6MT clutch spring rattle sound

[Rajmun340]

This thread is for manual transmission 6MT M3 owners and interested/helpful mechanics.

I've had this annoying clutch noise that i think is symptomatic. It's a spring oscillation rattle that you can hear from the driver seat. I am 99% sure that it is related to the compression springs in the clutch, there are six of them. They work in conjunction with the dual mass flywheel. The springs provide the right amount of counterforce coupled with the dual mass flywheel to critically damp (smooth) the reciprocating engine rotative force jolts. I am not a mechanic but I've driven enough manual cars over decades to be able to tell that much. This is none of the typical clutch issues that are usually listed under clutch diagnosis. The oscillating spring rattle lasts less than a second and happens everytime the car is moving with a gear engaged and the clutch pedal is pressed down to take the gear out (but it sometimes depends how fast you press the pedal). It is loud enough to be heard from the driver seat, there is no doubt that it's from the clutch, it is absolutely not from the pedal, not the engine, not anything else. It's the clutch. it happens in reverse gear too.

The recording is taken from the cabin with smartphone held by hand one feet over the center console. The car is slowly moving on large parking lot in first gear, I brake a little as if coming to a stop and press down the clutch pedal decisively to take the gear out ->*rattle sound* . I go again in first gear and repeat 6+ times over the 1 min duration of the recording. The soundtrack has been amplified by 3X to better hear.

You will hear the rattle at 3 secs, 7 secs, 12 secs, 28 secs, 34 secs, 40 secs, etc. Each rattle starts loud and very quickly dampens in less than half a second. Since i've owned this car from the factory, I know it was never abused (but i cannot account for how the car was driven by Bmw stealers employees).

I've driven another 6MT M3 and was able to detect the same rattle but it was much less. I would say it is abnormal. I suspect that some of the compression springs are broken in place and the remaining ones provide insufficient counterforce for the mass leading to underdamped oscillations when the clutch disc is physically disconnected from the car load. OR it could be the clutch release bearing making the noise while the springs are briefly oscillating. The concern is if this could lead to something more serious.

I also wonder if it could be related to another issue :
My engine+gearbox also has a bad tendancy to accelerate in slight hesitations when taking off. You'll have to take 100.00% my word on it that this is a machine issue, I am more than very experienced with manual transmission cars without being imodest about it. This is absolutely not about user error.

The clutch and the dual mass flywheel in our transmission are permanently connected and balanced as a single unit as per BMW technical doc. So it cannot be repaired.

Additional note : I was able to hear the rattle at the moment the engine is turned off (pressing the engine button). Since the gear is already in neutral and the foot is off the clutch pedal it cannot be the clutch release bearing, it has to be the compression springs inside the clutch.

[gingerbread]

sounds like some chatter from the twin disc clutch or gear lash in the trans

[Rajmun340]

Originally Posted by gingerbread

sounds like some chatter from the twin disc clutch or gear lash in the trans

It cannot be gear lash. When I put the gear in neutral and then turn off the engine, i can hear the same rattle just as the engine stops rotating. The crankshaft stops rotating very quickly the loss of inertia has the same effect on the clutch springs.
Could it be chatter caused by some broken or fallen hub springs weakening the damping response (underdamped takes longer to settle with multiple oscillations instead of no oscillation for critically damped)

[pbonsalb]

If you hear it on a 2008 car but not a later car it is because BMW used a heavier flywheel in later cars to dampen transmission sounds. The 2008s rattle more. Also, most twin discs rattle.

I don't think there is any way to inspect the clutch other than pulling the trans. I would not do that unless I was sure I had a problem.

[monterey]

My 08 makes all kinds of noises. I changed my fluids and now the noise/rattle is even MORE pronounced. I had a lightweight flywheel in my E36M3 and it sounds the same so I am assuming the 08 has a lighter flywheel.

[Rajmun340]

Originally Posted by pbonsalb

If you hear it on a 2008 car but not a later car it is because BMW used a heavier flywheel in later cars to dampen transmission sounds. The 2008s rattle more. Also, most twin discs rattle.

I don't think there is any way to inspect the clutch other than pulling the trans. I would not do that unless I was sure I had a problem.

