1 pdfsam motorcycle handling and chassis design foale

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1 pdfsam motorcycle handling and chassis design foale

  1. 1. Motorcycle Handlingand Chassis Design the art and science Tony Foale
  2. 2. WARNING – Important safety and legal notice.Building and/or modifying motorcycle chassis or parts is a serious undertaking and theconsequences of mechanical failure can result in serious injury. Some of thepractices and designs outlined in this book may be dangerous unless implemented byskilled and experienced people who have qualifications in engineering, design andfabrication. The author has tried to ensure the accuracy of the contents of this book,but some text, of necessity, represents the opinion and experience of the author andis not guaranteed to be fact. Therefore, the book should be read whilst mindful of thepotential risks of motorcycle modification, both in racing and other use.It is for this reason that the author makes no warranties, explicit nor implied,that the information contained herein is free of error nor that it will meet thepurpose of any specific application. The author disclaims any liability for anyand all forms of damage that result from any use of the contents in thispublication.Copyright © 2002 Tony FoaleAll rights reserved. Printed in Spain.First printing.ISBN for CDROMISBN for bookOrdering information is available on the internet at www.tonyfoale.com and by email atbook@tonyfoale.com.Comments or notifications of any errors are welcome and should be sent tobookrevision@tonyfoale.com.
  3. 3. ForewordThe motorcycle is a complex system that has long defied full analysis. For a very long time,motorcycle handing was hardly even considered a subject. Engines, whose performance couldbe measured in "objective" terms, therefore received the lions share of development. Enginedevelopment moved rapidly ahead of chassis, suspension, and tires, creating a succession ofdesign crises that required new thought for their solution. Examples might be Rex McCandlessstwin-loop swingarm chassis of 1950, Tony Millss wide, belted Dunlop Daytona tire of 1974, andthe present-day elaborations of Antonio Cobass large-section aluminium twin-beam chassis ofthe early 1980s. In each case, motorcycle performance had ceased to advance because ofspecific problems that could not be solved by traditional means.In general, the innovations that have broken these deadlocks have been creations of practicalpersons, not of theorists. The role of theory in motorcycle design has, if anything, suffered at thehands of history, for the strange forkless creations of ELF, Fior, and Bimota have come and gonewithout solving any actual problem.Yet motorcycle performance is at present again deadlocked, with no sunny uplands of easyprogress in sight. As motorcycles lean over farther on their wonderful tires, their suspensionsturn sideways, at a large angle to the bumps they are designed to absorb. As engine and braketorque is applied, motorcycles short enough to turn quickly, and tall enough for adequatecornering clearance suddenly lift the front or rear wheel, limiting maximum rates of accelerationand deceleration. While autos present 100% of the width of their tires to the pavement, themotorcycle offers only 1/3 of tread width at a time, severely limiting cornering grip. To makemotorcycles steer well, front tires must be of modest section, while rears, to apply engine power,must be large. With the forward CG position necessary for rapid acceleration, a powerfulmotorcycle must therefore overload its small front tire in cornering, while under-using its largerrear. The result is that as a machines power increases, its corner speed must decrease.Racing is the environment in which these problems hurt worst, and from which solutions havemost often come. Racing has, however, evolved from a sport into a conservative business. Thepractical men of racing are now too busy loading and unloading their beautifully painted transporttrucks to have much time for innovation. The theoreticians remain, as ever, divorced frompracticality, often ignorant of the real problems motorcycles confront.Yet the infinite refinement of the piston internal combustion engine did not create the gas turbine -only a careful consideration of theoretical heat engine cycles could make that leap. Therefore thepractical and theoretical sides need each other - but they have had little dialogue thus far.This book is a valuable step toward that dialog. Tony Foales first book was almost entirelypractical, and has been deservedly widely read. He is a man who can control a weld puddle andtwist safety wire. He also knows that refinement within existing thought must ultimately reach adead end. This has forced him to learn to walk with one foot upon practicalities and the otherupon theory. This new book is the result. Read on.Kevin Cameron. Technical editor Cycle World magazine. March 2002.
