Braking of Road Vehicles

By Andrew J. Day and David Bryant

Braking of Road Vehicles, Second Edition includes updated and new subject matter related to the technological advances of road vehicles such as hybrid and electric vehicles and "self-driving" and autonomous vehicles. New material to this edition includes root causes, guidelines, experimental and measurement techniques, brake NVH identification and data analysis, CAE and dynamic modelling, advances in rotor and stator materials, manufacturing methods, changes to European and US legislation since 2014, recent developments in technology, methods and analysis, and new and updated case studies.

This new edition will continue to be of interest to engineers and technologists in automotive and road transport industries, automotive engineering students and instructors, and professional staff in vehicle-related legislational, legal, military, security and investigative functions.

• Completely revised to keep up-to-date with the demands and requirements of a new generation of road vehicles
• Includes new chapters on Autonomous and Regenerative Braking, Brake-by-Wire and Electronic Braking Systems
• Addresses issues such as prediction of brake performance, component stresses and temperatures, and durability
• Discusses operational problems such as noise and judder, variable torque generation and variable deceleration

• Language: English
• Publisher: Elsevier Science
• Release date: Mar 21, 2022
• ISBN: 9780128220061

Andrew J. Day

Andrew Day is the former Dean of the School of Engineering, Design, and Technology, at the University of Bradford, UK and course leader of the university’s well-known Braking of Road Vehicles course (widely referred to as ‘The Braking Course’) for engineers in industry.

Book preview

Preface

Aimed as an introductory textbook about the science and technology of road vehicle braking, this Second Edition of the ‘Braking of Road Vehicles’ book has been revised, updated, and expanded to reflect recent technological developments and evolution in the original subject matter so that it continues to be a source of reference for both new and experienced engineers working within the industry.

David Bryant has joined the First Edition author Andrew Day as co-author of this Second Edition, and in its preparation, we have each drawn upon our many years’ experience of teaching and research relating to braking and automotive systems. One of our most influential sources has been our interactions with the many expert practitioners and hundreds of delegates through the annual ‘Braking of Road Vehicles’ short course for the industry at the University of Bradford since 1996 (the course itself started at Loughborough University in 1966). These people, with their thirst for knowledge, have contributed their own expertise and asked many questions, giving freely of their time. We extend our thanks to all colleagues, companies, and organisations who have supported and helped us to advance our knowledge over so many years; we have named some of them below but there are many more un-named to whom we also owe some debt of gratitude.

The field of vehicle braking continues to be a fascinating discipline which is constantly evolving and adapting to meet new challenges and requirements, so we have broadened and deepened each of the chapters to reflect the associated technologies and developments. What has become apparent to us whilst writing this Second Edition is the considerable influence that regenerative braking is now having upon all areas of road vehicle braking, whether it be advanced control systems and technologies, alternative materials, particulate emissions, or legislative requirements.

The structure of the book remains much the same as the First Edition, although there has been some re-ordering of chapters and content to improve the flow. The first six chapters introduce the fundamental engineering theory; Chapters 7–11 cover specialist topics; and Chapter 12 rounds off with a selection of case studies relevant to the individual disciplines covered within the book.

Chapter 1 (Introduction) has been updated to reflect how recent developments in vehicle design, in particular powertrains, have influenced all areas of braking science and technology.

Chapter 2 (Friction Pairs) covers recent developments including new and alternative materials and the significance of particulate emissions.

Chapters 3 and 4 (Braking System Design for Passenger Cars and Light Vans/Vehicle and Trailer Combinations) have been updated to provide additional clarity to the text.

Chapter 5 (Brake Design Analysis) includes several revised analyses and an alternative method of drum brake torque calculation with comparisons to more classical methods.

Chapter 6 (Brake System Layout Design) now includes content on brake actuation requirements, electro-mechanical and electro-hydraulic actuators, regenerative braking, and mixed-mode brake blending.

Chapter 7 (Electronic and Autonomous Braking Systems) has been updated to include new content on brake-by-wire technologies, autonomous braking, regenerative braking, and expanded text on the various electronic control strategies.

Chapter 8 (Thermal Effects in Friction Brakes) includes additional content on numerical modelling, and thermal localisations, such as hot banding and hot spotting.

Chapter 9 (Brake Noise, Vibration, and Harshness) has been substantially updated covering experimental analysis of brake squeal, judder, and creep groan, with additional theoretical, experimental, and numerical examples.

Chapter 10 (Brake Testing) has been updated to provide additional detail, examples, and clarity to the text including requirements and procedures for the measurement of brake particulate emissions.

Chapter 11 (Braking Legislation) has been significantly rewritten to incorporate recent changes to EU and UN legislation with comparisons drawn with other legislation in use worldwide.

Chapter 12 (Case Studies in Braking) has been extended with three new case studies on NVH and mixed-mode braking.

The book is dedicated to two Peters, Peter Newcomb and Peter Harding, who mentored and guided Andrew Day many years ago through his introduction to road vehicle braking. The original ‘Braking of Road Vehicles’ book, co-authored by Peter Newcomb and Bob Spurr and published by Chapman & Hall in 1967, was the classic definitive introductory textbook on the subject and provided generations of Brake Engineers with the foundations of their braking knowledge. Peter Newcomb advised Andrew throughout his PhD research, and we subsequently worked together for many years on all aspects of braking, including the ‘Braking of Road Vehicles’ short course. Peter Harding was a gifted Engineer and manager (and rock-climber) at Mintex Ltd., manufacturers of friction materials, who had the most remarkable knowledge of braking and friction materials gained from a lifetime in the industry. Much of the knowledge presented in this book started from them and has been assimilated by both of us over the intervening years. Although we cannot always remember the original sources, where possible we have attempted to reference them.