I am affraid not, it's a 2013 E92 M3 last month of production. It cannot be accentuated by a lighter flywheel since it is actually heavier. I can live with the annoying rattle but want to be sure it's not something broken. I recall that it did not rattle from the factory. Just that at some point the rattle was there. I guess i am looking for a twin disc clutch diagnosis but designs vary from one manufacturer to the next using different straps, pins location, etc.

Originally Posted by monterey

My 08 makes all kinds of noises. I changed my fluids and now the noise/rattle is even MORE pronounced. I had a lightweight flywheel in my E36M3 and it sounds the same so I am assuming the 08 has a lighter flywheel.

it's a 2013. I wonder if a couple springs broke and fell to the bottom of the housing. Given i hear the springs rattlings in transitions with either the clutch getting open (car coming to a stop) or already closed (shutting off the engine in neutral) it points to missing dismounted springs causing underdamping. The sound is clearly multiple oscillations of a sprung mass. If the springs were in the right quantity they would not allow even one oscillation which is what is called critically damped. I am only thinking but if all the hub springs were to fall off, the rattle would disappear and gear changes would be harsher with a thumping sound when letting go of the clutch pedal. Of course that would be a bit worse. Isn't that the thump that some 6MT owners complain about ?

Just a picture showing the difference between critically damped (well designed) , underdamped (unwanted oscillations), and overdamped (too slow)

http://nptel.ac.in/courses/112103022...3/images/1.png

[OlBloo]

There were some other people that were hearing noises from their clutch and their car caught on fire after a spring popped off.

I havent heard what their symptoms actually sounded like, but maybe someone knows the link
Hopefully you are dealing with something less drastic

[Rajmun340]

Originally Posted by OlBloo

There were some other people that were hearing noises from their clutch and their car caught on fire after a spring popped off.

I havent heard what their symptoms actually sounded like, but maybe someone knows the link
Hopefully you are dealing with something less drastic

i could not believe this at first. I am going to check the sources. thks.

[pbonsalb]

Originally Posted by OlBloo

There were some other people that were hearing noises from their clutch and their car caught on fire after a spring popped off.

I havent heard what their symptoms actually sounded like, but maybe someone knows the link
Hopefully you are dealing with something less drastic

I remember seeing the picture but don't recall the postmortem. I think Mike Benvo posted it. I recall just that one example but maybe there are others.

[deansbimmer]

Sounds like driveshaft and differential backlash, not clutch related. Very common.

But there were a few things in here that I can clarify.

First- In neutral the transmission main shaft is still rotating with the engine. You can still hear chatter in the transmission, and the driveshaft can still be motivated back enough to make a slight backlash sound upon shutdown. Pushing the clutch in disengages the transmission input shaft so if you want to try that test again, try it with the clutch in.

Second- I posted a thread where a clutch failure led to a fire. That was from broken spring straps. Those would make a much different sound than what the video dictated (think marbles in a can).

[ophano]

I haven't owned my 2010 long but it makes this exact same noise. I assumed it was relatively normal. Interesting discussion however.

[Solo_M_Tech]

My 08 makes the same sounds and the noises go away when I press the clutch pedal down. It's flywhel/clutch/trans noises combined. It's normal. 09+ cars have a heavier flywheel which reduces this noise but it can still be heard on some cars. It's just the nature of these cars to be noisy.

[deansbimmer]

The noise in the video does not sound like clutch noise at all. As the car ages, parts wear and slop becomes great enough to cause noise in the joints and diff, which reverberates through the driveshaft with a bell like sound when it happens. Tap on the driveshaft sometime. That is what you're hearing in the video.

[ekological]

Were you operating the vehicle in a manner to accentuate the noise? I can create the same noise if there is a load (engine braking or accelerating) on the drivetrain and I mash in the clutch pedal abruptly.

[Rajmun340]

Originally Posted by pbonsalb

I remember seeing the picture but don't recall the postmortem. I think Mike Benvo posted it. I recall just that one example but maybe there are others.

Wouldn't that trigger a safety recall ?

Originally Posted by deansbimmer

Sounds like driveshaft and differential backlash, not clutch related. Very common.

