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Design of

Design of

Human-Powered

Human-Powered

Vehicles

Vehicles

by Mark Archibald

by Mark Archibald

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© 2016, The American Society of Mechanical Engineers (ASME), 2 Park Avenue, New York, NY 10016, USA (www.asme.org)

All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher.

INFORMATION CONTAINED IN THIS WORK HAS BEEN OBTAINED BY THE AMERICAN SO-CIETY OF MECHANICAL ENGINEERS FROM SOURCES BELIEVED TO BE RELIABLE. HOW-EVER, NEITHER ASME NOR ITS AUTHORS OR EDITORS GUARANTEE THE ACCURACY OR COMPLETENESS OF ANY INFORMATION PUBLISHED IN THIS WORK. NEITHER ASME NOR ITS AUTHORS AND EDITORS SHALL BE RESPONSIBLE FOR ANY ERRORS, OMISSIONS, OR DAMAGES ARISING OUT OF THE USE OF THIS INFORMATION. THE WORK IS PUBLISHED WITH THE UNDERSTANDING THAT ASME AND ITS AUTHORS AND EDITORS ARE SUPPLY-ING INFORMATION BUT ARE NOT ATTEMPTSUPPLY-ING TO RENDER ENGINEERSUPPLY-ING OR OTHER PROFESSIONAL SERVICES. IF SUCH ENGINEERING OR PROFESSIONAL SERVICES ARE REQUIRED, THE ASSISTANCE OF AN APPROPRIATE PROFESSIONAL SHOULD BE SOUGHT.

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Library of Congress Cataloging-in-Publication Data

Names: Archibald, C. Mark, author.

Title: Design of human-powered vehicles / by C. Mark Archibald.

Description: New York : ASME Press, [2016] | Includes bibliographical references and index. Identifiers: LCCN 2016005632 | ISBN 9780791861103

Subjects: LCSH: Velocipedes--Design and construction. | Bicycles--Design and construction. | Human powered vehicles--Design and construction.

Classification: LCC TL400 .A73 2016 | DDC 629.227/2--dc23 LC record available at http://lccn.loc. gov/2016005632

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ABLE OF CONTENTS

ABLE OF CONTENTS

Acknowledgments ... ix Acknowledgments ... ix Preface Preface ... ... xixi Chapter 1: Rationale for Chapter 1: Rationale for Human-Powered VHuman-Powered Vehicle Design ehicle Design and Use and Use ...11

Recreation ...2

Competition ...2

Economics ...4

Fitness and Health ...5

Mobility ...8

Environment ...8

Chapter 2: Overview Chapter 2: Overview of Human-Powered Vehicles A Brief Historical Perspective ...13of Human-Powered Vehicles ...1313 Land Vehicle Applications and Functions ...16

Land Vehicle Configurations ...17

Chapter 3: General Structured Design of HPV’s Chapter 3: General Structured Design of HPV’s ... 2525 General Structured Design of HPVs ...25

Target Speeds for Tandem Bicycle ...30

Example Vehicle Design Specification ...31

Functional Requirements ...32

Outline of Design Process ...38

Chapter 4: Physiology Chapter 4: Physiology of Human of Human Power Generation Power Generation ...4141 Muscle Structure and Function ...42

Nutrition ...49

Body Systems during Exercise ...52

Maximal Oxygen Consumption ...57

Anaerobic Threshold...58

Appendix: Calculating the CO2 Production Rate as a Function of External Work ...58

T

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Design of Human-Powered Machines Design of Human-Powered Machines

Chapter 5: The Human-Machine

Chapter 5: The Human-Machine Interface Interface ... 6161

Comfort ...61

Mechanisms for Power Transfer to Human-Powered Vehicles ...63

Optimal Body Position for Leg Cranks ...64

Crank Length and Physiological Efficiency ...67

Hand Cranks ...69

Unusual Mechanisms for Human Power Transfer ...71

Safety ...72

Chapter 6: Manufacturing Chapter 6: Manufacturing Processes and Materials ... 77Processes and Materials ... 77

Wrought Metals—Overview ...78

Cast Metals—Note ...81

Non-Metal Materials—Overview ...81

Frame Materials and Manufacturing Processes...85

Steel ...86

Stainless Steel ...89

Aluminum ...89

Titanium ...91

Fiber Reinforced-Polymer Composites ...92

Other Frame Materials ...96

Frame Manufacturing Processes ...97

Brazing ...99

Bonding ...99

Other Frame Processes (Monocoque, etc.) ...100

Fairing or Shell Materials ...101

Fairing Hardware ...104

Summary ...104

Chapter 7: Road Chapter 7: Road Loads Loads ...105...105

Review of Equilibrium Equations ...105

SAE Vehicle Coordinate System for Vehicle Dynamics ...106

Static Loads on Level Ground ...107

Static Loads on a Grade ...110

Steady Motion Road Loads ...112

Basic Loads in a Steady Turn ...115

Acceleration and Braking ...117

Power-Limited Acceleration ...118

Traction-Limited Acceleration ...119

Inertia Coefficient ...121

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TTable able of of ContentsContents

Braking ...125

Braking Performance ...128

Chapter 8: Speed Chapter 8: Speed and Power Models and Power Models ... 133133 Drive Train Efficiency ...134

Aerodynamic Drag ...135

Rolling Resistance ...138

Frictional Losses in Wheel Bearings ...139

Changes in Potential Energy ...140

Changes in Kinetic Energy ... 140

Power Models ...141

Cubic Power Model ...144

Applications of Power Models ...146

Chapter 9: Aerodynamic Drag ... 151

Chapter 9: Aerodynamic Drag ... 151

Causes of Aerodynamic Drag ...151

Lift and Induced Drag ...157

Computing Drag Force ...158

Factors Affecting the Drag Coefficient ...159

Estimation of the Drag Coefficient ...162

Drag Coefficients for Various Vehicles ...163

Chapter 10: Bicycle Handling Performance ...167

Chapter 10: Bicycle Handling Performance ...167

Bicycle Stability ...167

Bicycle Handling ...173

Patterson’s Method ...175

Chapter 11: Multi-Chapter 11: Multi-TTrack Vehicle Handling Performance ...rack Vehicle Handling Performance ...195195 Multi-Track Vehicle Handling ...195

Definitions and Nomenclature ...196

Low-Speed Cornering ...197

High-Speed Cornering ...207

Lateral Load Transfer and Rollover Threshold...214

Summary of Multi-Track Handling Characteristics ...225

Appendix 11-1 Kinematic Solution of the Track Rod Steering Mechanism ...226

Appendix 11-2 Derivation of Rollover Threshold for Tadpole Trike ...228

Chapter 12: Drive Tr Chapter 12: Drive Train Design ain Design ... 233233 Gearing ...233

Recumbent Drivetrains ...236

Drive Train Configurations ...238

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Design of Human-Powered Machines Design of Human-Powered Machines

Drive Train Technologies ...243

Efficiency of Chain Drives ...246

Chain Drive Design ...251

Chapter 13: Land Vehicle Fram Chapter 13: Land Vehicle Frames and Structures es and Structures ... 253253 Functional Requirements ...253