  4. 4. AcknowledgementsIt is quite usual that the author of a book has various people to thank for providing help in itspreparation. In my case I have hundreds to thank. Prior to publication in book form, previewversions of the manuscript were made available on CDROM, in different stages of completion. Intotal about 250 CDs were distributed, and the feedback and notification of errors from a sizableproportion of those readers has proved to be an invaluable aid.The numbers make it impossible to name everyone, but you know who you are – thanks a lot,you made the job much easier.Another source of aid came from those who have supplied information, expert proof reading orcontributed ideas for topics without which this book would have been the poorer. This group issmall enough to thank individually and it gives me pleasure to be able to so. They are, inalphabetical order:Dr. Andreas Fuchs – Innovative Mobility. ( www.swissmove.ch )Arnold Wagner – Ecomobile. ( www.peraves.ch )Carlos Calleja. – ( www.moebius.es/~ccalleja/ )David Sanchez. – ( www.bottpower.com )David Searle – Motorcycle Consumer News. ( www.mcnews.com )Douglas Milliken – Milliken Research Associates, Inc. ( www.millikenresearch.com )Hubert Kleis and Rainer Diabold – 2D Meßsysteme GmbH. ( www.2D-Datarecording.com )Ian Drysdale – Drysdale Motorcycle Company. ( http://home.mira.net/~iwd )Keith Duckworth.Peter Hopkins – BMW UK. ( www.bmw-motorrad.co.uk/ )Peter McNally – Avon tyres. ( www.coopertire.com/avon_motorcycle/frames.htm )Dr. Robert Lewis – Advantage CFD. (www.advantage-cfd.co.uk & www.reynard-motorsports.com)Roberto Lot – University of Padova. ( www.mecc.unipd.it/~cos/DINAMOTO/indexmoto.html )Dr. Robin Sharp – Cranfield University. ( www.Cranfield.ac.uk )Dr. Robin Tuluie – MTS Systems Corp. ( www.mts.com )Ted Blais – Rokon. ( www.rokon.com )
  5. 5. PrefaceThe book “Mototorcycle Chassis Design” was first published in 1984 and was subsequentlyreprinted several times without under-going change. Although out of print for over 12 years or so,I know from personal inquiries that there is still considerable demand for a book on this subject.A new book was obviously well overdue, although much of the original material is as currenttoday as it always has been. After all, the laws of Newtonian Physics tend to be stable over time.During the nearly two decades since the original book, the motorcycle chassis has undergonegradual evolutionary change and there is no doubt that handling in general has improved. In the1970s. the main emphasis was on ever more powerful engines being fitted into flexible tubularframes unable to provide a reasonable level of handling or stability. Thankfully that has generallychanged. Forks, frames and swing-arms have become much more rigid, and in some caseslighter as well, at least at the sport bike end of the market. The change to radial ply tyres hasbeen of the utmost importance to this process of change. Despite the prophecies of manycommentators the front suspension of choice is still the telescopic fork, although generally muchimproved. For any number of reasons manufacturers have been reluctant to experiment withother forms in the marketplace. This probably has more to do with the product liability lawyersthan it has to do with the engineers. There have been two notable exceptions amongst the majormanufacturers. Although now out of production, Yamaha marketed the GTS with a suspensiondesign based on the work of James Parker. BMW changed over completely to the “Telelever”system, similar in principle to the design used by the British Saxon concern.I’ve had considerable feedback from readers of that first book and I’ve done my best toincorporate the many suggestions. Although greatly enlarged, most of the original subject matterremains. Many topics have under gone revision to improve clarity or remove ambiguity. Materialhas be added which explores in more depth those subjects which were only briefly mentioned inthe original book, mostly due to publishing space constraints. An example of this is thedescription of initiating a turn, this topic is central to an understanding of motorcycle behaviour.However, it was then covered only briefly, the content on this subject is considerably enhanced inthe current book. Completely new chapters have been added on various topics that just weren’tin the original. For example: tyres, aerodynamics, the important subject of anti-squat and a casestudy of improving a standard production frame for racing.Since the first book was published the sport of motorcycling in all forms has become much moretechnical and so in order to do the subject justice this book has had to become more technicalalso. Reviewers of the previous book praised the lack of drawn out explanations, I have tried tomaintain this characteristic where possible, but within the need for coverage in greater depth.This book is not intended as a handbook for chassis setup etc. rather it is an attempt to providethe reader with the background knowledge of how and why motorcycles react in the way that theydo. An understanding at this level will however, equip the reader to undertake his own design,modifications or setup with greater confidence. The acquisition of knowledge is rarely easy andrequires commitment, any book is purely a passive aid and the benefit to each reader will dependon the effort put into it. It is probably best to initially read it through quickly , ignoring some of thedetail to get an overall view and then to re-read it to gain a more in-depth appreciation of thesubject. It is also recommended that the reader looks at some of the appendices for backgroundinformation, prior to tackling the main text. In particular appendices 2,3 and 4.
