of pulsations ( 3 - 10 per sec) Pulsations attributed to slip-stick event between pad/caliper assembly & rotor

End of Stop Crunch / Grunt / Release Grunt:

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Terms to describe noise heard at end of stop as vehicle recovers from nose dive or when brake pressure released and suspension windup relaxes

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Potential Contributors to Groan

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Rotor Deformations - vane/channel surface ripples, grind/swirl surface finish patterns, etc. Pad thermal deformation Pad load pattern Friction film on rotor, mu variation Pad physical property values, shear and compression Mu versus speed, static / dynamic Caliper stiffness Bushing stiffness Etc.

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Dynamic Measurement of Instop Rotor Thickness Variation Showing How Vane Channel Surface Ripples Can Contribute to Groan

28th order from vane/channel pattern

Amplitude

Time,

Frequency, Hz

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Sec

500

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Signature of Creep Groan Vibration Taken With Accelerometer Mounted on Vehicle Suspension

( 192 Hz)

192 Hz

Time, Sec

Time

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To Reduce Roughness or Groan

EXCITATION
BRAKE SYSTEM Torque Variation

RESPONSE
SUSPENSION Transmission VEHICLE BODY ROUGHNESS/ GROAN

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Minimize excitation mechanism in brake Minimize vehicle response

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Increase Isolation / Damping in Suspension Add Damping to body components Shift Resonant Frequency of vehicle component

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Methods for Evaluating Roughness/groan Issues

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Vehicle tests (subjective driver ratings, with some accelerometer, torque wheel, strain gage, and microphone signature analysis)
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LACT DST Laurel Mountain Western ride Eastern Mountain ride Specialized procedures

Lab tests (quantitative comparisons, but difficult to duplicate vehicle conditions in lab setting)
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Off-Braking / On-Braking DTV procedures Rotor / friction sample low pressure wear Dyno wear tests, bath tub Static / Dynamic mu measurements In-stop torque variation and rotor thickness variation measurements

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Summary
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Brake noise and vibration issues arise from a complex interaction between the various brake and suspension components. Successful and more robust solutions come from:
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Understanding the system and individual components Systems approach Utilization of proper tools for given problem Careful interpretation of test and simulation results

Need to continue progress made on enhancing lab and vehicle test procedures, data collection and analysis, as well as modeling tools.

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