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What is a car flywheel and what does it do?

Ellie Dyer-Brown, 18 days ago

4 min read

  • Advice
  • Repair

Find out everything you need to know about your car’s flywheel, including what it does and why it sometimes needs replacing.

What is a dual mass flywheel?

Flywheels can be dual mass (DMF) or single mass (SMF).

As you might expect, dual mass flywheels consist of two flywheels. One is attached to the clutch, the other to the crankshaft.

What does the flywheel do in your car?

The purpose of single mass flywheels is to provide a direct contact between the engine and clutch assembly, storing rotational energy to prevent the car from stalling. 

Single mass flywheels:

  • Have greater warp and thermal resistance

  • Are cheaper to buy than their dual counterparts

Unfortunately, they result in more vibrations and noise, which can damage the transmission and engine.


Dual mass flywheels, on the other hand, are designed to specifically reduce noise and vibration. To put it simply, their purpose is to make the process of starting the engine, idling and changing gear as smooth as possible.

The two discs have a series of springs between them which act like a ‘mechanical sponge’, reducing the damage caused by engine vibrations. This also protects the gearbox and allows for more efficient use of engine power.

There some other functions of the flywheel, for example, it:

  • Reduces drivetrain stress

  • Makes engine speed more smooth

  • Balances the engine

  • Helps the engine start

What is the difference between a single mass flywheel and a dual mass flywheel?

A single mass flywheel is a mechanical device made out of a single piece of metal, so it has no moving pieces. 

Here are some key differences:

  • Dual mass flywheels are more expensive

  • Single mass flywheels can be resurfaced, whereas DMFs cannot

  • Dual mass flywheels are more prone to heat-related warping

  • DMFs weigh considerably more than SMFs

  • DMFs are better at evening out vibrations

  • DMFs provide better protection to the transmission

How does a dual mass flywheel work?

Dual mass flywheels work by: 

  • Storing energy from a piston firing pulse 

  • Transmitting this energy to the crankshaft

  • Allowing the crankshaft to rotate smoothly until the next piston pulse

Does my car have a dual mass flywheel?

DMFs are usually used in larger vehicles, particularly those with diesel engines. They are also sometimes used in high torque petrol engines.

Almost all four cylinder diesel engines will have a dual mass flywheel.

According to WhoCanFixMyCar data from 2021, the five vehicle models most likely to suffer from dual mass flywheel problems are:

  • BMW 3 Series

  • Vauxhall Insignia

  • Volkswagen Golf

  • Audi A3

  • Ford Mondeo

Do automatic cars have flywheels?

Automatic cars are not fitted with dual mass flywheels because most of them have torque converters instead. 

Torque converters perform the same role but in a slightly different way, using fluid coupling. This transfers the rotating power of the engine to the transmission, allowing the engine to spin somewhat independently.

Can a bad flywheel cause starting problems?

A car’s ignition system is directly affected by the flywheel, so it’s certainly possible a bad flywheel could cause starting problems; in fact, it’s likely to.

When the flywheel becomes worn or cracked, the starter will struggle to engage it properly, particularly if it has lost a significant number of teeth.

This is because, in the starter motor, a small gear (known as the Bendix gear) connects with the flywheel when you turn the key. The Bendix gear spins the flywheel; once the engine starts, it withdraws, allowing the flywheel to spin freely.

If the flywheel is missing numerous teeth, the gear will not be able to connect with it.

How do I know if my dual mass flywheel needs replacing?

There are some common symptoms you should look out for.


Top reasons a car engine flywheel needs to be replaced

  • A hard clutch pedal

  • Difficult gear shifting

  • Scratching noises from engine, especially at lower speeds

  • Jerking or shaking

  • Clutch slipping (without a burning smell)

Carry out a visual inspection

If you feel comfortable doing so, you could:

  • Check the springs for grease; there should be plenty, but this can become depleted due to overheating

  • Check the friction surface for signs of excessive heat such as scoring

  • Check for visual damage, particularly to parts inside the bellhousing

If you’re not sure whether your flywheel needs replacing, the best option is to have it looked at by a professional mechanic.

What does a damaged flywheel sound like?

If you have a dual mass flywheel, you will likely notice some unusual banging sounds when the vehicle is idling. You may also hear scratching noises, particularly at lower speeds.

How long will a noisy flywheel last?

If your flywheel is making an unusual noise, this indicates it has become severely worn and should be replaced as soon as possible. 

Flywheels usually last around 80,000 to 100,000 miles, if not more, but as soon as you notice symptoms of an underlying problem, you should have your car checked by a mechanic.

How long it will last depends on the extent of the damage, but this is likely to be a fairly short time frame as the issue will be exacerbated by further use.

Driving with a faulty flywheel puts both your safety and the safety of other road users in jeopardy.

What is the difference between a clutch and a flywheel?

The clutch and flywheel share similar responsibilities - they are the primary components used for transmitting power from the engine to the transmission - but they are also fundamentally different.


The clutch consists of an arm (also known as an actuator), a clutch disc and a release bearing. Conversely, the flywheel is a large metal disc that forms the interface between the transmission and the clutch mechanism. 

Clutches are designed to:

  • Disconnect the engine from the gearbox

  • Allow you to change gears

  • Prevent the engine from stalling

Do I need to replace my flywheel with my clutch?

The clutch and flywheel are closely related, so when one needs to be replaced, it’s often advisable to replace the other as well. 

When the flywheel has to be replaced, this involves removing the transmission, which is a labour intensive (and therefore expensive) job. That’s why it’s often worthwhile having the clutch replaced at the same time since you’re already paying for the labour that would ordinarily make a clutch replacement so costly.

Likewise, when your clutch needs replacing, if you have a single mass flywheel it’s worth having this component resurfaced at the same time. 

Problems with one of these components can often lead to problems with the other, so before you decide whether to have one or both replaced, check them thoroughly for any signs of wear.

How much does a dual mass flywheel and clutch replacement cost?

According to WhoCanFixMyCar data from 2021, the average cost for a clutch and dual mass flywheel replacement is £954.63.

MakeAverage cost

What are the benefits of a new dual mass flywheel and clutch?

If your DMF or clutch is on the way out, there isn’t really a lot you can do about it other than arrange for one or both to be replaced. 

Driving with a faulty clutch and dual mass flywheel is dangerous and will eventually lead to your car not working at all, so ignoring the problem isn’t an option.

However, there are at least a few benefits you can look forward to if you do go ahead with a replacement:

  • Improved vehicle performance

  • Smoother drive

  • Better value replacing both parts together

  • Your vehicle will last longer


Having trouble with your dual mass flywheel? WhoCanFixMyCar can help you find the right garage at the right price.

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