Our acknowledgments and thanks go to the following people and organisations:

John Baggs and Peter Marshall (both formerly of Ford), Eddie Curry (formerly of MIRA), and Dave Barton (Leeds University) for sharing their knowledge of passenger car braking system design and brake design analysis on which Chapters 3 and 5 are based.

Brian Shilton and Colin Ross (formerly of Wabco and Knorr-Bremse respectively), and Neil Williams and Paul Thomas (Meritor) for sharing their knowledge of commercial vehicle braking system design and brake design analysis on which Chapters 4 and 5 are based.

Jos Klaps and Ludwig Fein (formerly of Ford), Thomas Svensson (Ford), Mike MacDonald (formerly of Jaguar Land Rover), Neil Williams, Colin Ross, and Brian Shilton for sharing their knowledge of brake system layout on which Chapter 6 is based.

Ian Moore, Thomas Svensson (Ford), Pierre Garnier (Jaguar Land Rover), Deaglán Ó Meachair (BrakeBetter), and Colin Ross for sharing their knowledge of electronic braking systems on which Chapter 7 is based.

Marko Tirovic (Cranfield University) for sharing his vast knowledge of the thermal analysis of brakes on which Chapter 8 is based.

John Fieldhouse (formerly of Huddersfield University) who mentored David Bryant through his PhD research, for sharing his extensive knowledge on brake NVH over many years on which Chapter 9 is based. Also to colleagues at Bentley Motors and Jaguar Land Rover for their research collaboration on which many of the results and analyses are based.

Narcis Molina (Applus Idiada), and Rod McLellan for sharing their knowledge of brake and vehicle testing on which Chapter 10 is based.

Winfried Gaupp (formerly of TÜV Nord) for sharing his vast knowledge of braking legislation and especially for his substantial help and advice in writing Chapter 11.

Jos Klaps for sharing his wealth of knowledge about braking systems generally, and specifically steering drift as presented in Chapter 12, and the many diagrams and figures carried over from the 1st Edition.

John Readle for his patient and meticulous work reviewing and proofreading every aspect of the manuscript.

Jaguar Land Rover and Valx for the cover pictures.

Meritor, Link Engineering, and Arfesan for diagrams, images, and information relating to their products.

And finally, the many other people and organisations for the knowledge and information that we have gained from them over the years and used in the First Edition as well as this Second Edition.

Andrew Day, David Bryant

February 2022

Chapter 1

Introduction

Abstract

This chapter provides an introduction to brakes and braking systems for cars and commercial vehicles including trailers to achieve safe deceleration and stability under all operational conditions. It outlines how recent developments in advanced materials, design methods, electronic control, safety systems, legislative requirements, and mixed-mode regenerative braking have all enabled the functional requirements of modern automotive braking systems to be achieved in reliable and durable designs. The contents and layout of the book are introduced; each chapter starts from the fundamental principles and practice relating to brake system design, implementation and operation, and includes a discussion with examples of analyses and calculations.

Keywords

Brakes; Braking; Deceleration; Friction; Regenerative; Systems; Automotive; Vehicle; Safety; Legislation; Introduction

‘Never start anything you can't stop' applies to many aspects of modern life but nowhere does this maxim apply more appropriately than to transport. For road vehicles, whether intended for personal or commercial use, it is surprising how performance data still appears to concentrate on the capability of the powertrain to accelerate the vehicle and provide an attractive power-to-weight ratio to maintain speed, with scarcely a mention of the ability of the braking system to decelerate it quickly and safely.

Almost since the dawn of wheeled road transport, friction between a rotor (attached to the wheel) and a stator (attached to the vehicle body, chassis, or axle) has been utilised in some form to provide controlled vehicle retardation. Other methods have historically been employed, e.g. dragging a heavy object on the road behind the vehicle, or simply steering the vehicle into a conveniently positioned obstacle, but these do not offer much in the way of sustainability, consistency, or reliability. Using transmission-mounted retarders and/or the vehicle's internal combustion engine to provide retarding torque (engine braking) is standard practice in commercial vehicles to generate braking torque, whilst aerodynamically designed ‘air brakes' are found to be effective in taking on some of the duty of the friction brakes at high speeds in high-performance cars. But still, the conventional view of vehicle braking systems, even in the technologically advanced 21st-century world of road transport, is that brakes are ‘straightforward'; what could be simpler than pushing one material against another to create a friction force to absorb the energy of motion and slow the vehicle down?

When the first edition of this book was published less than 10 years ago, it stated that ‘the braking system of a modern road vehicle is a triumph of technological advances in three distinct scientific and engineering disciplines'. These were materials science and engineering, advanced mechanical engineering, and electronics and software engineering. Since then, there has been rapid development of a fourth discipline, viz., regenerative braking, where deceleration and kinetic energy conversion is achieved by non-frictional means, which has affected many aspects of the science and technology of vehicle braking as described in the relevant chapters.

Materials science and engineering have continued to deliver technologically advanced friction pairs that form an essential part of any road vehicle braking system. These advanced friction pairs provide reliable, durable, and consistent friction forces under the most arduous conditions of mechanical and thermal loading in operating environments where temperatures may exceed 800°C. The materials used are in many ways quite environmentally sustainable, e.g. the cast iron for brake discs or drums is a relatively straightforward formulation that utilises a high proportion of scrap iron, and many friction materials include in their formulation naturally occurring materials such as mineral fibres and friction modifiers, together with recycled components such as rubber in the form of tyre crumb.

