Flywheels are heavy rotating disks that have a high mass and are connected to a rotating shaft. Because they store angular momentum, flywheels offer a smooth delivery of torque, or turning power, from a motor or engine. The inertia of flywheels allows them to absorb and release energy with little change in speed. Flywheels have been used for many years in all kinds of machinery to ensure steady shaft rotation when an uneven torque is applied, such as in a reciprocating or piston-driven engine. Flywheels can also be used to store up kinetic energy that is released to provide short durations of additional power output in small motors. A flywheel is a common drive component in a vehicle's manual transmission.

A standard shift flywheel is mounted on the engine shaft and disengages briefly from a pressure plate mounted on the transmission shaft when the driver puts a foot on the clutch to shift gears. When the driver removes a foot from the clutch, a friction plate is sandwiched between the flywheel and the pressure plate, allowing the engine shaft to turn the transmission. Cars and trucks with manual or standard transmissions use flywheels of different weights depending on the size of the vehicle and the engine. A truck flywheel is heavier to store more energy and provide more acceleration.