High Speed Off-Road Vehicles: Suspensions, Tracks, Wheels and Dynamics

Dedication

To the late Professor David Crolla, whose advice, knowledge, encouragement and

humour are sorely missed.

To my wife Jacqueline, for her encouragement and tolerance of my many hours

communing with journals, papers, books and my computer.

Series Preface

The automobile is a part of our society, and tightly linked to many aspects of our daily lives. We see a wide variety of vehicles every day, passing us on the streets in our cities and on our motorways. There is of course a vast plethora of different vehicles used for different aspects in our daily lives, and in more special applications. Perhaps some of the more interesting and exciting applications are those that are far removed from our everyday lives. Two of the more famous and popular vehicle applications in the automotive sector are high speed vehicles such as race cars and off‐road vehicles such as large earth‐moving equipment. Being in the field for over 30 years, many of those years as a faculty member, I can testify to the fact that most people get very excited when they are inspecting a top‐rated race car or see these vehicles on the track. The same is true when standing next to or watching large earth‐moving equipment in operation. There is nothing quite like seeing an earth‐mover that is capable of effortlessly hauling the volume of several large sedans up a steep grade out of a mining operation. Of course, the combination of these two areas into High Speed Off‐Road Vehicles is an amalgamation that is not only extremely exciting from an engineering perspective, but one that presents unique challenges to vehicle designers that are not faced in many other automotive sectors.

High Speed Off‐Road Vehicles is an excellent and in‐depth review of vehicle performance in off‐road conditions with a focus on key elements of the running gear systems of vehicles. In particular, elements such as suspension systems, wheels, tyres and tracks are addressed in depth. It is a well‐written text that provides a pragmatic discussion of off‐road vehicles from both a historical and analytical perspective. Some of the unique topics addressed in this book include link and flexible tracks, ride performance of tracked vehicles, and active and semi‐active suspension systems for both armoured and unarmoured vehicles. The book also provides spreadsheet‐based analytical approaches to modelling these topic areas, providing insight into steering, handling and overall performance of both tracked and wheeled systems. The author further extends these analyses to soft‐soil scenarios and thoroughly addresses rollover situations. The text also provides some insight into more advanced articulated systems.

It is quite clear that this text is a unique and valuable addition to the Automotive Series whose primary goal is to publish practical and topical books for researchers and practitioners in industry, and postgraduate/advanced undergraduates in automotive engineering. The series addresses new and emerging technologies in automotive engineering, supporting the development of next‐generation transportation systems. The series covers a wide range of topics, including design, modelling and manufacturing, and provides a source of relevant information that will be of interest and benefit to people working in the field of automotive engineering.

High Speed Off‐Road Vehicles is written from a very pragmatic perspective, based on the author’s extensive experience, and provides an excellent introduction to off‐road vehicles. Simultaneously, it is a strong reference text for those practising design and analysis of such systems. No other text covers the concepts and vehicle systems that are presented in this book. It is an excellent read, very understandable and highly informative. The bottom line is that this book covers a very interesting topic area and is highly unique in its content, making this book a welcome addition to the Automotive Series.

April 2018

Thomas Kurfess

Acknowledgements

I would like to extend my gratitude to:

the UK Ministry of Defence, for sponsoring most of the work described in this book;

the many colleagues at DERA who contributed, and in particular Robert Gray, Robin Warwick, Peter Cox, Narinder Dhillon and Matt Williams; and to my nephew Peter Maclaurin for producing some of the line drawings.

the team at John Wiley and Sons, including Eric Willner and Anne Hunt (commissioning editors), Nithya Sechin and Blesy Regulas (project editors), P. Sathishwaran (production editor) and Elaine Rowan (freelance copy editor).

Introduction

To a large extent, this book reflects my time and experience working at the UK Ministry of Defence Military Vehicle Establishment at Chertsey, Surrey. During this time it was variously known as FVRDE (Fighting Vehicles Research and Development Establishment), MVEE (Military Vehicles Engineering Establishment), RARDE Chertsey (Royal Armaments Research and Development Establishment), DRA (Defence Research Agency) and DERA (Defence Engineering and Research Agency) before it was closed in 2002 and split between QinetiQ and DSTL (Defence Science and Technology Laboratory). For the purposes of the book, the establishment is generally called DERA. The term ‘high speed’ in the title of the book is used somewhat loosely, but is meant to exclude mainly unsprung vehicles. The vehicles described are largely military because they are the most common type of off‐road vehicles, although wheeled logistic vehicles spend much of their time on roads.

