64 Tiger Cub Motorcycle

FRS 106, Michael Littman – Spring 2012

Key Components


The Alternator is made up of the stator and the rotor.  The stator is six coil windings connected in series.  Each of the coils has a different number of windings.  The rotor is a six pole magnet that is attached to the engine sprocket.  As the rotor spins, a combination of the different number of coils of the stator produce different levels of voltage, which in turn power different parts of the bike.  The high voltage output, for example, could be used for powering the spark plug while the low voltage output could be used for the brake light.

On the left is the spinning magnet of our stator tester, which acts like a rotor. On the right is our stator, consisting of six different coils.

The Points

The points act as a physical switch to regulate the timing of the spark plug.  As the motor spins, the cam (the slightly oval piece in the middle of the picture below) spins with it.  The purpose of the cam is to open the points at the appropriate time.  It does this by pushing the hammer-like piece back each turn.  Adjusting the cam correctly is very important because the points must be precisely timed to spark the spark plug at the proper time and prevent pre-ignition.  At high speeds, small weights on the centrifugal advance mechanism also advance the timing of the points and makes the spark plug fire slightly earlier.  This compensates for the fuel having less time to burn as the engine reaches higher speeds.

The Coil

The coil consists of two coil windings.  It is a transformer that provides the step-up voltage obtained from the points needed to spark the plug.  When we received the bike, it used a battery ignition coil system.  Because a battery provides DC current, the old coil was a DC coil. Now, our bike uses an energy transfer system, so it uses an AC coil.  The coil operates on the principle of induction which is covered in the Physics Review section.

Spark Plug

The spark plug is what ultimately ignites the fuel in the engine.  The spark plug uses the high voltage current that is produced by the coil to spark across the “spark gap.”  It is important that the spark occurs at the right time to maximize efficiency.


The handlebar switch is the primary way of controlling the various electrical systems on the bike.  It is used to control all of the lights, with the exception of the brake light, and it is also used as the off button, or “kill switch” for the bike.  The kill switch grounds all power from the spark plug circuit which then stops any further ignitions.

We originally did not have a wiring diagram, so we tested each connection manually. After some searching, we found this diagram that tells us what each wire is connected to. On our bike, the horn button serves as the kill switch.  

Wiring Harness

All of these key components come together in the wiring harness, which is the network of wires that supplies power to each part of the bike. The next page of this website gives descriptions of the wiring harness and its construction.

Circuit Simulator

Here we have a software simulation of the electrical circuit that powers the bike. Many key components such as the alternator, points, coil, and spark plug appear in the circuit. A detailed description of the physics of this circuit appears in the physics/theory page of this website.

There are several parts that are necessary for the electrical system to function.  They are, briefly, the alternator which provides power from the motor, the points that regulate the timing of the spark plugs, the coil to increase the voltage from the alternator to the spark plug, and the wiring harness to distribute the power.

The basic flow of power in the circuit as it related to running the motor is as follows:
    1 – The alternator generates power through electromagnetic induction
    2 – When the points are closed, this electricity flows straight to ground.
    3 – When the points are open – they are precisely timed to open just as the engine piston is nearing the top of the cylinder – the electricity flows to a capacitor that is in parallel with both the points and the coil.  If there were no capacitor, then the electricity would arc across the points.
   4 – From the capacitor, the electricity flows through the primary coil and generates an increased voltage that is used to create a spark in the spark plug, thus igniting the fuel in the engine.
We built this part of the circuit in a circuit simulator to see how it works.  The opening and closing of the points is simulated by the middle switch.

Link to circuit simulation

may not work without java plugins.