Notes on chatter from light weight flywheels

[TiGraySpeed6]

Considering a lightweight flywheel?
Wondering about "Chatter" or the rattling noise that's often reported, noticeable at idle, in neutral & with the clutch out?

Frankly, when you start modifying your car, you're going to have compromises. Some may be completely acceptable, others may not. Things like Motor Mounts, or Single Mass Lightweight Flywheels, can and will introduce new sounds and vibrations that are good happy things to some people, but totally unacceptable to others. Perhaps with some understanding of why a lightweight single mass flywheel "chatters" you can make an informed decision before purchasing, and not freak out after installing and think your new toy is borked, or your install went horribly sideways.


So, what's the deal with LW FW chatter? Simply put, you've got a lot of stuff rattling around in the trans, commonly called backlash, as the gears bounce off of each other by pulses from the idling engine that are no longer being damped by the DM FW.


A piston engine's power delivery isn't smooth & seamless - Far from it actually and very much not smooth at idle which is what we're concerned about here. Think about the Balance Shaft assembly for a moment, and why it exists. Piston engines are actually pulses of power. Every fire of a spark plug is it's own unique power creation event, or pulse of power. Everything down the line from that explosion, from piston to the tire, has a bit of give, of slack, to it. You can't have everything tight or there would be too much friction and drive train losses would be, at the least, excessive.

To our point on the LW FW chattering noise, when that pulse hits the tranny it's transmitted right on through to the gears. It's important to remember that each plug fire is a unique event even tho they happen remarkably fast. In action, this means the gears are actually kind of tapping against each other, not a smooth continuous contact such as when your moving, under load & higher RPM's. Tap your fingers on the table in sequence- pinky, ring, middle, index, over & over again. That's what your engine is doing, albeit really really quickly, and that pulse is transmitted right through to the transmission. At idle these pulses can still be distinct.

Twenty-five-ish years ago (1985ish I think?) dual mass flywheels became a popular way to soften this effect. Simply put, a DM FW is just two flywheels in one, coupled together to allow some slight variation between the engine and trans. This is specifically to dampen that pulse, and especially beneficial on 4 cylinder engines that don't have the balancing available to V style engines such as 6 & 8 cylinder engines. The DM FW smooths that pulse & prevents the chatter, and does a damn fine job of it too.

They also bring weight & inertia to the party which makes them very popular as they tend to be easier to live with in the average, work day stop & go traffic the the average consumer deals with. Simply put, they're easy to drive.

An aluminum lightweight flywheel such as the Fidanza is a classic example of a single-mass flywheel - a single chunk of metal. As an attempt to compensate for the pulsing a sprung-hub clutch is typically used. Originally the Sprung hub clutch plate was used to soften the engagement point as you engage the clutch, but as it happens, it also provides a very small ability to soften the pulse. Unfortunately the sprung hub design only has a very small amount of movement compared to a DM FW and it's benefit is practically negligible on 4 cylinder engines.

ACT specifically has both sprung & unsprung hub clutches for the Speeds, with the unsprung tagged as "Race" clutches.

It should be noted that this chatter is only particularly noticeable at idle, in neutral & with the clutch out. It should not heard at any other time and raising the idle speed, even just a couple hundred rpm, should move things beyond the point where the pulses are noticeable.

Means of addressing the chatter symptom include the use of thicker, or heavier, transmission fluid to help control the backlash that you hear at idle. Changing to a straight 90 weight such as MT-90 can help. Depending on climate you may need to blend fluids to get the needed viscosity for cold winters or very hot summers.

I have not had a chance to test any blends yet with my Fidanza-LuK combo and it's chatter. Since I've just filled up with the Motorcraft unicorn juice so it will be a while before I do. Mine's also combined with an 80 duro trans mount so it's chatter central inside the car. Without question your choice of mounts can & will impact how audible the chatter is! Stiff mounts transfer the chatter to the frame rail directly, where the stock mounts isolate very well & will be significantly quieter than aftermarket mounts. Even a stock trans mount with something like the CorkSport inserts will isolate the chatter from the frame.


Here's a quote from ACT back when they were contributing to MSF-

Originally Posted by ACT Clutches ...Gear rattle ... is caused by the engine's torsional vibrations (violent rotational pulses) being transmitted to the gears in the transmission. The main problem is that it is impossible to replicate the torsional dampening of the stock dual mass flywheel with a light weight single mass flywheel or spring centered disc. There are a variety of different gear noises with different characteristics. Each one requires a specific type of dampening to dampen out the vibration. There is no free lunch to performance. If you like the more responsive lighter flywheel, or turn up the power, it may tend to be a bit noisy...

