Classic Motorcycle Electrics Manual E-Book

James Smith

• Herausgeber: Crowood
• Kategorie: Lebensstil
• Sprache: Englisch
• Veröffentlichungsjahr: 2015

Seitenzahl: 862

Beschreibung

Even the most hands-on of classic bike enthusiasts will often shy away from working on their bike's electrical system, believing they have neither the skill nor the knowledge for such work. Dr James Smith explains in Classic Motorcycle Electrics Manual that this need not be the case. Starting with basic electrical theory, the book demonstrates a wealth of electrical tips and techniques, providing a progressive and detailed guide to tasks ranging from simple repairs and upgrades, through to completely rewiring a classic motorcycle. Illustrated profusely with full-colour photographs and easy-to-follow wiring diagrams, this book will be an invaluable resource for all classic bike owners and restorers. The book covers: basic electrical theory; correct usage of a multimeter; comprehensive fault-finding techniques; making good electrical connections; fuses and circuit protection; dynamo and alternator charging systems; correct battery selection and maintenance; improving lighting and installing LEDs; selecting the right spark plug, and much more. Fully illustrated with 420 colour photographs and 167 CAD easy-to-follow wiring diagrams, this is an essential reference work for all classic bike owners.

Leseprobe

Classic Motorcycle Electrics Manual

James Smith

First published in 2015 by The Crowood Press LtdRamsbury, Marlborough Wiltshire SN8 2HR

www.crowood.com

© James Oliver Smith 2015

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publishers.

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.

ISBN 978 1 84797 996 4

To Mom and Dad, for providing me with more Lego and electronics kits as a child than any budding engineer could wish for. And also to my wife, Katie, for continuing to allow me to play with old motorbikes.

DisclaimerSafety is of the utmost importance in every aspect of an automotive workshop. The practical procedures and the tools and equipment used in automotive workshops are potentially dangerous. Tools should be used in strict accordance with the manufacturer’s recommended procedures and current health and safety regulations. The author and publisher cannot accept responsibility for any accident or injury caused by following the advice given in this book.

contents

acknowledgements

introduction

1  electrical theory

2  electrical components

3  tools for electrical testing

4  using a multimeter

5  fault finding techniques

6  soldering skills

7  making connections

8  wiring looms

9  fuses and circuit protection

10  lighting

11  switches, instruments and horns

12  batteries and chargers

13  dynamos

14  dynamo control boxes

15  alternator charging systems

16  ignition systems

17  cspark plugs

appendix: wiring diagrams

glossary

index

acknowledgements

Many thanks to all of the people who, knowingly or unknowingly, have provided me with endless ideas, inspiration, technical information and anecdotes for this book over the last two years. It has taken a while to compile everything together but I hope that you find the result to be a useful addition to your bookshelves.

I am indebted to Michael Peirce at NGK Spark Plugs (UK) Limited for the time and expertise he provided in ensuring that the spark plug chapter is fully up to date with the latest information concerning modern spark plug options for classic bikes. I am sure that this will help to dispel some of the common myths and misunderstandings about spark plugs. A number of the illustrations in the spark plug chapter are included courtesy of NGK. Also my thanks go to Lucas UK for their permission to reproduce and adapt some of the illustrations from their historic parts catalogues and service manuals.

Emmanuel Donati at TecMate International (www.tecmate-int.com) was kind enough to provide a sample of their latest ‘OptiMate 5 Voltmatic’ battery charger for testing and photographing. This new charger worked great on both my 6- and 12-volt bikes and I’m sure it will be at the top of many owners’ wish lists. Similarly Michael Hutchings of Dynamo Regulators Limited (www.dynamoregulators.com) kindly supplied one of his excellent DVR2 regulator units. The photos in Chapter 14 show this being installed on my 1949 Matchless and I look forward to it giving many years of service, just as the unit on my other bike has already done.

Steve Surbey at AMC Classic Spares (www.amcclassicspares.com) has, as always, been an amazing source of knowledge concerning all things AMC. He was also kind enough to allow me to photograph a number of items from his stock including various switches, lamps, ammeters and batteries.

My thanks also go to the following people who have kindly allowed me to reproduce their photos as illustrations in this book: Rob van den Brink for the photos of the old Lucas four-position lighting switches from his WD Norton website (www.wdnorton.nl); Theo-door Leenes for the numerous photos of the different dynamo and magneto models from his reconditioning web-site (www.theodole.nl); Rob Currie for his pictures of the Thorspark coil ignition installation on his AJS; Laurence Swain for some great shots showing the difference between earlier and later Lucas MCR2 regulators; Skip Brolund for the shots of an early Lucas three-brush dynamo unit; Andy Perkins of Pazon ignitions Limited (www.pazon.com) for the photographs of their electronic ignition system.

Finally, I am indebted to John Clement for his encouragement and efforts in helping ensure that this book is, I hope, without any significant errors.

introduction

Many years ago whilst at university, I worked as a weekend sales assistant at a local camera shop. On one particularly quiet Sunday afternoon, a well-spoken elderly lady entered the shop and asked the manager if he could fix her faulty camera. My colleague took the camera, carefully checked it over then opened the back cover to remove the battery. After weighing the battery carefully in his palm for a few seconds he announced, with a completely straight face, ‘Aha, here’s the problem, madam, your battery is empty!’