Secondly, advanced mechanical engineering design has continued to enable high-strength braking system components to be optimised to generate consistent and controllable braking torques and forces over a huge range of operational and environmental conditions. The use of Computer-Aided Design and Engineering methods has enabled stress concentrations to be identified and avoided, with the result that structural failures of brake components are extremely rare in any aspect of modern braking systems. The modern ‘foundation brake' i.e. the conventional friction brake unit, has been designed to dissipate the heat converted from the kinetic energy of the moving vehicle through the process of friction to the environment as quickly and effectively as possible. Design advances such as ventilated brake discs and sliding calipers have only been possible through the use of modern modelling and simulation techniques so that the underlying scientific principles can be applied effectively.

Thirdly, close and accurate control of braking systems and components through electronics and software engineering has positioned braking firmly in the area of active vehicle safety. In the late 1960s, the first antilock braking systems (ABS) demonstrated the safety benefits of maintaining directional control while braking under high deceleration and/or low adhesion conditions. It quickly became clear that ‘intelligent' control of the braking system had much more to offer, ranging from traction control where the brake on a spinning wheel could be applied to match the tyre/road slip to the available adhesion, through electronic braking distribution to maximise the brake torque depending on the adhesion conditions at each tyre/road interface, and most recently to electronic stability control (ESC) where judicious application of individual wheel brakes according to carefully developed and extremely sophisticated control algorithms could help mitigate the effect of potentially hazardous manoeuvres. It is worth noting that this required a change in legislation, in the sense that non-driver-initiated brake application, or ‘intervention' as it is known, had to be permitted before such active safety could be legally incorporated in production vehicles. Control systems incorporating automatically commanded braking are now commonplace on modern motor vehicles and provide a wide range of safety improvements for vehicles operating on the margins of stability or in abnormal conditions, and include for example, rollover stability control, trailer sway control, and torque vectoring by braking.

The fourth discipline, regenerative braking, requires the conversion of kinetic energy to and from a more easily storable form of energy, plus a device to which that energy can be transferred, stored, and re-used. The most widely adopted technology now (2022) is based on electric motor/generator and battery technology which has benefited from massive scientific and commercial development. Largely driven by changing consumer attitudes towards the environment and global CO2 emissions, together with government policies and financial incentives, powertrains-based solely on internal combustion engines powered by hydrocarbon fuels are rapidly being replaced in part or wholly by those incorporating compact, high-performance, electric motor/generator units, and battery storage systems with increasingly high energy and power capabilities. Approximately 20% of new cars registered in the United Kingdom in 2020 had some form of kinetic energy recovery within their powertrain which demonstrates clearly how regenerative braking can be combined with friction braking to create effective mixed-mode braking systems.

The increased complexity and sophistication of advanced braking systems will require extensive testing and verification but by using ‘model in the loop', ‘hardware in the loop', and ‘software in the loop' methods to replace the ‘traditional' rig and vehicle tests (which can be expensive, time-consuming and prone to variation due to environmental effects), this does not necessarily mean that design verification will take more time. Legislation must also keep pace with these rapid technological developments such that any safety improvement facilitated by the new systems can be embraced, but at the same time adequate control measures are put in place to prevent unintended consequences. The critical importance of braking system reliability and maintainability remains paramount.

Under normal operating conditions, regenerative braking can be the main mode of vehicle retardation for Hybrid Electric Vehicles (HEV) and Full Electric Vehicles (FEV); friction braking is used purely to supplement the braking force requirements in situations where the motor/generator, energy transfer, and energy storage systems cannot meet the performance requirements associated with downhill drag and emergency braking. This shift in both the performance requirements and operating conditions of the friction brakes has led to new challenges and opportunities for their design and implementation, which have influenced all areas of brake engineering. Brake blending together with the complexities of using brake forces at individual wheels to influence the vehicle dynamics (e.g. ESC) places increased demands on the control strategies, necessitating new methods of control intervention. Actuation systems have developed to a high level of sophistication, with electro-mechanical and electro-hydraulic brake actuators which facilitate mixed-mode blended braking whilst maintaining a consistent pedal feel. For example, the ABS/ESC modulator is fully integrated within many designs of actuator unit. The consequent reduced operational duty of the friction brake presents opportunities for lightweight rotor and stator materials with associated improved thermal management, while at the same time maintaining the capability of consistent and stable friction braking under all operating conditions. As a result, new formulations of friction material are likely to emerge; this, in turn, may mean that the nature of issues relating to brake Noise, Vibration and Harshness (NVH) broadens and adds complexity to an area that is still not fully understood. Particulate emissions from friction brakes are recognised as an increasing concern, but even with new friction pairs for mixed-mode braking, the generation, chemical composition, and particle size of brake particulate emissions will continue to be researched to ensure that threats to public health are avoided.

Alongside the remarkable technological advances that have emanated from these four areas of endeavour, it should be noted that the friction brake of a road vehicle is still a remarkably low-cost part of the overall vehicle, and the reliability and maintainability of the braking system on any modern road vehicle is extremely high. Despite the complexity and sophistication of the actuation systems, and the often environmentally challenging conditions under which the friction brakes have to operate, routine maintenance is mostly all that is required, and when replacement of, e.g. the brake pads or discs is required, the correct parts can be obtained and fitted quickly almost anywhere in the world.

