Chassis and Axles (Commercial Vehicle Technology) 3662608340, 9783662608340

Table of contents :
Preface
Contents
1 Chassis/Frames
1.1 Axle Configurations
1.2 Vehicle Layout
2 Suspension
2.1 Leaf Spring Suspension
2.2 Air Suspension
2.3 Level Control
2.3.1 Roll Control
3 Steering
3.1 The Various Types of Steering System
3.2 Wheel Suspension on the Steering Axle
3.3 Requirements for the Steering system and the Ackermann Condition
3.4 Realistic Ackermann Steering
3.4.1 Steering Assist and Power Steering Pump
3.5 Additional Steered Axles
4 Other Attachments
5 Axles
5.1 Axle Housing
5.2 Axle Drive
5.2.1 Central Axle Drive
5.2.1.1 Through-Drive Axle
5.2.2 Multi-Stage Axles
5.2.2.1 Hub Gear
5.2.2.2 Portal Axles
5.3 All-Wheel Drive Vehicles – Driven Front Axles
5.3.1 Hydraulically Driven Axles
6 Tires
6.1 Structure of a Tire
6.2 Various Types of Tire
6.3 Identification of a Tire
6.3.1 Regrooving Tires
6.4 Tire Pressure
6.4.1 Tire Pressure Monitoring System
6.4.1.1 Localization of the Wheel Electronics
Comprehension Questions
Abbreviations and Symbols
References
Index

Citation preview

Commercial Vehicle Technology

Michael Hilgers Wilfried Achenbach

Chassis and Axles

Commercial Vehicle Technology Series Editors Michael Hilgers, Weinstadt, Baden-Württemberg, Germany Wilfried Achenbach, HPCC2D-ENG, Daimler Trucks North America LLC, Portland, OR, USA

Michael Hilgers · Wilfried Achenbach

Chassis and Axles

Michael Hilgers Daimler Truck Stuttgart, Germany

Wilfried Achenbach Daimler Truck Portland, OR, USA

Commercial Vehicle Technology ISBN 978-3-662-60834-0 ISBN 978-3-662-60835-7  (eBook) https://doi.org/10.1007/978-3-662-60835-7 © Springer-Verlag GmbH Germany, part of Springer Nature 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Planung/Lektorat: Markus Braun This Springer Vieweg imprint is published by the registered company Springer-Verlag GmbH, DE part of Springer Nature. The registered company address is: Heidelberger Platz 3, 14197 Berlin, Germany

Preface

For my children Paul, David and Julia, who derive just as much pleasure from trucks as I do, and for my wife, Simone Hilgers-Bach, who has shown so much understanding for us.

I have been working in the commercial vehicle industry for many years. Time and again I am asked, “So you work on the development of trucks?” Or words to that effect. “That’s a young boy’s dream!” Yes, indeed it is! Armed with this enthusiasm, I have attempted to create as complete a picture of truck engineering as possible. So, in the course of time I started to write down as many technical aspects of commercial vehicle technology as possible. This booklet deals with the backbone of the vehicle, namely the chassis frame and the axles. Readers who are studying this subject (students and technicians) will find this booklet to be a good entry point and, as a result, may discover that commercial vehicle technology is a fascinating field of work for them. In addition, I am convinced that this booklet will provide added value for technical specialists from related disciplines who would like see the bigger picture and are looking for a compact and easy-to-understand summary of the subjects in question. My most important objective, though, is to familiarize the reader with the fascination of truck technology and make it fun to read. With this in mind, I hope that you, dear reader, have a lot of pleasure reading, skimming and browsing this booklet. At this point I would like to thank my superiors and numerous colleagues at the truck division of Daimler AG, who so readily gave me their support while I was working on this series of booklets. In particular, I would like to thank Mr Lothar Noll, who proofread the German version of this text, for his valuable comments. A very big thank you goes to my colleagues at Daimler Trucks North America who added all the content specific to the US to this book. Without this help, this booklet for the American market would never have come to existence. I would also like to thank my publisher, Springer Verlag, for their friendly cooperation, which has made the end result possible. vii

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Preface

Last but not least, I have a small request on my own behalf. It is my intention to maintain continual further development of this text. Dear reader, I would greatly welcome your help in this regard. Please send any technical comments and suggestions for improvements to the following email address: [email protected]. The more tangible your comments, the easier it will be for me to comprehend them and, where appropriate, integrate them into future editions. If you discover any inconsistencies in the content or you would like to express your praise, please let me know via the same email address. Hoping that you have a lot of fun and that everything is understandable. Weinstadt-Beutelsbach Stuttgart-Untertürkheim February 2019

Michael Hilgers

Contents

1 Chassis/Frames. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Axle Configurations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Vehicle Layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 Suspension. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1 Leaf Spring Suspension. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Air Suspension. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3 Level Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.1 Roll Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3 Steering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.1 The Various Types of Steering System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.2 Wheel Suspension on the Steering Axle . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.3 Requirements for the Steering system and the Ackermann Condition . . . . 19 3.4 Realistic Ackermann Steering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.4.1 Steering Assist and Power Steering Pump. . . . . . . . . . . . . . . . . . . . 23 3.5 Additional Steered Axles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4 Other Attachments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5 Axles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.1 Axle Housing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.2 Axle Drive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.2.1 Central Axle Drive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.2.2 Multi-Stage Axles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 5.3 All-Wheel Drive Vehicles – Driven Front Axles. . . . . . . . . . . . . . . . . . . . . 37 5.3.1 Hydraulically Driven Axles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 6 Tires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 6.1 Structure of a Tire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 6.2 Various Types of Tire. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

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6.3 Identification of a Tire. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 6.3.1 Regrooving Tires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 6.4 Tire Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 6.4.1 Tire Pressure Monitoring System. . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Comprehension Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Abbreviations and Symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

1

Chassis/Frames

The chassis1 is the basic structure of the vehicle. In a narrower sense, the chassis is only the support structure of the vehicle. In vehicle technology, though, what is often meant by the chassis is actually the frame (i.e. the support structure) complete with the suspension, steering and attachments that are directly fastened to the frame. Frame The frame is the backbone of the vehicle. The majority of today’s heavy trucks are designed with a so-called ladder frame as the supporting element. The ladder frame consists of two longitudinal members called frame rails attached by multiple crossmembers; hence the name ladder frame: two long loadbearing elements that are connected by a few rungs, looking something like a ladder.2 The frame rails are so-called C-channel beams, which have C-channel profile – see Fig. 1.1. The steel thickness of the C-channel beams is determined according to the specific application and the permissible gross weight of the vehicle. Different material thicknesses of the longitudinal frame members are used for various applications of the different vehicles. The design of a commercial vehicle that tends to be road oriented requires a more rigid design of the ladder frame to improve the handling characteristics, while off-road and construction-site oriented vehicles require more of a flexible frame design. Where there is very uneven ground, a flexible frame twists more readily, enabling better traction. In the case of medium-duty trucks (Class 6 and 7), C-channel beams with a thickness of between 6 and 11 mm are used. Heavy trucks have a C-channel beam with a thickness up to 15 mm. The height of the vertical section of the C-channel beams is in the region of 250 to 330 mm on standard heavy trucks. 1The

word chassis is a French word meaning framework, structural member or rack. so-called backbone tube chassis, which is used by a specialized manufacturer of heavy trucks and is supposed to provide particularly good off-road mobility, is a more exotic solution.

2The

© Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7_1

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1 Chassis/Frames Top flange

Web

Vertical section height

Bottom flange

Frame thickness

Frame track width

Fig. 1.1   C-channel beam and frame track width

The truck frame is a structure that is extremely rich in variants. Many properties of the vehicle result in changes to the frame geometry. Simple examples include different overall length of the vehicle or different tail overhang require different frames. Frames for long vehicles have additional crossmembers. The form and arrangement of the crossmembers take into consideration, where appropriate, which attachments are to be supported by the frame and at which points (tanks, exhaust aftertreatment systems). Frames for heavy load vehicles have various reinforcements, for example, the frame rails are reinforced by sheet metal inlays or C-channel beam inlays. The closing crossmember may need to be reinforced, if it is going to be supporting the coupling  towing hitch of a trailer coupling. These and many other considerations results in a huge number of frame variants. Figure 1.2 shows the ladder frame of a tractor unit. The photo in Fig. 1.2a shows, as a partial structure, the frame rails with two crossmembers. The CAD data3 in Fig. 1.2b shows the entire frame of the same  semitrailer tractor. Figure 1.2b is explained in more detail below: the so-called end crossmember ends off the frame. Various crossmember geometries are used between the (longitudinal members) frame rails. Viewed from rear to front, the following can be seen: a tubular crossmember, a C-channel crossmember and an underslung crossmember, which bulges out downwards, enabling it to pass under the transmission. Fig. 1.2c shows the frame of a Freightliner Cascadia 6x4 tractor. The vehicle and hence  the  frame is somewhat longer than its European counterpart  due to the rear axle tandem and the longer cab. The distance between the two frame rails is defined as the frame track width. This is decisive for the installation of the axle and for the packaging of all components arranged inside and outside of the frame rails.

3CAD

is short for Computer Aided Design. The geometry of the components is defined and visualized on a computer.

1 Chassis/Frames

3

Fig. 1.2   Basic structure of frames for long-haul semitrailer tractors a) and b) show a 4x2 semitrailer tractor in Europe (Mercedes-Benz Actros) and c) shows the frame of a 6x4 Freightliner Cascadia. a) is a photo b) and c) are CAD data. Pictures: Daimler AG

The bevels at the end of the frames in Fig. 1.2 are typical for tractor units. They create space when the semitrailer and the tractor are tilted against one another on ramps. The brackets on the frame rails provide reinforcement and mounting points for the fifth wheel. In the rear axle area, the frame track width must be narrower to provide enough space next to the frame for the suspension and dual tires. The space provided for the tires is measured so that various tire sizes can be used, while still leaving sufficient space to put on snow chains. In the front area, with some frame concepts the frame rails are splayed (curved outwards), to provide space for the engine and the radiator. The front end crossmember ends off the frame. In North America, it is common to put holes into the frame where attachments are to be fastened. The hole pattern of the frame rails is then customized to the specific vehicle. This frame concept is illustrated in Fig. 1.2c). Other concepts have a uniform hole pattern over the entire length of the frame rail, which allows further frame components and attachments to be flexibly bolted on within the hole matrix. This concept is common in Europe  see Fig. 1.2 a). With various bodies, a second frame – the so-called subframe – is mounted on the actual vehicle frame. The purpose of this is to increase the overall rigidity of the vehicle. Figure 1.3 shows how two frames are bolted onto one another on a dump truck.