Yielding ...254

Fatigue ...254

Inadequate Stiffness ...256

Finite Element Modeling of Frames ...256

FEA Modeling—Idealizations, Loads, Constraints, and Validation ... 258

Beam Elements ...259

Shell Elements ...262

Summary of Frame Analysis Using Shell Idealizations ...264

Solid Elements ...264

Boundary Conditions: Loads and Constraints ...265

Load Cases ...265

Vertical Drop ...266

Horizontal Impact (CPSC or ISO Frame Test) ...267

Maximum Acceleration ...269

Hill Climb ...271

Maximum Front Braking ...271

Maximum Rear Braking ...273

Steady-State Pedaling ...274

Other Load Cases ...274

FEA Load Verification ...274

Initial Design Using Beam Idealizations ...274

Detailed Frame Design ...275

Frame Compatibility with Components ...276

Bottom Bracket Shells ...276

Head Tubes ...277

Steerer Tubes ...278

Dropout Locknut-to-Locknut Dimensions ...279

Integral Derailleur Hangar Dimensions...280

Chapter 14: Bicycle Components Chapter 14: Bicycle Components ... 281281 Wheels and Tires ...282

Drivetrain Components...290

Headsets ...296

Stems ...298

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TTable able of of ContentsContents

Brakes ...299 Compatibility of Levers and Brakes ...304 Summary ...305

Index

Index ... 307307 About

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CKNOWLEDGMENTS

CKNOWLEDGMENTS

A

A

I am deeply indebted to the many people who—directly or indirectly—

assisted with this book. First, many, many thanks go to Aaron Williams,

who thoughtfully and thoroughly reviewed many of the chapters. The

book is much improved as a result of his help. Many of my students

have assisted in various projects over the years that contributed data for

this book. Among them, I would particularly like to thank Tyler Baker,

Cameron Daugherty, Gretchen Robinson, and Liz Casteel, each of whom

spent many hours conducting experiments and analyzing data that was

used in this text. I must also thank Dr. Mark Reuber, who set me on the

journey of writing this book and, of course, my family that put up with my

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P

P

REFACE

REFACE

Matlab Computer Programs Matlab Computer Programs

Several chapters in this book refer to short computer code written in

Matlab. Matlab is a technical computing environment that includes a

high-level programming language. The script files and functions

men-tioned in the text are available for download to all purchasers of this

book. Navigate to the ftp site ftp://dohpv.gcc.edu. To download the files,

log in as

anonymous

,

and enter your email address as your password.

At this time, all files require a valid Matlab license and installation. Many

universities with engineering or science programs have site licenses for

Matlab. If you are affiliated with a university, you may well have a license

available. Otherwise, see http://www.mathworks.com/pricing-licensing/

for purchasing Matlab. (Student licenses are quite reasonably priced,

and home licenses are only a little more expensive.) All files include a

help section that will provide instructions for use. In some cases,

multi-ple versions of a program may be provided. The functionality may be the

same, but usage may differ. For example, a program may be offered as

both a script and a function. Please make use of the programs and use

them to design your ideal vehicle. I hope you find both the text and the

programs useful and helpful in your design efforts.

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CHAPTER

CHAPTER

1

A

ATIONALE

TIONALE FOR

FOR

HUMAN-POWERED

HUMAN-POWERED

VEHICLE DESIGN

VEHICLE DESIGN

AND USE

AND USE

T

his book is about the design of vehicles with wheels that are powered by

hu-man muscles alone. These can provide affordable, sustainable, and healthy transportation to people around the globe. The term Human-Powered Vehicle, or HPV, is sometimes used to denote a sub-class of vehicles including only high-performance bicycles or tricycles equipped with aerodynamic fairings. More generally, the term refer to any semi-recumbent bicycle. But the term should properly refer to any means of carriage, conveyance, or transport that is powered solely by human muscles. Manufacturers of bicycles, canoes, kayaks, and scooters do not market their products as HPVs, but surely all of these qualify for the name. Hybrid human-powered vehicles such as mopeds and electric bikes use human power in addition to other sources. While these vehicles are outside our definition of HPVs, they are certainly similar, in both technology and philosophy.

Human-powered vehicles were srcinally designed for transportation, and that is still their most important use. HPVs today provide clean, quiet, and efficient transportation. In most developed countries, and in particular the United States, the primary transportation systems are powerful and inefficient, generating large amounts of air and noise pollution. HPVs may be chosen simply because it is pleasurable to travel quietly through the countryside, experiencing nature rather than blocking it out behind steel and glass. They may be chosen because in some cases HPVs provide mobility that no other vehicle can. Couriers in congested cities use bicycles because they are faster. Campers and fishermen in areas such as Minnesota’s Boundary Waters or Ontario’s Algonquin Park may choose a canoe because no other vehicle can traverse the lakes, rivers, and portages quite so

R

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Design of Human-Powered Machines Design of Human-Powered Machines

well. Many choose human power because it is significantly less expensive than other alternatives, or perhaps because it is good for their health. Athletes and those with a competitive bent can find many venues for racing. Perhaps the most compelling reason to use HPVs is sustainability: the environmental footprint of HPVs is typically much, much smaller than that of other modes of transportation. Despite these commonalities, HPVs are used by a variety of different people for a wide range of diverse reasons, including recreation, competition, cost, health, transportation, and concern for the environment.

This book is limited to design of land human-powered vehicles. There are many reasons why the design and use of such vehicles is beneficial. In developed coun-tries, using an HPV in lieu of an automobile (or in lieu of a second automobile for a family) can save $5,000 to $10,000 each year, while improving health and reduc-ing emissions of greenhouse gasses and pollutants. Greenhouse gas emission will be reduced by more than 4,000 kg per year due to the corresponding reduction in energy consumption of more than 17,000 kWh. In addition, infrastructure for cycling is far less costly than highways designed for automotive traffic. It is appro-priate to look more deeply into the benefits of HPV use.

Recreation Recreation

HPVs, including both land and water vehicles, are frequently used for recre-ation. Often a bike ride or a canoe trip is a social event with friends and family. Quiet streets and rural roads can offer excellent cycling. The number of bicycle paths is increasing in many parts of the United States as abandoned railroads are converted into rail-trails and as local, state, and national parks provide more bike trails and paths. These facilities provide scenic routes for day rides, and can provide a sense of security for young riders, their parents, and others who are concerned about riding in traffic. Increasingly, the trails are long enough to use for multi-day trips. Streams, rivers, lakes, and coastal waters provide a rich range of environments for canoeists, kayakers, and rowers. A quiet pond or small stream may be an ideal place to get away for a while with a small paddle craft, while white water offers kayakers thrills and challenges. Many regions of the country have waterways that are restricted to human-power, either through law or in practice due to the nature of the lake or river.