The braking system of any road vehicle is subject to extensive legislative standards and requirements in many regions of the world. In this book, the legislative framework focuses on the European Union/United Nations Legislation and Regulations 13 and 13-H (UN Regulation 13-H, 2015; UN Regulation 13, 2016), although comparison with other countries' legislation is made where appropriate. EU law states that all road vehicles are required to have a working braking system that meets the legislative requirements. Included in the braking system requirements are ‘service' and ‘secondary' braking systems so that the vehicle can be safely brought to rest even in the event of the failure of one part of the system, and a ‘parking brake' that can hold the vehicle safely on a specified incline. In Europe, vehicle manufacturers have to demonstrate that their vehicle meets the design and performance standards specified in the UN Regulations through a process of Type Approval. Once a vehicle is sold, the responsibility passes to the owner or user of the vehicle to ensure that the vehicle's braking system continues to meet legal requirements; usually this takes the form of a regular compulsory examination of the vehicle. The design and performance standards associated with Type Approval are regarded as minimum standards, and most vehicle manufacturers have their own ‘in-house' standards that exceed the ‘legal requirements', often by a considerable margin. For example, UN Regulation 13-H (2015) states that the minimum service braking performance defined by the ‘Type-0 test with engine disconnected' for a passenger car (category M1) is a mean deceleration of 6.43 m/s² for a driver pedal effort (brake pedal force) of between 6.5 and 50 daN. Car manufacturers would typically design for substantially more vehicle deceleration for this level of pedal effort but have to bear in mind the requirement for the secondary braking system to provide a deceleration of not less than 2.44 m/s² within the same range of pedal effort. Pedal effort is important because of the large range of physical capability of different drivers. Likewise, the parking brake is covered by a set of legislative requirements and standards, including operating force requirement.

Fundamental to the design of a braking system for a road vehicle (under UN regulations) is that a brake is required at every road wheel. The only exception is light trailers (Category O1: trailers with a maximum mass not exceeding 0.75 tonnes), which do not need to be fitted with wheel brakes, relying instead upon the brakes of the towing vehicle. In commercial vehicle parlance, the brake unit at the wheel is known as the foundation brake. This term, which is applied exclusively to friction brakes, is used throughout this book to define the wheel brake unit for all vehicles including commercial vehicles, passenger cars, and trailers. The function of the foundation brake is to generate a retarding torque i.e. one that opposes the direction of rotation of the wheel to which it is attached, which is proportional to the actuation force applied. There are two distinct types of automotive foundation brake in common use today, viz., the ‘drum' brake see Fig. 1.1A, where the stators are brake shoes fitted with friction material linings that are expanded outwards to press against the inner surface of a rotor in the form of a brake drum, and the ‘disc' brake see Fig. 1.1B, where the stators are brake pads that are clamped against the outer surfaces of a rotor in the form of a brake disc. Included in the definition of foundation brake are the mounting fixtures such as the ‘anchor plate', (also termed ‘torque plate', ‘spider', or ‘reaction frame'), which is firmly bolted to the axle or steering knuckle. The mechanism by which the force provided by the actuation system is applied to the stator elements (pads or shoes) is also considered as being part of the foundation brake.

Figure 1.1 (A) Drum brake; (B) disc brake.

The brake actuation system comprises the mechanical, electrical, and electronic components, which recognise and interpret the ‘driver demand', typically from the movement of the brake pedal and/or the force applied by the driver to it, and convert it into forces applied to the individual foundation brakes to generate the required brake torque. A traditional basic brake actuation system transmits the force applied to the brake pedal by the driver through various mechanical connections to the wheel brake unit. These mechanical connections have taken the form of cables, rods and linkages, hydrostatic, hydraulic, or pneumatic systems, and fall into two distinct categories, viz., those that rely upon the ‘muscular' energy of the driver, and those that rely upon a separate energy source under the control of the driver to provide the actuation force. The former usually has a ‘servo' or ‘booster' in the system to provide power assistance in order to reduce pedal efforts (termed ‘power brakes' in the United States). It still forms the basic system on lighter vehicles such as passenger cars and light vans, largely because it can provide vehicle retardation even if all other control and actuation assist systems fail. The latter type of system is used on heavy commercial vehicles in the form of pneumatically actuated (also known as air brake) systems. Power hydraulic braking systems are also fitted to some types of commercial vehicle, sometimes in the form of combined ‘air over hydraulic' systems but these are not considered further here.

The fundamental scientific principles of the design and analysis of road vehicle friction brakes and their associated brake actuation systems were established many years ago. The basic performance of the braking system for any road vehicle is always specified in terms of the required brake force at each wheel. This depends upon the vehicle's design specification so this is always the starting point for braking system design. The design of the brake, actuation system, and associated components, although addressed in this book, are usually completed in detail by specialists, and from the vehicle manufacturer's point of view, braking system design has tended to become a process of specification and selection. Some vehicle manufacturers have in the past contracted the braking system design out to ‘full service suppliers' with the specialist skills and knowledge to design and deliver a vehicle braking system that meets their requirements. But increasingly, the importance of the braking system to the overall safety of the vehicle, the need for close integration of the braking system with other vehicle control and management systems, and the sensitivity of the customers to braking system performance have encouraged most vehicle manufacturers to retain a substantial interest in the braking system design. This has meant that a detailed knowledge of brakes and braking systems is valuable to the vehicle manufacturer and it is the purpose of this book to address this.