But there were a few things in here that I can clarify.

First- In neutral the transmission main shaft is still rotating with the engine. You can still hear chatter in the transmission, and the driveshaft can still be motivated back enough to make a slight backlash sound upon shutdown. Pushing the clutch in disengages the transmission input shaft so if you want to try that test again, try it with the clutch in.

Second- I posted a thread where a clutch failure led to a fire. That was from broken spring straps. Those would make a much different sound than what the video dictated (think marbles in a can).

Good idea i will try shutting it down with pedal in and report.
I will lookup your broekn spring straps thread. If i don't find it, would be good if you had the link.

Originally Posted by ophano

I haven't owned my 2010 long but it makes this exact same noise. I assumed it was relatively normal. Interesting discussion however.

Yes since it is a twin disc clutch i am tempted to think it is normal but still, it shouldn't make that sound if it was working as the design intended.

Originally Posted by ham_sammich

My 08 makes the same sounds and the noises go away when I press the clutch pedal down. It's flywhel/clutch/trans noises combined. It's normal. 09+ cars have a heavier flywheel which reduces this noise but it can still be heard on some cars. It's just the nature of these cars to be noisy.

thanks for reporting.

Originally Posted by deansbimmer

The noise in the video does not sound like clutch noise at all. As the car ages, parts wear and slop becomes great enough to cause noise in the joints and diff, which reverberates through the driveshaft with a bell like sound when it happens. Tap on the driveshaft sometime. That is what you're hearing in the video.

No, it sounds like it's the clutch very much, just not one of the typical clutch noises that you would be used too (clutch bearing, hiss). I does not sound like differential noise at all. The sprung mass oscillation sound is unmistakable, there is no coiled type spring in the diff. The audio captures the first oscillation each time but not the decaying tail sound like a quick serie of exponentially decaying oscillations.

Originally Posted by ekological

Were you operating the vehicle in a manner to accentuate the noise? I can create the same noise if there is a load (engine braking or accelerating) on the drivetrain and I mash in the clutch pedal abruptly.

Yes, i tried to accentuate the noise, i also scaled up (amplified) the recorded soundtrack when making the video. Yes, the noise occur when there is a load being applied or removed (engine braking or accelerating). And yes, you have to push down the clutch pedal quite swiftly if you push very slowly it will not make the noise. I think that the loudness is proportional to how many springs have already fallen off (there are 6) and the system is more underdamped as a result.

[deansbimmer]

As you rapidly push in the clutch and the loaded drivetrain suddenly unloads, everything slaps back and forth. You will notice you don't get the sound when you clutch in slowly, or in a neutral load driveline? You're calling it "sprung mass oscillation" which is fine since the driveline's stored energy is essentially behaving like a spring in this context, but 100% of the sound does not originate from the flywheel or clutch. These are unsprung clutches. The transmission gears, driveshaft joints, diff (ring to pinion play) are the only parts with enough mass and tolerances to become audible here...

I get asked about this sound all the time. You can duplicate it in just about any M3, I can even duplicate it in my DCT.

[Rajmun340]

Originally Posted by deansbimmer

As you rapidly push in the clutch and the loaded drivetrain suddenly unloads, everything slaps back and forth. You will notice you don't get the sound when you clutch in slowly, or in a neutral load driveline? You're calling it "sprung mass oscillation" which is fine since the driveshaft is essentially behaving like a spring in this context, but 100% of the sound does not originate from the flywheel or clutch. The transmission gears, driveshaft joints, diff (ring to pinion play) are the only parts with enough mass and tolerances to become audible here...

I get asked about this sound all the time. You can duplicate it in just about any M3, I can even duplicate it in my DCT.

