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We've been asked by many site visitors for some clarification on this topic.

Dual-mass flywheels are fitted to some modern manual-transmission 4WDs. What was wrong with the tried and proved solid flywheel and sprung clutch plate? 

dmf Let’s look at the principal purpose of a flywheel. In any reciprocating engine the power strokes come in pulses as each cylinder fires in turn.

A heavy flywheel is used to store energy from each piston’s firing pulse and keep the crankshaft rotating smoothly until the next piston stroke.

However, as engine outputs and peak torque levels increased in the late 1980s, in pursuit of better fuel economy and lower emissions, the levels of vibration also increased.

Vibrations are caused by the fact that an engine doesn’t generate constant torque; it comes in increasing and decreasing amounts from top dead centre to bottom dead centre, at which points there’s no torque generated at all.

Also, a crankshaft may look rigid, but as with all structures there’s innate elasticity. As an engine fires, combustion pressure microscopically distorts the crankshaft torsionally and it springs back again as the pressure decreases.

dmf The torsional ‘spikes’ caused by engine vibration can cause manual-transmisson ‘rattle’ and even damage. In the case of an automatic transmission these
vibrations are damped out by the fluid coupling in the torque converter.

Manual-transmission driveline vibrations can also be transmitted to the chassis and body, causing ‘drumming’.

Historically, the springs in the clutch disc absorbed most of these vibrations, but clutch disc technology was found wanting as engine performance – especially from turbo-diesels – increased dramatically from the late 1980s.

Higher demands on noise, vibration and harshenss (NVH), as well as greater measures to protect transmissions dictated more efficient torsional vibration dampers. There was also a need to reduce gearbox synchroniser wear, caused in part by the heavy clutch plate masses needed to handle greatly increased input torques. Enter the dual mass flywheel (DMF).

There are several different DMF designs, but in essence all have two main discs, connected by captive springs between them. The two halves rotate together at the same average speed, but with ‘back and forth’ relative movement between them. The springs act in the same way as the springs in a clutch disc centre, but have much greater oscillation capacity, for more effective vibration damping. The clutch disc in a DMF is solid and therefore much lighter than a sprung-centre clutch disc.

In addition to the vibration-reducing characteristics of a DMF are lower fuel consumption, due to lower rpm levels when idling, easier engine operation at lower rpm and, where fitted, smooth ‘stop/start’ performance; lower shifting forces, due to reduced inertia of the clutch disc; iimproved driving comfort, thanks to strong vibrational damping of sudden load changes; and quieter operation, thanks to decoupling of vibrations throughout the entire rpm range.


The not-so-good side of a DMF

A DMF is a factory-built, composite part that cannot be reconditioned. When the springs fail or the friction surface wears that’s it. Unlike a solid flywheel a DMF’s worn friction face cannot be machined smooth again. Also, unlike a solid flywheel, a DMF is an expensive component.

A 4WD with a ‘chipped’ engine that’s also used for heavy towing may wear out its DMF long before a less stressed powertrain does.

When faced with a DMF replacement some 4WD owners opt for a solid flywheel kit instead. After-fitment experience is mixed, with many people loving the more positive take-up of a conventional arrangement and prepared to put up with some transmission vibration. At the other extreme, some people have experienced transmission damage.