The end-user of the braking system on any vehicle is the driver, whose expectations are quite straightforward; they should be able to apply the brakes in a smooth and controllable manner to generate an equally smooth and controllable vehicle deceleration that is consistent throughout all conditions of vehicle operation and environments. In the friction brake, this requires remarkable stability in the frictional performance of the brake friction pair, viz., the friction material and the rotor, over a wide range of operational and environmental conditions. Most drivers are very sensitive to changes in the braking response of the vehicle, so brake ‘pedal feel' is a major attribute in a successful road vehicle to the extent that poor brake response can adversely affect vehicle sales. Added complexity now comes where regenerative and friction braking are combined in mixed-mode braking. This requires either partial or full decoupling of the brake pedal and actuator to facilitate blending of the brake torque, and this blending has to be transparent to the driver because of the sensitivity previously noted. ‘Smooth and controllable' also includes the NVH characteristics of vehicle braking systems; drivers (together with passengers and other road users) generally do not like their brakes to make a noise, or create uncomfortable vibration while applied, so attention to the noise and vibration aspects of a brake installation is very important for the vehicle manufacturer to avoid customer dissatisfaction.

This book covers the design, implementation, and operation of brakes and braking systems for cars and commercial vehicles with associated trailers, which represent the majority of road vehicles. The principles described do apply to other types of road vehicle, though for some other types (e.g. motorcycles) specific aspects are significantly different to those presented here. Examples of analyses and calculations are included, together with some examples of ‘things that can go wrong' and their likely causes. It starts with a consideration of the science and technology of friction as applied to friction materials and vehicle foundation brakes; this is because an understanding of friction is still considered to be fundamentally important in effective road vehicle braking system design including those incorporating regenerative braking. The decelerating road vehicle, including the specific configurations of two-axled rigid vehicles and multi-axle vehicle and trailer combinations, is then analysed to establish an understanding of the requirements of the braking system to achieve the levels of vehicle deceleration, stability, driver effort, and performance that are needed to achieve safe braking under all operational conditions. The dynamic distribution of brake force at each axle (and wheel) is then analysed taking account of longitudinal and lateral weight transfer, and parameters such as adhesion utilisation and vehicle braking efficiency are defined and developed so that safe and legally compliant braking system designs for different types of road vehicles and towing combinations can be generated. Friction in tyre/road contact is also considered and the importance of the tyre/road ‘grip' (adhesion) is explained and analysed.

Friction brake design focuses on automotive disc brakes and drum brakes. After developing the basic mechanical theory of these two types of brake, the performance characteristics of each are explained and discussed. Brake torque calculation, whether for friction brakes or regenerative braking systems, enables the actuation system to be designed to provide the required distribution of braking forces at each axle, and an eight-step procedure to design the brake system layout is outlined. Electronic braking methods and practice are described, which include the application of advanced technologies in modern road vehicle braking systems, in particular individual brake control and well-established features such as ABS and ESC. The implementation of regenerative braking and autonomous braking technologies is considered in terms of the basic principles in the eight-step system design procedure.

One of the most important operational challenges in friction brake design relates to frictional energy transformation, heat transfer, and the temperatures generated during braking. The thermal analysis of brakes is explained, including the calculation of temperatures reached in the brake components during braking, the effect of vehicle speed, load, and deceleration, and the importance of sizing the foundation brakes to withstand thermal and mechanical loads. Friction brake-related NVH is a major concern in all road vehicles, ranging from brake squeal to brake judder in passenger cars through to heavy commercial vehicles. The phenomena of brake noise and judder are introduced, and experimental and theoretical approaches to minimising their propensity at the design stage are described. The underlying theories are explained together with practical approaches to brake noise and judder reduction.

Since all designs ultimately require experimental verification, component, system and vehicle testing principles, procedures, and technologies are explained for all parts of the braking system. Aspects of braking legislation that influence vehicle braking system design are explained and discussed. Because European and other regions' legislation is based on the system of type approval, legal requirements, design analysis, and validation test procedures are explained, and comparisons are drawn with the self-certification system used in the United States and elsewhere. The book ends with an extended selection of case studies that are intended to illustrate the experimental verification of braking system design and how and why the actual performance of brakes and braking systems can vary from the design.

Refeerences

UN Regulation 13-H 2015. Regulation No 13-H of the Economic Commission for Europe of the United Nations (UN/ECE) - Uniform provisions concerning the approval of passenger cars with regard to braking [2015/2364]. Official Journal of the European Union L335, 1‐‐84.

UN Regulation 13, 2016. Regulation No 13 of the Economic Commission for Europe of the United Nations (UN/ECE) - Uniform provisions concerning the approval of vehicles of categories M, N and O with regard to braking [2016/194]. Official Journal of the European Union L42, 1‐‐262.

Chapter 2

Friction pairs

Abstract

This chapter introduces the principles and practice of dry sliding friction in road vehicle brakes. It describes how brake friction pairs are designed, manufactured, and used to generate consistent and reliable retarding torque to decelerate the vehicle. After some historical context and an introduction to the basic science of sliding friction, automotive brake friction pairs comprising conventional (resin-bonded composite) friction materials operating against cast iron rotors (brake drum or disc) are discussed in detail. The types, properties, and functions of the constituents are explained, an outline of processing practice is given, and examples of thermophysical properties are discussed. The coefficient of friction (μ) between the rotor and the stator is the most important brake design parameter and the influence of operating parameters especially temperature, on μ and other properties is discussed. Health and environmental issues are explained, including particulate emissions and brake dust. Finally, an overview of new generation friction pairs and materials for improved performance and lighter weight, especially for new vehicles with hybrid and electric powertrains and regenerative braking, is presented.