Competition Competition

Racing HPVs has likely existed as long as human-powered vehicles them-selves. It is easy to imagine a group of tough and intrepid cyclists racing their

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Rationale Rationale for for Human-Powered Human-Powered Vehicle Vehicle Design Design and and UseUse

high-wheeled ordinary cycles over roughly paved roads in the nineteenth cen-tury. As bicycles became more advanced, the competition undoubtedly became faster, but perhaps no keener. Today, bicycle racing is an extremely popular sport in many parts of the world, especially Europe. Competitors can find venues for racing a variety of human-powered land and water craft, and several competitions have involved aircraft. HPV organizations provide many venues for racing re-cumbent vehicles, including very fast streamliners. Often these events showcase technological developments and design innovation. Many are local or regional events, sponsored by clubs. Of the more traditional races, the most well known is the Tour-de-France, an event restricted to diamond-frame bicycles. Two notable races that permit recumbent bicycles and streamliners are the Race Across Amer-ica and the World Human-Powered Speed Challenge. The Race Across AmerAmer-ica is one of the toughest races in the world. Competing individuals or teams start in California and race to New Jersey, with minimal sleep. The team record is slightly over 5 days for a faired recumbent bicycle, while the individual record is a little over 8 days. The World Human-Powered Speed Challenge held in Battle Moun-tain, Nevada, has hosted most of the land HPV speed records in recent years. On September 17, 2015, Todd Reichert set the men’s world record for the 200 meter flying start time trial with a speed of 137.9 kph1 (85.71 mph). This is quite remark-able, considering that top speeds for conventional racing bikes are usually under 50 kph (31 mph) and for recreational cyclists around 30 kph (19 mph).

For vehicle engineers, racing is a means of validating and proving new de-signs and design modifications. Competitive cyclists tend to be strong and to ride frequently. They demand the best performance from each vehicle system and often ride vehicles to the limits of performance. Components and systems that continue to operate and function well throughout training and racing generally function well for many years of less rigorous use. In recent years, cycling compo-nent manufacturers have competed to develop better, lighter, race-worthy parts and systems. The most successful designs become top-tier components seen on the best competition vehicles. Lower-tier components benefit as the best tech-nologies trickle down through product lines. The bicycle or HPV consumer is the ultimate beneficiary of this process, as the quality of lower-end components has increased significantly over the last few years wit hout a concomitant increase in cost.

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Design of Human-Powered Machines Design of Human-Powered Machines

Economics Economics

While the physical challenges and excitement of racing appeal to some, econom-ics attracts more people to HPVs than perhaps any other reason. Human-powered vehicles, especially bicycles, are substantially less expensive to purchase, own,

and operate than other vehicles. In the United States, a significant number of people with limited or no income use bicycles for transportation simply because they are affordable. This group includes students, of course, but it also includes many of our nation’s poor. It is not hard to find a rideable bicycle at a garage sale or thrift store for less than $25.00. What many people with incomes well above the poverty level do not realize is just how large transportation costs can be, partic-ularly with a transportation infrastructure that favors personal automobiles. The cost differential can be calculated relatively easily.

Consider a commuter that lives seven miles from her workplace. Additional driving brings her yearly average up to 15,000 miles. She bought the car after graduation from college for $18,000, paying $4,000 down and financing the rest at six% interest. On average, the car gets 22 miles per gallon, and her average price for fuel is $2.60. Maintenance costs her on average five cents per mile. She is more fortunate than many city workers, as she has free parking both at work and home. Including insurance at $350 per year, her total operating costs are very close to average, about 45 cents per mile. See Table 1-1 for more details and assumptions. She decides to investigate how much money she would save if she sold her car and bought a bicycle. Her bicycle would cost $1500, plus an additional $250 for clothing and accessories. She would spend about $725 each year on bicycle maintenance, sports foods and drinks, and accessories. Because she makes some long-distance trips that would not be practical for the bike, she spends about $400 per year on automobile rental. Since she rides regularly, she also cancelled her $216 gym membership.

Annual cost for both car and bike are plotted in Figure 1-1. The first year, she would save over $8,000, primarily because of the large down payment on the car. During loan repayment, she would save over $5,900 per year, but after the loan is paid off, her annual savings is still almost $2000. Each year she places the savings in a certificate of deposit earning four percent interest. At the end of 10 years she would have over $50,000 in the bank thanks to her bicycle commute.

The example is quite realistic, and the savings are realizable. In this case, the yearly savings is more than 12% of the United States median family income. For many people, this is a very significant amount. Different scenarios may result in different savings, but in virtually all cases, the savings are large. In some cases, the savings after 10 years can approach $100,000.

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Rationale Rationale for for Human-Powered Human-Powered Vehicle Vehicle Design Design and and UseUse

Fitness and Health Fitness and Health

Human-powered vehicles used on a regular basis can significantly improve health and fitness. Health problems related to sedentary lifestyles affect a signifi-cant portion of the world’s population. In 2006, Dr. Barry Popkin, in a presentation to the International Association of Agricultural Economists, announced that the number of overweight people around the globe exceeds the number of hungry

people.2 This is particularly a problem in developed countries such as the United

2Story from BBC NEWS: http://news.bbc.co.uk/go/pr/fr/-/2/hi/health/4793455.stm

Pub-lished: 2006/08/15 09:06:27 GMT, accessed 2007/08/20. Table 1-1 Table 1-1

Data for cost comparison of bicycle and automobile Data for cost comparison of bicycle and automobile

IInnppuut t VVaarriiaabblleess VVaalluuee UUnniitts s IInnppuut t VVaarriiaabblleess VVaalluuee UUnniittss

Initial Cost of Auto 18000 $ Initial cost of bike 1500 $

Financed amount 14000 $

Cost accessories/

clothing 250 $

Loan period 4 years Maintenance 150 $/year

Interest 6 %APR Clothing maintenance 75 $/year

Gas Mileage 22 MPG Auto rental (trips) 400 $/year

Annual Miles driven 15000 miles Sports drinks/snacks 500 $/year

Gasoline price 2.60 $/gal Gym Savings 216 $/year

Insurance 350 $/year Sold at 10 years

Maintenance 0.05 $/mile Salvage value 0 $

Sold at 10 years

Salvage value 1500 $

Savings interest rate 4 %APR

Assumptions:

Assumptions: Auto and bike replaced with identical vehicle

Difference placed in savings Auto loan is paid off on schedule Bicycle is bought with cash Bicycle is not insured

Notes:

Notes: Auto rental covers transportations costs for trips that would be driven in

auto only

Annual bike mileage is usually less than comparable auto mileage Savings based on the entire difference placed in savings account

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Design of Human-Powered Machines Design of Human-Powered Machines

States. Sedentary lifestyles can lead to obesity, with many associated health risks including diabetes and cardiovascular disease. In contrast, regular physical activ-ity can reduce the likelihood of obesactiv-ity and increase overall health.

Many people in the United States recognize the benefits of regular exercise. People who exercise regularly tend to live longer, more active lives. Regular aero-bic exercise keeps the cardiovascular system in good shape, prevents or reduces high blood pressure, and reduces levels of potentially harmful LDL cholesterol. Long-term regular exercise may also raise HDL (good) cholesterol levels. Moder-ate levels of exercise improve the immune system (although very intense exer-cise may actually impair immune system function.) Exerexer-cise also elevates mood and feelings. Brain levels of endorphins, serotonin, and dopamine are raised with either brief, intense exercise or longer, moderate exercise. These benefits have been shown to reduce the effects of depression and lead to improved feelings of well-being.Long-term exercise, coupled with a good diet, is effective for weight loss. Even brief, but regular, periods of exercise can be beneficial, and cycling is particularly effective. Generally, exercise does not make overweight people hungrier.

