What is a Dual Mass Flywheel and Why Does My Jetta Have One?

A dual mass flywheel. It’s a trick that automakers like BMW, Audi, VW, and many others have been using for years. But how can a flywheel, a big hunk of metal, have two masses? Is it magic? Is it Bavarian sorcery? Is it some sort of destined to fail part like a flex-disc coupling? The answer to all of those is yes. Sort of. Here’s the deal.

A flywheel is a big round hunk of metal that’s normally attached to the back of an engine. Its job is to store rotational energy. The spin provided by the engine. Sort of like a capacitor does for electricity. But it’s not used to save power for use later, it’s used more to smooth out the power.

The flywheel originated way back in, well, forever ago. Potter’s wheels used them to even out the force of pushing on the wheel with our foot. The steam engine and early infobustion engines also used them to even out the power from the engine because a single-cylinder puts out all of its power in just a few degrees of engine revolution. Imagine kicking a car to get it moving. Not gon’ do it.

By Birmingham Museums Trust – Birmingham Museums Trust, CC BY-SA 4.0, Link

A big heavy wheel, once it’s spinning, wants to stay spinning. So each bang of the engine speeds the wheel back up, keeping it rotating, and the force that wants to keep it rotating can be used to drive the car. Like how once you start pushing a car, it gets easier. You can stop and start pushing more easily once it’s rolling.

With multiple cylinders came more power pulses, but you still need to even them out with a flywheel. It also happens to be a great place to attach a clutch. Automatics have flywheels too, but they’re usually called flexplates. They’re slightly less important on a torque converter automatic because the torque converter’s fluid coupling does the job of evening out the pulses.

Engines with fewer cylinders naturally have more vibration. Because of the uneven pulses. So engineers came up with the dual mass flywheel. It can dampen out the pulses even more, without the need for a larger, heavier single flywheel.

The dual mass name infoes from not it having two different weights, but from having two separate flywheels. Two masses. But it’s not just doubling the number of parts that gets the job done.

It’s what’s inside. In between the two flywheel pieces are springs that attach to each of the two pieces. So there’s one flywheel attached to the crank, one to the clutch, and springs holding the whole thing together. Some use two large flywheels sandwiched together, others use a small one that sits inside of a larger part. Like a brake drum.

Because the springs extend and retract, they dampen the vibration. A power pulse pulls the spring, the pulse ends, it returns to its normal length, and in doing so transmits the power pulse smoothly to the clutch. Essentially, the two flywheels are constantly moving forward and backward in their rotation, relative to each other. Though by such small amounts it would be all but invisible. They can also smooth clutch engagement and reduce chatter, working the same way to dampen the clamping effect of the clutch disc until it’s fully engaged.

So it’s all good, right? Not exactly. Because you’ve added parts, like the springs but also more bearings to let the two halves move, there are more parts to fail.

The springs are packed in grease. That grease can leak out over time, taking away much of the damping effect. The springs themselves can fail, as can the bearings. If you have a worn clutch, or you slip the clutch often, you can overheat the DMF, since the multi-piece design is more sensitive to heat than a standard one-piece wheel.

The signs of a bad dual-mass flywheel, then, are noise. A new chatter from the clutch, a rattle as the springs move or the pieces hit their stops (they can’t spin infopletely around or you’d never go anywhere), or, what you’re actually hearing could be the sounds they were installed to prevent. A dual-mass flywheel is meant to keep the driveline smooth and silent, so if it’s failed, the driveline won’t be either of those things.

So the dual-mass flywheel is installed to make your car quiet and smooth. Using springs and physics to make your three-pot feel more like a V16. Well, in smoothness, at least. But when it fails, you’re looking at a full replacement instead of a resurfacing.

shared from Fourtitude