  6. 6. There are a wide range of technical topics discussed within a relatively small book and so insome cases a prior knowledge of the basics has had to be taken for granted. Naturally someparts of the general text are more technical than others, but there should be little problem for anyinterested enthusiast in gaining a better understanding of the principles involved. It is notnecessary to understand every last detail to derive benefit.To cover the subject adequately it is impossible to completely avoid mathematics, I have tried tokeep this as simple as possible. The level of mathematics used is deliberately kept at a levelbelow that requiring a knowledge of calculus, in the hope that the book will be of use to the widestrange of readers. A multitude of diagrams and graphs from both data logging and computersimulation have been used to demonstrate various phenomenon without a great number offormulae.Even in this age of much greater technical understanding, there are still many aspects of designand handling setup that can better be described as art more than science. Hence, the book titlehas been changed to reflect this. All engineering design is the art of compromise, the best bike isthe one whose designer has achieved the best overall compromise for the intended purpose,whether that be racing or commuting. We often hear that competition machines are built with nocompromises, in fact the opposite is true. Highly focused machines such as racers are probablysubject to the biggest compromises of all. Throughout the book I have tried to emphasize theconflicting requirements that always compromise any design or setup decision. Nowhere is thismore evident than when selecting suspension characteristics, this is demonstrated at every racemeeting where much time is spent making minute adjustments to achieve the “optimum” setup.Many points in the text are illustrated with example photographs. It has been a policy to use olderexamples where possible to acquaint younger readers with some of these machines and also todemonstrate that much of what is regarded as being new has in fact been around for aconsiderable period. Most readers will in any case be familiar with photos of modern examplesfrom the general motorcycle press.The first book was co-authored by Vic Willoughby, undeniably the doyen of motorcycle technicaljournalists. When I was a teenager (many years ago) I would read his weekly articles many timesover and there’s no doubt that these played a great part in the motivation for me to start designingand making my own chassis. Many years later I was privileged enough for him to write articlesdescribing some of my work. We became friends and I was honoured when he agreed to helpwhen I approached him with the idea for the original book. He was in his retirement then but stillhad enormous energy for the task. Unfortunately, Vic passed away in November 2000 and I haveundertaken this new book solo, and so must take sole responsibility for any errors.I would however, like to dedicate this book to Vic. without whom the original would never havepassed the idea stage.Tony Foale, SpainMarch 2002
  7. 7. Dedicated to the memory of Vic Willoughby.
  8. 8. 1Contents 1 Function and history Some basic definitions ................................................... 1-1 Function .......................................................................... 1-3 History............................................................................. 1-4 Front suspension.......................................................... 1-16 Rear suspension........................................................... 1-23 Spring types.................................................................. 1-29 Load Compensation ..................................................... 1-30 2 Tyres Weight support ............................................................... 2-2 Suspension action .......................................................... 2-4 Tyre stiffness or spring rate............................................ 2-8 Contact area ................................................................. 2-11 Area when cornering .................................................... 2-15 Friction (grip) ................................................................ 2-15 Braking & driving .......................................................... 2-17 Cornering ...................................................................... 2-17 Mechanisms of grip ...................................................... 2-17 Under- and over-steer .................................................. 2-27 Construction ................................................................. 2-32 Materials ....................................................................... 2-34 Summary ...................................................................... 2-34
  9. 9. 2 Contents 3 Geometric considerations Basic motorcycle geometry ............................................ 3-1 Trail................................................................................. 3-1 Rake or castor angle (steering axis inclination) ............. 3-5 Wheelbase.................................................................... 3-15 Wheel diameter ............................................................ 3-16 Other considerations .................................................... 3-18 Angular motions............................................................ 3-22 4 Balance and steering Balance........................................................................... 4-1 Steering .......................................................................... 4-3 Gyroscopic effects only .................................................. 4-9 Gyroscopic with tyre camber force only. ...................... 4-12 Gyroscopic with tyre camber and steer forces............. 4-14 Tyre forces only – no gyroscopic effects. .................... 4-18 Body lean only – no steering........................................ 4-20 Conclusions: ................................................................. 4-23 5 Aerodynamics Drag ................................................................................ 5-1 Evolution of the racing fairing......................................... 5-9 Internal air flow ............................................................. 5-10 Lift ................................................................................. 5-11 Airflow evaluation ......................................................... 5-18 Side wind stability (traditional view) ............................ 5-21 Steady state directional stability................................... 5-24 Dynamic directional stability ......................................... 5-28 Summary ...................................................................... 5-31