Regular cycling or HPV use provides all of these health benefits. Gym member-ships and fitness clubs are quite popular, and work well for many. Others, how-ever, find it difficult to make time for a workout, or drop out after a few sessions. Recreational bicycling or HPV riding is an enjoyable way to exercise, but—as with

Figure 1-1

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Rationale Rationale for for Human-Powered Human-Powered Vehicle Vehicle Design Design and and UseUse

gym memberships—it can be difficult to make time for it. An excellent alternative is to use HPVs for commuting or errands around town. The incremental time required to complete a trip by HPV is much smaller than that required for an exercise program, and can be done on a daily basis. Some consider HPV trans-portation a means of getting time for free. If a commute takes 20 minutes by automobile and 50 minutes by HPV, the trip adds 30 minutes to the commute, but provides 50 minutes of exercise. Compared to spending an hour in the gym, this provides an extra 20 minutes per day. Using HPVs as transportation leads to a much healthier lifestyle than that of driving every trip. Counterintuitively, blood levels of toxins from automobile exhaust products are higher in motorists than in cyclists in the same environment—another health benefit of cycling.

Bicycle and HPVs are also much safer than many people realize. Based on sta-tistics for fatal accidents, for every hour of operation a motorist is approximately twice as likely to die in an accident as a cyclist. By comparing on an hourly basis, compensation is made for the different number of miles traveled and the different speeds of automobiles and bicycles. The likely cause is the substantial difference in kinetic energies. A light car at 45 mph has about one hundred times the kinetic energy of a cyclist, while an SUV at interstate speeds has about a thousand times the energy.

Most bicycle accidents are falls, in which the rider is not seriously injured. HPVs can be made even safer than bicycles. A long wheelbase recumbent bicycle with under-seat steering is perhaps the safest type of non-faired bicycle. The risk of a forward tip-over is negligible, and the energy of a frontal collision is absorbed by the bicycle and the rider’s legs (rather than his head). A fairing, or shell around the rider, can provide additional protection. Shells are used to reduce aerody-namic drag and to enclose vehicle systems which sometimes include rollover protection. A well designed shell will also provide protection against abrasions in the event of a fall. The safety advantage of enclosed HPVs was illustrated in 2003 when Sam Whittingham experienced a front tire blowout at 82 mph during a speed record attempt. He slid, spun, and went airborne before coming to rest 250 yards away. He was shaken, but walked away from the accident. It is quite

possi-ble to design everyday HPVs and velomobiles to be exceptionally safe.

There is a well-established body of literature documenting the health benefits

of cycling. Johan de Hartog3 and colleagues conducted a study in the Netherlands

investigating the health benefits and risks of cycling as compared to car driving. 3Johan de Hartog, J., Boogaard, H., Nijland, H., and Hoek, G., 2010, “Do the Health

Benefits of Cycling Outweigh the Risks?”, Environmental Health Perspectives, 118(8), 1109–1116.

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Design of Human-Powered Machines Design of Human-Powered Machines

They found that the beneficial effects of exercise exceeded the health risks due to both accidents and exposure to air pollution. Cyclists in general lived longer than car drivers. A number of other studies support the claimed health benefits of

cycling.4,5 These studies indicate that the general perception in the United States

that cycling is inherently dangerous is probably inaccurate.

Mobility Mobility

Human-powered vehicles provide excellent mobility for local trips. Mobility is the ability to get to a destination in an efficient and relatively quick manner. Road HPVs are particularly adept at local trips in both rural and urban areas, with mo-bility sometimes approaching or exceeding that of motor vehicles. For example, in heavily congested areas, bicycles are often faster than automobiles. The bicycle messenger business exists primarily because of this fact. Mountain bicycles can provide outstanding mobility in areas without roads. Canoes and kayaks excel in waterways that other boats cannot navigate, such as shallow, rocky waters or wa-ters choked with vegetation. They are also easily carried across portages, making them ideal for wilderness trips in regions with many lakes and rivers.

The majority of automobile trips in the United States are short, and many trips are very short—less than one or two miles. Most of these trips could be completed by human-power with little or no lost time. In areas that have limited parking for automobiles and in congested areas, bicycles are much more convenient, often allowing the operator to ride right up to his or her destination.

Environment Environment

Environmental concerns provide a compelling reason to use human-powered vehicles. In the developed world, transportation systems account for a very sig-nificant fraction of air pollution. Emissions from motor vehicles include toxins and greenhouse gasses (GHGs.) In the United States, the prevalence of personal automobiles and the relatively low cost of motor fuel have led to a transportation infrastructure that is predominantly based on highway vehicles. These vehicles are generally inefficient, resulting in more pollution per passenger-mile than other transportation modes, such as rail. A second consequence is the rapid growth and 4Pucher, J., Dill, J., Handy, S., 2010, “Infrastructure, Programs, and Policies to Increase

Bicycling: An International Review”, Preventive Medicine, 50, S106–S125.

5Yang, L., Sahlqvist, S., McMinn, A., Griffin, S. J., and Ogilvie, D., 2010, “Interventions to

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Rationale Rationale for for Human-Powered Human-Powered Vehicle Vehicle Design Design and and UseUse

development of suburban and rural areas. This has put increased pressure on wetlands and wildlife habitat, as well as encouraged longer commutes from home to workplace. The result is a combination of increased air pollution and decreased natural resources. Automobiles are also noisy, which can also be considered a type of pollution of the environment.

Transportation accounts for a significant fraction of air pollution and green-house gas emissions in the United States. On-road mobile sources account for 51% of all carbon monoxide emissions, 29% of all hydrocarbon emissions, 34% of all nitrogen oxides, and 10% of all particulate emissions. Cars and motorcycles contribute more than half of the on-road mobile sources for carbon monoxide and hydrocarbons, while gasoline trucks and diesel vehicles account for most of the on-road nitrogen oxides and particulates. Emissions include combustion products and fuel evaporation. One third of the greenhouse gas emissions are produced by mobile sources. For every liter of gasoline burned, 2.3 kg of carbon dioxide equiv-alents are released into the atmosphere (19.4 lb CO2E per US gallon burned). Greenhouse gas concentrations have risen sharply since the start of the industrial

Many scientists are concerned about the significant increase in the

con-centration of CO2 and other GHGs in the atmosphere. Since the preindustrial

era, atmospheric concentrations of CO2 have increased by nearly 30 percent

and CH4 concentrations have more than doubled. There is a growing

interna-tional scientific consensus that this increase has been caused, at least in part, by human activity, primarily the burning of fossil fuels (coal, oil, and natural gas) for such activities as generating electricity and driving cars.

Moreover, in international scientific circles a consensus is growing that

the buildup of CO2and other GHGs in the atmosphere will lead to major

vironmental changes such as (1) rising sea levels that may flood coastal and river delta communities; (2) shrinking mountain glaciers and reduced snow cover that may diminish fresh water resources; (3) the spread of infectious diseases and increased heat-related mortality; (4) possible loss in biological diversity and other impacts on ecosystems; and (5) agricultural shifts such as impacts on crop yields and productivity. Although reliably detecting the trends in climate due to natural variability is difficult, the most accepted cur-rent projections suggest that the rate of climate change attributable to GHGs will far exceed any natural climate changes that have occurred during the last 1,000 years.