Keywords

Brake; Material; Coefficient of friction; Rotor; Stator; Pad; Lining; Composite; Carbon; Ceramic; Manufacture; Properties

2.1 Introduction

2.1.1 History

The brakes of road vehicles have relied upon friction for hundreds of years. The use of frictional forces generated between two bodies in sliding contact to provide a retardation mechanism for moving bodies can be traced back in history almost to the origins of human endeavour (Dowson, 1979). Wheel bearings were first noted 5000 years ago, but the use of wheel brakes cannot be traced back this far; it is almost certain that frictional retardation was first invoked by dragging, e.g. a log behind a horse-drawn cart when descending a steep hill. Parking while ascending, to allow the horse to rest, would be achieved by a ‘sprag' to prevent rolling back. A mechanism that pressed a friction pad against a rotating wheel was a subsequent technological advance, in the United Kingdom, probably dating from the 1700s on horse-drawn carriages. Braking devices of this basic form were then utilised on railway carriages and trucks, and then on the first ‘horseless carriage' road vehicles in Europe in the later 1800s (Newcomb and Spurr, 1989).

Fundamental to the operation of the friction brake is the dynamic or sliding coefficient of friction (μ) between the rotor and the stator components of the brake, the ‘Friction Pair'. The symbol μ is universally used for the coefficient of friction; in this book, it is used specifically to represent the dynamic or sliding coefficient of friction when the bodies in contact are moving relative to each other. The static coefficient of friction, when the bodies in contact have no relative motion between them, is represented here by μs. The iron tyre on a wooden cartwheel worked well as a rotor surface against materials such as wood, leather, and felt, but these were sensitive to the environment, e.g. mud and rain, and as vehicle speed, size, and weight increased, the amount of energy to be dissipated increased so that the operating temperatures increased beyond the limits of these materials. Recognising the importance of temperature stability of the coefficient of friction, resin-bonded composite friction materials were invented over 100 years ago to extend the operating range and durability of the friction material. Modern friction pairs comprising conventional resin-bonded composite stators (pads or linings) and cast iron rotors (brake discs or drums) have developed so far that few drivers nowadays give a moment's consideration to the function of the friction pair when they apply the brakes.

2.1.2 Frictional contact and sliding friction

Sliding friction between two dry contacting surfaces is often known as ‘Coulomb friction' after Charles Coulomb (1736–1806), but despite its everyday nature the friction forces involved can usually only be estimated from previous experience and experimental evidence. In brake friction pairs, the friction surfaces are often coated with ‘transfer layers' as a result of the sliding process. For example, a rotor friction surface may have superimposed on it layers of material transferred from the stator material and vice versa. This is a self-healing process that mitigates damage by abrasion, adhesion, or deformation during frictional sliding.

A friction surface, even if it appears to be geometrically smooth, is rough on the microscopic scale, with a distribution of asperities across it. One explanation of the genesis of friction is the interaction of microscopic asperities between the two surfaces, but when two such surfaces are forced together it is very difficult to say where and how contact between them occurs. Model systems have been studied in which the materials and surfaces were scientifically controlled in the expectation that once the friction of such systems was understood, more and more complicated systems could be examined. Although the scientific understanding of friction has benefited from such research, the extremely complicated nature of braking friction, involving high energy, high temperature, high speed, and high pressure, remains an inexact science that relies upon specialist knowledge and understanding. Whilst from an engineering point of view a constant coefficient of friction between two sliding bodies may seem a reasonable assumption, working with friction brakes requires an understanding that the coefficient of friction is likely to vary and of the reasons why.

The basic empirical laws of friction, known as Amontons' laws after Guillaume Amontons (1663–1705), are stated below. Coulomb introduced the fourth law which stated that the friction force is independent of sliding speed but whereas Amontons' laws of friction represent a good practical basis for brake friction pairs, the Coulomb law does not, for reasons explained later.

1. Friction force is independent of the nominal or apparent area of the surfaces in sliding contact.

2. Friction force F is proportional to the normal force N between two bodies in sliding contact, i.e.:

(2.1)

where μ is the coefficient of friction between the rotor and the stator.

3. The friction force always opposes the direction of sliding (i.e. the relative motion).

A simple physical explanation of the first two laws relates to the difference between the ‘real' area of contact AR (based on the total surface areas of the microscopic asperities in contact) and the ‘apparent' (or ‘nominal') area of contact AN (indicated by the overall size of the contact interface) at any friction interface. The real area of contact AR is very much less than, and independent of, the apparent area AN, but is proportional to the normal force between the two bodies in sliding contact. A simplified model of the contacting surfaces based on the idea of contacting asperities is shown in Fig. 2.1; a series of elastic hemispheres on one surface pressing against another perfectly flat rigid surface.

Figure 2.1 Idealised model of frictional contact.

Each hemisphere can be considered to adhere to the flat surface so that when sliding occurs a shear force is generated that is proportional to the area of contact between it and the rigid surface. The sum of all the areas of the hemispheres in contact with the rigid surface equates to the real area of contact:

(2.2)

If the constant f (N/m²) denotes the specific friction force (i.e. the tangential friction force per unit real area of contact), then for any individual contact ‘i' :

(2.3)

Making the assumption that the area of contact between each hemisphere and the rigid surface is proportional to the normal force between them:

and therefore

(2.4)

Since therefore μ must also be constant.

This simple theory assumes that the surfaces adhere to one another at the real areas of contact when pressed together by a normal force and that no adhesion remains when the normal load is removed. Any variation in μ is attributed to variations in f arising from differing degrees of contamination of the surfaces. For a full understanding of the phenomenon of friction, it would be necessary at the very least to be able to determine AR and f, but the more this is investigated the more complicated it appears and there is no easy way of doing this for the types of friction material pairs used in modern automotive braking systems. Hence it remains necessary to measure rather than calculate the frictional properties of any friction material, although skilled formulators are able to estimate friction (and wear) behaviour based on their knowledge, expertise, and experience.