  10. 10. Contents 36 Suspension principles Springs............................................................................ 6-1 Damping ......................................................................... 6-8 Sprung and unsprung mass ......................................... 6-21 Basic suspension principles ......................................... 6-21 Other factors................................................................. 6-31 Lateral suspension ....................................................... 6-42 Summary ...................................................................... 6-497 Front suspension Head stock mounted forks ............................................. 7-1 Alternatives to the head stock mounted fork................ 7-13 Hub centre steered ....................................................... 7-14 Double link.................................................................... 7-18 McPhearson strut based .............................................. 7-27 Virtual steering axis ...................................................... 7-318 Rear suspension Effective spring rate........................................................ 8-3 Chain effects................................................................. 8-11 Wheel trajectory............................................................ 8-15 Structural ...................................................................... 8-16 Single or dual sided ...................................................... 8-19 Summary ...................................................................... 8-26
  11. 11. 4 Contents 9 Squat and dive Load transfer .................................................................. 9-1 Squat and dive................................................................ 9-4 Shaft drive....................................................................... 9-4 Chain drive.................................................................... 9-13 Aerodynamic squat....................................................... 9-25 Braking reaction (rear).................................................. 9-26 Dive (front) .................................................................... 9-29 Dynamic effects............................................................ 9-37 Summary ...................................................................... 9-47 10 Structural considerations Fatigue .......................................................................... 10-1 Structural efficiency ...................................................... 10-1 Triangulation................................................................. 10-2 Beam frames ................................................................ 10-5 Triangulated frames ..................................................... 10-9 Tubular backbone....................................................... 10-11 Structural comparison ………………………………….10-12 Fabricated backbone .................................................. 10-14 Monocoque ................................................................. 10-15 Structural engine ........................................................ 10-17 Conventional multi-tubular ......................................... 10-20 Twin-spar .................................................................... 10-23 Other types ................................................................. 10-27 Summary .................................................................... 10-28 11 Engine Mounting
  12. 12. Contents 512 Braking The basics .................................................................... 12-1 Effects of CoG height ................................................... 12-7 Generation of torque .................................................... 12-9 Hardware .................................................................... 12-10 Discs ........................................................................... 12-11 Calipers....................................................................... 12-14 Pads............................................................................ 12-15 Linked brakes ............................................................. 12-15 ABS............................................................................. 12-1713 Materials and properties Typical properties of some common materials ............ 13-3 Frame ........................................................................... 13-5 Wheels .......................................................................... 13-8 Fuel tank ..................................................................... 13-13 Brake discs ................................................................. 13-13 Bodywork.................................................................... 13-1314 Stability & control Under-/over-steer ......................................................... 14-2 High-siding.................................................................... 14-7 Stability under braking.................................................. 14-9 Instabilities .................................................................. 14-10 Damping ..................................................................... 14-1415 Performance measurement Track side ..................................................................... 15-1 Laboratory..................................................................... 15-9 Strength analysis .......................................................... 15-9 Measurement and simulation ..................................... 15-12 Future development ................................................... 15-14
  13. 13. 6 Contents 16 Practical frame building Welding ......................................................................... 16-1 Distortion....................................................................... 16-3 Gussets......................................................................... 16-5 Jigging .......................................................................... 16-6 Tube profiling ................................................................ 16-8 Tube types .................................................................... 16-9 Tube sizes .................................................................. 16-10 Frame finishes ............................................................ 16-11 Design layout .............................................................. 16-12 17 Case study Measurement................................................................ 17-2 Main frame.................................................................... 17-2 Engine mounting........................................................... 17-5 Results .......................................................................... 17-5 Material ......................................................................... 17-5 Swing arm..................................................................... 17-5 Forks ............................................................................. 17-6 Caution ......................................................................... 17-6 Tuning ........................................................................... 17-6 18 Future developments The status quo .............................................................. 18-1 Future possibilities........................................................ 18-2 Active suspension......................................................... 18-2 Rheological Fluids ........................................................ 18-4 Two wheel drive (2WD) ................................................ 18-4 Two wheel steering (2WS) ........................................... 18-8 Feet-Forward motorcycles. (FF)................................ 18-12
