Classic Motorcycle Electrics Manual

James Smith

• Publisher: Crowood
• Year: 2015
• ISBN:
  9781847979964
• Topic: Technology & Engineering
• Subtopic: Bicycles & Motorcycles
• Index: Technology & Engineering

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.

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 fr...