HSRI Report No. HuF-6 Brake Force Requirement Study: Driver- Vehicle Braking Performance as a Function of Brake System Design Variables R. G.Mortimer, L. Segel, H. Dugoff, J.D. Campbell, C.M, Jorgeson, R. W. Murphy Highway Safety Research Institute University of Michigan Huron Parkway and Baxter Road Ann Arbor, Michigan 481 05 April 10, 1970 The contents of this report reflect the views of the Highway Safety Research Institute which is responsible for the facts and the accuracy of the data presented herein, The contents do not necessarily reflect the official views or policy of the Department of Transportation. This report does not constitute a standard, specification or regulation. I. Report No. 2. Government Access~onNo. 3. Rec~p~ent'sCatalog Xo. 1 4. T~tleand Subt~tle 1 5. Report Date 1 April 10, 1970 Brake Force Requirement Study: Driver-Vehicle 6. Perforrn~ngOrgan~zat~on Code Braking Performance as a Function of Brake System Design Va~iables 7. Author(s) 8. Performing Organ~zat~onReport No. R.G. Mortimer, L. Segel, H. Dugoff, J.D. Campbell, C.M. Jorgeson, R.W. Murphy HuF-6 9. Performing Organ~zat~onName and Address 10. Work Uut No. Highway Safety Research Institute University of Michigan Ann Arbor, Michigan 48105 11. Contract or Grant No. FH-11-6952 13. Type of Report and Per~odCovered 12. Sponsoring Agency Name and Address Final Report Federal Highway Administration Julv 1. 1968-A~ril10. 1970 -- - National Highway Safety Bureau 14. Sponsonng Agency Code Washington, D.C. 20591 15. Supplementary Notes 116. Abstract The objective of this study was to define those brake characteristics, within the space bounded by the relationship between brake pedal force and vehicle deceleration, which lead to acceptable driver-vehicle performance. A driver-vehicle braking test was performed in which the deceleration/pedal force ratio, the pedal displacement, the surface-tire friction, and driver character- istics (age, weight) were systematically varied in order to determine the influ- ence of these variables upon minimum stopping distance and other performance variables. The tests that were performed on a low coefficient of friction sur- face showed that high values of deceleration/pedal force gain result in large number of wheel lockups and lower mean deceleration in bringing the vehicle to a stop, compared to intermediate or low deceleration/pedal force gain levels. Tests conducted on intermediate and high coefficient of friction surfaces showed that high and intermediate deceleration/pedal force gains produced greater mean decelerations and greater frequencies of wheel lockups than lower gain systems. The frequency of loss of lateral control was significantly greater with the high deceleration/pedal force gain brakes on all surfaces than with lower gains. There'were minor benefits of 2.5 inch pedal displacement compared to zero inches. Potential brake failures and their effects upon pedal force requirements were analyzed. The implications of the findings for a vehicle braking standard were shown in terms of deceleration/pedal force gain and pedal force. I 17 Key Words 'BRAKING, DECELERATION, STOPPING DISTANCE, Availability is unlimited. Document 1 DRIVER BRAKING, PEDAL FORCE, PAVEMENT may be released to the Clearinghouse FRICTION, BRAKE FAILURE. for Federal Scientific and Technical Information, Springfield, Va. 22151 fer sale to the public. 19. Secur~tyClass~f (of thls report) 20. Securlty Classif.(of thls page) 21. No, of Pages 22. Pr~ce unclassified I unclassified I xx + 200 I TABLE OF CONTENTS List of Tables List of Figures Acknowledgements Introduction .................... Summary of Tasks .................. 1. Literature Review ............. 2 . Foot Force Capability of Drivers ...... 3 . Driver Braking Performance as a Function of Pedal-Force and Pedal-Displacement Levels . 4 . Driver Braking Practice .......... 5 . Failure Analysis .............. 6 . Recommendations .............. Tasks ........................ 1. Literature Review ................ Introduction ..................... 5 Pedal Force as a Function of Design Parameters .... 9 Brake System Design ............... 9 Pedal Force Design Goals .......... 9 Pedal Force/Brake Torque Relationship .... 9 Brake Proportioning ............. 12 Design Practice and Trends ............ 13 Brakeusage* .................... 15 Brake Usage Measurements ............. 15 Fade and Fade Testing .............. 16 The Phenomenon of Fade ........... 16 Thermal Analysis .............. 17 Lining Materials .............. 18 Fade and Pedal Effort ............ 18 Brake System Degradation ............. 19 Hydraulic System .............. 19 Wear of Friction Elements .......... 20 Skidding as RelatedtoBraking............ 20 Importance of Braking Control in Accidents ... 