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As defined by Webster's, a flywheel is a mechanical device used to store rotational inertia. All engines have some sort of flywheel device, which is crucial to smooth operation and easy starting.
What It Is
Flywheels are large, metal discs that are bolted to the back of an engine's crankshaft, between the engine and transmission. Manual-transmission cars use them as a friction surface for the clutch.
Principle
Objects in motion tend to stay in motion. After a flywheel has been accelerated by the engine, it will resist any attempt to reduce or increase speed, thus stabilizing RPM.
Flywheel Size
Because they have a significant mechanical advantage, larger flywheels will act as though they are heavier than they really are. This can be helpful for large-displacement, low-cylinder-count engines that operate at low RPM
Weight
Increasing a flywheel's weight is the easiest way to enhance its damping ability. This is usually done by making it thicker, but changing to a heavier material like chrome-moly is also common.
Heat Sinking
Thicker flywheels are used in heavy applications for reasons other than damping. Starting a heavily loaded vehicle from a stop creates a great amount of heat in the clutch, which can be more easily absorbed and dissipated by a larger flywheel.
Source:
Resource:
Flywheel Resurfacing (Video)
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