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1 Chassis/Frames

Fig. 1.3   Connection of the vehicle frame (at the bottom) to the subframe of a dumper body. The vehicle frame shown here does not have a uniform hole pattern, but holes only where they are required. (Photo: Michael Hilgers)

Similar to the frame illustrated in Fig. 1.2c) the frame shown in this photograph does not have a uniform hole pattern. In this case, holes are made in the frame only where holes are required to connect frame sections and fasten attachments to the frame. You will come across both concepts: frames with a standard hole pattern and frames with ­vehicle-specific hole patterns.

1.1 Axle Configurations The vehicle rests on two or more axles. The axle configuration describes how many axles the vehicle has and what tasks the axles perform. The first digit of the axle configuration specifies how many wheels or twin wheels the vehicle has. The second digit specifies how many of the wheels are driven. A forward slash is followed by the number of the steered wheels. A vehicle with the wheel formula:  8×4 4 (1.1)

1.2  Vehicle Layout

5

Fig. 1.4   Examples of various axle configurations

has 8 wheels or twin wheels (i.e. four axles). Of these, two axles are driven and two axles are steered. Alphabetic character combinations provide additional information: • • • •

NLA describes a trailing axle that can be steered or unsteered. DNA stands for a twin-tire trailing axle. ENA is a single-tire trailing axle that can be steered or unsteered. VLA is the leading axle.

Figure 1.4 shows examples of various axle configurations.

1.2 Vehicle Layout The frame carries the drivetrain, the cab and the body. In addition, numerous frame attachments, such as the diesel tank, AdBlue  or DEF -tank, battery box, mudguards and numerous components of the pneumatic system and brakes, are also arranged on the

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1 Chassis/Frames

Spare wheel Crossmember Frame rail

Fuel tank AdBlue tank

Battery box Brake system

Fig. 1.5   Example of the frame layout by way of the example of a light truck. (Illustration from a Nissan brochure [7])

frame. The axles with the suspension also have to be fastened to the frame. The spatial arrangement of these components constitutes the so-called vehicle layout. Since there are numerous variations of vehicle configurations with differing wheelbases, overhangs and very differing equipment variants for the various heavy trucks, a production series is made up of a multitude of different vehicle layouts. With some designs the spatial arrangement of the components and assemblies is particularly difficult because the space on the frame is limited. In particular, long-haul vehicles with a short wheelbase and a large tank volume requirement are worthwhile mentioning in this regard. Figure 1.5 shows the layout of a light truck with a comparatively generous amount of space on the frame.

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Suspension

For the cargo, the suspension consists of the tires and the suspension of the axles. For the driver, the overall suspension has additional spring elements consisting of the cab mounting and the suspended seat, which drivers really appreciate. Two different basic principles of axle suspension are employed: leaf spring suspension and air suspension. Leaf spring suspension is often also called steel spring suspension because leaf springs are usually made of steel. Both air suspension and leaf spring suspension systems are not only offered for trucks but are also available for full trailers and semitrailers. Another type of suspension, namely coil spring suspension, can be found in the van segment and on special heavy trucks (Unimog).

2.1 Leaf Spring Suspension Leaf spring suspension usually consists of steel springs that are arranged between the axle beam and the vehicle frame. Steel spring suspension is a low-cost solution and it is robust. The spring suspension system often consists of several leaf springs stacked on top of one another – these are called multi-leaf springs, or spring packs. On light vehicles and even heavy vehicles, mono-leaf (single-leaf) spring suspensions are sometimes used on the front axle, which has to bear less load. This saves weight in comparison to a ­multi-leaf spring suspension. Leaf springs have traditionally been designed as flat leaf, however modern designs called taper leaf have become more commonplace, which take on a parabolic shape of the spring. Taper leaf leafs (parabolic leafs) are offered both in monoleaf (single) or multiple leaf spring packs. The typical nomenclature taper leaf is used to describe parabolic leaf spring designs of any number of leafs.

© Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7_2

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Fig. 2.1   Leaf springs on the dual-tire rear axle of a medium-duty truck. (Photo: Daimler AG)

Flat leaf and multi leaf are interchangeable to describe the older technology of stacked flat leaf spring packs. For weight reasons, leaf springs made of other materials than steel, for example, GFRP springs are also used1 because the latter are significantly lighter in weight than steel springs. However, these are associated with increased costs. Figure 2.1 shows the leaf spring suspension of a rear axle for a distribution truck. Two leaf springs (stacked) assume the spring function in this case. If the load is light, only the upper leaf spring is active. If the vehicle is laden, the upper leaf spring is pressed onto the lower leaf spring and both springs contribute to the spring suspension. Technically, in comparison to air suspension, leaf spring suspension has the advantage of also contributing to the guidance of an axle. Furthermore, the leaf springs used in the heavy construction site segment enables significantly greater spring deflection, so that on difficult terrain a steel-sprung vehicle has better traction than an air-sprung vehicle. The disadvantage of steel spring suspension is that the vehicle level changes if the vehicle is laden. The available suspension travel depends on the load of the vehicle. Ride height control cannot be achieved with steel alone.

1GFRP = Glass-Fiber

Reinforced Plastic.

2.1  Leaf Spring Suspension

9 V-Rod

Stabilizer

Steel spring pack

Prop shaft Brake cylinder Through-drive axle Shock absorber Drum brake

Fig. 2.2   Suspension of a tandem axle with steel springs and drum brakes. (Illustration: Volvo Trucks)

Tandem Axle With a tandem axle, one spring pack can assume the spring function for both axles. Figure 2.2 shows a leaf sprung rear tandem axle for a heavy truck. Comparing Figs. 2.1 and 2.2 it is evident that the leaf springs in Fig 2.2 are made for higher permissible vehicle weight: heavy trucks require more solid steel springs than medium-duty trucks, of course. Figure 2.3 shows a design solution in which both axles of the rear tandem axle have their own steel springs. The two steel springs are coupled by way of a tiltable connecting piece so that they mutually support one another. This achieves axle load compensation: if one of the axles has a high degree of deflection, the other axle is pressed via the moving adapter, which executes a rocking movement, in the direction of the ground. The ­non-deflected axle thereby relieves the load on the heavily loaded deflected axle. Axle load compensation systems are also used for two front axles. Figure 2.4 shows the front axle load compensation system for vehicles with two steered front axles.

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Fig. 2.3   Sketch of a design solution for a tandem axle with axle load compensation. The smaller sketches in the lower half of the illustration show how the axle load of a heavily loaded axle is transferred onto the less heavily loaded axle

The linkage transmits the steering function to the second axle

Steering booster (Ram)

Direction of travel

Axle load compensation

Fig. 2.4   A drawing of the front axle load compensation system of the Mercedes-Benz Actros/Arocs. The steering movement is transmitted via a linkage from the first to the second axle. (Illustration: Daimler AG)

2.2  Air Suspension

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2.2 Air Suspension With air suspension, the suspension is done by air springs or air bags that provide a connecting element between the axle and the vehicle frame, and bear the weight of the vehicle. The air springs are filled with compressed air. Air suspensions provide superior ride comfort and are considered a premium compared to a pure steel leaf suspension in North America. A higher degree of suspension comfort can be achieved with air suspension. In addition, air suspension has the advantage that the suspension comfort and suspension height are independent of the load condition. For the rear axle there are air bellow (or air bag) concepts with one bellow per side (two bellows per axle) and concepts with two bellows on each side (four bellows per axle). These are called two-bellow air springs and four-bellow air springs. The air-sprung front axle is equipped with one air bellow on each side. Figure 2.5 shows the air bellow on the front axle of a heavy long-haul truck. Figure 2.6 shows an air-sprung rear axle with four air bellows. Fig. 2.5   The front-axle air bellows of a Mercedes-Benz Actros (Actros 3). (Photo: Daimler AG)

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2 Suspension

Fig. 2.6   Air suspension with four bellows for the rear axle of the Mercedes-Benz Actros from 2011 (Actros 4). (Photo: Daimler AG)

The air bellows of an air suspension system are able to transmit forces in vertical direction only (along the z-axis). Guidance of the axle in x- and y-direction must be performed by additional components (so-called control arms). Alternatively, a leaf spring and an air springs can be combined: In North America it is common to feature an air spring on top of a leaf spring. Leaf spring provides axle guidance and approximately 50% of spring rate; air spring provides additional spring rate and height maintenance based on load. In addition to comfort, level control is the most important and most helpful function of air suspension.

2.3 Level Control The level control is a mechatronic system for air-sprung vehicles. Its basic functionality consists of being able to change, within certain limits, the height of the vehicle frame and therefore also of the cargo area, and the height of the fifth wheel coupling relative to the road surface. When it is at so-called vehicle level the vehicle frame is in the basic position designed for the vehicle in drive mode. The air spring of the air suspension system can be filled with additional air to lift the vehicle frame or the air in the bellows can be reduced to lower the vehicle.

2.3  Level Control

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This functionality of the level control is important especially for vehicles that have to pick up exchangeable containers, commonly found in Europe but not used in North America. The vehicle is lowered to drive the frame under the container. Once the vehicle frame has been maneuvered under the container that is going to be picked up, the vehicle (with the container over it) is lifted so that the container legs can be folded away. With tractor semitrailers combinations the level control is also very helpful for hitching and unhitching. With the tractor semitrailer combination, though, it is sufficient if the rear axle(s) of the tractor semitrailer combination has/have a level control system. Most tractor units are therefore partially air sprung, in other words, the rear axle is air sprung and therefore has a level control function, while a steel spring is used on the front axle. Operator control of the level control system can be performed via a switch on the cockpit instrument panel, the buttons on the multifunction steering wheel or by way of a separate control unit, depending on the vehicle manufacturer and vehicle equipment. The technical basis for the level control system is primarily the air suspension with its air bellows. Various solenoid valves allow air to flow into or escape from the air bellows. In solely pneumatic systems (North American market) adding or removing air for hitching and unhitching is achieved with separate valve that are operated manually. More advanced systems use electronic valves to operate the system and sensors are used to measure the height of the frame above the axle beam (common in Europe). Pressure sensors determine the air pressure in the bellows. Information from the sensors, the operator control unit which records the drivers input and other vehicle data, for example, the speed, are processed in a control unit that actuates the level control.

2.3.1 Roll Control So-called passive roll control has been used on trucks for several years [3]. The road control system is an advanced system (usually optional) that reduces the roll of the vehicle. To do so the system quickly changes the compression and rebound stages of the shock absorbers, thereby reducing the roll of the vehicle. In contrast to the passive roll control for trucks, active roll control – as known from passenger cars – actively changes the movement of the vehicle (rotary movement about the x-axis and y-axis), thereby reducing the pitch and roll motions. Due to the large dimensions of a truck, the power required for sufficiently fast active control is not available.

