BYOM Part 9: Motorcycle Electrical System

The Motorcycle Electrical System:

Some people who are perfectly happy with the metal
working parts of the bike get nervous at the electrics,
though they are really fairly straightforward in theory.

If you don't know what you're doing, you may want some professional help. We'll look at the theory and the practice here.

The system is made up of the starting system, the charging system, the ignition system, a circuit breaker, and the electrically operated accessories.

The starting system is what works when the start switch is first flicked, and consists of a battery, start switch, solenoid and the starting motor. The charging system recharges the battery when the machine is running and is simply the alternator and a voltage regulator. The ignition system is obviously the ignition switch and ignition coil, the kill switch, the electronic control unit and the spark plugs themselves.

Finally all the other electrically operated accessories are connected into this system, such as the lights, horn, turn signals and various warning lights.

Aside from the components themselves, you'll need ton fasten them together with wire, which will involve
choosing the right wire, a bit of soldering and some

Wire is measured by Gauge 0, 2, 4 etc, with the larger numbers being the smallest diameter wire. The largest size of wire, 0000 Gauge, has a diameter of about 12mm, and the measurement of each gauge is arrived at by multiplying this diameter by 0.890522 to get the next gauge diameter!

It's thought that the Gauges came about in the eighteenth century as a way of describing the number of times a copper rod had to be drawn to make a given wire. So Gauge would have been the original rod of copper, 1 Gauge would have been drawn once, 2 Gauge twice and so on.

When choosing wire it helps to remember that the
resistance is inversely proportionally to the current, by physical law. Put more simply, at the same voltage (which is the situation on a bike, using a battery) we need to decrease resistance to increase current (amps). This is why we use fatter wire for higher amperage applications (there is less resistance to the free flow of electrons), and why when the distance of the wire run increases, we choose thicker diameter wire than we had chosen for short runs of the same current. Heavier wire, like 4 Gauge, would be used for heavy duty applications on a bike like the big cables to the battery, and smaller sizes, say 22 Gauge, is used for the finer applications, like indicator lights.

How the relationship works is shown in the table below similar ones are in almost every catalogue.

motorcycle wiring diagram

here are a couple of key quality indicators for wire.

First of all, the higher the numbers of individual strands of metal that makes up the wire, the better quality the wire, so always go for higher numbers of strands. Secondly there are two grades of suitable insulation, known as TXL and GPT. The TXL is a newer product with better heat and friction resistance.

The ignition system

The illustration below is a simplified ignition system wiring diagram. The thick line from battery to solenoid represents a heavy gauge wire (e.g. Gauge 4). From the battery to the 30A breaker, on to the ignition and terminal 30, then to the solenoid itself from terminal 87, we would use 12 Gauge wire.

ignition system

The breaker is nothing more than a fancy fuse - it
disconnects the current by breaking a pair of contacts and then resets, rather than blow out like a fuse would. Newer breakers are solid state so don't really break a pair of contacts in the way we think of, but they do exactly the same job. The starter relay is nothing more than a switch that allows the action of passing current to take place a little further away from the push buttons and a little nearer where it is needed.

The Battery

This usually sits in the oil tank below the seat, though
sometimes moves around the bike when space is tight.

Because space is an issue there are various sizes of battery.

If you've bought a pre-fabricated oil tank, the decision has been made for you and you simply need a battery to fit. The two figures a battery carries are cold cranking amps and amp hour rating. The first is how much power the battery generates when cold and being called upon to deliver 7.2 volts for 30 seconds ' bigger ratings mean more power and underpowered batteries may lead to starting problems with big engines. The second measure is an amount of time that the battery will provide a given power and voltage.

Because the battery provides the electricity, stores the electricity and stabilizes the voltage provided by the alternator, it is important to have a good battery and keep it maintained. This includes charging it with a smart charger that alters the charge it delivers as the battery becomes fully charged, and keeping the battery clean since dirt can conduct electricity between the poles of the battery and speed up discharge. Remember that when connecting the battery, the earth needs to be in contact with unpainted metal, since paint insulates to some extent.

The Starter

As I mentioned above, the starter needs to be matched with the primary. They are rated in kilowatts (kW), and with your engine and primary in mind, you will be able to get good advice.

The Alternator and the Regulator

The alternator sits within the primary and, like all
alternators, takes some of the vehicle's motion to generate electricity. This is done by moving magnets and coils of wires. The circular alternator we use in a bike has unmoving coils of wire (the stator) around which a bank of magnets whirl to make electricity that is then fed via a regulator. Once the battery is charged and all the electrics are being powered, spare electricity is just wasted so there's no benefit in generating lots of electricity. Depending on what kind of electrics you have, an alternator and matching regulator rated at 32amps should be plenty. Like so many other parts, these bits can be bought in a kit.

The Electrically Operated Accessories

To continue the kit theme, it's easy to buy the electrical wiring as a complete harness, color coded and with or without terminals and with or without switches, displays, bulbs etc. The diagram below uses some of the factory colors (yellow, white, orange, green red and tan) to illustrate how easy a basic circuit can be.

motorcycle circuit diagram

When you buy additional switches remember that you need something that is rated for the job (measured in amps in this case), just like the wire and all the other components.

Take care over all the connections you need to make soldering and shrink-wrapping two wires that need to be joined results in a solid, secure wire in a way snapping them together with quick connectors doesn't. There seems little point in saving time here, given how much time you put into the whole build. And remember, if the wiring is to be neatly hidden within the frame then the wiring needs to be figured out before the final build, rather than as a finishing touch. I suggest thinking about it from the start (where do I want the switches to end up) and I will mention it again at the end of the build sequence below.