Now of course it was not the weight of the tiny battery that had indicated to my colleague that it was flat. Even after several decades in the camera business there was still no way he could possibly have determined whether the battery was ‘empty’ just by looking at it or weighing it in his hand. His magical electrical skills that had so impressed the old lady were actually nothing more than reading a few simple signs; something the lady could have easily done herself. The dim display on top of the camera had been the real giveaway and the whole battery weighing charade was merely a distraction to help pass a long Sunday afternoon at work.

And so it is with classic motorcycles too. An aura of mystery seems to have developed over the years concerning our old bike’s electrical systems which, I think, is completely unfounded. Perhaps it is because we cannot see an electric current in the same way that we can spot an oil leak that leads many to believe that this ‘electric-trickery’ is best left to an initiated few. Many skilled owners have completed a full mechanical restoration of their cherished motorcycle, only to then pay someone else to sort out the electrics.

In reality, there really is no mysterious ‘black art’ to motorcycle electrics – honest! This is especially true when it comes to our simple old bikes that have none of the complications, computers or gadgetry of more modern machines. If you are competent enough to change a spark plug and are willing to learn a few basic techniques, then you are more than qualified to become a classic motorcycle electrician.

I therefore hope that you will find this book both an interesting read and a useful reference for when you are out in the shed or garage with your bike. It is written from a purely practical sense of what is likely to go wrong, how to spot it and then what to do about it. That is not to say that we won’t indulge in a little electrical theory along the way, but we’ll restrict this to just enough to help us understand what is actually going on inside our bike’s wiring loom. Our aim will be not only to understand what is happening, but also why it is happening. In this way we’ll be in a much better position to apply the same skills and logic to any new problems you may encounter with your motorcycle.

It is worth taking the time to read through the first few chapters to begin with in order to get up to speed with the basics, learn some useful background information and become familiar with the important terminology. After that, feel free to dip in and out of the book as your interests (or problems with your bike) dictate. I am sure that the simplicity of a classic bike’s electrical systems will soon become apparent. Before long and with a little practice, you will inevitably become that person to whom everyone in your local riding group or owners’ club section turns when ‘The Prince of Darkness’ next makes a visit.

James Smith

Hong Kong, May 2015

1

electrical theory

The logical place to begin this motorcyclist’s electrical handbook is to define what exactly this strange ‘electricity’ stuff actually is. We can’t normally see it, it doesn’t have a smell, it’s silent and we’re going to try our best to avoid touching it (at least as far as getting an electric shock is concerned). So that doesn’t really give us much to go on.

Yet while electricity may at first seem somewhat mysterious, it is actually rather predictable and we can easily visualize what is going on inside our bike’s wiring loom with a few simple analogies. This first chapter aims to provide you with enough background information to be able to understand not only what is happening inside your bike’s electrical system, but also why this is happening. It is this understanding of why something does (or doesn’t) happen that will make the discussions in the following chapters much more straightforward.

WHAT IS ELECTRICITY?

Put simply, what we call ‘electricity’ is just the flow of charged particles around a circuit. But what is a ‘charged particle’, what makes it suddenly decide to ‘flow’ and what exactly constitutes a ‘circuit’? To answer these questions and properly understand what electricity really is, we’ll first need to brush-up on a little background physics.

The structure of an atom

Every part of a classic motorcycle is made up of atoms, just like the rider sat on top of it and the road beneath its wheels. For years, atoms were considered to be the smallest fundamental particle of which everything in the universe was made. The word itself comes from the Greek word atomos, which roughly translates as ‘something that cannot be cut in two’.

Advances in science soon revealed that atoms weren’t quite as indivisible as we had previously thought. We now know that they are in fact made up of even smaller particles called protons, neutrons and electrons. There are other particles too, many with strange sounding names and even stranger properties. Some of these can themselves be broken down into even smaller particles, but none of these are particularly relevant when it comes to a motorbike’s electrical system. So we’ll leave these to the physicists and stick with just the basic protons, neutrons and (perhaps most important of all), electrons.

You will no doubt be at least vaguely familiar with the simplified view of an atom illustrated in fig. 1.1. At the centre of the atom is the core or nucleus, which is made up of a mixture of positively charged protons and charge-less neutrons. The number of protons defines what type of element the atom is and is referred to as its atomic number. If it has one proton then it’s a hydrogen atom, two protons and its helium, eight gives us oxygen, twenty-nine and it is copper, and so on.

A neutron is roughly the same size and mass as a proton. While the number of neutrons in the nucleus doesn’t affect what element the atom is, it does affect some of its chemical properties. Different versions of an element may be found with different numbers of neutrons in the nucleus and these are what we call ‘isotopes’ of the basic element.

Orbiting around the central nucleus of the atom is a collection of negatively charged electrons. These are much smaller than both the protons and neutrons (about 1/1800th of their size), but the negative charge of an electron is about equal in magnitude to the positive charge of a proton. Most atoms have an equal number of protons and electrons such that their charges cancel each other out, giving the atom an overall neutral charge.

If we think of the nucleus of the atom as being like the sun at the centre of our solar system, then the electrons are like the planets orbiting around it. Just as each planet has its own particular orbit around the sun, so the electrons have fixed orbits at specific distances out from the atomic nucleus. These electron orbits are known as ‘shells’ and each shell can contain one or more electrons.

