Crank Hub Issue Master Thread: What You Need To Know!

[FrankMstein]

Originally Posted by Adcampo

It could have been an over rev on my part.

That's called the money shift for a reason.
DO NOT take it to a $tealer. Find an indy shop in the area THAT HAS DONE ONE BEFORE and it will be a lot less. Ask around.

[wheela]

I don't work in automotive, but I am a mechanical engineer with >20 years engineering experience (primarily manufacturing, now product development). I have a lot of thoughts to share on this, so please bear with me!

I agree that extreme loading from fast DCT shifts is probably factor. I also agree that the extra rotational inertia from turning the heavier harmonic dampener and a mechanical water pump are probably factors why this is more prevalent on s55 vs. n54 or n55. But I suspect the biggest rotational loading on this joint probably comes from crank shaft twist (against the reaction force rotational inertia of the balancer, valve train, wtc.), which is probably much worse if you lug the engine. Also, as you tune the engine and start making more torque, crank shaft twist would increase and would also be worse as. But given the relatively low-ish rate of failure, there's still something we're missing.

Personally, I suspect it's variability in the friction disks, where over time some portion of them (but not all of them) start to crush/stress relieve. This would directly reduce clamping force, eventually to the point where the forces described above are greater than the friction and the hub/sprocket spins.

How could some friction disks start to crush though? Somebody mentioned they're made of a diamond sintered metal. I know fatigue doesn't normally happen to materials in compression (most fatigue formulas ignore compressive forces for this reason). But if you search compressive fatigue, you'll find that research shows if the compressive force is high enough (> yield strength) and stress concentrations are present, fatigue cracks can grow under purely compressive cylical stress (but still much slower than in tension).

Back to the diamonds... They are much harder than the metal they're sintered into, and are distributed through the bulk of the friction disk. Since they're so much harder than the metal, any deformation in the disk when its torqued will all turn into strain concentrated in the metal matrix, potentially exceeding its yield strength. All those diamonds have sharp edges, creating stress concentrations in the metal matrix. Now we have high stress and stress concentrations - the conditions needed for compressive fatigue to happen. If fatigue is happening, then the metal matrix will start to break down causing the disk to take a crush and relieve stress through crack growth, reducing the clamping force (without the crank bolt having to loosen up). It would be easy for the BMW engineers to miss this if it's happening, because "everybody knows fatigue doesn't happen in compression"...

Depending on how much variability they have in the diamonds (size distribution, shape, concentration in the matrix, etc.), some disks (maybe even some entire lots of disks?) may have greater stress concentrations in them than others, making them more prone to fatigue and stress relieving, reducing clamping force, and causing your crank hub to spin.

So, I suspect it may come down to the particular friction disks installed in your car (basically a gamble). Which is kind of what we see - somewhat random occurrence across all power levels, but seemingly at a higher average random occurrence with higher power levels.

Thoughts, comments, suggestions?

[wheela]

Another thought occurred to me with the friction disks. With all the back and forth shear stress that thing is under from crank twist, and the fact that any strain in the disk would be isolated to the metal matrix (I doubt the diamonds have any meaningful deflection), I wonder how much heating it gets from all that internal friction straining back and forth from crank twist under extended heavy driving? Could it get hot enough for the metal matrix to start to creep and stress relieve that way? Think of ultrasonic welding or friction stir welding, this is basically how the metal is heated in those processes...

That could even help explain some of the observations where people say the hub spun backing out of their driveway, or starting the car. If they drove it really hard the time before, and the disks had already fatigued over time close to the point of failure, they'd have heated that much more from the stress being concentrated over even less area of metal matrix in the disk (fatigue cracked portions of the metal matrix wouldn't support stress very well). The resulting thermal expansion in the disk would temporarily supply some level of extra clamp force, perhaps enough to prevent the hub spin. Once you shut the car down for the night and let everything cool off, any clamp force you had on the previous drive from thermal expansion would be gone, leaving you with less clamp force starting your car the next morning than you had on your previous drive when that disk was nice and hot. If the disk was already on the cusp of failure, that loss of clamp load from thermal expansion on the previous drive combined with the reduced clamp force from the fatigued and/or creeped disk could be enough for the hub to spin under such unchallenging conditions.

Thoughts, comments, suggestions???

[FrankMstein]

The trans modes 1, 2, 3 all have different increasing clamping forces from what I hear.
"The resulting thermal expansion in the disk would temporarily supply some level of extra clamp force, perhaps enough to prevent the hub spin. Once you shut the car down for the night and let everything cool off, any clamp force you had on the previous drive from thermal expansion would be gone, leaving you with less clamp force starting your car the next morning than you had on your previous drive when that disk was nice and hot. If the disk was already on the cusp of failure, that loss of clamp load from thermal expansion on the previous drive combined with the reduced clamp force from the fatigued and/or creeped disk could be enough for the hub to spin under such unchallenging conditions."
Also the opposite, lack of warming up the engine/oil should allow more slippage in the friction disk of the CH.