3

Steering

The task of the steering system is to enable the driver to change direction as needed. In Europe, regulations that a steering system for commercial vehicles (and also the steering system of other road vehicles) must meet are set out in ECE-R 79 [2]. This regulation does not apply in North America.

3.1 The Various Types of Steering System Multi-axle wagons and carriages were and still are constructed with the so-called single-pivot turntable steering system (or turntable steering). With this type of steering system, a rigid axle is pivoted at its mid-point and rotates under and through the vehicle. The single-pivot steering system requires a lot of space and the vehicle is susceptible to tipping at large steering angles. In addition, disturbance forces that affect only one of the two wheels of the steering axle (pothole) have a long lever arm that is equal to half the track width of the axle. Full trailers often have a single-pivot steering system. The three-wheeler is simpler than single-pivot steering. The first automobile was designed as a three-wheeler [4]. A single steered wheel is for the purpose of selecting the direction of travel. Easily maneuverable, light and low-cost mini-vehicles, designed in the form of a three-wheeler, are used nowadays in various countries in southern Europe and Asia. The three-wheeler is relatively unstable and is suitable only for vehicles with a low gross weight and low speed level. The tank steering system (or skid steering system) is used on armored tanks, other tracked vehicles and construction vehicles. With this steering system, the wheels or chains are accelerated or decelerated with differing intensity on the two sides of the vehicle. As the steering is instigated by driving and/or braking, it is also called drive or brake steering. As a result of the differing wheel rotation speeds and chain speeds, torque is

© Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7_3

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3 Steering

created about the vertical axis and the vehicle turns. ESP interventions work on the same principle. The advantage of the skid steering system is that the running gear parts do not move laterally relative to the chassis and that extreme maneuverability is possible. However, the skid steering system does cause high loads to be imposed on the vehicle and on the undersurface, and it has significant weaknesses in terms of comfort. The articulated steering system is another widespread type of steering used in the design of vehicles. The vehicle has two vehicle parts – both with axles – which are connected by an articulated joint. If the vehicle articulates at the joint, the axles are counter-rotated and cornering is provoked. The articulation is hydraulically enforced. Articulated steering systems are suitable for construction vehicles, such as large wheel loaders and construction site dumpers, without on-road operation. A detailed explanation of articulated steering and the specific advantages and disadvantages are provided by [5]. Articulated steering is less suitable, though, for high speeds and operation with trailers. The steering system of choice for demanding vehicles that are used on the roads is the Ackermann steering system—see subsect. 3.4. Ackermann steering on the front axle has asserted itself on passenger cars, trucks and buses.1 It can be implemented as a space-saving solution that enables a high degree of comfort together with a high degree of safety. The vehicle remains stable even at large steering angles (in contrast to the articulated steering and single-pivot steering systems). There is only a small disturbing force lever arm for unilaterally acting forces. The maintenance and reliability of the Ackermann steering system meet the expectations of a modern commercial vehicle. Several axles are steered on vehicles with three or four axles to enable a smaller turning circle.

3.2 Wheel Suspension on the Steering Axle Various terms are required to understand the kinematics of the front axle and steering system. These terms and their effect on the handling performance are explained below. Toe-in describes that the front wheels do not stand exactly parallel in straight-ahead position, but slightly converge at the front with a positive toe-in. The toe-in value is specified either by way of the toe-in angle or as the spacing difference between the front wheels at the front and rear (measured at the rim flange). If the spacing at the front edge of the wheels is less than at the rear edge of the wheels, the toe-in is positive. If the spacing between the wheels in straight-ahead travel is less at the rear than the front, the toe-in is negative. The term toe-out is also used for this. Figure 3.1 illustrates toe-in.

1Having

the steering on the rear axle is often the first choice for forklifts, wheel loaders, etc., because with this concept the attachments, such as stacker forks and buckets, can be more easily brought into position at the front. For road vehicles, the pure rear axle steering system is prohibited in regions where ECE-R 79 [2] applies (i.e. Europe).

3.2  Wheel Suspension on the Steering Axle Fig. 3.1   Toe-in and toe-out angle

Toe-in angle

17

φ

front

Direction of travel Toe-in

rear

As a result of a positive toe-in, the wheels are pressed inwards by the toe-in force. This reduces the juddering tendency of the front wheels and improves (in combination with other measures) the straight-ahead running of the vehicle. The camber is the angle of inclination of the wheels out of the vertical. The wheels do not stand exactly upright, but lean either outwards or inwards. If the wheel tilts outwards at the top (as in Fig. 3.2), it is called a positive camber. If there is a negative camber, the wheel is tilted inwards. The camber can be used to influence the lateral stability of the tires. The steered wheels of the double-pivot steering system rotate about the kingpin during the steering movement. The kingpin is spatially inclined. The variables kingpin inclination and caster angle describe the spatial inclination of the kingpin. The kingpin inclination describes by what angle the kingpin is inclined out of the vertical and tilted to the vehicle. As a result of the kingpin inclination, the vehicle is minimally lifted during steering. As a result, the vehicle weight acts against the steering angle and generates a steering restoring torque, which attempts to bring the wheels back to straight-ahead position. The caster effect also exerts a restoring force on the steering system. The caster angle describes by what angle out of the vertical the kingpin is inclined to the front or rear. As a result of the caster angle, a distance is generated between the center of tire contact and the point of intersection of the axis of rotation (of the kingpin) with the road surface, the so-called caster. The kingpin inclination and caster are illustrated in Fig. 3.2. If there is a positive caster and the wheels are steered, as a result of the lateral support force, which takes effect at the tire contact point, torque is generated about the axis of the kingpin that counteracts the steering angle. The caster is the lever arm by means of

18

3 Steering

Camber angle α

Inclination of the wheel in plane yz

top

Direction of travel Caster angle δ

Inclination of the axis of rotation/kingpin in plane xz

Scrub radius rs

Road (Positive) caster

Kingpin inclination β

Inclination of the kingpin in plane yz

Trace point–piercing point distance

Fig. 3.2   Illustration of the important geometric terms relating to the front axle and steering axle, kingpin inclination, camber, scrub radius and caster. The terms are explained in the text

which a force, transversely to the roll direction of the steering tire, seeks to impose a rotational movement. This is demonstrated particularly well by the caster effect of shopping carts: the wheels of the shopping cart are mounted so that an intense caster effect is created between the axis of rotation and the point of contact of the wheel. The restoring forces ensure that the wheels align themselves automatically in straight-ahead position. If the friction value between the road and tires changes, the steering restoring torque also alters. The required steering force changes. In [6] it is attempted to utilize this relationship in order to draw conclusions as to the friction value of the road during travel. The scrub radius describes the distance transversely to travel direction (in direction y) between the point of contact of the wheel and the wheel rotation point. The camber, the position of the kingpin with the kingpin inclination and the rim offset determine the scrub radius. If the steering pivot point (the extension of the kingpin) is further inwards than the point of contact of the tire, as is the case in Fig. 3.2, it is called a positive scrub radius. If the pivot point is further outwards, it is logically a negative scrub radius. The scrub radius is the lever arm by means of which a longitudinal force applied to the tire (the braking force) introduces a rotational movement into the steering.

3.3  Requirements for the Steering system and the Ackermann Condition

19

Tab. 3.1  The effects on the vehicle characteristics due to the various geometric variables acting on the front wheels Toe-in Stabilization of straightahead running

X

Juddering tendency

X

Steering return Tire wear

Kingpin inclination

Camber

Scrub radius

Caster

X

X

X

X

X

X

X

X X

When designing the front axle and steering system, the variables shown here must be mutually optimized to suit the required vehicle behavior. Tab. 3.1 shows which variables particularly influence the various characteristics of the vehicle. In addition, the behavior of the vehicle in lateral dynamic situations (cornering) is also intensely influenced by the rigidity of the axles, frame and characteristics of the axle suspension. The steering gear, steering ratio and the friction in the overall system are also of great significance for the steering experience.

3.3 Requirements for the Steering system and the Ackermann Condition The steering system is one of the assemblies that typify the character of a vehicle. Differences in the steering are easily identified even by inexperienced drivers.2 A functioning steering system is essential for a vehicle to be safe to operate. Special attention is therefore paid to the steering and to the functional safety of the steering system. The steering system must meet numerous requirements. The vehicle should have as small a turning circle as possible, yet the installation space for the steering is limited. The steering forces should be adequate and road bumps should not be felt on steering wheel, but at the same time the steering should provide the driver with a good feel for the road. The steering linkage (steering shaft) from the cab to the front axle must be designed on cab-over-engine vehicles so that the cab can be tilted. The steering shaft must also be able to compensate for the relative springing movement between the cab and the chassis and designed with enough collapsibility for crash protection. The so-called Ackermann condition is an important geometric design condition for a steering system: it requires that when cornering, that the axis’ of rotation of all wheels intersect at one point, the center of the curve. This ensures non-slip rolling of the wheels.

2However,

it is also the case that drivers normally become accustomed to different steering systems very quickly, as long as the steering behavior remains within acceptable boundaries.

20

3 Steering

h δi

δa

r sa si

rs

fs

rs

Fig. 3.3   The Ackermann condition and the geometric conditions. The Ackermann condition for a multiaxle vehicle is shown in the small box: each steered wheel having a different Ackermann angle. The tandem axle is considered as one axle located at the center of the tandem axle

The Ackermann condition provides the desired angle for each individual wheel. For design reasons, the actual angle frequently differs from this. Figure 3.3 shows the geometry for the Ackermann condition. δi and δa describe the steering angles of the inner cornering and outer cornering wheels, si and sa being the radii of the track circles that the two front wheels describe. The distance between the intersection points of the axes of the kingpins with the road surface is designated by fs. rs is the scrub radius and r is the wheelbase of the vehicle. The following applies to the angles:

sin δi =

r si + rs

(3.1)

sinδa =

r sa − rs

(3.2)

An assessment possibility for the smallest achievable turning circle results from the Eq. 3.2, if δa;max describes the maximum steering angle of a vehicle:

3.3  Requirements for the Steering system and the Ackermann Condition

Direction of travel

21

Direction of travel

Fig. 3.4   The steering trapezium

Sa,min =

r r + rs ≈ sin δa,max sin δa,max

(3.3)

The actual turning circle is (possibly even distinctly) larger because the vehicle has an overhang at the front and at the rear, which is not taken into consideration here. With the auxiliary variable h, the following applies to the steering angles of both wheels:

h r

(3.4)

h + fs r

(3.5)

cot δi =

cot δa = and therefore3:

cot δa − cot δi =

fs r

(3.6)

The actual angles during cornering deviate from the calculated angles, because the tires and the running gear have elasticities that lead to slip angles.