You description of when it happens is correct. I thought about your theory but it does not work. I agree that torsion on the drive shaft could provide a stored energy spring effect released when quickly disengaging the clutch but when in neutral only the gearbox input shaft is connected to the clutch not the driveshaft to the diff. And since the noise is heard when shutting off with already in neutral, the drive shaft is not involved. The oscillations rate is about 10-15Hz and quickly damped (5-7 cycles max). the spring effect sounds quite flexible with not a significant amount of mass being flung back and forth. The clutch coiled springs are mounted on the primary clutch disc hub. that hub is tied to the gearbox input shaft via splines and to the secondary disc through pins (don't know about ZF oem design specifics, that is based on another vendor description). When the clutch is engaged the springs serve no purpose and all the parts are solidary rotating together. When disengaged the hub springs separate on one side the primary disc and on the other side the primary hub + secondary disc + gearbox shaft + (gearbox + driveshaft + diff + rear axle ) the last term only if gear is not neutral. The lighest of these two sides is the primary disc which is believe is what is being heard. For such a relatively light part to oscillate the springs are providing insufficent reactive force. From there comes my missing springs hunch.

There is the possibility that the heard oscillations always existed silently. but still.. i suspect broken springs.

[Rajmun340]

deansbimmer, I found your thread :

http://www.m3post.com/forums/showthread.php?t=1342757

I can guarrantee you that my M3 (factory owner) was never abused, never launched full force. If this is a manufacturer defect, design issue, then I think it would call for a NHTSA investigation.

[deansbimmer]

Originally Posted by Rajmun340

You description of when it happens is correct. I thought about your theory but it does not work. I agree that torsion on the drive shaft could provide a stored energy spring effect released when quickly disengaging the clutch but when in neutral only the gearbox input shaft is connected to the clutch not the driveshaft to the diff. And since the noise is heard when shutting off with already in neutral, the drive shaft is not involved. The oscillations rate is about 10-15Hz and quickly damped (5-7 cycles max). the spring effect sounds quite flexible with not a significant amount of mass being flung back and forth. The clutch coiled springs are mounted on the primary clutch disc hub. that hub is tied to the gearbox input shaft via splines and to the secondary disc through pins (don't know about ZF oem design specifics, that is based on another vendor description). When the clutch is engaged the springs serve no purpose and all the parts are solidary rotating together. When disengaged the hub springs separate on one side the primary disc and on the other side the primary hub + secondary disc + gearbox shaft + (gearbox + driveshaft + diff + rear axle ) the last term only if gear is not neutral. The lighest of these two sides is the primary disc which is believe is what is being heard. For such a relatively light part to oscillate the springs are providing insufficent reactive force. From there comes my missing springs hunch.

There is the possibility that the heard oscillations always existed silently. but still.. i suspect broken springs.

This is where a strong working knowledge of our specific system is advantageous to quick diagnosis. Our OEM clutch is not a sprung clutch. The radial hub springs you describe do not exist on our clutch design, therefore they cannot be the cause of sound as you suspect.

Also, while it sounds good on paper that in neutral the driveshaft cannot be affected by the engine or transmission, in reality there is enough energy transfer allowed into an idle driveshaft via friction of fluid, stacked gears+syncros, to cause a slight audible transfer in certain load/unload scenarios.
These are amplified by the hollow steel driveshaft which acts as the sounding board.

[monterey]

the video sound reminds me of the blair witch project lol.

[Rajmun340]

Originally Posted by deansbimmer

This is where a strong working knowledge of our specific system is advantageous to quick diagnosis. Our OEM clutch is not a sprung clutch. The radial hub springs you describe do not exist on our clutch design, therefore they cannot be the cause of sound as you suspect.

Also, while it sounds good on paper that in neutral the driveshaft cannot be affected by the engine or transmission, in reality there is enough energy transfer allowed into an idle driveshaft via friction of fluid, stacked gears+syncros, to cause a slight audible transfer in certain load/unload scenarios.
These are amplified by the hollow steel driveshaft which acts as the sounding board.

The hub springs are not radial they are ortho-radial.
When shutting off the engine in neutral there is no physical connection between the gearbox input shaft and the driveshaft. Therefore friction fluid, stacked gears+synchro do not come into play whatsoever. The engine cranckshaft comes to a quick stop which results in the hub springs oscillating the gearbox shaft which ties to nothing else (neutral).

[deansbimmer]

I’m not here to force knowledge on anyone if they don’t want it. Look up the PN for the clutch. Find pics of the actual part, not a generic realoem clutch image. Im speaking from real first hand experience from working on these M3’s daily, not from google.