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Every car you see on the road has some sort of flywheel, and many of them have a dual-mass flywheel or DMF. It’s a crucial component, adding weight and momentum to the engine’s crankshaft to keep it spinning and smoothing out vibrations for quieter, more refined cars.

Different weights and sizes of flywheel are used depending on the type of engine and the car it’s installed in; as powerful, torquey turbo and diesel engines have become more popular in smaller cars, the dual-mass flywheel has stopped them being snappy, jerky or prone to breaking gearboxes prematurely. It’s also used to make cars more refined overall by damping small vibrations and encouraging the use of higher gears at slow speeds, for better economy.

If you think about how your car’s engine and gearbox are connected, the flywheel is connected to the clutch – and in the case of the dual-mass flywheel, there is almost always a clutch attached, as its purpose is to smooth out the torque and power of modern engines when putting drive to the road.

As a result, you will generally be aware of the DMF in smaller diesel manual cars where the vibration of the engine and amount of force it can generate when pulling away is enough to jolt and shake the car even when driven normally, and low speed refinement used to be poor. The DMF made these diesels more civilised and appealing from the mid-1990s on.

In technical terms, it’s a torsional damper – it performs the same task as the shock absorbers in your car, but on a spinning motion rather than a vertical motion. As the name implies, there are two masses – two flywheels – sandwiched together with a set of springs or rubber that allow an averaging of the rotation and torque coming from the engine (which is usually a set of pulses at high speed) and the gearbox’s rotation, which is usually smooth at a given road speed.

2The effect of dual-mass flywheens on vibration and refinement

The upshot of this is a car that is quieter and smoother to drive. It’s so effective that you’ll find dual-mass flywheels in cars with semi-automatic gearboxes or CVTs, and in those places it will often last a long time.

In a manual car, however, the driver can rev the engine hard and release the clutch abruptly. The dual-mass flywheel will attempt to smooth this difference, where a solid flywheel will usually cause the tyres or the clutch to slip. Over time, the DMF wears out from constantly absorbing that shock – but it is working all the time, even when not abused, to reduce vibration and noise.

Symptoms can include rumbling or squawk noises when chainging gear, shuddering when pulling away and in extreme cases, severe vibration and rattles when the car is in neutral.

On a test drive, if you’re particularly aggressive with the accellerator and clutch you may cause the DMF to ‘yelp’; if it does that with normal inputs then it’s starting to fail.

Some signs of dual-mass flywheel failure are shared with engine running problems, clutch, or gearbox failure but as investigation needs that whole area of the car stripping down, it’s more a case of being prepared for it being needed.

This handy guide from Sachs has some good tips on diagnosing and understanding dual-mass flywheel problems.

On many cars you can replace the dual-mass flywheel with a conventional one, but you may need to change the clutch, starter motor and other components too. Replacement of the flywheel can cost around £800 including labour, but they typically last as long as a clutch and the work involved to replace both components together is almost the same as replacing either one. For example, a common fitment – the Ford Focus 1.6TDCi which uses a joint-venture engine shared with many different marques – costs around £179-279 for the part.

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