3The same relationship is also found, if analogously to Eq. 3.2, expressions for cos δ and cos δ are a i set and cot = cos is used. sin

22

3 Steering

Steering wheel

Steering gea r Steering shaft Pivot of the kingpin Steering knuckle

Steering arm

Pitman arm

Drag link

Steering arm

Axle beam

Tie rod Steering knuckle

Fig. 3.5   Illustration of the mechanical components of a commercial vehicle steering system. The steering system also includes the power steering pump, power steering fluid reservoir, hydraulic lines and mechatronic components, which are not shown here

3.4 Realistic Ackermann Steering Commercial vehicles are constructed – as already described above – with Ackermann steering. Figure 3.4 illustrates the operating priciple of the so-called steering trapezium. The kinematics of Ackermann steering cause the inner cornering wheel to execute a larger steering angle than the outer cornering wheel. The behavior of the steering angles in the steering trapezium can be described by means of three variables. The trapezoidal angle stipulated by the geometry of the steering arm in straight-ahead position αg, the distances between the kingpins fa and the length of the steering arm rs define what steering angle δa occurs at the outer wheel, if δi is predetermined. One finds that the ratio of the angles can meet the Ackermann condition of the Eq. 3.6 for only one steering angle. For all other steering angles, the Ackermann condition is met only approximately. For the design of the steering trapezium, it must be considered within what steering range the Ackermann condition is to be particularly well met.

3.4  Realistic Ackermann Steering

23

Figure 3.5 shows the mechanical steering system of a modern truck. The steering wheel movement is transmitted via the steering shaft to the steering gear. The steering shaft is divided by one or two universal joints. This is required in order to provide an angled steering shaft that is usually needed as the result of the installation spaces available. In addition, the steering shaft with the universal joints allows the steering wheel to move relatively to the steering gear so that the cab with the steering wheel can be tilted. A steering wheel that is adjustable for the driver can also be realized in this way. A so-called cardan error (gimbal error)  causes an altering steering wheel gear ratio during the steering movement. The cardan error causes sinusoidal-like fluctuations in rotational speed on the output side. With two universal joints in the steering shaft, the two universal joints can be arranged so that the errors  (almost) compensate each other – this is often referred to as phasing the U-joints. The steering gear transmits the turning movement of the steering shaft to a rotary movement of the pitman arm. This moves the drag link back and forth. As a result, the steering knuckle is in turn rotated via the steering arm, to which the wheel is attached via the wheel hub bearing. The steering gear in a heavy truck is usually a recirculating-ball steering system. On light trucks, rack-and-pinion steering systems (as usual in passenger cars) are also used. The steering gear and the geometric conditions of the pitman arm and steering arm result in a transmission ratio between the steering wheel angle and the steering angle of the wheel. Typical steering ratios on long-haul trucks are between 1:17 and 1:23.

3.4.1 Steering Assist and Power Steering Pump Truck steering systems are equipped with hydraulic assistance to reduce the steering forces that the driver has to apply to operate the steering wheel. A power steering pump is driven by the truck’s internal combustion engine. In conventional systems, the hydraulic power steering pump generates a flow rate that is dependent on the engine speed. With straight-ahead travel, the hydraulic fluid flows at a low circulation pressure through the power steering pump, steering gear and power steering fluid reservoir. If the driver, turns the steering wheel, hydraulic pressure is provided via a torsion bar and a valve actuator on one side of a double-acting piston in the steering gear. This piston amplifies the force from the steering wheel and the driver perceives power assistance. The energy required to circulate the hydraulic fluid must be provided by the vehicle. The power steering pump is usually designed so that even when there is a high friction value between the tires and the undersurface, stationary steering is still possible. With this particular load incidence, a high level of assistance is required from the hydraulics. When driving, the required power steering assistance is significantly less and only a fraction of the power that the power steering pump provides is used for the power steering assist. To reduce fuel consumption, modern hydraulic pumps can variably adapt the flow rate of the hydraulic oil to the driving situation (low flow rate during straight-ahead travel).

24

3 Steering

If the hydraulic assist fails, the driver has to provide the entire force required to turn the steering wheel from his own muscular strength. In Europe, limit values are defined for the forces required for the steering wheel. If these limit values are not attained without hydraulic assistance, the vehicle must be equipped with a redundant auxiliary power system. Traditionally, vehicles that have a redundant power steering system have a second hydraulic circuit that is not driven by the engine, but is driven via the wheels/axle/propeller shaft, and builds up auxiliary hydraulic pressure when the vehicle is rolling. An alternative concept replaces the second heavy and expensive hydraulic circuit with electrically driven assistance, in order to accomplish steering assistance electrically, if there is a hydraulic failure of the primary steering circuit. An electrical second circuit is weight-saving and cost-saving in comparison to a hydraulic dual-circuit steering system. An electric motor that acts on the steering gear can also be used to implement additional functions, for example, to improve the maneuverability and alter the characteristic of the steering system [8].

3.5 Additional Steered Axles Vehicles with four axles frequently have two steered front axles. Vehicles with three axles can be constructed with only one steering axle or even a second steered axle. This second steered axle can be constructed as a leading axle or as a trailing axle. Examples are shown in Fig. 2.4. In comparison to rigid axles, steered additional axles have two advantages: the turning circle of the vehicle becomes smaller – an advantage that is important in particular in urban traffic and when maneuvering – and the tire wear is reduced. The behavior of the steered trailing axle is dependent on the driving situation: at low speeds, for example, up to about 25 km/h, the wheels of the trailing axle rotate in the opposite direction to the front axle, thereby reducing the turning circle. By increasing the speed, the steering angle of the additional axle is reduced. It might still contribute to the steering of the vehicle, but has smaller steering angles. As of a defined limit speed (e.g. 45 km/h) the trailing axle is held in a straight-ahead position and no longer contributes to steering. Hence at higher speeds, the stable driving behavior of a three-axle vehicle without a steered additional axle is obtained. The steering function of the trailing axle is also disabled when reversing or if the electronics of the steered additional axle have detected a fault.

4

Other Attachments

As already mentioned in subsect. 1.2, many other components are fastened to the frame along with the large assemblies, such as the cab, body, axles and drivetrain (that get their own booklets in this series). The fuel tank system is a noticable attachment to the chassis. Large tanks are needed, especially for long-haul transport. The trucking company can then fill up their vehicles where diesel is cheaper (Fig. 4.1). Tanks with a maximum volume of 300 gal (1136 l) make it possible to undertake journeys of more then 2000 miles without refueling. There are trucking companies that optimize their routes so that their vehicles travel at regular intervals through states and countries with a low diesel price. A large fuel tank volume also makes it possible for refueling procedures to always be performed at the trucking company’s premises, relieving the driver and the trucking company’s accounting department of the payment procedure elsewhere (Fig. 4.2). The one-sided mounting requires well thought-out tank holders. An L-shaped tank support is usually bolted onto the frame. The tank lies on this tank support and is held by several fastening straps. On the inside of the tank, baffle plates can be used to ensure that the sloshing movement of the liquid is suppressed. While baffles can be found on European trucks, they are not used in North America. A battery carrier holds the starter battery. Starter batteries (220 Ah) weigh about 50 kg and two are required to provide the voltage of 24 V in Europe. In North America, two to four batteries (12 V) will be used depending on the amperage current requirements of the components on the vehicle. In many vehicle configurations, the battery carrier or battery box sits on the side of the frame. Two batteries are usually adjacent to one another in this position. If only a restricted amount of installation space is available, they are sometimes also arranged one on top of the other. The battery box supports the heavy weight of the batteries. In the case of tractors, the installation space on the side of the frame is limited and is preferentially used for the exhaust system and fuel tank(s). So the batteries might be © Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7_4

25

26

4  Other Attachments

Fig. 4.1   Utilization of the entire installation space between the axles of a semitrailer tractor to provide the maximum volume for the tank system. (Photo: Daimler AG)

Filler neck

Maximum utilizable volume up to overflow point Fuel pump nozzle shuts off

Suction line

Measuring range of the fuel tank level sensor

Geometric volume of the fuel tank

Display indicates reserve Fuel level sensor indicates zero

Filter

Fuel suction zero = vehicle halts

Fig. 4.2   The utilizable volume of the tank is considerably less than the geometric volume of the tank. The levels shown on the sketch do not reflect the real proportions, but serve only to illustrate that the total tank volume is made up of various non-usable fractions

located between the frame rails or at the rear of the vehicle. This is common in Europe— see Fig. 4.3 showing an example of a battery box at the rear of a tractor unit. In addition, on cab-over-engine, tractor units the load distribution of the vehicle is also improved with a battery at the rear the axle, when it is on the road without a semitrailer.

4  Other Attachments

27

Fig. 4.3   Battery box and compressed air reservoir at the rear of a tractor semitrailer combination. (Photo: DAF)

The compressed air system and the brake system consist of many components that are accommodated on the frame. The compressed air reservoirs, which have a large volume, require a lot of space. With the numerous small components of the brake system, it is less a matter of their space requirement, but more one of repositioning the various components for each vehicle variant (length, wheelbase, fuel tank volume, special equipment). Fenders prevent excessive dirt and spray – whirled up by the wheels – from hindering other road users. Side paneling and side underrun protection cover the sides and provide improved safety for other road users. Some vehicles have a spare tire, though not common in North America. Wheel chocks for securing the parked vehicle is a regulation item and is usually mounted on the vehicle frame. All of these parts require brackets. On account of the heavy weight of some of them and the wide overhang of the attachments, these brackets are subject to high loads and must be robustly constructed.