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Design of Human-Powered Machines Design of Human-Powered Machines

era, leading to increasing average global temperatures. The US EPA summarized

the likely or possible environmental changes, some of which are already visible:6

Significantly increasing the number and use of human-powered vehicles used for transportation can alleviate the environmental damage from the transportation sector. This is a practical and effective solution that could be easily and quickly implemented. HPVs produce no pollutants during use, and a small fraction of the greenhouse gas emitted by automobiles. Figure 1-2 shows the greenhouse gas emissions of automobiles on a per-mile basis as a function of miles-per-gallon. The average automobile fuel mileage is about 22 mpg. The average automobile emits 418 grams of greenhouse gas every mile. An HPV driver exhales carbon dioxide, but in much smaller amounts. Carbon dioxide emissions for a typical city bicycle with a 77 kg rider are plotted as a function of speed in Figure 1-3. For a given trip, greenhouse gas emissions are reduced by about two orders of magnitude.

Any alternative transportation system must not only reduce emissions, but must also be affordable and use existing technology. Bicycles and HPVs use exist-ing infrastructure—roadways and bicycle paths—and would require no additional capital outlay. In fact, HPVs inflict less damage to roads than automobiles, so it is conceivable that infrastructure costs could actually decline with increased HPV 6SOLID WASTE MANAGEMENT AND GREENHOUSE GASES A Life-Cycle Assessment of

Emissions and Sinks, 3rd EDITION, US EPA, September 2006. Figure 1-2

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Rationale Rationale for for Human-Powered Human-Powered Vehicle Vehicle Design Design and and UseUse

use. Cost to consumers could also be significantly reduced, as discussed above. Human-powered vehicles fill a unique role in sustainable transportation alterna-tives. No other option can provide quantifiable reduction in air pollutants and greenhouse gas emissions with available and affordable technology that uses ex-isting infrastructure. HPVs are, for the present, critical to achieving a sustainable transportation system.

Figure 1-3

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CHAPTER

CHAPTER

2

VERVIEW OF

VERVIEW OF

HUMAN-POWERED

HUMAN-POWERED

VEHICLES

VEHICLES

A Brief Historical Perspective A Brief Historical Perspective

Humans are mobile. People like to move, to travel, to roam. In the dawn of human history, there were no vehicles, and all transportation was by our own— human—power. Probably the very first vehicles were floating logs or vegetative mats carried by currents or waves. When an early human first kicked, paddled, or poled one of these, human-powered vehicles came into existence. Simple paddled or rowed watercraft was quite likely the earliest human-powered vehicles. By the time of the ancient Greeks, human-powered boat technology had advanced to large rowed vessels—biremes and triremes—that were up to 40 m (130 ft) long and powered by a crew of up to 170 men. The ancient Greek trireme, with three rows of rowers, probably was capable of seven or eight knots for short bursts.1

The technology, as always, was driven by need—in this case the need for a fast, powerful military vessel.

For millennia, human muscles, along with wind and currents, were a domi-nant source of power for boats. For land transportation, animal power as well as human power was extensively used prior to the industrial revolution. Wheeled vehicles probably date back at least 5,000 years. While these may have been pri-marily pulled by animals, surely some were pulled by humans—perhaps creat-ing a primitive version of the rickshaw. With the advent of steam and internal combustion engines, and later electric motors and batteries, most high-power transportation modes switched from muscle to alternative power sources. This transition, which occurred during second half of the eighteenth century, marked

1 Abbot, Allan V. and Wilson, David Gordon, 1995, Human-Powered Vehicles, Human

Kinet-ics, Champaign, IL, Chap 2.

O

O

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Design of Human-Powered Machines Design of Human-Powered Machines

a blossoming of new technologies, including the bicycle, the automobile, and the soon-to-fly airplane. Muscle power (both animal and human), steam power, and power from the new internal-combustion engine were all viewed as viable options for transportation and industry. Electric power was also rapidly develop-ing, although the power grid was still extremely limited and batteries were heavy and low-powered. Human mobility was increasing significantly, and vehicles of all types were rapidly developing. These heady times saw the bicycle develop from a rudimentary hobby-horse type device into a practical, clean, and fast means of personal transportation.

The bicycle srcinated with the Draisenne (Figure 2-1), invented by German Baron Karl von Drais early in the nineteenth century. This device had two wheels, the front one steerable. It was propelled by kicking the ground, similar to a young child’s bicycle. By 1880, the bicycle had evolved into the ordinary, or high-wheel, bicycle of familiar lore. This vehicle, sometimes called a penny-farthing, was driven by cranks connected directly to the front wheel. In order to obtain high speeds, the wheel diameter was increased to quite large sizes—up to 1.5 m (60 in). The large wheel allowed higher speeds, but was limited by rider leg length. These bicycles, with a dangerous tendency to pitchover (throwing the rider over the handlebars during hard braking), were soon replaced with the safety bicy-cle, driven by a chain drive which permitted higher gearing and hence smaller wheels. This lowered the center of gravity and moved the front wheel forward, significantly reducing pitchover accidents. The chain drive, in conjunction with the invention of the pneumatic tire, created a strong demand for bicycles in the last decade of the nineteenth century. The bicycle industry experienced rapid and extensive growth, and spawned many technological improvements that carried over to the automobile and aviation industries in the early twentieth century.2

The late nineteenth and early twentieth centuries were exciting times for the bicycle industry. Cycling was immensely popular. Cycling was the most advanced form of transportation for local trips, and bicycle racing was quite popular. Cycling technology advanced dramatically, with many new developments in structural de-signs, components, and systems. Recumbent seating, tricycles, and tandems were all tried in a variety of configurations. However, by the 1930s international racing rules prohibited all but conventional upright frame designs. The bicycle industry by and large followed the racing standards, and the upright bicycle design became the standard.

2 Herlihy, David V., 2004, Bicycle, The History, Yale University Press, New Haven and

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Overview Overview of of Human-Powered Human-Powered VehiclesVehicles

Although the basic configuration of the modern multi-speed bicycle did not change dramatically during the twentieth century, cycling technologies contin-ued to develop. Advancements in materials, processes, and components were dramatic. Perhaps the bigger story of that century is the shift in transportation modes and public perception of bicycles. At the turn of the twentieth century, bicycles were seen primarily as affordable, fast transportation. As the automobile became more affordable—most dramatically in the United States, and to a lesser extent in Europe—and the network of roads (srcinally paved for cyclists!) im-proved, public perception of the bicycle shifted from a desirable primary mode of transportation to a sporting device or child’s toy. Recreational cycling increased in the United States at the end of the twentieth century, perhaps due to the in-troduction of the mountain bicycle in the 1980s. Cycling as a primary transpor-tation mode in the United States remains low, but is experiencing slow growth. In Europe, particularly northern European countries, cycling for transportation is much more common than in the US, and bicycle sales exceed automobile sales. In both Europe and the US, regional planners are much more likely—and often required by law—to consider infrastructure investments for cyclists than they were several decades ago. This is in contrast with China, where the bicycle has been a ubiquitous and affordable transportation mode for quite some time. As China’s economic growth continues, there is a mode shift away from bicycles to private automobiles.