The saying goes that opposites attract and this is definitely true when it comes to charged particles. The positively charged protons in the nucleus and the negatively charged electrons whizzing around it feel a strong attractive bond within the atom.

Fig. 1.1 The simplified planetary model of an atom with a core of protons and neutrons, and with smaller electrons orbiting around the outside. (Not to scale.)

Fig. 1.2 The atoms and electrons in an insulator and conductor.

These attractive forces are, however, quite short-range and so extend for a relatively small distance out from the nucleus. Electrons orbiting close to the inner core tend to be held quite tightly, while those that are further away experience a weaker attractive force. These outer electrons therefore find it much easier to break free from the attractive bonds of the nucleus and leave the atom.

In some materials the outer electrons are held so loosely and are so close to their counterparts in neighbouring atoms that they are constantly switching positions. Rather than continually orbiting a single nucleus, they are instead shared between neighbouring atoms and can move around within the material by hopping from one atom to the next. Their motion is, however, very random, so while they may appear to move around a lot compared to the nucleus, they don’t actually get very far in any particular direction.

This effect is often described as the material having a ‘sea of free electrons’ flowing around the atoms. It is this sea that give metals the ability to conduct electricity. Conductors have lots of free electrons, whereas electrical insulators have only a few or maybe none at all. Without a ready supply of free electrons, insulators are unable to conduct electricity.

Let’s now imagine that the free electrons in a conducting material all suddenly feel some urge to move in a particular direction. Their motion will still be quite random as they jump between neighbouring atoms, but overall they might tend to drift in one direction more than any other. This is what electricity is: simply the drift of free electrons in a particular direction within a conducting material.

The next logical question to ask is what would make all of the electrons suddenly decide to start drifting in a particular direction? Clearly some sort of invisible force would need to be applied, but what could this be? As you might well have already guessed, this is where a battery comes in rather handy. The battery is what supplies us with the invisible force that urges the free electrons to move together through a conductor in a certain direction.

The battery provides what is known as an ‘electromotive force’ or EMF (electro- because it acts upon electrons and -motive because it makes them move). Hence an EMF is a force that makes electrons move, and as we now know, that’s what gives us the effect we call electricity. While you may not have come across this particular term before, you are almost certainly familiar with its meaning as it is simply the technical name for what we normally refer to as the battery’s voltage. A 12-volt battery supplies us with an electromotive force equal to 12 volts.

What is a circuit?

Free electrons are always present in a conductor irrespective of whether or not an EMF is applied across it. The electrons are not created by the battery, nor are they used up by the bulb. The EMF of the battery merely provides the necessary push to make the existing electrons move.

Fig. 1.3 Even the simplest electrical circuit must include these four basic components.

In order for electrons to be able to flow from the battery towards the headlight, there must also be a path by which they can flow back from the headlight to the battery, thereby completing the circuit. Any electrical system must be comprised of at least four components: a source of EMF (the battery), a load (the bulb), a conductor connecting the battery to the bulb (the supply wire) and another conductor connecting the bulb back to the other terminal of the battery (the return wire).

This is the simplest electrical circuit there is, and you’ll be pleased to learn that many of the circuits on your classic motorcycle are not a great deal more complicated. Add in a simple switch and you have the basis of a headlight circuit, or change the bulb to something that makes a loud noise and you have your horn circuit.

We’ll come back to these circuits in later chapters. For now it is sufficient to understand that any circuit must include each of these four basic components. There must always be a complete circuit for the current to flow around, although of course few of the circuits on our classic bikes will be so neatly laid out in practice.

Motorcycle earth

The ‘earth’ or ‘ground’ of a vehicle has nothing whatsoever to do with the road beneath its wheels. Instead these terms are used to denote a common electrically conductive connection between the various components of the vehicle formed by the metal frame, engine and bodywork.

Fig. 1.4 With the addition of a switch, we already have the basis of the majority of electrical circuits on a classic motorcycle.

By connecting the battery’s earth terminal directly to the metal frame of a vehicle, every connected metal component therefore becomes a potential earthing point. This cuts the amount of wiring almost in half since, rather than requiring an individual return wire back to the battery, each electrical load can instead be earthed to the nearest available point on the engine, frame or metal body of the vehicle. This saves both cost and weight, and it also vastly simplifies the wiring loom since there are far fewer wires required.

Take a motorcycle’s headlamp as an example: a wire goes from the live battery terminal to the headlight bulb via the appropriate switches. But rather than having a second wire completing the circuit back to the battery, the other side of the bulb is instead connected to the metal headlamp shell. The headlamp shell is bolted to the front forks, which join via the metal head bearings to the motorcycle’s frame. Since the frame is already connected to the battery by a short length of wire, the circuit is complete. The various metal components of the motorcycle thereby act as a very thick conductor joining the earth terminals of the bulb and battery.

Fig. 1.5 We can reduce the number of wires required by using the metal frame of a motorcycle as the connection back to the battery.

Voltage

If we define electricity as the flow of electrons through a conductor, then the electrical voltage is the force that drives it. The speed at which the electrons move through the conductor (ignoring the random part of their movement) is proportional to the magnitude of the applied EMF. Increasing the voltage provides a greater force on the electrons, which causes them to flow more quickly through the conductor.