5

Axles

The axle performs numerous fundamental functions in the vehicle. The typical functions of axles are: • Support • Spring suspension • Rolling • Braking • Steering • Driving The first three functions of support, spring suspension and rolling are exercised by all axles of the vehicle. All axles on most vehicles also contribute to the braking function. The functions of steering and driving are assumed only by some of the axles that are installed on the vehicle, depending on the vehicle configuration – see Fig. 1.4. Consequently, there are driven axles, non-driven axles, steered axles and unsteered axles. In addition, it is differentiated between front and rear axles. Front axles have single tires, while rear axles usually have a greater load capacity and frequently have dual tires. The type of axle suspension (steel spring or air) and the brake (drum/disc) generate further axle variants. Figure 5.1 shows the various features that define an axle. The envisaged axle load for a specific axle is taken into consideration in the design of the mechanical load capacity of the axle beam. The axle loads that are allowed in Germany are defined in 96/53/EC (see also [9]). In addition to the geometric requirements, there are further requirements for driven axles that result from the drive function. The vehicle speed v results as follows from the engine speed, where the axle ratio is iaxle, the transmission gear ratio is igear and the tire radius is rdyn: © Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7_5

29

30

5 Axles

Front axle

Rear axle

Driven

Steered

Steered

Unsteered

Single tire / twin tire

Non-driven

Leading axle / trailing axle Liftable / non-liftable Disk brake / drum brake Steel spring / air spring

Fig. 5.1   The various features of axles. A multitude of different axle variants is generated by combining the various features. The maximum permissible axle load and the different ratios of the axle drives for driven axles are further variant generators (not taken into consideration in this illustration)

vvehicle = 2 · π · nengine · rdyn ·

1 1 · igear iaxle

(5.1)

For design of the drivetrain, the desired maneuvering speed, the vehicle speed in top gear at a required engine speed and the climbing ability of the vehicle are taken into consideration in defining iaxle. Typical axle ratios on trucks are between iaxle = 2 and iaxle = 6. A detailed explanation of the drivetrain design can be found in [10]. The torque passing through the axle results from the engine torque and the transmission gear ratios. The specific application of the vehicle, the gross combination weight and the torque curve of the engine determine what load (what load spectrum) the axle drive will actually experience in operation. The conventional truck axle is a rigid axle (Fig. 5.2). The main components of this form of axle are the axle beam (or axle housing), the wheel bearing and, in the case of a driven axle, the axle gear. The axle beam connects the two wheel bearings so that a force acting on one side of the axle (for example, a pothole) also influences the wheel of the other side. Figure 5.3 shows an axle with independent suspension. There is no axle beam. As a result the two wheel tracks are separated and independent. The wheels on both sides of the axle can deflect independently of one another.

5 Axles

31

b) A Mercedes-Benz VL4 front axle Axle beam

Wheel bearing

Axle gear a) A Mercedes -Benz HL6 rear axle Fig. 5.2   Axles for heavy trucks. a Shows a driven rear axle with disk brakes. b Shows a non-driven front axle with disk brakes. The front axle embodies the steering function. The axle is mounted to the suspension on the flat surfaces of the axle beam, the so called spring seats.

Fig. 5.3   Independent wheel suspension for the front axle of a heavy truck. (Illustration: Volvo Trucks AG)

To date, independent wheel suspension has not been very widespread in the commercial vehicle sector. For comfort and vehicle handling reasons, buses frequently have independent wheel suspension and some special vehicles are designed with independent wheel suspension. However, interest in independent suspension from time to time pops up in the truck sector. Figure 5.3 shows an independent wheel suspension for heavy trucks that was exhibited as non-standard equipment in 2012.

32

5 Axles

5.1 Axle Housing The axle housing is the backbone of the axle. It assumes the function of supporting. The axle housing has a connection surface (the spring seat, number 8 in Fig. 5.6), which enables connection of the axle to the suspension. The wheel bearing and the brake are located at each end of the axle housing. In the case of driven axles, the moving parts such as the axle drive and axle shafts, are mounted inside the axle housing. The axle housing can be a cast part or can be assembled from formed sheet-metal parts, cast and/ or forged parts.

5.2 Axle Drive All driven axles have a center gear. In addition, dual stage axles also have a so-called hub drive that is seated in the hub, close to the wheel.

5.2.1 Central Axle Drive A center axle drive is always required for driven axles to transmit the turning movement of the propeller shaft to the axle shafts. As the engine on trucks is mounted longitudinally, the axes of rotation of the engine, transmission and propeller shaft run at right angles to the axis of rotation of the wheels.1 The axle gear has the task of turning the direction of power transmission through 90°. The so-called bevel drive axle drive is normally used: a pinion or bevel gear that sits on the propeller shaft engages with the so-called ring gear. If the axis of rotation of the pinion and ring gear intersect, the pinion is lying centrically on the ring gear. If the pinion is arranged eccentrically, it is called a hypoid gear and a hypoid axis. Figure 5.4 is a diagram of the bevel gear system. The teeth between the bevel gear and ring gear are usually designed in the form of a spiral bevel gear with a hypoid gear. Because of the higher contact area of the gear teeth and the rolling-sliding movement on the tooth contact surface, hypoid gearwheels offer significant advantages: the teeth have a higher load capacity, a longer service life and a lower noise level. In addition to the ring gear and pinion, the central axle drive also comprises the differential bevel gear. The differential gear or transverse differential ensures, when cornering, that the outer cornering wheel turns faster than the inner cornering wheel. This

1In

the case of trucks this always applies because there are only so-called rear longitudinal configurations—see Fig. 1.4. In the case of passenger cars and vans there is also so-called front wheel drive with transverse engine installation, whereby the axis of rotation of the crankshaft runs parallel to the axis of rotation of the drive wheels.

5.2  Axle Drive

33

Taper angle of the ring gear

Spiral gear

Ring gear Taper angle of the pinion

Pinion

Ring gear

Spur gear

Axle offset or hypoid gear

Pinion

Fig. 5.4   Illustration of a central bevel gear on a drive axle. b and c show the difference between a spur gear and a spiral gear. d illustrates a hypoid gear

is necessary because the roll paths of the two wheels are different when cornering.2 The design described below has established itself for the differential gear system (though other designs are conceivable): the differential, consisting of four gearwheels arranged at right angles to one another, is accommodated in a housing that is fastened to the ring gear and that co-rotates with the ring gear. Figure 5.5 shows the wheel layout of a simple central drive system complete with pinion, ring gear and differential. The torque of the ring gear is distributed to the two drive shafts by this gear system. If the wheels (drive shafts) are rotating at equal speeds, the differential gearwheels do not counter-rotate and also do not cause any internal friction. The speed difference between the drive shafts results mechanically due to the fact that with less friction, the wheel rotates faster. If the frictional forces on both sides of the axle are equal, than the torque is equally distributed (i.e. 50:50) to the two sides.

2In

fact, the roll paths of the wheels not only differ when cornering, but they also differ a little when traveling straight ahead, because the wheel circumference, the degree to which the tires are inflated and the tire slip of the individual wheels differ.

34

5 Axles

Bevel gear Ring gear

Housing

Axle shaft

Axle shaft

Differential gearwheels

Fig. 5.5   Wheel layout of the central axle drive complete with differential

Despite all the benefits, the differential has an unattractive side effect: the total tractive power of the vehicle is determined by the wheel with the lowest coefficient of friction. If a wheel is at an extremely low coefficient of friction, only that wheel will still rotate. In fact, on the spot, the wheel spins. Even if the other wheel provides good grip on a surface, the vehicle will not budge an inch. This is because the axle side that encounters a high coefficient of friction lacks support at the opposite end. The differential delivers the entire rotation only to the wheel with the low coefficient of friction. This problem arises when (at least) one wheel encounters a very low coefficient of friction, for example, when one wheel is on black ice. There are several technical solutions for this problem, for example, a differential lock and anti-slip control (ASR). ASR provides a remedy in that it brakes a spinning wheel to decelerate. As a result, the wheel with a high coefficient of friction transmits tractive power again. It supports itself on the braking force of the spinning wheel and the vehicle then moves. Another possible solution for ensuring tractive force on differing coefficients of friction is the differential lock. This locks the differential and enforces equal wheel speeds on both sides of the axle. Differing rotational speeds for the two axle sides cannot occur. Both wheels and the axle shafts rotate together just like a rigid body. When cornering on normal road surfaces, the locked (inactive) differential forces the tires to rub, causing a

5.2  Axle Drive

35

sharp increase in tire wear. The differential must therefore be free-moving (unlocked) when the vehicle is moving normally.

5.2.1.1 Through-Drive Axle Axles with so-called through drive are required to provide two driven rear axles (or two driven front axles for that matter). The drive torque is divided on a through-drive axle. For one thing, the wheels of the first axle (or through-drive axle) are driven, secondly, a proportion of the drive power is output via a transfer case to a second flange. A second axle is driven by this output flange via an intermediate propeller shaft. An inter axle differential enables the two axles to rotate at different speeds. For use on difficult terrain, the effect of the inter axle differential can be switched off by means of an inter axle differential lock. Figure 5.6 shows a multi-stage through-drive axle with an additional outer hub gear.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Tires Valve Valve extension Wheel rim Brake disk internally vented Radial shaft seal Axle housing Rear spring seat Axle housing with bearing cap Drive bevel gear Through-drive pinion housing Countershaft gear (driven) 12-point nut Cover (through-drive housing) Shift cylinder (through-drive lock)

16 17 18 19 20 21 22 23 24 25 26 27 28

Drive shaft Coupling sleeve (through-drive lock) Countershaft gear (driving) Differential case (interaxle differential) Output shaft gear Shifter fork (differential lock) Rear axle shaft, right Coupling sleeve (differential lock) Dog clutch Threaded flange Through-drive shaft with coupling flange Ring gear Differential case

29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Differential bevel gear Differential side gear Combination cylinder Brake caliper Brake pad Wheel fastening bolt Internal ring gear with carrier Planetary gear Oil drain plug Rear axle shaft, left Sun gear Oil filler plug End cover Bell hub with planet carrier Rear wheel hub

Fig. 5.6   Illustration of a through-drive axle. This is a through-drive axle for a tandem axle on a heavy truck. A two-stage axle with planetary hub reduction gear is shown. The axle has disc brakes, an inter axle differential and a transverse differential with differential locks. (Illustration: Daimler AG)

36

5 Axles

5.2.2 Multi-Stage Axles There are drive axles for which the axle gear ratio is implemented with two steps. There are two-stage axles, in which a two-stage gear reduction is executed in the central axle drive, and two-stage axles in which one gear reduction is implemented in the central drive and another gear reduction in the hub. The two-stage design in the central drive can be realized with additional spur gearing or with planetary gearing. The two-stage arrangement in the central drive can be designed to be switchable so that a selection can be made between two gear ratios in the axle. In niche specialty application (heavy haul) in North America, double reduction axles are used, in which the first gear step takes place outside of the axles center drive in a set of helical gears that connect the axle input shaft to the pinion gear. By its very nature, the weight and inner friction of the axle is higher for a two-stage axle than for a single-stage axle.