The book mainly describes the running gear systems of vehicles, that is, the suspension systems, tracks, wheels and tyres and their effects on vehicle performance in off‐road conditions. The book does not review vehicle power trains, except for describing the mechanisms used for providing the differential track speeds required for steering tracked vehicles. The engines used in wheeled vehicles, logistic and armoured, are typically uprated commercially available engines. For main battle tanks (MBTs), more specialist units are required because of the need to combine high power (up to about 1100 kW) with very compact dimensions. Apart from the Abrams tank, which uses a gas turbine engine, the diesel engine technology used is still fairly conventional. The transmissions of tracked military vehicles tend to be specialist because of the need to combine the drive engagement and ratio‐changing functions with the system used for providing differential speeds to the tracks. The drive engagement and ratio‐changing systems remain largely conventional. The units also need to be mounted transversely. As described in Chapter 7, electric drive and steering systems are now being investigated for these functions.

Much use has been made in the book of spreadsheet analysis using Microsoft Excel, and particularly the Solver routine for solving equations of motion. Although the Excel/Solver approach can be somewhat laborious at times, an advantage is that the equations of motion need to be written from first principles, particularly those for tracked vehicles, requiring careful analysis of and good insight into the systems being studied.

The book is partly descriptive of past and present systems and partly analytical. It is not an academic book, or intended to be so, but hopefully some of the methods shown will be of use to vehicle designers.

Chapter 1 describes the suspension systems that are and have been used on tracked vehicles. In particular, the properties of the hydrogas unit used on the Challenger tank are examined in detail.

Chapter 2 describes vehicle track systems, both link tracks, and the flexible tracks that are increasingly being used. Performance aspects considered include rolling resistance and the noise and vibration caused by link tracks.

Chapter 3 examines the ride performance of tracked vehicles, including human response to vibration, terrain profiles, wheelbase filtering and computer modelling. Pitch response to braking is also considered.

Chapter 4 examines the potential advantages of active and semi‐active suspension systems and describes two DERA test vehicles and their ride performance.

Chapter 5 describes the driveline and suspension systems of wheeled vehicles, both unarmoured and armoured. Interconnected suspension systems are also described.

Chapter 6 considers the suspension performance of wheeled vehicles including the use of quarter‐car models and the effect of using the different ISO 2631 and BS 6841 HRV filters. Also described are some ride performance measurements of a logistic vehicle.

Chapter 7 examines the steering performance of tracked and wheeled vehicles. The Magic Formula, widely used for describing the force–slip characteristics of pneumatic tyres, is used here to describe the force–slip properties of a track system in a skid steering model. Results are shown for steering response and also for the power flows through the double‐differential steering system. Similar models are shown for comparing the steering performance of a skid steered and an Ackermann steered wheeled vehicle. The effects of torque vectoring are also considered.

Chapter 8 examines the soft‐soil performance of wheeled and tracked vehicles. Most predictive methods are empirically based, particularly for wheeled vehicles, because of the difficulty of directly modelling the behaviour of a pneumatic tyre in a soft yielding soil. The results of DERA field trials with single pneumatic tyres and a track rig are described together with the predictive models developed. A tractive force–slip relationship for a tyre in a soft cohesive soil is also developed from the field trial results.

Chapter 9 describes the effects of limited‐slip differentials on the traction and steering performance of vehicles. Relationships are developed to describe the effects of frictional limited‐slip differentials on tractive performance on different surfaces and soils. Results are compared with those for free and locked differentials. The effects on steering performance on a road surface are also examined.

Chapter 10 describes some previous, experimental and current articulated vehicles, both tracked and wheeled. The traction forces required to steer skid and articulated tracked vehicles are compared. Similarly, the traction forces required to steer articulated wheeled vehicles on hard and soft soils are compared with those for skid and Ackermann steered vehicles.

Chapter 11 examines the basic relationships that determine the likelihood of a vehicle to rollover. Methods to reduce the likelihood of rollover are reviewed. A study of a rollover incident with a logistic vehicle is described. A model is developed to predict the rollover angle of the vehicle and compare it with the measured vehicle tilt angle.

The author has made every reasonable effort to trace copyright holders and to obtain permissions to reproduce images in the book. Apologies are made if suitable permissions have not been obtained; copyright holders should then contact the publishers so that corrections can be made in any further print editions.

1

Tracked Vehicle Running Gear and Suspension Systems

The running gear systems used on high speed, mainly military, tracked vehicles provide four essential functions:

the transmission of drive to a relatively large number of road wheels;

the distribution of the weight of the vehicle over a relatively large area;

a large suspension displacement to allow high speeds over rough terrains; and

a particular requirement of military armoured vehicles, the running gear system should occupy the minimum space in the overall vehicle envelope in order to maximise internal hull volume (as will be shown in Section 8.4, this is a particular attribute of tracked vehicles compared to wheeled vehicles of similar soft‐soil performance).

In addition, the running gear must be of minimum weight, reliable, easy to maintain, and compared to some other vehicle components, relatively cheap to produce.