‘Tribology' is the generic name for the science of friction, lubrication, and wear. Braking friction mostly constitutes ‘dry' friction in which there are three main mechanisms that may be considered to contribute to the generation of friction and wear in a brake friction pair (Spurr, 1976):

1. Adhesion. Components of the friction material adhere to asperities on the mating surface, and the friction force is generated by the action of shearing those junctions.

2. Abrasion. Components of the friction material abrade the mating surface, physically removing parts of any transfer film and the mating surface material. The avoidance of excessive wear of the mating surface can be achieved by carefully balancing those constituents of the friction material that contribute to transfer film generation.

3. Deformation. The friction material is deformed by the action of interfacial shearing. The high hysteresis of the friction material causes the work of deformation to appear as heat, which is then dissipated by conduction back into the friction material and transfer across the friction interface into the mating body. The proportion of heat that flows each way influences heat energy transfer from the brake friction interface.

There are other processes and mechanisms that contribute to the tribological behaviour of particular brake friction pairs, but these three mechanisms are sufficient for a practical understanding of brake friction science.

2.2 The friction pair

2.2.1 Rotor and stator

A modern automotive brake friction pair comprises a stator of friction material mounted on a mechanically strong and durable backplate (disc brake) or shoe (drum brake) that rubs against a rotor that rotates with the wheel to which it is attached. There are many different types of friction pairs in use today. Some of these are relatively humble (cast iron brake shoes are still used on railway stock, mating on the treads of steel wheels) while others are exotic, such as carbon fibre composites (carbon–carbon (C–C)) on aircraft and Formula 1 racing cars, or carbon–ceramic (C–SiC) as used on high-performance road cars. Sintered metal friction materials are used in some industrial applications and in high-duty, high-performance applications, such as rally cars and motorcycles, while cork- or paper-based friction materials are still used in low-duty applications, usually oil-immersed, typically in automatic transmissions and clutches. However, the most widely used brake friction pair is still a resin-bonded composite friction material stator operating against a cast iron rotor.

Grey (flake graphite) cast iron has been used for brake discs and drums for over 100 years. It is well-established, has adequate strength for braking purposes, stable mechanical properties, good machinability, and is readily available at a low cost. It is mechanically and thermally durable despite its low ductility and Young's modulus (compared with steel) and has good tribological compatibility including wear resistance with many types of friction material. It has very good heat transfer properties, especially thermal conductivity and specific heat, with a high maximum operating temperature (MOT) see Chapters 6 and 8. Its relatively high hysteretic damping properties are valuable in brake noise, vibration, and harshness (NVH) suppression, see Chapter 9. The graphite flakes that are characteristic of grey cast iron microstructure are formed during the slow solidification of the material from its molten state. ‘Spheroidal Graphite' (SG) cast iron, in which the graphite takes a spherical form, has higher ductility but reduced heat transfer properties, while ‘Compacted Graphite' (CG) cast iron has microstructure and properties somewhere between grey and SG cast irons. SG and CG are mostly used for rotors only when ductility is important, e.g. for greater crack and fatigue resistance than grey cast iron. A description of the typical elements (additional to iron) in brake disc cast iron compositions is shown in Table 2.1. Refer to Chapter 8 for design analysis and material properties relating to cast iron rotors.

Table 2.1

The type of cast iron used for the brake rotor, its precise metallurgical composition, microstructure, and manufacturing processes, must be carefully controlled. For example, small (trace) amounts of elements such as titanium and vanadium have been found to drastically affect the friction and wear performance of certain types of resin-bonded composite friction materials, while much larger amounts of titanium in a cast iron not only affect the friction and wear performance but also can render it almost unmachineable (Chapman and Hatch, 1976). The surface finish of the rotor produced by different machining processes can affect the friction and wear performance of the brake pad or lining.

Although conventional resin-bonded composite friction materials may be operated against rotors made of materials other than cast iron, their performance may be significantly different. An example is the dependence of the coefficient of friction of a conventional resin-bonded brake friction material on the mating material, as illustrated in Table 2.2. These data were generated on a small sample pin-on-disc test machine using 10 mm diameter friction material specimens, see Chapter 10.

Table 2.2

The use of resin-bonded composite friction materials operating against cast iron rotors in road vehicle brakes is starting to change as ‘new generation' powertrains with regenerative braking, especially on hybrid and electric vehicles (HEV/EVs), emerge as a result of mainly environmental concerns. New generation friction pairs, utilising materials such as aluminium, metal matrix composites, carbon fibre composites, and ceramics are beginning to appear. These present both opportunities and challenges to designers and materials engineers and are discussed later.

2.2.2 Functional requirements

Whichever materials are chosen for existing and future generations of road vehicles, the functional requirements of a brake friction pair include:

• Consistent and reliable frictional force: A road vehicle must meet the legislative demands of braking when sliding in a controlled manner against a defined mating surface. This encompasses all aspects of vehicle usage: speed, temperature, mechanical and thermal loadings, and environmental conditions, such as humidity and moisture, dirt and dust. As an example, stainless steel is now extensively used for motorcycle brake discs, but when first introduced the ‘wet' performance was found to be substantially lower than the dry performance, sometimes by up to a factor of 3, from 0.6 in dry conditions to around 0.2 in wet conditions. To overcome this inconsistency of braking performance, manufacturers now provide slots and/or grooves to help clear water film from the friction surface, and have also developed special formulations of friction material for operation against stainless steel discs.

• Durability: The effective life of both parts of the friction pair must be in line with vehicle manufacturers' service targets. They must wear in a steady and controlled manner, and not suffer from degradation of performance as wear occurs. The friction material has traditionally been the sacrificial part of the friction pair because it was cheaper to replace a brake pad or lining than a cast iron disc or drum. Both stator and rotor wear in use, and the replacement of both parts of the friction pair is increasingly seen as being necessary to ensure consistent braking performance, especially as new types of lightweight brakes start to become specified on road vehicles with regenerative braking.