5.2.2.1 Hub Gear In addition to the central drive, which is always required, two-stage axles with hub gears have a second gear reduction on the hub. This gear reduction can be designed in the form of a planetary gear set or spur wheel gearing. The advantage of the final gear reduction being close to the wheel is that the last stage of the torque increase does not take place until it is really close to the wheel. The central drive and the drive shafts from the central drive to the hub are therefore subjected to a smaller torque and can be accordingly designed to be lighter. The two-stage axle also allows smaller ring gear diameters to be selected (with the same or a higher overall gear ratio of the axle), with the result that you need a smaller axle box, thereby gaining ground clearance under the axle. The disadvantage of an axle with hub gear is that you need two hub gears and a central drive in the axle. For this reason, axles with hub gears are expensive and heavy. Figure 5.6 shows a two-stage axle with hub gear in the form of a planetary gear set. 5.2.2.2 Portal Axles So-called portal axles are made for extreme off-road vehicles to improve ground clearance. With these axles the drive shaft runs above the axis of rotation of the wheel to increase the ground clearance. At the wheel hub, the rotational movement must be transmitted to the axis of rotation of the wheel, which means that a gear reduction close to the hub is required. Figure 5.7 shows a gear reduction close to the wheel in the portal axle of a Unimog. Inverse portal axles are used on urban buses. These are portal axles on which the drive shaft runs below the axis of rotation of the wheel. This makes it possible to design the center aisle of the passenger compartment so that it is at a low height above the road surface, saving the passengers a few steps when they board the bus.

5.3  All-Wheel Drive Vehicles – Driven Front Axles

37

Fig. 5.7   The portal axle of a Mercedes-Benz Unimog: the drive shaft lies above the axis of rotation of the wheel. A gear reduction transmits the rotation of the drive shaft to the wheel. The distance between the wheel center (2) and the axle (3) is the gain in ground clearance with this design. (Illustration: Daimler AG)

The portal axle:

1. Hub reduction gear 2. Wheel center 3. Axle center

5.3 All-Wheel Drive Vehicles – Driven Front Axles Driven front axles are required for all-wheel drive vehicles. The special challenge in this case is that the axle must combine the steering function and the drive function. Only a small proportion of the trucks produced are vehicles with a driven front axle. The classic driven front axle is mechanically driven by a propeller shaft. The propeller shaft comes out of the transfer case. In the same way as with driven rear axles there are single-step axles and hub reduction axles with a secondary gear reduction in the hub.

5.3.1 Hydraulically Driven Axles For several years there have also been hydraulically driven axles (HAD – Hydraulic Auxiliary Drive) [16] in the commercial vehicle sector. Vehicles with additional hydraulic axles have a normal rear axle drive with a mechanical connection via a propeller shaft between the engine and the main drive axle, and a front axle that is hydraulically driven when required. Installed in the vehicle is an engageable hydraulic pump that is driven by a power takeoff. The vehicle has hydraulic wheel motors on the front axle. If support of

38

Wheel hub motor

5 Axles

Hydraulic pump on the power takeoff

Wheel hub motor

Oil cooler, reservoir

Fig. 5.8   Hydraulic auxiliary drive in the vehicle. (Illustration: Daimler AG)

the front axle is required, the hydraulic pump delivers hydraulic fluid to the front axle and the wheel hub motors of the front axle contribute to the tractive force. The entire system also requires a fluid reservoir for the hydraulic circuit, an oil cooler (the fluid heats up intensely) and a valve block with a control unit to control the system. Figure 5.8 shows the HAD installation situation in the vehicle. If the traction potential of the conventional drive is sufficient, the system is switched off. Only moderate additional fuel consumption is incurred in normal operation through the increased friction in the front axle. In contrast, the conventional all-wheel drive, with a transfer case, propeller shaft to the front axle and axle drive on the front axle, causes significantly higher additional fuel consumption. Other advantages of this hydraulic auxiliary drive in comparison to a conventional all-wheel drive are that the system is lighter and less expensive. However, the hydraulic auxiliary axles that are available nowadays are generally intended to be short-term traction aids. They switch off at a certain speed and on difficult terrain cannot provide the performance of a all-wheel drive with a conventional mechanical front-axle drive.

6

Tires

According to Michael Schumacher, in Formula 1 motor racing the tire is the component that most frequently decides whether a race will be won or lost. In the commercial vehicle business, although it is not a matter of winning or losing, the tire is pretty important for the performance of the vehicle. The tire makes the (only) connection between the vehicle and the road. All forces that act on the road and from the road on the vehicle must be transmitted by the tire – see Fig. 6.1. The tire determines the transmissible transversal and longitudinal forces, and it decisively influences the comfort of a vehicle. The tire contact patch on a truck tire is about as big as a size A4 sheet of paper. In addition, the tire makes an important contribution to the economy of a vehicle. In the long-haul transportation segment, tires account for 2.3% of the total costs [11]. The fuel consumption and therefore the diesel costs are also influenced by the choice of tire. In addition, the tires are responsible for more than a quarter of all breakdowns of trucks [15] – see Fig. 6.2. Depending on the application, tires roll over a wide range of different surfaces. From perfectly asphalted freeways to off-road tracks with sharp rocks, trucks are moved on all kinds of surfaces. Even a perfectly asphalted road changes its appearance continually: it can be hot and dry or wet with rain, the road can be covered with snow or ice and in the fall the road can be dirty and covered with leaves. The temperature of the tire is also subject to large fluctuations. A different tire is required as the optimal tire for each of these areas of application. Hence the choice of tire is usually a compromise. In fact, there are several different tire types, each of which are focused on a different application. The various tire types differ in terms of their tread, rubber blend and inner structure. Figure 6.3 shows examples of the treads of various tires.

© Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7_6

39

40

6 Tires

Contact pressure

Braking Cornering

Accelerating

Fig. 6.1   The tire is the only contact between the vehicle and the road

Tires

Electronics Motor Miscellaneous Average of the years 2007–2012 Fig. 6.2   Evaluation of the breakdown statistics of the ADAC (the German automobile club) Truck Service for 2007 to 2012: tires are responsible for about a quarter of all truck breakdowns [15]

6.1 Structure of a Tire A tire consists of various rubber blends, steel cord and different plastic fibers (nylon, aramids). The tread of a tire is the surface that rolls on the roadway. The tire has a tread profile to improve the grip and to enable the tire to be self-cleaning. Under the tread there are

6.2  Various Types of Tire

Long-haul transport tires

41

Winter tires

Regional tires

Construction site tires

Off-road tires

Fig. 6.3   Examples of various types of tire for heavy trucks. (Photos: Continental Truck Tires 2008)

several layers of supporting materials that increase the stability and strength of the tire. The carcass is the load-bearing structure of the tire. It consists of steel cords that are embedded in rubber. Thickening at the inner edge of the tire, the so-called tire bead, provides a connection to the wheel rim. The tire bead consists of peripheral steel wires that are embedded in the rubber. On the inside the tire is coated with a special rubber blend to prevent the diffusion of air and moisture. This layer is called the liner or innerliner. Retreading When the tread is worn out, truck tires are often retreaded. First of all it is checked whether the basic structure of the tire is in good order. Any minor damage is repaired as necessary. The old tread of the tire is buffed or scraped off. A new rubber layer is then applied to the tire. Several processes are available for retreading tires [14]. The cold method involves bonding a previously constructed length of tread rubber to the buffed tire casing. The hot method involves applying a length of pre-cured rubber, without tread, to the buffed tire casing. It is then placed in a mold and cured, resulting in the vulcanization (hardening) of the tread rubber, creating a tread pattern, and bonding it to the tire casing.

6.2 Various Types of Tire There are tires that are intended for use on the road and special off-road tires. Tires that are a compromise between on- and off-road use are also offered. Long-haul transport tires place great value on optimization of the rolling resistance, providing customers with fuel economy. Super-wide tires are a variant of the long-haul transport tire. In this case the dual tires on the drive axle are replaced by an extra-wide single tire. In principle, the single tire takes up less flexing energy than the twin tire and therefore affords lower rolling

42

6 Tires

resistance.1 In addition, super-wide tires are lighter compared to dual tires. ­Super-wide tires are not suitable for distribution transport because super-wide tires are subject to greater wear when cornering. Off-road tires are very resilient and have a rough tread. The tire must be robust enough to withstand damage to the tread and sidewall caused by stones and gravel. A rough tread prevents the tire tread from clogging up too quickly with dirt in off-road conditions. Winter tires are specifically focused on providing good coefficients of friction at low temperatures, i.e. good driving and braking properties on snow and ice. Winter tires for trucks usually have many small tread blocks. As a result, when the tire is rolling, a movement of the tire tread is created that leads to self-cleaning of the gaps in the tread. Small kerfs are cut into the tread blocks of winter tires. If the tread blocks deform during the driving-off procedure, the kerfs, or sipes, open and form a multitude of gripping edges that provide increased traction on winter surfaces. There are regulations that differ from country to country for the use of winter tires. In Germany, the following applies: the equipment on vehicles must be adapted to the weather conditions. In other countries winter tires are explicitly required. On account of the numerous braking and accelerating procedures, short-haul and regional transport tires are highly focused on abrasion resistance. In addition to the differentiation between the areas of application, with commercial vehicle tires it is also differentiated between specific tires for the various axles: there are tires for steered axles, drive axles, semitrailers and full trailers. Tires for steering axles and semitrailer axles usually have a tread with a strong longitudinal orientation as compared to tires for drive axles. The rolling resistance of drive tires is therefore usually somewhat greater than the rolling resistance of steering axles and semitrailer tires. Figure 6.4 illustrates an attempt at segmenting the various areas of application of tires for trucks. The development of a tire by the manufacturer and selection of the tire by the customer must balance conflicting priorities between economy, driving safety, comfort and environmental friendliness. Under these headings, numerous tire properties can be grouped that have to be taken into consideration during development of the tire and when selecting the correct tire. In Fig. 6.5, the target dimensions for the development and selection of a tire are shown. In particular, the requirement of low rolling resistance usually conflicts with the desired requirements of traction, high mileage and low noise emission. The tire manufacturer can alter the tire in several ways to constitute the desired tire properties: it can optimize the composition of the rubber, the so-called blend; it can vary

1In

fact, the quantities of super-wide tires sold are small. Tire manufactures do not foucs on that type of tire and presumably the theoretical potential of this tire type is not realized.

