There are two basic types of flywheel, mechanical and electronic. Mechanical flywheels have been used in internal combustion engines and machinery for many years and are both the simplest and most common form of flywheel in use. The electronic flywheel is not actually a true flywheel but it produces a comparable effect on electric motors, the electronic flywheel is often used when limitations of weight or available space make use of a true mechanical flywheel impractical, such devices are sometimes referred to as inertia simulator circuits.

The flywheel is a mass, usually a metal weight, fitted to a rotating power transmission shaft or directly to the motor spindle. When power is fed to the motor it will not react as quickly as the same motor without a flywheel fitted, providing slower acceleration up to the voltage/speed required. This is due to the motor having to work a little harder to start the extra weight of the flywheel moving and get it spinning. Once the flywheel is spinning at the desired speed it has an amount of stored kinetic energy in proportion to the speed and weight of the flywheel. The kinetic energy stored in the flywheel during running will resist `stuttering' rapid changes in speed normaly caused by small spots of dirt on the track or wheels used for traction current collection. When the voltage to the motor is reduced (or removed completely) the stored kinetic energy in the flywheel will try to keep the motor spinning producing a smoother more gradual braking curve. (most small DC motors have very low mass/weight and will stop very rapidly when the power feed is removed) The flywheel gives the model more momentum resulting in more prototypical behaviour and a different `driving experience'. It does not take long to learn the acceleration and braking characteristics of a model with flywheel(s) fitted and once mastered they are much easier to drive in a prototypical manner than models without flywheel(s). I intend to fit flywheels to all of my chassis designs although some of the prototypes will not incorporate a flywheel in initial developments they will be built with provision for fitting flywheel(s) as required.

The electronic flywheel is simply a low capacity temporary power supply which stores a small amount of electrical charge in one or more capacitors whilst the locomotive is being fed power from the track. In the event of a failure in the traction power feed due to a break in continuity, IE: a dirty spot on the rails or locomotive wheels causing the traction current to stop, The electronic flywheel circuit will provide traction current for a short period of time. The length of time is governed by the capacitance of the `flywheel' circuit, thus the time can be extended by increasing the rating and number of capacitors used. (only really limited by available space in the model) When the traction feed current returns the locomotive will continue to be powered from the track and the `flywheel' circuit will recharge.
This type of circuit is said to give `Simulated Inertia' If the traction current feed is stopped the flywheel circuit keeps the train moving for a short time, it will decelerate gradually as though the brakes have been applied instead of stopping abruptly. The acceleration and braking characteristics of the locomotive can be fine tuned to suit the particular model by changing capacitor and resistance values in the circuit governing speed and deceleration characteristics. The only limitation being the amount of space available for the circuit and capacitors inside the model. Most electrically powered `Diesel, Electric, DMU, EMU' models will have space available for such a circuit within the body, Steam locomotives often only have enough space in the cab or smokebox but steam locomotive classes with a tender can have the circuit hidden in the tender affording plenty of space in 3mm scale or larger models. Smaller models 2mm / Ngauge will require the use of very small components (Surface Mount) and can only realistically be fitted into the larger types of loco. Small locos can run with a brake van or support coach semi-permanently attached with the `flywheel' circuit in this vehicle, although this arrangement requires connections between loco and `flywheel' vehicle it is possible and would be well suited to branch line Push-Pull operations with the coach being semi-permanently connected to the loco as per the prototype.