  14. 14. Contents 7Appendices A1 Experiments with rake and trail Rake..............................................................................A1-1 Trail...............................................................................A1-7 Conclusions ..................................................................A1-8 Post script.....................................................................A1-8 A2 Glossary of terms A3 Units conversion A4 Gyroscopic effects A5 Basic physics of motorcycles Basic Trigonometry ......................................................A5-1 Units of angle................................................................A5-2 Velocity .........................................................................A5-3 Acceleration..................................................................A5-4 Mass .............................................................................A5-4 Momentum....................................................................A5-5 Newton’s laws...............................................................A5-6 Force and weight ..........................................................A5-7 Moments, couples and torque......................................A5-8 Centripetal & centrifugal force......................................A5-9 Addition and resolution of velocities and forces ........A5-10 Work, energy and power ............................................A5-12 Nomenclature and sign conventions..........................A5-13 Normalization..............................................................A5-14 A6 Analysis of mechanisms A7 CoG and mass distribution of rider A8 Typical data
  15. 15. 1-11 Function and historySome basic definitionsBefore getting into much detail we need to consider some definitions of terms that are often bandedabout loosely and misunderstood as a consequence.HandlingBy this, we mean the ease, style and feel with which the motorcycle does our bidding. It depends mainlyon overall geometry, chassis stiffness, weight and its distribution, tyre type and size. It may come as asurprise to some people to learn that the rider has a major influence on the handling characteristics of amotorcycle. Rider responses have a large effect on the overall interaction of the dynamic forces thatcontrol the motion of the machine.RoadholdingThis means the ability of the machine, through its tyres, to maintain contact with the road. It dependsmainly on tyre type and size, suspension characteristics, weight and its distribution, and stiffnessbetween the wheels to maintain their correct relationship to one another. In the days of relatively narrowtyres, roadholding and handling generally went hand-in-hand, indeed, the terms were usedinterchangeably. However, nowadays the requirements are sometimes contradictory and a compromisemust be struck, depending on the intended use of the machine.A big enemy of tyre grip and hence roadholding is dynamic variation in the vertical load at the roadinterface, there are many factors that contribute to such variation and we shall see that suspensionparameters are important as a means of providing control over this aspect.StabilityThere are many types of stability or instability that can influence a motorcycle. There’s balance stability,aerodynamic stability etc. Formal definitions of stability in control systems exist but they are too involvedfor a book of this nature, although we’ll look at these aspects a bit closer in a later chapter. For ourpresent purposes we mean:The ability to maintain the intended manoeuvre (i.e. continue in a straight line or round a corner) withoutan inherent tendency to deviate from our chosen path. This implicitly includes the absence of wobblesand weaves.The ability to revert to the intended manoeuvre when temporarily disturbed by external forces (e.g.bumps, cross winds and so on).Handling, roadholding and stability are affected by many parameters and the interaction between them.The subject is complex but not magic, and – judging from some chassis designs – has not always beenwell understood. However, relatively simple laws of physics are always obeyed. This book will try toremove the mysteries and consider the main parameters involved and study their various effects. It mustbe emphasized that there is much cross-coupling between these effects – there is no ’correct’combination, no ’perfect’ design. Any motorcycle embodies several essential compromises.
  16. 16. 1-2 Function and HistoryLinear and angular motionsIf we are to study the behaviour of any type of vehicle we first need to consider just how it can move.The linear motions are easy to visualize, firstly the machine can move in a forward direction and theengine and brakes are responsible for controlling this. Road undulations and hills cause motion in avertical direction and sidewinds can result in sideways movement. It is the angular motions that aresomewhat less familiar to most people. The overall angular movements can be completely described byconsidering the motions about three separate axis. These axis are at right angles to one another andare known as roll, pitch and yaw.Fig. 1.1 Showing the threeprincipal axis of rotation.Yaw is the angular motion abouta vertical axis.The pitch axis is horizontal andpasses sideways through thebike.The roll axis is also horizontaland is orientated fore and aft.Roll is probably the most familiar of the three and is the most obvious motion that occurs when we leanthe bike over for cornering. Fig. 1.1 shows the roll axis passing through the CoG. However, as we shallsee later the location of this axis depends on our frame of reference.Yaw is the movement about a vertical axis and occurs as we steer around a bend, it can also be causedby various disturbances such as sidewinds.Pitch is the motion about an horizontal axis that passes sideways through the machine, we get this underbraking and acceleration, as well as from road irregularities.Due to the large roll angles involved with cornering, the pitch and yaw axis of the machine move relativeto the global vertical and horizontal coordinates. For this reason it is important to be careful whenspecifying the axis system that we are using. There are several such systems that are used in vehicleanalysis but for our purposes the two most important will be the machine coordinates and earthcoordinates, initially defined in terms of the original direction of travel, before performing somemanoeuvre.