20 Braking Dynamics ................ 21 Road-Tire Friction Coefficients ......... 22 Skid Control by Braking Modulation ....... 23 Testing of the Vehicle-Tire-Brake System ....... 25 Deceleration Performance ........... 25 Stopping Distance ........25 Deceleration Measurement ......25 Directional Control ............ 26 Control of Braking Tests ........26 Driver Response ................... 27 Static Driver-Vehicle Relationships ....... 27 Driver Transient Response Characteristics ...28 Driver-Brake Pedal System Dynamics ....... 31 2 . Foot Force Capability of rivers. ........... 32 Introduction ..................... 32 Method ....................... 34 Apparatus .................. 34 Pilot Study ................. 38 Procedure .................. 38 Results.. ................. 38 Mainstudy ................. 40 Procedure .................. 40 Subjects ................. 41 Results ....................... 41 Discussion ...................... 42 3 . Driver Braking Performance as a Function of Pedal-Force ana Pedal-Displacement ........... 50 Introduction ..................... 50 Method ........................ 52 Subjects ..................... 52 The Test Vehicle ................. 52 General Description ........... 52 The Brake System ............ 55 Brake Proportioning .......... 58 B;ake System Parameters ..,..... 60 Speed Control System ....... 61 Data Collection Instrumentation .... 63 Independent Variables ............ 70 Dependent Variables ............. 71 Procedure: Pilot Studies .......... 71 Hydroplaning .............. 71 Procedure: Braking Test ........... 74 Performance and Subjective Data Recording 75 Experimental Design ............. 76 Results ...................... 76 Braking Distance ............... 76 Deceleration ................. 77 Braking Time ................. 84 Forty Percent Decrease in Speed ....... 85 Wheel Lockup Frequency ........ 86 Wheel Lockup Duration ............ 86 Proportion of Wheel Lockup Time to Total Braking Time .............. 95 Loss of Lateral Control ......... 95 Ratings of Controllability .......... 99 Ratings of Pedal Force ............ 99 Between-Subject Performance Comparison ...100 Correlation Between Maximum Pedal Forces Measured in the Vehicle and the Buck ....100 Subject Age and Weight ............ 103 Discussion .................... 103 Deceleration Measures ............ 103 Loss of Control Measures ........... 104 Subjective Measures ..............106 Driver-Vehicle Braking Efficiency .......106 Derivation of the PFG Envelope ........109 Development of a Revision to MVSS-105 .....112 4 . Driver Braking Practice ...............114 Introduction ....................114 Method .......................115 Apparatus ...................115 Procedure ...................120 Subjects ...................120 Results ......................120 Discussion .....................124 5 . FailureAnalysis ..................125 Introduction ....................125 Failure Modes ...................125 Line Pressure Failure .............125 Power Boost Failure ..............133 Brake Fade ..................136 Factors Influencing the Partial Failure of Brake Systems ...................140 Consequences of Failure ..............147 Effects on Vehicle Performance ........147 Influence of Partial Failures on Driver- Vehicle Braking Performance ..........148 General Discussion ..............157 Recommendations .....................162 Appendices ........................ Appendix I . Derivation of Constant Pedal Dis- placement/Deceleration Character- istic ............. Appendix I1 . Instruction to Test Subjects ....166 Instructions-Practice Run .......... Instructions-Official Run ...........168 Appendix I11 . Vehicle Instrumentation for Deceleration Recording ....... 175 Deceleration Measurement ........... 175 Deceleration Calibration ........... 177 Brake Line Pressure ............. 181 Brake Line Pressure Calibration ....... 181 Sample Data ................. 181 Instructions to Driver ............ 183 Trip Sheet ................. 184 References ....................... 185 vii LIST OF TABLES -Table Page 2.1. PILOT STUDY MEDIAN RIGHT AND LEFT FOOT FORCE BY THREE GROUPS OF SUBJECTS FOR "STANDARDtf AND "INDUCED" MOTIVATION INSTRUCTION. DATA ARE IN POUNDS. .................... AGE DISTRIBUTION OF FEMALE AND MALE SUBJECTS. WEIGHT DISTRIBUTION OF FEMALE AND MALE SUBJECTS CUMULATIVE PERCENT RIGHT FOOT FORCE DISTRIBU- TION: 276 FEMALE DRIVERS ........... CUMULATIVE PERCENT RIGHT FOOT FORCE DISTRIBU- TION: 323 MALEDRIVERS. ........... CHARACTERISTICS OF THE TEST SUBJECTS (DRIVERS). PEDAL FORCE GAINS ............... PILOT TEST: MEAN BRAKING DISTANCE (FEET) ON DRY AND WET FOR POWER AND MANUAL BRAKE. .... PILOT TEST : MEAN DECELERATION (ft/;ec2) ON DRY AND WET FOR POWER AND MANUAL BRAKE. .... PILOT TEST: MEAN TIME (SECONDS)