Figure 2-1

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Design of Human-Powered Machines Design of Human-Powered Machines

Land Vehicle Applications and Functions Land Vehicle Applications and Functions

Human-powered vehicles meet a diverse variety of transportation needs around the globe. Unlike the United States, where cycling, along with canoeing and kayaking, are seen primarily as recreational activities, many people choose human power as the most viable transportation alternative. The choice of trans-portation mode, is—and always has been—based on the balance of needs and resources. For many, human-powered vehicles are simply the best option. Fac-tors that affect mode choice include the available transportation infrastructure, performance and functional needs, the local environment, public transportation policy, and economics. In some areas of Africa, public transportation is very lim-ited, the road network is poor, and private automobiles are very expensive. Bicy-cles are highly valued for transportation. In Copenhagen, Denmark, high costs of living and taxes make automobiles quite expensive; an excellent infrastructure for cycling and public transportation exists; and cycling is perceived as a viable and affordable—and socially acceptable—option. As a result, 38% of trips into downtown Copenhagen are by bicycle, and 50% of trips within the central city are by cyclists. In Cairo, flatbread is delivered by cycle couriers that reportedly balance up to 50 kg of bread while they work their way through congested traffic.3

The entire bike messenger business, common in many US cities, exists because cycling is often faster than automobile travel in congested urban traffic.

Land human-powered vehicles are used primarily for personal transportation: one person getting from one place to another. Some vehicles can carry passen-gers. Passengers may be young children in a vehicle designed and marketed for family transportation or they could be paying customers in a pedicab. Tandems allow both riders to pedal; some vehicles allow three or more riders to contribute to propulsion. In some cases, the passenger is a medical patient, and the vehicle is a human-powered ambulance. Several of these are in operation in Namibia and other African nations. Rather than carry people, some vehicles are cargo haulers, designed to transport relatively heavy loads. Several commercial models are avail-able that are designed to carry over 90 kg of cargo. Many industrial factories use work cycles to move people or light goods around large plants. Trailers are also widely used with cycles to transport both children and cargo.

Bicycle courier, or bike messenger, services are offered in many urban areas, where cyclists can deliver light parcels or letters much quicker than alternative transportation modes. These cyclists are able to negotiate around heavy auto-motive traffic and congestion in order to make prompt deliveries. Somewhat

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Overview Overview of of Human-Powered Human-Powered VehiclesVehicles

larger parcels are routinely carried in panniers, trunks, baskets or backpacks by cyclists during the daily routines of life, such as shopping, commuting to school or work, or a picnic in the park.

Many bicycles are used for recreation or competition. These cycles are often designed to be fast and sporty, with little consideration for cargo, weather pro-tection, or reduced maintenance cycles. Professional bicycle racing is a very pop-ular sport in some areas of the world, and famous races, such as the Tour de France, draw very large viewing audiences worldwide. Recumbent bicycles and multi-track vehicles are not generally allowed in these races, so alternative venues have developed specifically for these vehicles. The Race Across America—a race in which contestants ride across the United States from the Pacific to the Atlantic coasts—permit both recumbents and tandems, but they compete in different classes from the upright single bicycles. The annual World Human-Powered Speed Chal-lenge, held at Battle Mountain, Nevada sees the fastest human-powered vehicles in the world—sleek recumbent streamliners that have demonstrated speeds exceeding 38 m/s. The current record, set on September 17, 2015 by Todd Reichert is 38.3 m/s (85.71 mph). Recreational riders cycle for pleasure: the sounds, sights and smells of spring ride along a country road can be immensely rewarding. Some recre-ational riders also ride for fitness, reaping health and fitness benefits in an enjoy-able sport. More adventurous riders use human-powered vehicles for extended tours lasting from a few days to many months. Touring cycles are usually rugged, with reliable tires and equipment suitable for varied and perhaps hostile terrain. They generally have good cargo capacity and are designed to be comfortable for long hours of riding.

Land Vehicle Configurations Land Vehicle Configurations

The arrangement of elements, such as wheels, seat, drive train, and steering system, define the configuration of a vehicle. For land human-powered vehicles, the number and position of the wheels and the body position of the rider are the most significant configuration options. Bicycles have two wheels, tadpole tricycles have three wheels with one in the rear and two in front, delta tricycles have one wheel in front and two aft, and quadricycles have four wheels. Body positions are most often identified as upright or recumbent, although there is considerable variation within each classification. The word recumbent means “lying down.” In cycling, the more accurate term would be semi-recumbent, referring to leaning back in a reclined position. The term is shortened in the vernacular to “recum-bent,” and sometimes the even shorter ““recum-bent,” and is often used as a noun to denote any vehicle in which the rider sits in a semi-recumbent attitude.

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Design of Human-Powered Machines Design of Human-Powered Machines

Vehicle configuration is determined during the conceptual stage of design. In the vast majority of cases, the configuration is immutable once the design is in production. There are exceptions, however, and some vehicles are designed to be convertible by the user. A few production bicycles can be converted from a long wheelbase recumbent with the front wheel in front of the cranks to a short wheel base, for example. Aftermarket kits are available to convert some bicycle models into tricycles. (The author once had the opportunity to ride a bespoke vehicle that could convert between a recumbent and upright riding position. The conver-sion could be made on-the-fly, while riding the vehicle. It was quite fun to ride, if of questionable utility.) Configuration design is an important task that must be completed early in the design sequence. The following descriptions provide ex-amples of some fairly typical configurations, along with some general comments.

The familiar upright bicycle has two wheels, and the rider sits on a saddle in an upright position or a forward-leaning crouch. Steering is generally direct, with several different styles of handlebars. Typically, the rear wheel is driven, and the front wheel is steered. The differences between upright bicycles are gener-ally more in the details than in configuration. Mountain bicycles, city bikes, and racing bicycles, while different, all share a similar configuration. Road bicycles usually have drop-style handlebars and the rider is positioned in a crouch that minimizes air resistance and provides good power transfer. These bicycles usually use narrow tires and caliper brakes. Mountain bicycles position the rider in a more upright position, with straight or riser handlebars. They usually have wide-range gearing and can accommodate wide tires. Linear pull, disc, or cantilever brakes are used as these types better accommodate the wide tires. Touring bicycles usu-ally have wide-range gearing, a longer wheelbase, and ample fittings for racks and bottle cages. They may have any style of handlebar, but are designed for heavy use and relative comfort for long hours in the saddle.

Figure 2-2 shows an upright bicycle designed to be folded and transported in a suitcase. This bicycle requires some disassembly prior to transport, but can be easily checked on an airline or train as ordinary luggage. Two companies known for their high-quality travel bikes are Bike Friday and Airnimal. Other folding bi-cycles are designed to fold quickly and be stored in a very compact space. These are frequently used by urban commuters that either have little storage space or use multi-mode transport, carrying bikes onto the subway or bus. Some of these models fold into a compact package within seconds. Brompton and Dahon are examples of companies that specialize in folding bicycles.

