The flywheel sits bolted to the crankshaft flange and serves two critical functions: storing rotational energy to smooth out power pulses between combustion events, and providing the friction surface for the clutch (or the ring gear for the starter motor in automatics with flexplates). Its design directly affects engine smoothness, shift quality, and throttle response.

Solid (Single-Mass) Flywheels: A single chunk of cast iron or steel. Simple, durable, and cheap to resurface. The mass is chosen to match the engine's firing order and displacement. A heavier flywheel smooths idle and low-RPM driving but resists revving quickly. Lightweight aftermarket flywheels (often chromoly steel or aluminum with a replaceable friction insert) drop rotational inertia by 30–50%, improving throttle response at the cost of rougher idle and easier stalling.

Dual-Mass Flywheels (DMF): Introduced widely in the late 1980s (LuK was the pioneer), DMFs split the flywheel into two halves connected by arc springs. The primary mass bolts to the crank; the secondary mass faces the clutch. The spring set between them absorbs torsional vibration before it reaches the gearbox. This is why modern turbodiesels almost universally use DMFs — their high torque and wide combustion pressure spikes would destroy synchros and cause gear rattle without the damping.

Real-world example: The BMW N47 diesel is notorious for DMF failure. Symptoms include a rattling noise at idle that disappears when the clutch is depressed (because decoupling the gearbox removes the load on the secondary mass). Replacement runs $1,500–$2,500 because the clutch is always replaced simultaneously.

Rule of thumb for rotational inertia: Moment of inertia scales with mass × radius². A flywheel with half the mass but the same diameter retains 50% of the original inertia. But if you reduce the outer diameter by 20% at the same mass, inertia drops by about 36% (0.8² = 0.64). This is why lightweight flywheels concentrate remaining material at a smaller radius — maximizing the inertia reduction per pound removed.

Flexplates (automatic transmission equivalent) are thin stamped steel with no significant energy storage — the torque converter's fluid coupling handles the smoothing instead. They carry the ring gear and bolt to the torque converter.

Failure modes: Solid flywheels crack from heat (clutch slipping) or lose surface flatness. DMFs fail when the arc springs fracture or the internal grease leaks, leading to free play between the masses. Once a DMF shows more than about 1–2° of freeplay beyond spec, it's done.