• Strength: Rotor and stator materials must be mechanically and thermally strong enough to withstand the loads experienced during use. This includes for example the attachment of the friction material by adhesive bonding or mechanical means (riveting) to the stator, and having an acceptable compressibility so that the brake pedal movement is not excessive.

• Tribological compatibility: This can mean that the mating surfaces should not be damaged by the action of frictional sliding, and wear should be kept within accepted limits (with no expectation that mating surface wear is zero). But it can also refer to erratic friction or wear behaviour which might lead to NVH problems as explained next.

• Low NVH propensity: In the context of road vehicle brakes, NVH relates to phenomena including squeal, judder, and groan, see Chapter 9. Friction pair designers aim to minimise or preferably avoid frictionally initiated instabilities that cause vibrations in the brakes, suspension, or other parts of the vehicle system leading to noise and/or vibration. This links back to tribological compatibility.

• Environmental acceptability: The manufacture of brake friction pair materials requires high technology materials and processes which must meet environmental standards and regulations. In many parts of the world, emissions from road vehicles are increasingly tightly controlled by legislation covering the emission of carbon dioxide and other pollutants including particulates. At present, debris or waste from vehicle brakes is not included in the legislation but for environmental reasons some regions (the European Union (EU) for example) are preparing to address ‘whole vehicle' emissions, including particulate emissions from friction brakes, see Chapter 11.

• Cost-effectiveness: A major advantage of conventional brake friction pairs (resin-bonded composite friction material/cast iron) is that they are cost-effective for both manufacturers and customers. Some new types of friction pair are based on expensive high technology materials and manufacturing processes, and as a result these are presently limited to use in high performance and prestige road vehicles.

• ‘Feel': The stator plays an intrinsic role combined with the brake actuation and transmission system in determining the subjective (brake pedal) feel and progression of the brake pedal in modern disc brake installations. Deformation of the pad assembly is influenced by the caliper design, the compressibility of the friction material, and any backplate shim, and these, together with the pad coefficient of friction, are important in defining the relationship between pedal travel and deceleration. From the driver's perspective, the pedal feel should be consistent and progressive to give confidence in the brake system performance and should be comparable with other vehicle models in the manufacturer's portfolio, see Chapter 6.

2.3 Resin-bonded composite friction materials

2.3.1 Formulation and design

Modern resin-bonded composite friction materials are specially formulated to give good friction and wear braking performance, usually against cast iron rotors. The basis of their formulation is usually a polymeric binder (resin) and a fibrous matrix that provides most of the mechanical strength necessary to withstand the generated frictional forces.

Asbestos, once used universally as the fibrous matrix in friction materials, is a proven carcinogen. Five of the six types of asbestos were banned in the EU from 1991 and a complete ban came into force in 2005 (The European Commission bans White Asbestos). Most European friction material manufacturers have not used asbestos in friction materials since the 1980s having started to develop non-asbestos formulations in the 1970s based on a range of substitutes including cotton, mineral, metal, and organic fibres. These are considered to have fewer known health and environmental disadvantages. Fillers, friction modifiers, solid lubricants, etc., are included to tailor the mechanical, thermal, friction, and wear properties, and often only a small amount of one constituent can have a substantial effect on the overall friction and wear performance of the friction material. The binder is a thermoset polymer most commonly based upon a phenolic resin and may include other related organic compounds to achieve the desired chemical, processing, and thermophysical properties. The information about modern friction materials discussed below concentrates on friction materials for disc brake pads. Large drum brakes on commercial vehicles tend to use friction materials that are similar to disc brake pads in terms of the technology of formulation and manufacture, while the drum brake linings used on many mass-produced cars can be simpler.

Resin-bonded composite disc brake pads are generally considered to fall into three generic classes: ‘Non-Asbestos Organic' (NAO), ‘low steel', and ‘semi-metallic' (precise nomenclature varies from manufacturer to manufacturer, and also in different countries). Each class includes many different proprietary formulations designed to match the operational duty and requirements specified for particular brakes and vehicles, usually to operate against a cast iron rotor. Of these three classes, NAO friction materials are usually the most expensive. They tend to have lower μ levels (typically 0.3–0.4), and have superior wear characteristics up to around 220°C, although wear can increase dramatically at higher μ levels and higher duty. They tend not to use significant quantities of iron or steel in the formulation, are relatively clean in operation (generating low levels of visible wear products), and have low noise propensity, particularly in terms of avoiding brake squeal. Low steel friction materials, as the name suggests, use iron and/or steel in the formulation. They have higher μ levels than NAO materials (typically 0.35–0.5) and are considered to provide good ‘pedal feel'. Fade and high-speed/duty performance is considered to be good, but noise propensity tends to be higher. Wear characteristics may not be as good as for NAO materials and they are not so clean in operation (brake dust is discussed later). Semi-metallic friction materials tend to be simpler formulations compared with NAO and low steel materials and have a ferrous metal content of up to 40% by weight. These friction materials are the lowest cost of the three classes and tend to have low μ levels (typically 0.25–0.35). High wear can be experienced at high speed and low temperatures, but wear and fade performance are better at higher temperatures. They tend to be prone to noise and judder compared to NAO materials and, because of the high metallic content, higher heat transfer through the disc brake pad can cause high brake fluid temperatures in a hydraulic system, potentially affecting operation. The formulations of commercial friction materials are regarded as highly confidential and are proprietary to the manufacturers. Typical compositions are shown in Table