6.3  Identification of a Tire

43

Harshness of the application Legend Heavy

Segment

Off-road

- Important features of the segment for tires

Construction

- Development focus of the tire

- On-road and off-road - High resistance to damage - Heavy payload - High risk of damage

Distributor

- Stop and go traffic: many braking and acceleration procedures - Many bends - Maneuvering

Light

- Robust - High durability

Winter tires

- Very different weather conditions: ice, snow and cold, but also "normal" roads

- Steering behavior and traction - Good braking behavior on various surfaces

Long-haul transport

- Long distances - High mileage - A lot of freeway - Low fuel consumption - Comfort

Low

High Annual mileage

Fig. 6.4   Various tire categories and their areas of application

the shape of the tread; and it can design the inner structure of the tire, the carcass, so that it provides the desired behavior of the tire.

6.3 Identification of a Tire The specifications shown in Fig. 6.6 identify a tire: 1. The width of the tire is specified in millimeters. In the case of older formats, tire sizes in inches can still be found. A tire with the identification number 12 R 22.5 is 12 in. wide. 2. The ratio between the sidewall height and the width of the tire, the so-called cross-section ratio, is specified. A ratio of 80 means that the height of the tire is 80% of its width. 3. An alphabetic character provides information about the construction of the tire: R stands for radial, no specification means a bias tire (also called cross-ply tire or diagonal tyre). The radial tire is dominant in truck usage. 4. The nominal diameter of the wheel rim is specified in inches (1 in. = 2.54  cm).

44

6 Tires

Safety • Braking • Traction (on good roads/when wet/in the winter) • Steering behavior/lateral stability

• Robustness (resistance to damage)

Economy • Price • Service life • Regrooving capability • Retreading capability

• Handling

Comfort

• Rolling behavior • Damping of the road surface excitations

• Rolling resistance (= consumption)

• Noise

• Recycling • Material usage in production

Environment Fig. 6.5   Various requirements of tires

tubeless 1) Tire width w in mm 2) Ratio of height to width 3) Construction of the tire 4) Nominal diameter of wheel 5) Tire load capacity, if single tire 6) Tire load capacity, if dual tires

w

7) Speed code 8) Alternatively permissible load capacity and speed

h

9) Indicates a tubeless tire

Re. 2) Explanation of the cross-section ratio h:w

Fig. 6.6   The most important specifications on a tire

6.3  Identification of a Tire

45

Tab. 6.1  Load capacity index for commercial vehicle tires Index

147

148

149

150

151

152

153

154

155

156

Load capacity (kg per tire)

3075

3150

3250

3350

3450

3550

3650

3750

3875

4000

Tab. 6.2  Speed approval for commercial vehicle tires Letter

F

G

J

K

L

M

N

P

Q

R

Speed (km/h)

80

90

100

110

120

130

140

150

160

170

5. An index number specifies how great the load capacity of the tire is. Tab. 6.1 defines the meaning of the index numbers. 6. In a dual tire arrangement the load capacity of a tire is lower. There is therefore a second index number that describes the load capacity of the tire for dual tires. 7. An alphabetic character specifies the speed for which the tire is approved. The speed specification code is shown in Tab. 6.2. 8. A second tire load capacity specification with a different speed approval can be indicated on the tire. 9. It is specified on the tire whether it is a tubeless tire or a tire that has to be used with a tube (tube type). Further information can usually be found on a tire: • The manufacturer’s name and the designation of the specific tire type. • Designation of the recommended area of applications, for example, regional, or designation by M+S or by a snowflake to signalize that the manufacturer recommends the tire for winter operation. • The ply rating is a specification of the strength of the tire carcass. There is a specification for the tread and for the sidewall. Historically, this rating describes how many ply the substructure has been made from. Modern materials attain higher strengths, so the ply rating no longer describes an actual number of plies. • The tire load capacity, according to the U.S. standard in pounds (lbs), and the maximum inflation pressure in psi are indicated on the tire. Tab. 6.3 provides help with conversions into SI units. • DOT identification (DOT stands for the U.S. Department of Transportation) provides information about the time of manufacture of a tire. The last four digits designate the production week and year of production. If the last four digits are 0709, the tire was produced in week 7 of year 2009. The other characters of the DOT identification are codes that represent the tire production plant, the tire size and the ­manufacturer-specific tire type.

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6 Tires

Tab. 6.3  Conversion from pounds (lbs) and psi to SI units SI unit 1 lb

1 pound

1 psi pound-force/square inch aThe

= 453.59237 g 1 psi =

0.45359237 kg · 0.980665 m/s 0.0254 m · 0.254 m

2

= 0.45359237 kg

a

6.8948 · 103 Pa

2

conversion of psi to SI units is done with a value of 9.80665 ms for normal gravitational acceleration

Details of the regulations that relate to the identification of tires for commercial vehicles can be found in [12].

6.3.1 Regrooving Tires There are tires that are specified by the tire manufacturer as being regroovable, in accordance with [12], the designation marked as regroovable. With regrooving, when a tire is worn the rubber between the steel belt and the tread base is incised to deepen the (worn) tread and extend the service life of the tire. The basic strength of the rubber over the steel belt is reduced by the regrooving process. After regrooving, cord material below the grooves should have a protective covering of tread material at least 3/32-inch thick (more than 2 mm). Regrooving must be performed by qualified and specialized personnel. Regrooving dates back to a time when bias-ply tires were in widespread use. The market shift to radial tires has made retreading the primary method of extending life from a tire. Regrooving is no longer a common practice in the United States.

6.4 Tire Pressure If the tire pressure is too low, it can cause cost-efficiency losses for the trucking company. Low air pressure causes increased flexing in the tire; increased flexing means higher rolling resistance and increased fuel consumption. In addition, increased abrasion results from the increased flexing. The inverse conclusion: if the air pressure is correct, tires wear more slowly. However, correct air pressure also has a safety aspect: the danger of a burst tire is less, if the air pressure is correct. Increased flexing with underinflation is converted into thermal energy. A tire that is underinflated will heat up more intensely. This thermal load damages the tire and can cause tires to burst or blowout. A burst tire is also a ­cost-efficiency nuisance for the trucking company. Replacement of the damaged tire costs money, the driver and the vehicle spend unproductive time dealing with it and the customer may be dissatisfied about the late delivery.

6.4  Tire Pressure

47

For practical purposes, most trucking companies adjust their tire pressures to the correct pressure on their own premises. It is often difficult for the driver to adjust the air pressure once he is on the road. Not every filling station has suitable tire inflation equipment for trucks. In addition, the air pressure must be checked when the tires are cold, because the specifications of the tire manufacturer for the correct tire pressure are for cold tires. Drivers entering filling stations for refueling arrive with tires that are warm from driving. When the air pressure is checked at filling stations, there is a tendency to underinflate the tires. Many trucks have a tire inflating hose in their vehicle tool kit. With this, the driver can use air from the compressed air system to inflate the tires. The driver also needs a pressure gauge to measure the actual air pressure after inflation. A more convenient method of checking the tire pressure is provided by a tire pressure monitoring system that is installed in the vehicle. This system also monitors the tire pressures while driving.

6.4.1 Tire Pressure Monitoring System With a directly measuring tire pressure monitoring system (TPMS = Tire Pressure Monitoring System) the tire pressure is measured directly. Each tire has a so-called wheel electronics or a wheel sensor for this purpose. This includes a pressure sensor and other functions, such as a transmitting antenna, logic and an acceleration sensor. There are several methods of placing the wheel electronics in the tire. You can glue them from the inside into the tire (inner surface of the tread), screw them to the valve on the inside of the wheel rim or use a retaining strap, which runs around the circumference of the wheel rim, to fasten it in the rim base. Vulcanization of the wheel electronics into the tire rubber is being investigated for future development. There are also systems where the electronics are positioned outside the tire and sit on one of the wheel nuts that hold the wheel. With this system, a tube leads to the air inflation connector of the tire, transmitting the air pressure to the sensor. In all cases, the wheel electronics rotate with the wheel. A radio connection between the wheel electronics and the vehicle is therefore required. The wheel electronics measure the air pressure and temperature, and transmit them together with an ID to an on-board antenna. The wheel data is processed in a control unit and sent to the vehicle’s instrument panel or to a separate display to provide the driver with information about the tire pressures. The wheel electronics have a battery as their energy source. To conserve this battery, the electronics switch to idle mode when the vehicle is at a standstill; neither continuous determination of the tire pressures nor radio transmission of the data are required when the vehicle is a standstill. The acceleration sensor of the wheel electronics detects the motion status of the vehicle and activates the air pressure measurement and radio transmission when the vehicle is in motion.

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6 Tires

6.4.1.1 Localization of the Wheel Electronics To be able to accurately display to the driver which tires contain too little air, the air pressure monitoring system must be able to assign the different wheel electronics, which also transmit an ID, to the different tire positions. With simple systems, assignment of the wheel electronics ID to the tire positions is performed manually. With advanced systems, the different wheel electronics are automatically assigned to their installation positions. Localization of the electronics by signal strength comparison A technical concept that accomplishes assignment of the sensor signals to the tire positions is as follows: Several antennas are positioned on the vehicle. All of the antennas receive the signals of all the wheel sensors, however, with a varying signal level. From the different signal levels at the different antennas of the system the logic of the control unit is able to determine which wheel sensor is seated at which wheel position on the vehicle. The more axles and tires a vehicle has, the more antennas are required to dependably determine the position assignments. The position resolution via the signal levels of the wheel electronics is not sufficient to differentiate between the two wheels of dual tires. However, with dual tires you can make use of the fact that the wheel rims are installed the opposite way around, i.e. turned through 180°, to each other. If the installation position of the sensor is fixed relative to the wheel rim (for example, by installing the wheel electronics on the valve), the wheel sensors of a pair of dual tires have different directions of rotation. If the wheel sensors are able to detect the direction of rotation by way of two-dimensional acceleration measurement, the tire pressure monitoring system can assign the sensor signals from the two wheels of dual tires to the correct tire in each case. Localization of the wheel electronics by comparing the wheel rotation signals Another technique of assigning the signals that the control unit receives from the wheel electronics to the corresponding wheel positions is based on a comparison of rotation patterns. With the aid of the acceleration sensors of the wheel electronics, each wheel electronics unit can signal when a full revolution of the wheel has taken place. The brake system also receives continuous information about the wheel rotations via the ABS sensor and the rotor. As each wheel rotates at a slightly different rotational speed, it can be determined, from the comparison of the revolution information from the wheel electronics and brake system, which wheel electronics unit is located at which wheel position. It also applies in this case that it cannot be differentiated between the two tires of a pair of twin tires by this technique. In this case, again, the direction of rotation information can be used.