Voltage is known by a variety of different terms depending upon the context and it is useful to be able to understand and make use of the correct terminology. We have already mentioned that the voltage supplied to a circuit by a battery is known as the electromotive force (EMF). Another term for voltage is ‘electrical potential’, although this is seldom used in practice. When we measure the difference in voltage between two points in a circuit, however, we normally refer to this as the ‘potential difference’ (rather than the voltage difference) between the points.

Historically the term ‘electric tension’ has also been used to describe a voltage. While this term is now mostly obsolete, it is still used in relation to some automotive electrical systems. The high-voltage ignition coil, for example, is often known as the ‘high-tension’ (HT) coil and the wire going from the coil to the spark plug is referred to as the ‘HT lead’.

Irrespective of whether we are talking about an electromotive force, potential difference, electrical potential or electric tension, all of these quantities are measured in the same units of ‘volts’. We use the uppercase letter ‘V’ to denote volts and hence voltages may be written as either 12 volts or 12V.

Current

A current is a flow of electrons within a conducting material. However when we talk about current we usually want to be a little bit more precise about how many electrons are flowing and how fast they are going in order to know how much useful work they will be able to perform.

The term ‘current’ is therefore mostly used to specify the rate at which electrons are flowing through a conductor. Current is measured in ‘amperes’, which we usually shorten to ‘amps’. It is given the symbol upper case ‘A’, so we can write 10 amps or 10A.

HOW FAST DOES ELECTRICITY TRAVEL?

As soon as we turn on the headlight switch, the front lamp of our bike begins to shine. So we might assume that the electricity must travel rather quickly through the wires from the battery to the bulb and back again in order to have this immediate effect. And we’d be right, but also at the same time, we’d also be wrong.

It all depends upon how exactly we define ‘electricity’. The effect of turning on the switch travels through the wires at almost the speed of light so that the headlamp appears to illuminate at the exact moment the switch latches. But the actual electrons themselves are moving surprisingly slowly through the wires of the bike. So slowly in fact that they would have difficulty keeping up with a snail crawling along the outside of the cable!

The easiest way to think about this is to consider a long tube filled with a single row of marbles. If we try to insert another marble into one end of the tube, the furthest marble will almost instantly fall out of the other end. The marbles themselves (representing our electrons) move relatively slowly along the tube (our wire). However, the effect of inserting another marble at one end (representing electrons flowing into the wire from the battery) is experienced almost instantly at the other end as the marble drops out (or our bulb lights up).

To put this back into the context of our bike’s electrical system, the electrons travelling through the wire between the battery and the headlight bulb move at around 0.22mm per second. They will therefore take around 2.5 hours to complete a single circuit from the battery to the bulb and back again. In contrast, a common garden snail travelling at around 1mm per second will have completed more than four of these circuits in the same time.

Fig. 1.6 We can think of a wire full of electrons as being like a long tube filled with marbles.

Resistance

In our model of a typical conductor, we have pictured a ‘sea’ of loosely bound free electrons moving randomly between the various atoms. When an EMF is applied across the conductor, these electrons all begin to drift in a single direction, giving us the flow of charge that we know as electricity.

Even under the effect of the strongest EMF, the electrons cannot move in a straight line from one end of the conductor to the other. There are far too many atoms in the way and so collisions are unavoidable. Each collision slows down the electrons and impedes their progress through the conductor. It is this restriction of the flow of electrons that gives rise to the resistance of the conductor.

We can think of a conducting material as being a bit like one of those old ‘penny falls’ arcade machines where you insert a coin at the top and it falls between various pegs before hopefully dislodging a cascade of pennies at the bottom. The coin you insert doesn’t fall directly to the bottom, but instead it bumps and bounces off various fixed pegs that make its path very random. This is exactly how we might visualize an electron moving through a conductor, bumping and bouncing off the various atoms that get in its way. The penny falls under gravity in the same way that electrons move under the influence of an EMF; neither gets from one end to the other via a very direct route.

A number of different factors can influence the amount of resistance the current experiences. Different conductors have different atomic structures, which may provide more or less resistance to the flow. Another factor is the temperature of the conductor. At an atomic level, heat can be thought of as being vibrations of the atoms. The atoms in a hotter material will vibrate back and forth much more than those in a colder material. The more an atom is vibrating, the more of an obstacle it presents to the electron flow and hence the greater the material’s resistance. Therefore the resistance of conductors increases when they are heated, such as in the case of a glowing filament in a lightbulb.

Resistance is measured in ‘ohms’, for which we use the Greek symbol omega (Ω). We can therefore write 500 ohms or 500Ω. Electrical conductivity is the opposite of resistance. A material with high conductivity has a low resistance, and conversely a material with a high resistance has low conductivity.

WHAT IS THE BEST CONDUCTOR?

The element with the best electrical conductivity (lowest resistance) is silver, although this is usually too expensive to use in wires. Therefore most wires are made from copper, which is only about 5% less conductive than copper, but costs significantly less. Third place goes to gold, which is 35% less conductive than silver. This is also too expensive to use for wires, but it is sometimes used as a coating on connectors since it does not oxidize over time, unlike copper. However a solid gold wire would have a higher resistance than a standard copper wire.