Recumbent bicycles also have two wheels, but the rider sits in a seat rather than on a saddle. Recumbent bicycles tend to be more specialized than upright bicycles. The fastest bicycle in the world is a recumbent. The most comfortable

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Overview Overview of of Human-Powered Human-Powered VehiclesVehicles

bike in the world is a recumbent. The safest bicycle in the world is most likely a recumbent. However, these may be three very different bicycles. This illustrates the great diversity of configurations for recumbent cycles. Most recumbent bicy-cles have rear-wheel drive, although some use front-wheel drive to avoid lengthy chains and idlers. Steering may be direct, meaning the handlebar connects di-rectly to the fork, or indirect, meaning the fork and handlebar are connected via a linkage of some type. There are many options for handlebar type and position. Some have the handlebar mounted under the seat, while others may curve over the rider’s legs or extend nearly to the rider’s chest. The last is sometimes colloqui-ally called the “begging hamster” position. The wheelbase has great significance for recumbent bikes as it affects rider position, handling, and vehicle structure. Long-wheelbase recumbents often have the front wheel located in front of the crank. They tend to be larger and heavier than comparable short-wheelbase mod-els, but may be easier to ride and somewhat more stable. Short-wheelbase bikes may have sportier handling and be lighter and more compact. The crank is usually in front of, or above, the front wheel. Seat construction and position can vary greatly between vehicles. Seats can be rigid molded composites, made of mesh slings, or a combination of the two. Position varies from a fairly upright sitting position to very reclined. In general, the more upright positions are more comfort-able, and are used on recreational vehicles, while the more reclined seats reduce aerodynamic drag and are frequently used on performance bikes. Hard or rigid seats are also often used on high-performance vehicles, and provide a firm back to resist hard pedal forces. Sling seats provide both breathability and some suspen-sion, and are generally considered more comfortable. Many seats have mesh back rests and a padded, rigid seat bottom. The position of the cranks relative to the seat also affects rider position. The most aerodynamic vehicles place the cranks

Figure 2-2

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Design of Human-Powered Machines Design of Human-Powered Machines

above the seat bottom and have very reclined seat backs. Lower crank positions and more upright seating are used when comfort is more important.

The following figures illustrate several examples of recumbent bicycles. Fig-ure 2-3 depicts a long-wheelbase bicycle with over seat direct steering, a padded hard seat bottom and mesh seat back. This bicycle would likely be well suited for rides through the countryside, where maneuvering is less important than a com-fortable high-speed cruise. A short-wheelbase bike is shown in Figure 2-4. This bicycle also has direct steering, but uses a pivoting handlebar mast that allows easy mounting, dismounting, and comfortable hand positioning. Note the hard-shell seat. The vehicle in Figure 2-5 is a low-racer streamliner, designed to be fast and aerodynamic. The entire bicycle fits within a compact, streamlined fairing. It has front-wheel drive and a steeply reclined hard shell seat. This bicycle would be very impractical in urban traffic, but can be quite fast on the race track. Note that in the figure, only one-half of the fairing is depicted in order to show the bicycle inside the fairing.

Gunnar Fehlau presents a nice overview and comparison of many configuration options for recumbent bicycles.4 His book is easy to read, and includes a brief

de-scription of each option, along with bullet lists of advantages and disadvantages. Tricycles have three wheels, with either upright or recumbent rider position-ing. Tadpole tricycles have two wheels in front and one wheel in back. Usually, the front wheels are steered and the rear wheel provides drive power. Tadpole trikes tend to provide the best handling response, but the steering mechanism is

4 Fehlau, Gunnar, 2006, The Recumbent Bicycle, 2nd Ed., Out Your Backdoor Press.

Figure 2-3

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Overview Overview of of Human-Powered Human-Powered VehiclesVehicles

more complicated. Underseat steering via handlebars and an Ackerman linkage (described in Chapter 16) can provide very nice steering response, although di-rect steering via handle bars connected to the kingpins is also a common option. The example shown in Figure 2-6 is a recumbent tadpole tricycle with direct steering. Like most tricycles, tadpoles are stable when stopped or moving very slowly. This is a distinct advantage for riders with balance problems, or riders that spend a great deal of time at very low speeds or hauling very heavy loads up hills. Performance tadpole trikes usually have long drive trains, often routed below the seat with idlers. Low seating provides more stability during high-speed cornering,

Figure 2-4

Figure 2-4 Short-wheelbase recumbent bicycle

Figure 2-5

Figure 2-5 Short-wheelbase low-racer streamliner. This bicycle has front-wheel drive

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Design of Human-Powered Machines Design of Human-Powered Machines

although not all tadpoles have low seating. A few upright tadpole tricycles have been produced, although most models are recumbents.

In contrast to the tadpole configuration, delta tricycles have two wheels in back and one in front. Steering is much simpler than the tadpole, but drive systems can be more complex. Delta trikes with rear wheel drive can be driven by only one wheel or both wheels. If both rear wheels are driven, a mechanism to allow the outside wheel to speed up during turns prevents wheel slip and the resul-tant handling problems. This can be done by clutches or a much more expensive differential. Some delta tricycles are driven with only one rear wheel, avoiding the problem altogether. These vehicles produce a turning moment under heavy pedaling which can be a problem in low-speed, high-torque situations. In some conditions—such as a hard, steep climb on a wet road—this can cause the front wheel to lose traction and control. Front-wheel drive is another option, but this usually results in pedals that swing during maneuvering or twisting chains. Delta tricycles have a tendency toward oversteer as speed increases. This results in poorer handling (and in extreme cases less stability) than tadpole trikes.

Quadricycles, or quads for short, have four wheels. Quads can have better sta-bility and handling than either tadpole or delta tricycles. In addition, there are es-sentially no restrictions on wheelbase, since the wheels do not interfere with either the seat or the cranks. However, quads have both the drive train complexities of the delta trike and the steering complexities of the tadpole. This usually results in a more complex and potentially heavier vehicle. Accurate wheel alignment both front and rear is required for good performance. There are a great many configuration

Figure 2-6

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Overview Overview of of Human-Powered Human-Powered VehiclesVehicles

variations possible with quads. Figure 2-8 shows a recumbent quadricycle designed to use either hand cranks or leg cranks. This is a multi-configuration design: the leg cranks can be replaced with stirrups for handcycling, or the hand cranks could be replaced with a conventional handlebar for leg cranking only. A minor drivetrain modification allowed both drive options to be used simultaneously.

The examples in this chapter illustrate some of the many configurations that are possible with land human-powered vehicles. The number and position of the wheels, position of the rider, number of riders, drivetrain type and wheels driven, handlebar position and steering system design, and other options define the con-figuration. An understanding of vehicle systems and configuration options is very beneficial to the design engineer at the outset of a new vehicle project.

Additional information for the interested reader:

For an excellent history of the bicycle, see:

Herlihy, David V., 2004, Bicycle, The History, Yale University Press, New Haven and London.

The following book contains illustrations of early bicycles and (mostly) compo-nents. Originally published in Japan, there is essentially no text, only figures.

100 Years of Bicycle Component and Accessory Design Authentic Reprint Edition of The Data Book, Van der Plas Publications, San

Francisco, 1998. Figure 2-7

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Design of Human-Powered Machines Design of Human-Powered Machines

Figure 2-8

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CHAPTER

CHAPTER

3

ENERAL

ENERAL

STRUCTURED DESIGN

STRUCTURED DESIGN

OF HPV’s

OF HPV’s

D

esigning a new human-powered vehicle from scratch is both stimulating and rewarding. It may be quite challenging, and the road from idea to realization can be more convoluted than linear. A pre-established structure, or plan, for the design process can be helpful to the designer, in much the way an outline aids a writer. This chapter defines and describes a general structure for HPV design. It is a broad template, applicable to many types of human-powered vehicles. The objective is to provide an outline or plan for the design process. Each section includes a description of tasks that should be completed, followed by a brief out-line, which can be used as a checklist.

General Structured Design of HPVs General Structured Design of HPVs

The design process for human-powered vehicles is similar to that for any other product. The design progresses through stages, from formulation of the problem statement to prototype testing. These stages are depicted in Figure 3-1 and de-scribed throughout this chapter. The central column in Figure 3-1 represents the eight stages of design. These are essentially the same steps used in the design of any product. The left column represents requirements that must be completed at each stage. These are specific to human-powered vehicle design. A large portion of this book is devoted to these topics. The right column illustrates the iterative nature of design. The bubble labeled “verify mission” is a reminder of the impor-tance of keeping a sharp focus on the mission of the vehicle.