  17. 17. Function and History 1-3FunctionThe functions of a motorcycle frame are of two basic types: static and dynamic. In the static sense theframe has to support the weight of the rider or riders, the engine and transmission, and the necessaryaccessories such as fuel and oil tanks. Although less obvious, the frame’s dynamic function is criticallyimportant. In conjunction with the rest of the rolling chassis (i.e. suspension and wheels) it must provideprecise steering, good roadholding, handling and comfort.For precise steering the frame must resist twisting and bending sufficiently to keep the wheels in theirproper relationship to one another regardless of the considerable loads imposed by power transmission,bumps, cornering and braking. By proper relationship we mean that the steering axis must remain in thesame plane as the rear wheel, so as to maintain the designed steering geometry in all conditions withoutinterference from frame distortion.Clearly, however, no steering system can be effective while the wheels are airborne, hence theimportance of good suspension, especially at the front. Good handling implies that little physical effortshould be required for the machine to do our bidding, so avoiding rider fatigue. (This requirement islargely a function of centre-of-gravity height, overall weight, stiffness, steering geometry, tyre sizes andthe moments of inertia of the wheels and overall machine/rider combination.)Comfort is important to minimize rider fatigue also, and requires the suspension to absorb bumps withoutjarring the rider or setting up a pitching motion. All these criteria the frame has to fulfil for the expectedlife of the machine, without deterioration or failure and without the need for undue maintenance.It should be borne in mind, however, that all design is a compromise. In any particular example, theprecise nature of the compromise will be governed by the use for which the machine is intended, thematerials available and the price the customer is prepared to pay.HistoryTubular framesOver the years designers have been repeatedly criticized for their seeming reluctance to depart from thediamond-pattern frame inherited from the pedal cycle. However, since the earliest motorcycles werevirtually pushbikes with small low-powered engines attached at various places, that was the logical frametype to adopt, particularly so long as pedal assistance was required.Until the general adoption of rear springing several decades later, this diamond ancestry (including itsbrazed-lug construction) was discernible in most frame designs. This was hardly surprising as theframe’s depth suited the tall single-cylinder engines that were popular for so long. In any case themotorcycle, like the pedal cycle, was after all a single-track vehicle in which the use of an inclinedsteering head was a convenient way to provide the front-wheel trail necessary for automatic straight-linestability.Once pedals were discarded, the frame with the closest resemblance to the ancestral pedal type was thesimple diamond pattern in which the engine’s crankcase replaced the cycle’s bottom bracket to span thelower ends of the front tube and seat tube. For many years both before and after the first world war thistype of frame was the overwhelming choice of the established manufacturers. An earlier variant of thediamond pattern was the single-loop frame in which the front tube and seat tube were bent from a singlelength, which passed underneath the engine. An improvement on both was the cradle frame. In this, the
  18. 18. 1-4 Function and Historybottom ends of the single front and seat tubes were spaced farther apart and rigidly connected by abrazed-in engine cradle, from the rear of which the tubes reached upward to the wheel-spindle lugs.Cradle frame, successor to the diamond bicycle pattern . The In the duplex cradle frame the cradle tubes are alsocradle tubes are extended rearward to the wheel spindle lugs. extended upward to the steering head.A straightforward development of this layout was the duplex cradle frame, in which the cradle tubes werecontinued upward to the steering-head lug as well as to the rear spindle lugs. Both types of cradle framesuited the upright single-cylinder engine by providing room for the narrow crankcase to be slung verylow, engines with wider crankcases had to be mounted higher, so raising the centre of gravity.In the design of these early frames, torsional and lateral stiffness seem to have been given a low priority.However, there were some commendable efforts between the wars to ensure the all-important torsionaland lateral stiffness through triangulation of the frame structure. In the Cotton, the four long tubesconnecting the steering head directly to the rear spindle Iugs were triangulated in both plan view andelevation, and the machine was renowned for its excellent steering. Triangulated in both plan and elevation, the straight- tube Cotton frame was renowned for its steering.A few attempts were made to stiffen the support of the steering head by incorporating it in one end of acast H-section frame member, this type of structure replaced the front down tube in the Greeves and thetop tube in some BSAs.
  19. 19. Function and History 1-5A 1960s. picture of the author racing a Greeves Silverstone with cast aluminium H-section front down member,incorporating the steering head. The under-frame was an open channel section fabricated from steel sheet, a bent steeltube backbone completed the frame loop.Bolted-up from straight tubes (for easy repair) and relying on the power plant for some of its stiffness, theFrancis-Barnett was fully triangulated from the steering head to the saddle, though the rear end wastriangulated only in the vertical plane. Another frame to depend on the engine for part of its stiffness wasthe open Scott. In this the rear end was triangulated fully but the steering head only laterally – whichwas much the more important plane from the steering viewpoint. The front brake of the day was hardlypowerful enough to tilt the head significantly in the fore-and-aft plane; and even if it did, that would nothave impaired the steering nearly so much as would twisting the head sideways. The Scott too earnedan enviable reputation for its steering.