The best conducting metals (with their conductivity expressed as a percentage of that of copper) are as follows: silver (106%), copper, gold (69%), aluminium (60%), tungsten (30%), zinc (28%), nickel (24%), iron (17%), platinum (16%), tin (15%) and steel (12%).

Although copper is the default choice, wire may also be formed from alternative metals for use in applications where other requirements take priority over conductivity. Aluminium wires are used in some aircraft because of the need to minimize weight. Similarly the filaments in halogen bulbs are made from tungsten wire since this has a particularly high melting point. Copper wires may also be tinned for use in harsh environments, for example in marine applications, since the tin coating helps to protect the copper strand beneath from corrosion. However, the majority of wires that we will come across on our motorcycles will be of the standard bare copper variety.

Fig. 1.7 Is the traffic flowing to the left, or is the space between the bikes flowing to the right?

Conventional current

Imagine a long line of vehicles all queuing up in a bumper-to-bumper traffic jam. Every now and then the car at the front of the queue moves forward a few metres, then the second vehicle pulls forward, then the third, and so on along the full length of the queue. The traffic is moving slowly forwards, but what if for a second we forgot about the vehicles themselves and instead focused on the short stretch of empty tarmac between them? This gap would start at the front of the traffic jam and, as each vehicle pulls forward, it would move gradually backwards towards the rear of the queue.

So which way is the flow going? Well it all depends upon whether you are watching the traffic or the empty stretch of road, each of which is moving at the same speed but in opposite directions. This is exactly what happens when a current flows in a conducting material. If we watch the free electrons then we would say that the flow is in one direction towards the positive battery terminal. Yet if instead we were to focus on the empty ‘hole’ left by each electron as it moved from one atom to the next, we might say that the flow is going in the opposite direction towards the negative battery terminal.

If we assume that each atom initially has no charge (i.e. it has the same number of positive protons and negative electrons), then when it loses an electron it becomes unbalanced and acquires a positive charge. So the empty space moving between neighbouring atoms is actually a flow of a positive charge through the material in the opposite direction to the electron drift.

We therefore have two different ways of thinking about the flow of electricity; negative electrons flowing one way or positive ‘holes’ moving the opposite way. This may at first seem a little confusing, but thinking back to the traffic jam analogy will help show that they are just two different ways of looking at exactly the same scenario. Neither way of thinking is particularly right or wrong; they are both just useful ways to help us visualize something complicated in terms of something that is a little more familiar. Whether you picture negative charges going one way or positive charges going the other, the result is just the same.

It is, however, more technically correct to think of the electrons leaving the negative terminal of the battery (remember that the reason it is negative is that it has an excess of negatively charged electrons). The electrons then flow around the circuit towards the positive battery terminal as opposites attract (remember that the positive battery terminal is positive because it has a deficiency of negatively charged electrons). This is what we call the ‘electron flow’ model as it is based upon visualizing the path that the electrons take.

Traditionally however, electricity was considered to flow from the positive battery terminal, around the circuit and back to the negative battery terminal. This is what we call the ‘conventional current’ model and it goes back to the days before we knew as much as we do now about what goes on at a sub-atomic level within a conducting material.

In practice, it makes no difference to how a motorcycle’s electrical system works if we think in terms of the electron flow or conventional current models. Therefore go with whatever seems to make the most sense to you, but also be prepared for someone else to adopt the opposite approach.

Series and parallel circuits

When two loads are connected to the same circuit, they can be connected in either ‘series’ or ‘parallel’ configurations. Take for example the pair of bulbs that make up a motorcycle’s twin headlights. In series configuration, we would have a wire from the battery to the first bulb (via the switch), then another wire between the first and second bulb, then a third wire back to the battery (fig. 1.8a). This is quite a neat and simple circuit with the two bulbs connected one after the other, i.e. in series with each other.

However there are some significant disadvantages with this type of circuit. First, each of the two loads receives only a proportion of the total battery voltage. If the two loads are the same (identical bulbs for instance) then they will each receive half of the battery voltage. So when using a 12-volt battery, each bulb wired in series will receive only 6 volts. Not only would it be confusing to use 6-volt bulbs in a 12-volt vehicle, it would also be inefficient as each bulb would require twice the current to give the same light output.

Fig. 1.8 A pair of headlamp bulbs wired in series and parallel configurations.

Fig. 1.9 Series and parallel circuits give very different results when one component in the circuit fails.

Another problem with connecting bulbs in series is what happens if one of these were to blow. The broken filament would break the circuit for both bulbs and hence both lights would stop working, even though only one has actually blown (fig. 1.9a). These two reasons mean that series circuits are not used very often in vehicles, and especially not when it comes to wiring the headlights.

The preferred option is to wire the two loads in parallel so that each has its own connections to the battery, thereby giving them both access to the full battery voltage (fig. 1.8b). Each bulb therefore receives 12 volts when it is connected in parallel to a 12-volt battery.

Wiring the two loads in parallel also means that if one of them were to fail, the other would still remain connected to the power supply (fig. 1.9b). For a headlight circuit this would mean that if one bulb were to blow, the other would continue working so that you wouldn’t lose all lighting. Clearly this is a much preferable arrangement.