Comprehension Questions

The comprehension questions serve to test how much the reader has learned. The answers to these questions can be found in the sections to which the respective question refers. If it is difficult to answer the questions, it is recommended that you read the relevant sections again. A.1 Suspension (a) What are the differences between air suspension and steel spring suspension? (b) What functions are enabled by air suspension? A.2 Steering (a) What is Ackermann steering? (b) Where is turntable steering used? (c) Which components establish the connection between the steering wheel and the wheel? (d) How are dual circuit steering systems implemented? (Two answers) A.3 The Ackermann condition (a) What does the Ackermann condition state? (b) How does one deal with a tandem axle subject to the Ackermann consideration? (c) Can the Ackermann condition be met? A.4 Frame (a) What is the ladder frame? (b) Which dimensions determine the frame rail of the ladder frame? (c) What is the hole pattern? A.5 Axles (a) What are the various functions that the axle performs? (b) What is a two-stage axle? (c) For what purpose does one need a through-drive axle? (d) What advantages does the portal axle offer? © Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7

49

50

Comprehension Questions

A.6 Axle drive (a) What is the function of the central drive? (b) What does the differential do? A.7 Tires (a) What types of tire are there? (b) How can one give a tire a second (or third) “life”? (c) What does 315/80 R22.5 mean? A.8 Tire pressure monitoring (a) Which variables do the wheel electronics of the tire pressure monitoring system sense? (b) How does fully automatic assignment of the signals of the wheel electronics to the wheel positions function? (c) How does the system differentiate between the signals of two twin tires? A.9 Terms Explain the following terms: (a) Hypoid gear, (b) Hub gear, and (c) Ring gear.

Abbreviations and Symbols

The following is a list of the abbreviations used in this booklet series. The letters assigned to the physical variables is in conformity with normal usage in engineering and the natural sciences The same letter can have different meanings depending on the context. For example a lower case c is a very busy letter. Some abbreviations and symbols have been subscripted to avoid confusion and improve the readability of formulas, etc.

Lowercase Latin letters a acceleration b linear measure, often width bar bar, unit of pressure—1 bar = 105 Pa c coefficient, proportionality constant f coefficient or correction factor g gravitational acceleration (g = 9.81 m/s2) g gram, unit of mass h linear measure, often height h hour, unit of time hp Horsepower, unit of power (not an SI unit)—1 hp = 735.5 W i gear ratio, ratio of rotational speeds k kilo  = 103 = a thousand times kg kilogram, unit of mass km km, unit of length—1 km = 1000 m km/h kilometers per hour, unit of speed—100 km/h = 27.78 m/s kW kilowatt, unit of active power—1 kW = 1000 watts kWh kilowatt-hour, unit of energy l length m mass m meter, unit of length © Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7

51

52

Abbreviations and Symbols

m milli  = 10−3 = one thousandth mm millimeter, unit of length—1 mm = 10−3 m n rotational speed p pressure psi poundforce per square inch, unit of pressure (normal in the USA, no SI unit) r linear measure, often radius rpm revolutions per minute; angular speed s linear measure (distance) t time t ton, unit of mass—1 t = 1000 kg v velocity x typical designation for one of three spatial coordinate axes y typical designation for one of three spatial coordinate axes z typical designation for one of three spatial coordinate axes

Uppercase Latin letters ABS Anti-lock braking system ADAC German automobile club ASR Acceleration skid control BGL Bundesverband Güterkraftverkehr, Logistik und Entsorgung e. V. (German federation for road haulage, logistics and disposal) CAD Computer-Aided Design DIN Deutsches Institut für Normung (German institute for standardization) DNA Twin-tire trailing axle DOT Department of Transportation (U.S.) ECE Economic Commission for Europe of the United Nations ENA Single-tire trailing axle ESP Electronic Stability Program F Force FG Weight force GFRP Glass-Fiber Reinforced Plastic HAD Hydraulic Auxiliary Drive ID Identifier = identification number or similar J Joule, unit of energy K Kelvin, unit of temperature on the Kelvin scale M Torque M Mega  = 106 = million MJ Megajoule, unit of energy—one million joules MW Megawatt, unit of active power—one million watts N Newton, unit of force

Abbreviations and Symbols

53

NH3 Ammoniac NLA Trailing axle OEM Original Equipment Manufacturer P Power (equals energy per time) TPM Tire Pressure Monitoring system SI Stands for International System of Units T Temperature (in Kelvin or °C) TCO Total cost of ownership incurred over the usage duration of the vehicle or of another asset TPMS Tire Pressure Monitoring System TÜV German technical inspection association V Volume V Volt, unit of electrical voltage VLA Leading axle W Mechanical work or mechanical energy Wkin Kinetic energy Wpot Potential energy W Watt, unit of active power Wh Watt-hour, unit of energy—cf. the more common kWh

Lowercase Greek letters α angle ß angle γ angle δ angle µ coefficient of friction, sometimes also µk coefficient of adhesion µ stands for micro = 10−6 = a millionth ρ density φ angle ω angular speed ω rotational speed

References

1. ECE-R 93 Regulation No. 93. Uniform provisions concerning the approval of: I. front underrun protective devices II. vehicles with regard to the installation of a front underrun protective device of an approved type III. vehicles with regard to their front underrun protection 2. ECE-R 79 agreement on the adoption of uniform regulations for wheeled vehicles, equipment and parts that can be installed and/or used in wheeled vehicles, and … Regulation No. 79, Revision 2, Uniform provisions concerning the approval of vehicles with regard to steering equipment 3. Hesse, K.H., Becher, H.O., Sieber, A.: Fahrwerkregelung in Nutzfahrzeugen. Nutzfahrzeuge, Mannheim, June 1997. VDI Rep. 1341, (1997) 4. Kaiserliches Patentamt (Imperial Patent Office) Berlin: Patentschrift No 37435, Fahrzeug mit Gasmotorenbetrieb (1886). granted to Benz & Co in Mannheim 5. Dudzinski, P.: Lenksysteme für Nutzfahrzeuge. Springer, Berlin (2005) 6. Degerman, P., Anund, O.A.: Friction Estimation using self-aligning torque for heavy trucks. Chassis.tech, 2nd International Munich Chassis Symposium, Munich, Germany, June 7 and 8, 2011 7. Nissan Center Europe GmbH: Nissan Atleon. Product brochure—status September, 2010 8. Gaedke, A., et al.: Driver assistance for trucks—from lane keeping assistance to smart truck maneuvering. Chassis.tech, 6th International Munich Chassis Symposium. Springer Vieweg, Berlin Heidelberg New York (2015). Proceedings published by Pfeffer P. 9. Hilgers, M.: Entire Vehicle. Commercial Vehicle Technology. Berlin/Heidelberg/New York: Springer (2021) 10. Hilgers, M.: Transmissions and Drivetrain Design. Commercial Vehicle Technology. Berlin/ Heidelberg/New York: Springer (2021) 11. Bundesverband Güterkraftverkehr Logistik und Entsorgung (BGL) e. V.: Kostenentwicklung im Güterkraftverkehr – Einsatz im Fernbereich – von Januar 2007 bis Januar 2008 12. ECE Regulation No. 54: Uniform provisions concerning the approval of pneumatic tires for commercial vehicles and their trailers (2003) 13. Continental: Lkw-Reifen: Die technischen Grundlagen (2006). Printout—available for download on the Internet. 14. Unspec.: Das zweite Gesicht. Lastauto Omnibus 4, 34 (2011). Articles on the retreading of truck tires 15. ADAC TruckService: Pannenstatistik: Die häufigsten Ursachen von LKW-Pannen (2013). Publications of the ADAC (2008–2013) http://www.adac.de 16. Frick, P.: MAN HydroDrive—Serienerfahrungen. Getriebe in Fahrzeugen 2006. VDI Rep., 1943 (2006) © Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7

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Index

A Ackermann steering, 16 Air bellows, 12 Air suspension, 7 All-wheel drive vehicles, 37 Arched crossmember, 2 Armored tank steering, 15 Articulated steering, 16 Attachments, 2 Axle, 29 hydraulically driven, 37 multi-stage, 36 Axle beam, 30 Axle drive, 32 Axle formulas, 4 Axle housing, 30 Axle load compensation, 9, 10 Axle ratio, 29

Center axle drive, 32 Chassis, 1 Control arm, 12 Coupling head, 2 Crossmember, 1 C-shaped beam, 1 C-shaped crossmember, 2

B Baffle plate, 25 Battery support, 25 Bevel differential, 32 Bevel gear, 32 Bias tire, 43

F Fastening strap, 25 Flexing, 46 Four-bag air spring, 11 Frame, 1 Frame track width, 2 Front axle, driven, 37

C Camber negative, 17 positive, 17 Carcass, 41 Caster angle, 17

D Differential, 33 Differential lock, 34 DOT identification, 45 Drivetrain design, 30

E End crossmember, 2

H Hole pattern, 3 Hub gear, 32, 36 Hypoid axle, 32

© Springer-Verlag GmbH Germany, part of Springer Nature 2021 M. Hilgers and W. Achenbach, Chassis and Axles, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-60835-7

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58 I Independent wheel suspension, 31 Index number, 45 Interaxle differential, 35

J Juddering tendency, 19

K Kingpin, 17 Kingpin inclination, 17

L Ladder frame, 1 Leading axle, 24 Leaf spring suspension, 7 Level control, 12 Long-haul transport tires, 41 Longitudinal member, 1

M Multi-leaf springs, 7

P Pinion, 32 Pivot axis steering, 15 Planetary gear set, 36 Portal axles, 36 Power steering pump, 23

R Radial tire, 43 Regional tires, 42 Regroovable, 46 Regrooving, 46 Retreading, 41 Ring gear, 32 Roll control, 13

S Scrub radius, 18 Shopping carts, 18 Single-pivot steering, 15

Index Skid steering, 15 Slip angles, 21 Starter battery, 25 Steel spring suspension, 7 Steering, 15 Steering gear, 23 Steering return, 19 Steering shaft, 23 Steering trapezium, 22 Straight-ahead running, 19 Subframes, 3 Super-wide tires, 41 Support structure, 1 Suspension, 7

T Tandem axle, 9 Tank, 25 The Ackermann condition, 19 Three-wheeler, 15 Through-drive axle, 35 Tire pressure, 47 Tire pressure monitoring system, 47 Tire wear, 19 Tires, 39 Toe-in, 16 Toe-out, 16 Trailing axle, 24 Transverse differential, 32 Tread, 40 Tubular crossmember, 2 Turning circle, 21 Turning radius, 20 Two-bag air spring, 11

V Vehicle layout, 5 Vehicle level, 12 Vertical section height, 1

W Wheel electronics, 47 Wheel formula, 4 Wheel motor, hydraulic, 37 Winter tires, 42