If you’ve ever studied physics and heard of a guy named Newton, you’ve probably come across the concept of inertia before.


noun 1. 
a tendency to do nothing or to remain unchanged 2. a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.   Simply put, Inertia is the resistance of any physical object to any change in its state of motion, including changes to its speed and direction. It is the tendency of objects to keep moving in a straight line at constant velocity. Where a rotating mass like a flywheel is concerned, the constant velocity is an angular velocity or the rotational rpm of the mass/flywheel.

Flywheels and inertia:

In the case of an automotive flywheel, inertia is often referred to as a way to describe how much energy (rotational kinetic energy) or power the flywheel can deliver to the driveline to move the vehicle. The engine delivers it’s power to the flywheel and starts it spinning, converting the energy of combustion to rotational energy. When the clutch is engaged, the flywheel transfers that rotational energy to the driveline to drive the wheels and move the vehicle forward. In the most basic form, the amount of energy/power (KE) a flywheel can deliver to drive a vehicle is based on it’s inertial mass (I), multiplied by it’s rpm/angular velocity (w2).


The formula looks like this:




  • ω is the angular velocity (RPM), and
  • I is the moment of inertia of the mass about the axis. The moment of inertia is the measure of resistance to torque applied on a spinning object (i.e. the higher the moment of inertia, the less it will spin when a given force is applied).


The moment of inertia for a solid cylinder is,




The BIG question:

Often, the big question where lightened flywheels are concerned is can they deliver enough energy/power to move the vehicle forward like a heavier cast iron or steel flywheel can? This is especially important when “launching” the vehicle, either from a stop sign or from the line at a drag strip. Looking at the information and formula above, you can easily see it is simply a question of math. When working with a lighter mass, in order to keep the energy/power the same, you simply need to increase the rpm (velocity) of the flywheel slightly.

What else does a flywheel do?

A flywheel is a rotating mechanical device usually in the form of a disc or cylinder that is used to store rotational/kinetic energy and also smooth out delivery of power from a motor to a machine. The amount of rotational/kinetic energy stored in a flywheel is often referred to as it’s inertial energy or inertia and is most simply described as a calculation of mass (weight) x rpm (velocity). The inertia of the flywheel opposes and moderates fluctuations in the speed of the engine and stores the excess energy for intermittent use. In automobile engines, the flywheel smooths out the pulses of energy provided by the combustion in the cylinders and provides energy for the compression stroke of the pistons. Energy is transferred to a flywheel by applying torque to it from an engine, thereby increasing its rotational speed, and hence its stored energy. Conversely, a flywheel releases stored energy by applying its torque to a mechanical load (transmission, driveline or axles), thereby decreasing its rotational speed. Other common uses of a flywheel include:
  • Providing continuous energy when the energy source is discontinuous. For example, flywheels are used in reciprocating engines because the energy source, torque from the engine, is intermittent.
  • Controlling the orientation of a mechanical system. In such applications, the angular momentum of a flywheel is purposely transferred to a load when energy is transferred to or from the flywheel.
Flywheels are typically made of steel or aluminum and rotate on conventional bearings; these are generally limited to a revolution rate of a few thousand RPM.