Just as loads can be connected in series or parallel, so too can batteries. When two batteries are connected together in series (fig. 1.10a), their individual voltages are added together. Therefore, connecting two 6-volt batteries together in series allows us to power a 12-volt electrical circuit. When batteries are connected together in parallel (fig. 1.10b), the overall voltage remains the same as per the individual batteries, but the capacity of the battery is increased. So by connecting two smaller 6-volt batteries together in parallel we form a larger capacity 6-volt battery. This will be discussed in more detail in Chapter 12.

Fig. 1.10 Batteries may be connected together in series or parallel configurations to give either a higher voltage or greater capacity.

AC and DC voltages

So far we have only discussed direct current (DC) voltages as supplied by a battery or dynamo as this is the type of electricity used by the majority of motorbike systems. There is, however, another type of electricity called alternating current (AC) with which you will be more familiar from the mains voltage supplied to your home.

The difference between AC and DC current is to do with how the electrons move within the wires. In DC current we have already seen that the electrons all flow (albeit very slowly) in a single direction around the circuit. However with AC current, the electrons flow in one direction for a while, then in the other direction, then back the other way again, and so on. They are constantly alternating their direction many times per second, hence the name of this type of current.

So why do we have these two different sorts of electricity? The answer comes from the way that we generate the electricity in the first place. Direct current can be considered the original form of useful electricity since it was the first to power a circuit by way of an electrical cell (i.e. a battery). All batteries are DC devices; there is no such thing as an AC battery. As well as using the chemical energy stored in a battery, it is also necessary to be able to generate electricity using mechanical energy. This is where electrical generators (also known as dynamos) come in.

Inside a dynamo is a coil of wire that rotates inside a fixed magnetic field. Each time the wire cuts through the magnetic field, a small electric current is induced. Actually a dynamo is not truly a DC device like a battery as the electrical output varies depending upon the position of the wires within the magnetic field and so changes as the coil rotates. But if we have a sufficient number of separate coils, which are each aligned at a slightly different angle, then the output is smoothed out and becomes much more like a proper DC current.

Fig. 1.11 The constant direct current (DC) voltage produced by a battery or dynamo versus the fluctuating alternating current (AC) voltage of an alternator.

The problem with DC dynamos is that they’re not very efficient because of the way that we need to connect to the rotating coils of wire. With fixed connections the wiring would soon become twisted and tangled after only a few turns of the generator. Instead, a pair of stationary contacts are used that connect to each of the separate coils inside the dynamo one by one as they rotate past. This works reasonably well, but isn’t an ideal arrangement as anyone with a dynamo-powered bike will be only too aware.

A different sort of electrical generator was therefore invented known as an ‘alternator’. Rather than having rotating coils of wire inside a fixed magnet, these devices have a rotating magnet inside a fixed coil, thereby doing away with the complicated wiring connections between the stationary and rotating parts of the circuit. As the name suggests, alternators produce an alternating current and this is where this second form of electricity originates from.

An AC alternator is much more efficient and is able to produce much more electricity than a DC dynamo. This is why the power we consume in our home is of the AC variety and is also why all modern vehicles have alternators rather than dynamos. The problem is that our motorcycle’s battery can only be charged with a DC voltage (an AC voltage would put some electrons in, take some out, put some in and so on, thereby never actually charging the battery). However, it is fairly straightforward to convert an AC voltage into a DC one. All vehicles with an alternator charging system will therefore also have a rectifying device that converts its output from AC to the DC voltage required to power the bike and recharge the battery.

RMS voltages

While on the subject of AC voltages, we should also consider how exactly we measure their magnitude. Since the current is changing directions back and forth many times per second, the voltage is similarly going from zero to a peak, back to zero again and then to another peak in the opposite direction. So what do we take as being the voltage of the supply if the voltage is currently changing?

The obvious answer might be to take the maximum or peak voltage, however this is not particularly useful because the actual voltage is much less than the peak for the majority of the time. It is also not a true reflection of the power output of the supply. Instead we calculate a special type of average value called the ‘root mean square’ (RMS) voltage.

When we measure AC voltages we must ensure that the multimeter is set to the AC mode. The result displayed on the meter will then be given in terms of an RMS voltage. For classic motorcycles, the only time we’ll be dealing with AC or RMS voltages is when we are working with an alternator charging system (seeChapter 15). Owners of earlier bikes with dynamo charging systems need not concern themselves with such complications since they will only ever be working with DC voltages.

CALCULATING THE RMS VALUE OF AN AC VOLTAGE

In order to be able to determine the RMS value of an AC voltage, we first need to know what the waveform looks like. Let’s assume that we have a nice smooth sine wave that is typical of the output from an alternator or of the mains power in our homes. As the voltage is constantly changing, we’ll need to take a series of instantaneous voltage measurements throughout the AC cycle. For this example we’ll take twelve measurements as this divides the wave pattern up nicely (a greater number of measurements would give us greater accuracy, but twelve is enough).

At each of these twelve equally spaced positions we make a voltage measurement. Fig. 1.12 shows an example of an AC voltage signal that peaks at 10 volts and which has been divided up into our twelve measurement segments. Since the positive (above the line) and negative (below the line) parts of the wave are equal, we can actually just focus on the positive half.

Once we have determined the voltage at each of our measuring points, we then square each of these individual values (i.e. multiply them by themselves) to give us our ‘square’ values (fig. 1.12a). We add together these voltage-squared values and then divide by the number of values to give us our ‘mean square’ value (fig. 1.12b). The final step is to take the square root (√) of this value to calculate the ‘root mean square’ value (fig. 1.12c), which works out as 7.1 volts for our example with a 10 volt peak.