  20. 20. 1-6 Function and HistoryFully triangulated at the front but only vertically at the Triangulated fully at the rear but only laterally at therear, this Francis-Barnett frame could be easily repaired front, this early Scott frame relied on the engine forby renewing any of the bolted-in tubes. some of its stiffness.When “plunger” sprung rear ends began to take over from the unsprung variety, many manufacturerssimply opened up the rear part of their cradle frames to accommodate the spring units. In most casesonly a thin wheel spindle held the two spring units together and so with many designs there was a lot ofdifferential movement between the two sides, resulting in the rear wheel twisting out of line with the restof the machine. Often, despite the welcome increase in comfort and sometimes in roadholding, therewas a deterioration in handling and stability compared to their unsprung forebears. In fact the Nortonplunger sprung frame earned the nickname of “garden gate”. Its general handling was thought to be akinto riding that particular piece of garden furniture.A revolution was started in 1950 when the works Norton racers were supported by the McCandless Bros.designed “featherbed” frame. It is hard to over-estimate the influence that this design has had onsubsequent chassis development. This shows just how easy it was to fit plunger The legendary Norton “featherbed” frame. The crossing springing in place of the then standard unsprung over of the tubes at the steering head facilitated the use rigid rear end. This design reduced the amount of a flat-bottom tank but impaired stiffness. A head of re-tooling required. steady was used which connected the steering head and top of the engine to overcome this.
  21. 21. Function and History 1-7Even now, over half a century later, many current designs still show a direct lineage back to it. Theenormous improvement over its predecessor, hinted at by its nickname, probably owed less to any onedesign feature than to a combination of several. Its duplex-loop layout had mediocre structural efficiencybut provided adequate (though not exceptional) stiffness and the general layout was such as to give afairly even weight distribution and a relatively low centre of gravity (considering the upright position of thecylinder). The front fork was one of the more robust telescopics of the period, and the steering geometryprovided light, responsive handling. As with many landmark developments the featherbed’s successprobably owes much to being the right product at the right time rather than having any overwhelmingtechnical superiority.Generally speaking steel has been the most used material for tubular frames although both titanium andaluminium have been used. BSA tried titanium in the 1960s. for their works moto-X bikes, and the1980s. saw various makers using aluminium alloys for both road and racing frames.BeamsAn entirely different approach to the problem of achieving adequate resistance to twisting and bending isto use a large-diameter tube as the main frame member, thus combining a high degree of stiffness withsimplicity and light weight. Provided it is of sufficient section, the tube does not necessarily have to becircular, though this is the best shape for torsional stiffness. Indeed, when the NSU Quickly popularisedthis type of frame at the start of the moped boom in the early 1950s the tube – or beam, to use anothername – was made from left and right half-pressings seam-welded together, giving an approximately ovalsection.Clearly, however, a plain beam could not connect the steering head directly to the rear-wheel spindle asdid the top four tubes in the Cotton frame. Hence it was bifurcated at the rear to accommodate thewheel, and the resulting open channel section of the two arms was closed by welding-in a U-shape stripto restore strength. Welding the beam from two halves in this way made it possible to incorporate anynecessary curvature in a vertical plane. NSU used curved beam frames not only on their moped but alsoon their Max roadster, their 250 cc world championship-winning Sportmax catalogue racing single, theworks racing 250 cc Rennmax twin and 125 cc Rennfox single. In this unsprung beam frame for a moped the open Welded from left and right hand pressings, this NSU channel section of the rear fork arms is strengthened by beam frame was shaped to accommodate pivoted fork a welded-in U-shape strip. rear springing and to support the engine at the top and rear.
  22. 22. 1-8 Function and HistoryPivoted-fork or swinging-arm rear springing removed the need to bifurcate the rear end of a frame beambecause with this type of suspension it is the swing-arm pivot, not the wheel spindle, to which thesteering head has to be stiffly connected. (Naturally, the swing-arm itself should continue the torsionaland lateral stiffness back to the spindle.) In the NSU Max and racers, the pressed-steel beam wascurved downward at the rear to make a direct connection from steering head to rear pivot.Winner of the world 250 cc championship in 1953, this NSU Rennmax twin had a curved beam frame welded from left andright halves (MCW)Because of its extremely large cross-sectional area (sufficient to accommodate a separate 2½ gallon fueltank inside), the Ariel Leader (and Arrow) frame was probably the stiffest and most outstanding of thebeam type. Predictably when pressed into racing service, its steering proved well up to the extrademands.