1.1 General Arrangement

Figure 1.1 shows the running gear of the Warrior Infantry Fighting Vehicle (IFV) and is typical of modern practice. Trailing suspension arms carry rubber‐tyre road wheels and operate transverse torsion bars which run across the floor of the vehicle. Rotary vane hydraulic dampers are incorporated into the pivots of the front, second and rear road wheel stations. Link tracks run under the road wheels and around hull‐mounted drive sprockets and return idlers. Track pretension is adjusted by means of oil‐filled rams reacting against the idlers, which are carried on short pivoting arms. The drive sprockets are front‐mounted but could be at the rear of the vehicle, depending on the position of the power pack. Small diameter rollers support the top run of the track. The track link pivots are rubber‐bushed and the links are fitted with replaceable rubber road pads to minimise road damage and reduce noise and vibration.

Figure 1.1 Warrior running gear layout. Source: Courtesy of Ministry of Defence.

Figure 1.2 shows the arrangement on the Leopard 2 Main Battle Tank (MBT). Rotary friction dampers are built into the front three and rear two axle arm pivots. The vehicle is fitted with rubber‐bushed double‐pin tracks (see Chapter 2).

Figure 1.2 Leopard 2 running gear layout. Source: Courtesy of ATZ.

1.2 Transverse Torsion Bars

Modern high‐strength spring steels, used with suitable presetting, shot peening and corrosion prevention techniques, allow nominal shear stresses of up to 1250 mPa to be used with a reasonable fatigue life [1.1, p. 226]. Suspension torsion bars are only loaded in one direction and so can be ‘preset’. To preset a torsion bar, it is wound up to induce partial yielding in the outer layers of the bar. On release, the outer layers take on negative shear stresses and torques opposed by positive stresses and torques in the inner layers of the bar (Figure 1.3).

Figure 1.3 The principle of presetting a torsion bar.

The relationship between the various variables that affect the maximum shear stress in the bar can be explored by setting up a suitable spreadsheet. The vehicle will be considered as a notional MBT with a sprung mass of 600 kN and an effective torsion bar length of 2.13 m. The variables that can be considered are the axle arm length (initially taken as 450 mm), the number of road wheels (initially taken as 12) and the stiffness of the bar. The latter can deduced from the ratio of wheel loads at full bump and at static FB/FS, initially taken as 3:1, and the required static to bump suspension displacement ΔSB, taken as 350 mm. This gives a heave natural frequency of about 1.2 Hz, which is typical for an MBT. The shear modulus C is set at 76 mPa [1.1, p. 226]. The diameter of the bar is left open.

This gives a maximum shear stress qmax of 1326 mPa, which can be considered too high for a good fatigue life. Increasing the arm length to 500 mm increases maximum torque on the bar, but also reduces maximum wind‐up angle; qmax reduces to 1258 mPa. This may be acceptable depending on the duty cycle. Measurements show that the front wheels nearly always have the most severe duty, largely because of the pitching motion of the vehicle; this can be controlled by an adequate measure of damping.

Softening the suspension to give a FB/FS value of 2.5 and with axle arm length R at 450 mm increases qmax to 1371 kPa. With the stiffer suspension, increasing the number of wheels to 14 reduces the value of qmax to 1276 kPa. With the 0.5 m wheel arms, qmax reduces to 1211 mPa. If the length of wheel arm can be further increased to 0.55 m without causing interference between the arms, then qmax further reduces to 1155 mPa.

Another possibility is of course to simply reduce the static to bump displacement to, say, 325 mm with 500 mm wheel arms, 14 wheels and the stiffer suspension; qmax is then 1158 mPa. Some of the different possibilities are summarised in the table overleaf.

The factors that reduce maximum shear stress are longer wheel arms, stiffer suspension and increased number of wheels. As maximum shear stress is reduced, the weight of the bars increases in a virtually linear relationship. This is for the ‘spring’ part of the torsion bar, that is, neglecting the end fittings which are usually splines.

Suspension bump displacement, and hence maximum torsion bar stresses, is normally limited by some form of bump stop acting on the suspension arm as shown in Figure 1.1. However, bump stops are not fitted on all or some of the wheels of the Alvis Stormer and Scorpion family of vehicles; the wheels are allowed to bottom through the top run of the track onto the hull sponson and trackpads. This apparently crude strategy works well in practice; it saves weight and reduces torsional loading on the axle arms.

If it is not possible to obtain satisfactory values of shear stress with hull width torsion bars, then two strategies can be used to effectively lengthen the bars. One is to approximately double the length of the bar by ‘folding’ it back. This arrangement was used on the Second World War (WW2) German Panther tank as shown in Figure 1.4. The vehicle used eight interleaved wheels per side, both to improve soft‐soil performance and to reduce loading on the rubber tyres of the wheels. Apart from the extra complication, another disadvantage of this arrangement is the possibility of mud and stones becoming stuck between the wheels; at low temperatures this could freeze and immobilise the vehicle. Maximum shear stresses in the torsion bars were limited to a mere 200 mPa because of the qualities of the available steel and the somewhat unrealistic – for a wartime tank – design life of 10 000 km. Factors tending to increase stress levels were the very soft suspension (a pitch frequency of only 0.5 Hz) and the very short axle arms; the latter was a requirement of the interleaved wheels. The static to bump displacement was only 200 mm, tending to reduce stress