We have thereby shown that the RMS value is 71 per cent of the peak voltage (but we usually round this off to 70 per cent for simplicity). This applies to all AC signals with a sinusoidal waveform.

Fig. 1.12 Calculating the RMS value of a 10V peak AC voltage.

ELECTRICAL CALCULATIONS

Ohm’s Law

The voltage, current and resistance present in a circuit are all very closely related. Higher voltages result in greater currents, whereas higher resistances decrease the current. These three quantities can be brought together in a simple equation that is known as Ohm’s Law. This states that the voltage (V) is equal to the current (I) flowing multiplied by the resistance (R) of the conductor, or in mathematical terms:

This simple equation forms the basis of many of the electrical calculations we may wish to perform on our bike’s electrical system. Ohm’s Law also provides us with a couple of useful insights into electrical flow. Firstly it states that voltage and current are directly proportional to each other. Therefore, if we keep the resistance of a circuit constant but double the voltage (for example by swapping the original 6-volt battery to a 12-volt one), then the current will also double. Similarly if we halve the voltage (by changing to a 3-volt battery) then the current will also be halved.

Fig, 1.13 Voltage is the pivot point of the Ohm’s Law seesaw: the current will go up when the resistance goes down, and vice versa.

Another way of looking at this is to examine what would happen if we were to keep the voltage constant but vary the resistance. If we double the resistance (for example by having two bulbs in the circuit rather than just one), then we would halve the current. But if we were to install a bulb with lower resistance, the current would go up. Current and resistance are therefore indirectly proportional; as one goes up the other goes down in the same ratio.

Work and power

Fig. 1.14 The Ohm’s Law triangle provides an easy way to remember and rearrange the formula to calculate voltage, resistance or current.

The triangular diagram shown in fig. 1.14 is often used to help remember Ohm’s Law and to easily rearrange it to get the required form for either voltage, current or resistance calculations. To use this, you place a finger over whichever quantity you wish to calculate and then read the arrangement of the other two quantities. For example, if you wish to find the voltage, cover the ‘V’ with your finger as shown in fig. 1.14b, which leaves you with ‘I’ next to ‘R’ (i.e. I × R). If we want to find resistance, cover the ‘R’ with a finger as in fig. 1.14d, leaving ‘V’ over ‘I’ (i.e. V ÷ I).

Moving electrons around an electrical circuit is hard work for a battery and can be thought of in much the same terms as physical work (for example, the work your motorcycle’s engine is doing while propelling you down the road). The difference between a high and low voltage battery is how quickly they can perform work and it is this rate of doing work that we know as power.

Electrical power is directly analogous to engine power. A higher voltage battery is the same as a bigger engine as both can do work at a much faster rate, and are therefore more powerful. Power is measured in watts irrespective of whether we are talking about it in its electrical or mechanical form (brake horsepower (bhp) is just an imperial version of watts). Larger powers are measured in kilowatts (kW); one kilowatt is equal to one thousand watts.

The formula for calculating electrical power is nice and simple as power (P) is simply the product of the current (I) and the voltage (V):

This form of the equation allows us to calculate the power of an electrical system if we know both the current and power, but we can also rearrange it to make either of the other two terms the main subject:

Fig. 1.15 The power triangle provides us with an easy way to rearrange the power equation.

Which we can simplify to:

Which we can simplify to:

Older model bikes tend to have 6-volt electrical systems, which require twice the current to match the power output of a 12-volt electrical system. Power losses in the wires and switches are proportional to the current squared, so by doubling the current these losses are increased by a factor of four. Both of these results are particularly significant when it comes to maximizing the efficiency of a bike’s electrical system, and especially if we are trying to get the most light possible out of a headlamp.

Kirchhoff’s Laws

Most of the circuits that we will encounter on a classic motorcycle aren’t going to be any more complicated than the basic circuits shown in fig. 1.4. Parallel circuits are sometimes a little trickier since they introduce junctions in the wiring at which electrical current has a choice of which way to flow. However they are still fairly straightforward once you are aware of a few important electrical rules.

When we are analysing circuits, we need to be able to determine how the voltage supplied by the battery gets divided between the various components. For parallel circuits, we also need to be able to work out what happens to the current when it reaches a junction in the wiring.

To do this, we rely on two rules that are known as Kirchhoff’s Laws (named after the nineteenth-century German physicist Gustav Kirchhoff). Kirchhoff’s First Law tells us what happens to a current when it reaches a junction in a circuit. Kirchhoff’s Second Law tells us about how the battery voltage is consumed by the various loads.

Kirchhoff’s First Law of currents

Whenever there is a junction in a circuit, the electrons have a choice as to which way they will flow. Electrons (and hence current) can be thought of as being lazy and will always try to take the path of least resistance. Hence, when given the choice between flowing through a headlight bulb or via an accidental short circuit, they will always take the easier short-circuit option.

The loads connected to most of the electrical junctions we will encounter tend to be equally matched (for example, the left and right headlamp bulbs) and hence some electrons will choose one path and some will choose the other. When we think in terms of electrons it should be apparent that whatever number of them flow into a junction, the exact same number must also flow back out. The electrons can’t magically disappear into the junction, nor can additional ones suddenly appear from nowhere.