  23. 23. Function and History 1-9The Ariel Leader (and Arrow)frame derived great stiffnessfrom the large cross-sectionalarea of the beam, whichenclosed the 11 litre fuel tank.To make a direct connection between steering head and rear fork pivot, the tubular beam on this early 125 cc Honda GPracer was curved through some 90 degrees. (Salmond)Other frames – such as that on the early grand-prix Hondas and some Reynolds one-offs – used alarge-diameter circular tube, similarly curved, to achieve the same effect. The Honda frame, however,like its duplex successor on the grand-prix fours of the early 1960s, was structurally incomplete withoutthe engine, which was attached at the cylinder head and gearbox.
  24. 24. 1-10 Function and History Making a direct connection with a straight tube is usually impracticable, even with a flat-single engine(although the first frame to be constructed by the author in the 1960s. achieved it). Nonetheless, withthat type of engine or say, a sloping parallel-twin two-stroke, the rear end of the tube can be broughtclose to the fork pivot, as it was in the Foale frame for a Yamaha TZ350. In that case, the 50 mm. gapwas bridged by a small welded-in box section.The first chassis built bythe author in the early1960s for a small 2-stroke 125cc engine.Features a 76 mm.diameter backboneframe connecting therear fork pivot directlyto the steering head.Note also the leadinglink forks with Greevestype rubber pivotbushes, which also actas the springs. An early 1970s frame by the author for 250 and 350 cc racing TZ Yamahas. The straight tubular beam is connected to the rear- fork pivot by a box section and stiffened at the steering head by a folded gusset. The front down tubes are there solely to support the engine weight and are bolted at the top to the main frame. Such a construction makes for easy engine installation and removal.
  25. 25. Function and History 1-11On a Reynolds frame for a 250cc Moto Guzzi flat single however, the gap was appreciably greater andwas spanned by a channel-section light-alloy fabrication, bolted through two cross-tubes welded into themain tube (which doubled as an oil tank) and the box-section sump welded to its underside.A substantially similar layout was used on Norton’s unfinished Moto Guzzi-inspired experimental 500 ccflat single in the mid-1950s. In that case the oil was contained in a 114 mm.-diameter main tube and awelded-on underslung box that also supported the crankcase, while the fork pivot was bolted between alight-alloy gearbox plate on the left and an aluminium casting on the engine.Believed to have nowbeen completed andresiding in the SammyMiller museum, thisNorton frame was beingprepared for 1956 butwas never finished bythe factory. Oil wascontained in thebackbone and theunderslung box, whichsupported thecrankcase. Surprising,this box section was notused to support theswinging fork pivot,which was held betweenlight alloy plates.(MCW)When a tall, bulky engine (such as a 1-litre twin-cam four abreast) has to be accommodated, an evenbigger gap has to be spanned. A self-defeating scheme adopted by some frame builders was to bridgethe gap with a pair of bolted-on light-alloy plates, which could make nonsense of the tube’s torsionalstiffness, depending on detail design.An alternative arrangement was incorporated in the Foale frame for Honda and Kawasaki fours, in whicha pair of tubular triangles splayed out from the rear of the tube to the sides of the fork pivot, so providinggood support in both planes.Another approach to the problem of accommodating a large engine is to split the beam around eachside, thus we come to the “twin spar” frame initially popularised in the 1980s. These frames have beenmade in both steel and aluminium, but aluminium is currently the material of choice. Designed andconstructed properly this frame type can be made quite stiff.
  26. 26. 1-12 Function and HistoryNote how the head stock is supported by the two side beams of this 1997 NSR 250 racing Honda. Frame material isaluminium alloy.Ner-a-CarAlthough its chassis comprised two full-length, channel-section sides in pressed steel, cross-braced frontand rear, the Ner-a-Car of the 1920s defies classification with the beam-type frames if only because itlacked a conventional steering head to connect to the rear-wheel spindle. Indeed, the steering kingpinwas set in the middle of the front axle and housed within the front hub – hence the term: hub-centresteering. The axle itself was horizontal, shaped like a U (closed end forward) and pivoted in lugsprotruding downward from the chassis sides, with stiff coil springs providing a short suspension travel,which thus varied the kingpin inclination. The chassis members were bowed outward at the front for tyreclearance on full lock, which was nonetheless severely restricted.Although the chassis’ resistance to bending in a horizontal plane must have been high, its torsionalstiffness was doubtful. The Ner-a-Car’s quite exceptional stability most likely stemmed from its ultra-lowcentre of gravity, allied to an uncommonly long wheelbase – 1500 mm. for the unsprung version, 1740mm. with quarter-elliptic, pivoted-fork rear springing, and its hub-centre steering, which is often moretolerant of torsional flexure.

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