A dual-mass flywheel is essentially two flywheels in one that provides some spring cushioning to dampen torsional crankshaft vibrations and shock loading when the clutch is engaged.

The two-piece assembly has a front primary flywheel and a rear secondary flywheel. The primary flywheel bolts to the crankshaft just like an ordinary solid flywheel, and has the ring gear for the starter. The clutch is bolted to the secondary flywheel, which is separate from the primary flywheel, but attached to it with a series of coil springs mounted sideways between the two flywheels. The springs allow just enough movement between the flywheels to dampen the power strokes from a diesel engine so the vibrations are not felt throughout the drivetrain when the clutch is engaged. The cushioning effect also reduces shock loading on the transmission for longer gear life and less noise.

On gasoline-powered passenger car applications, a dual-mass flywheel can provide the same kind of drivetrain cushioning and dampening for a smoother, quieter ride. A dual-mass flywheel also helps smooth clutch engagement in vehicles with high torque output engines. It’s sort of like relocating the springs from the hub in the flywheel and clutch disk further out so they can handle greater loads. Splitting the mass of the flywheel also reduces the rotating mass of the clutch assembly when shifts are made for smoother gear changes and reduced drivetrain noise.

Some dual-mass flywheels use a slightly different design to achieve essentially the same end. Instead of relying solely on springs between the two flywheels to dampen vibrations and shock loading, a planetary gear set is used to control the motions between the primary and secondary flywheels.

In spite of the functional advantages that dual-mass flywheels have over conventional one-piece solid flywheels, some dual-mass flywheels have proved to be troublesome and prone to premature failure. The dual-mass flywheels flexplate in early Ford 6.9L F-series diesel trucks have experienced a high rate of failure because of a weak spring design in the OEM flywheel.