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Recently in chat, a discussion arose about a dual mass flywheel. I am blissfully ignorant regarding how a dual mass flywheel actually functions and what the delta is between a dual mass flywheel and a standard flywheel.

I understand the basics of a flywheel and why you want one on your vehicle but diving any deeper than that is not within my knowledge base.

Can someone please illiterate the delta between the two flywheel types?

Are there other types of flywheels that deserve an honorable mention?

Are there benefits to having a dual mass flywheel?

Please post a diagram or other visual learning aid to assist in my enlightenment.

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  • Another "type" you might consider is a lightweight flywheel. Here is an excellent question about those.
    – JPhi1618
    Commented Sep 28, 2016 at 18:51

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A dual mass flywheel (or DMF) is a flywheel that is split into two halves (hence the name...), with a spring or springs between them to dampen out sudden changes in torque and rotation speed, and thus smoothen out vibrations in the drivetrain - making for a smoother drive, particularly in diesel engined vehicles, and the newer two and three cylinder engines that aren't as smooth as a properly balanced larger engine.

As usual, Wikipedia provides a nice diagram (red: crankshaft side, blue:transmission side):

enter image description here

The key disadvantage of them is that they wear out, and are very expensive to change - often meaning they get replaced with conventional solid flywheels by owners of older vehicles who are less concerned about refinement.

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The dual mass flywheel is typically misunderstood by a number of people. It's function is not to provide a smooth ride but is actually designed to protect the gearbox from destruction.

My car, a run of the mill Volkswagen 1.9 TDI from twelve years ago, develops a staggering 250ftlb of torque. If this was fitted with a solid flywheel, this is more than enough torque to, if I'm harsh with the throttle and clutch, destroy the gearbox within 20,000 miles.

Because the dual mass flywheel is relatively expensive when compared with solid flywheels that will fit the car, for example from the early VR6 engines on the previous models, I am aware of a number of people within the owners clubs who have opted to go this route when replacing their dual mass flywheel. Typically following this a trashed gearbox follows some months later.

So, the dual mass flywheel is effectively a consumable / sacrificial part designed to prevent the considerable expense of a gearbox rebuild.

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    It's worth noting that at least VW mounts them nowadays not only in diesel engines, but pretty much all high-torque gasoline engines as well.
    – Bartek Banachewicz
    Commented Sep 28, 2016 at 11:01
  • "High torque"? or typical VW gear box? In my experience, all VW parts turn into consumable/sacrificial parts after that 60k mile warranty runs out.
    – justinm410
    Commented Sep 28, 2016 at 15:21
  • High Torque, mines on 135k miles and doesn't miss a beat. My neighbors' Passat bit the dust at 267k miles when a seal in the turbo blew.
    – Steve Matthews
    Commented Sep 28, 2016 at 15:28
  • Can I ask where you got this info? All the mechanics I go to recommend using the single mass and say if you break the gearbox it's likely 'driver error'. ( disclaimer I recently switched to a single mass and may very well may have a broken gearbox XD)
    – Jesse Reza Khorasanee
    Commented Sep 30, 2020 at 21:50
  • There are a few articles online that do warn of gearbox damage should you switch to a solid flywheel. To be frank, manufactures fit them for a good reason. Given that most modern cars are designed by accounts, they wouldn’t go to the additional expense if they weren’t needed. Many mechanics will suggest going to a solid flywheel as it makes them able to charge less for the job whilst pocketing a higher profit (solid flywheels are about 20% of the price of a DMF)
    – Steve Matthews
    Commented Sep 30, 2020 at 22:13
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Adding to what Nick and Steve have said. Consider the millisecond-by-millisecond operation, and that the rated torque is an average over the engine cycle.

In fact the engine is producing large spikes of torque during each cylinder's combustion event. Rather than transferring this into the gearbox the DMF averages the torque output, reducing the peak and spreading it over a larger range of crank position.

This is of course, good news for drivetrain components.

This chart nicely illustrates my point:

enter image description here

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DMF is basically torsional vibration isolator. It does not do anything with damping. It is not damping element. DMF isolates vibrations coming from engine and preventing to reach to gearbox. You may want to look this thesis

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