Although this chapter divides the design process into eight distinct stages, this is for organizational simplification only. The design process is continuous and

G

G

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Design of Human-Powered Machines Design of Human-Powered Machines

contains many iterations, changes, and modifications. Frequently, stages overlap chronologically. For example, some parts may be in detailed design stage or even prototype fabrication stage while other parts are still in the parametric design stage. Nonetheless, the general workflow follows the stages described here and depicted in Figure 3-1. 1. Define Vehicle Mission 2. Product Design Specification 3. Develop Vehicle Concepts Concept Selection 5. Parametric Design Geometry Analysis Testing 6. Detailed Design Drawings Manufacturing Components 7. Prototype Fabrication 8. Performance Evaluation 4. Customer Info Define Design Parameters Determine Target Values Gross Geometry Drive Train Frame Geometry Special Features Configuration Verify Mission Iterate As Required Iterate As Required Iterate As Required Figure 3-1

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General General Structured Structured Design Design of of HPV’sHPV’s

STEP 1. Define the mission of the vehicle

STEP 1. Define the mission of the vehicle A clear mission statement for the proposed vehicle is invaluable. It is far too easy for a design to gradually shift direc-tion mid-stream, resulting in a vehicle that may not be what the customer wants. A well-formulated mission statement forces the designer to focus on the most im-portant aspects of the project, and maintains that focus throughout the design pro-cess. This is the best way to ensure that the customer gets what he or she orders.

A mission statement should clearly and concisely state the primary functional requirements for the vehicle. For example, a vehicle designed for year-round commuting in northern temperate might have the following mission statement:

This vehicle shall provide reliable year-round single-person transporta-tion in northern urban and suburban regions.

This statement is brief and contains quite a bit of information. The word reli-able indicates that the vehicle will not break down very frequently. The vehicle must be practical for the transportation needs of a single individual. This probably implies that cargo space be included. Other items can be inferred, such as lights and reflectors for night transport. Northern urban and suburban regions will un-doubtedly include weather and environmental hazards such as rain, snow, poor pavement, and probably salt. While much of this could be inferred from the state-ment, additional sentences can clarify and remove ambiguity from the statement. This vehicle shall provide reliable year-round single-person transpor-tation in northern urban and suburban regions. Expected usage includes personal transport, commuting, shopping, and recreation. The operator must be provided reasonable protection against the elements, and vehicle maintenance should be minimized. The vehicle should be comfortable, easy to operate, and easy to propel. Expected environmental conditions include cold, heat, rain, snow, and salt. The vehicle should be safe and legal for day or night travel.

a. Obtain

a. Obtain customer informationcustomer information If the user of the vehicle is not the de-signer, it is quite important to consult actual or typical customers and potential users of the vehicle. This should be done early in the design, and continue throughout the design process. The mission statement should be developed in conjunction with customer input.

STEP 2.

STEP 2. Develop vehicle design Develop vehicle design specificationsspecifications Design specifications define the attributes of a successful vehicle. In engineering design, these are more gen-erally called product design specifications, or, collectively, the PDS. The design specifications should clearly identify the qualitative and quantitative attributes

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Design of Human-Powered Machines Design of Human-Powered Machines

required or desired in the final design. They are used as a reference throughout the entire design process. For example, a specification for the maximum load capacity of a vehicle is used in vehicle performance calculations, and structural analyses, and vehicle testing.

The vehicle design specification should include the purpose, function, and intended market for the vehicle; functional performance requirements; a-priori design constraints; and any additional requirements. Specifications should be specific with regard to outcomes without limiting design options. That is, the re-quirements should be clearly identified without placing any constraints on how they will be physically realized. This gives the designer guidance without limiting creativity.

Developing the vehicle design specifications requires gathering information, identifying parameters, and setting target values.

1. Information Sources

1. Information Sources The product design specification should not be written without adequate information. Sources of information include customer data (Step 1), codes, standards, and warranty or historical data. Information should also be gathered from vehicles with similar missions or technologies. Particular emphasis should be placed on customer desires, whether the customer be a single individual or a population. This infor-mation is obtained with user surveys and interviews. QFD is particularly useful for this. Historical and current solutions should be thoroughly vestigated as well. Many designers have made the mistake of re-inventing old technologies, sometimes with the srcinal faults or limitations. Vehi-cles that serve similar missions should be investigated and ridden if possi-ble. The new design should show a distinct advantage over other vehicles in performance, cost, comfort, or other attributes.

2. Design Parameters

2. Design Parameters Design parameters should be identified while veloping the vehicle specifications. A well-defined set of critical design parameters provides a template for the new vehicle design. Three classes of parameters are defined: control variables, envelope variables, and per-formance metrics. Identifying these parameters from scratch for each design is time consuming and runs the risk of omissions. The alternative— to start with a slate of pre-determined parameters—reduces design time and potentially improves the quality of the final vehicle.

a. Design Control Variables

a. Design Control Variables include the parameters over which the de-signer has complete control. Examples include wheelbase, castor an-gle, wheel size, etc. In general, these variables should not be specified

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General General Structured Structured Design Design of of HPV’sHPV’s

in the PDS unless there is a strong a-priori reason for doing so. The val-ues of these parameters will be determined during the design process, and will depend on the conceptual design solutions.

b. Design Envelope Variables

b. Design Envelope Variables describe attributes over which the de-signer has no direct control, but whose values are required in the design process. These typically depend on the operator and environmental conditions. Examples include limits on operator size and weight, design vehicle speed envelope, etc. Many of these will be specified in the PDS,

either with a target value or a range of values. c. Performance metrics

c. Performance metrics are used to evaluate how well a proposed de-sign meets the mission and dede-sign specifications. Objective, quantifi-able performance metrics are said to be hard, while subjective metrics are soft. Examples of hard metrics include vehicle weight, coefficients of drag and rolling resistance, power required to operate the vehicle, etc. Aesthetics, ride quality, etc. are soft metrics. Performance metrics should be included in the vehicle design specifications. For many ve-hicles, the performance specifications are among the most important. 3. Determine target values

3. Determine target values Values must be determined for the parameters identified in Step 3. The starting point is always the design specifications— the design envelope variables and performance metrics. These are incor-porated into the product design specification, which is the driving force for vehicle design. The importance of this step cannot be overemphasized! A

poor job at this stage will result in ambiguity in later stages of the design. The result will likely not meet the customer’s desires, or will fail to perform as required.

The target values used in the specifications should be stated in a way that directly relates to analytical and experimental results. During the design process, the performance of proposed concepts is evaluated and compared to the specification. A specification that is expressed in a form that cannot be evaluated with reasonable accuracy is not useful. Consider vehicle top speed. For most vehicles, setting a numeric specification for vehicle top speed simply does not make sense. There are many factors that affect top speed, including the strength of the driver, road and wind conditions, and grade. Most human-powered vehicles will never operate at their theoretical top speed. (Exceptions are vehicles for which the achievement of that speed is critical to the success of the vehicle.) Much more important than top speed is the overall efficiency of the vehicle, which can be expressed in a variety of ways. For a hard specification, the

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