[Koke382]

Great writeup now when newbs ask me why my clutch rattles I can tell them how stuppid they are and point them to this thread. Lol

Sent from my SPH-L900 using Tapatalk 2

[pperrotta03]

I remember my grandpa telling me about the DM FW that he put in one of his trucks. Very good writeup. I am about to get an ACT flywheel. I will also be sending tha newbs here. Much appreciated info!

[2012FailWheelDrive]

Thanks for the info!

[Mandinca]

Great explanation. I'm probably replacing my clutch on stock MS6, how will I know if my DMFW is past it ?
I read that nobody resurfaces DMFW's and that most just hit them with sandpaper to scuff the surface.

Anything to add ?

[TiGraySpeed6]

Just my opinion, but if you're over 70K miles, plan on replacing the FW just to be safe- it most likely does not have another 70K of life left in it.

As for how to tell if it's worn out when you can touch it and inspect it, of course look for the normal wear & tear on the friction surface, and for the dual mass part, watch this video, cause saying "you'll know it's done" just doesn't cover it...

Dual mass flywheel worn new vs Old - YouTube

Dual mass flywheel worn new vs Old - YouTube

[Mandinca]

Thanks. I'm assuming the first flywheel is toast. WHat do you think about switching to a lightweight FW- any recommendations other than those in the OP?
I can't afford to replace my DM FW with an OEM one since they're $1000 +

[Roddiy]

ACT 6-puck with Pro-lite flywheel here, this bitch likes to chat more than my ex-girlfriend, no fucks given though, because racecar.

[Mandinca]

Yeah, trying to avoid chatter. I get enough of that at home.

[TiGraySpeed6]

Just nature of the beast. Lighter = more chatter, but essentially all single mass will chatter to some degree or another.

I run a Fidanza, 17lbs, and it chatters. Much worse with the AC on. I've still got the Balance shaft, and run the JBR 80a Trilogy. I'm at stock idle.

[jm211]

Rockauto sachs flywheel for 426$ SACHS Part # K7060601
Is this dmfw and compatible?


They also show the luk for $180 part10064

[TiGraySpeed6]

I'd be leary of the Sachs FW on RA.

The only place I can find in just a quick search that lists that part number for the Speed is RA, who has a reputation of listing any Mazda part as fitting any Mazda vehicle.

Cross referencing on the Sachs website doesn't appear to list either the 3 or the 6

SACHS | Online Catalog

[Mandinca]

Yeah, considering there is a thread on here devoted to Rock Auto shit that doesn't fit....i'd be leary too.
Car-Part.com--Used Auto Parts Market might be able to locate a used one.

[TiGraySpeed6]

After just over 5,000 miles on the JBR Trilogy I'm currently testing out the CorkSport inserts for the trans mount. Although mounts were addressed in the original version, I've added the following text to the OP for a bit of extra clarity-

Without question your choice of mounts can & will impact how audible the chatter is! Stiff mounts transfer the chatter to the frame rail directly, where the stock mounts isolate very well & will be significantly quieter than aftermarket mounts. Even a stock trans mount with something like the CorkSport inserts will isolate the chatter from the frame.

[Mulenax]

Originally Posted by TiGraySpeed6 After just over 5,000 miles on the JBR Trilogy I'm currently testing out the CorkSport inserts for the trans mount. Although mounts were addressed in the original version, I've added the following text to the OP for a bit of extra clarity-

pretty sure I will eventually run the same luk/fidenza setup as you and I have the jbr RMM/ and Corksport trans mount inserts in already. oh well, because racecar!

[TiGraySpeed6]

I actually ended up selling my JBR side mounts from the trilogy. The trio worked great and exactly as advertised, but was too much to live with in a 6 for my needs. Because old man daily driver....

Don't know why, but the 3's handle mounts much better than the 6's. For some reason the 6 transfers a shit ton more NVH into the cabin compared to a 3. For the purposes of this thread, that means that gear chatter from a SM FW is pretty loud.

I'm currently running the JBR 80a RMM, CS insert in the old original trans mount and a new OEM PMM. This package is much easier on the NVH, so for my old man self it's much more civil to live with as a daily driver, and the gear chatter volume has been reduced dramatically- by about 2/3's at least, if not more.


Also, I've added the following quote to the OP, from ACT back when they were contributing to MSF-

Originally Posted by ACT Clutches ...Gear rattle ... is caused by the engine's torsional vibrations (violent rotational pulses) being transmitted to the gears in the transmission. The main problem is that it is impossible to replicate the torsional dampening of the stock dual mass flywheel with a light weight single mass flywheel or spring centered disc. There are a variety of different gear noises with different characteristics. Each one requires a specific type of dampening to dampen out the vibration. There is no free lunch to performance. If you like the more responsive lighter flywheel, or turn up the power, it may tend to be a bit noisy...