Fig. 1.16 Kirchhoff’s First Law states that the current flowing into a junction must be equal to the current flowing out of that junction.

If we wanted to state Kirchhoff’s First Law a bit more scientifically, we would say that the sum of the currents flowing into a junction is equal to the sum of the currents flowing out of the junction. Fig. 1.16 helps to illustrate this: on the left we have a single current flowing into the junction from the battery (which we’ve called IA) and on the right we have two currents flowing out to the headlight and tail lamp (IB and IC). If IA coming in to the junction is 8 amps and we know that IB is 6 amps, then IC must of course be equal to 2 amps. That gives us 8 amps going in and 8 amps coming back out.

Kirchhoff’s First Law states that what goes in must come out. Currents cannot be created or destroyed in a circuit, they can only split up to travel along parallel circuits and then join back together again. Therefore the current flowing into and out of a junction must always be exactly the same.

Fig. 1.17 We can apply Kirchhoff’s First Law to this circuit to determine what current will flow at each of the labelled points.

As an example of the application of Kirchhoff’s First Law, let’s consider the simple circuit illustrated in fig. 1.17. We have a single 12-volt battery connected via a switch to two bulbs wired in parallel, one a headlight bulb and the other a stop light bulb. The question is how much current will be flowing at each of the five locations, labelled A to E, around the circuit when the bulbs are lit. To work out the answer, we’ll assume that the resistances of the two bulbs are 5 and 35 ohms respectively.

Kirchhoff’s Second Law of voltages

Fig. 1.18 Kirchhoff’s Second Law tells us that the full EMF of the battery is consumed by the combination of loads around the circuit.

A simple circuit is shown in fig. 1.18a in which a 12-volt battery is powering a single light bulb. If we were to use a multimeter to measure the voltage difference between the left side of the bulb and the battery’s negative terminal we would get a reading of around 12 volts. If we were to measure the voltage difference between the right side of the bulb and the battery’s negative terminal we would get a reading of 0 volts. So the full 12 volts of electrical energy supplied by the battery has been consumed by the bulb. If we replaced the battery with a 6-volt version we would find that the bulb consumed the full 6 volts, and similarly if we substituted a 24-volt battery the bulb will consume the full 24 volts (at least until it blows).

It is therefore clear that the voltage drop across the bulb is a function of the circuit in which it is connected and clearly we would want this to be the same as the bulb’s rated voltage. Whenever a load is connected to a power source, the load will always consume the full EMF supplied to it. This may seem fairly straightforward for a simple circuit with a single load, but what would happen if we were to have two or more loads connected together as part of the same circuit? How would the EMF supplied by the battery be consumed by the various different components?

The circuit shown in fig. 1.18b is a little more complicated since it now has a second bulb connected in series with the first. Again if we were to make multimeter measurements of the voltage at the left and right sides of the bulbs we would get readings of 12 and 0 volts respectively. But what reading would be get if we were to take a voltage measurement at the wire between the two bulbs? Does the first bulb consume all 12 volts or does the second bulb do this, or do they somehow share the 12 volts between them?

In many cases, the two loads will not be identical so we require a method to determine how they will split the available EMF between them. Let’s consider the circuit shown in fig. 1.19 in which we have a small brake light bulb connected in series with a larger headlight bulb (this doesn’t relate to any real motorcycle circuit). The electrical loads imposed by the two bulbs on the circuit will obviously be different depending upon their individual resistances. To answer this question, we first need to know the resistance of each of the two bulbs. Let’s assume that the headlight bulb has a resistance of 5 ohms and the brake light bulb a resistance of 30 ohms.

The two loads are in series so we can determine the total load resistance by summing the individual resistances together (see the next section), giving us a total of 35 ohms (fig. 1.19a). Using Ohm’s Law we can then determine that the current flowing around the circuit is 0.34 amps (fig. 1.19b).

Fig. 1.19 We can use Ohm’s Law to calculate the voltage drop across each of the loads in a series circuit.

We can therefore take a shortcut in the calculation and avoid the need to determine the current first (fig. 1.19e). Instead we calculate the proportion of the total resistance attributable to a particular load, and then multiply this ratio by the available EMF in order to find the component’s voltage drop. This gives us exactly the same results as the original method, but is significantly quicker.

What we have seen here is an example of Kirchhoff’s Second Law in action. The law states that the sum of the voltage drops around a circuit must be equal to the sum of the EMFs supplied by the battery. In our examples so far we have only looked at a circuit with a single battery, but the rule applies just the same if multiple batteries are used (for example, if we have two 6-volt batteries powering a 12-volt circuit). The only difference is that we first need to sum all of the EMFs together to determine the total EMF available to the circuit.

When it comes to parallel circuits, Kirchhoff’s Second Law still applies but we need to apply it slightly differently. When two bulbs are connected in parallel to the battery, as in fig. 1.8b, each will have access to the full EMF of the battery. In such cases they do not share the EMF as would be the case had they been connected in series (as per fig. 1.8a). Kirchhoff’s Second Law still applies around each circuit, however a parallel load arrangement must be considered as two circuits in one.

Fig. 1.20 Kirchhoff’s Second Law tells us that the voltage drops around a circuit must be equal to the EMF of the battery.

Take for example the circuit shown in fig. 1.20