[chris719]

Originally Posted by wheela

Another thought occurred to me with the friction disks. With all the back and forth shear stress that thing is under from crank twist, and the fact that any strain in the disk would be isolated to the metal matrix (I doubt the diamonds have any meaningful deflection), I wonder how much heating it gets from all that internal friction straining back and forth from crank twist under extended heavy driving? Could it get hot enough for the metal matrix to start to creep and stress relieve that way? Think of ultrasonic welding or friction stir welding, this is basically how the metal is heated in those processes...

That could even help explain some of the observations where people say the hub spun backing out of their driveway, or starting the car. If they drove it really hard the time before, and the disks had already fatigued over time close to the point of failure, they'd have heated that much more from the stress being concentrated over even less area of metal matrix in the disk (fatigue cracked portions of the metal matrix wouldn't support stress very well). The resulting thermal expansion in the disk would temporarily supply some level of extra clamp force, perhaps enough to prevent the hub spin. Once you shut the car down for the night and let everything cool off, any clamp force you had on the previous drive from thermal expansion would be gone, leaving you with less clamp force starting your car the next morning than you had on your previous drive when that disk was nice and hot. If the disk was already on the cusp of failure, that loss of clamp load from thermal expansion on the previous drive combined with the reduced clamp force from the fatigued and/or creeped disk could be enough for the hub to spin under such unchallenging conditions.

Thoughts, comments, suggestions???

The discs are almost always cracked when removed on cars that spin the hub. In fact, I think I've seen pics of two that were fractured slightly but had not spun yet and were found during preventative one-piece CH installation.

[Tangent]

Originally Posted by wheela

...
I agree that extreme loading from fast DCT shifts is probably factor. ...

The one thing which makes me disagree is people only look at the fact that about twice as many cases of SCH are on DCT cars and stop there. Problem is there are roughly four times as many DCT F8xs on the road as manuals which would seem to indicate it's actually twice as likely to happen on a manual car than DCT. That's not to say extreme loading from shifts aren't still to blame, plenty of people dump the clutch in manuals and/or shut off auto rev-matching and do a horrible of it themselves before again dumping the clutch...

[wheela]

Originally Posted by Tangent

The one thing which makes me disagree is people only look at the fact that about twice as many cases of SCH are on DCT cars and stop there. Problem is there are roughly four times as many DCT F8xs on the road as manuals which would seem to indicate it's actually twice as likely to happen on a manual car than DCT. That's not to say extreme loading from shifts aren't still to blame, plenty of people dump the clutch in manuals and/or shut off auto rev-matching and do a horrible of it themselves before again dumping the clutch...

Very good point. The sch doesn't seem to be as common on n54's, but I've seen it on multiple manual transmission n54's.

[FrankMstein]

https://f80.bimmerpost.com/forums/sh...&postcount=311

[m3lifecrisis]

FrankMstein so after investigating this subject for so long where do you currently stand? Is it (statistically) a non-issue which only happens to drivers with bad habbits (cold booting the car, constant kick-downs, lugging the engine etc.) or just pure luck as in: bad part batches.

I didn't get your point about this considered a US-issue to Europe? Is there really less numbers in Europe or you were being ironic in the sense that people in Europe don't want to recognize the issue?

Also, posted this elsewhere but isn't CBC mechanically pointless? You are securing the weak link (bolt) to something which isn't fixed (the hub) and can still move as long as the bolt starts to back out. Like securing something by adding extra attachment but... to itself! I'm probably missing something as it can't be that obvious/dumb?

[wheela]

Originally Posted by m3lifecrisis

FrankMstein

Also, posted this elsewhere but isn't CBC mechanically pointless? You are securing the weak link (bolt) to something which isn't fixed (the hub) and can still move as long as the bolt starts to back out. Like securing something by adding extra attachment but... to itself! I'm probably missing something as it can't be that obvious/dumb?

I don't think you're missing anything. The only case I see cbc helping is if the bolt is somehow able to work loose WITHOUT the hub or sprockets also turning. Not sure this is even possible UNLESS you're already losing clamp force through some other mechanism (assuming stock hub without some kind of pinning/keying or spline to stop rotation relative to the crank). CBC is like welding the head of a bolt to the washer underneath it so the bolt won't come loose (at least that's my analogy for it). Even if the friction discs are starting to fail reducing clamp load (which is just a theory) to the point where the bolt could start to back its self out, failure would still be imminent even with a cbc due to the reducing clamping load eventually allowing the bolt, cbc, and hub all to spin together as one peice. There are a lot of people running CBC that have not spun a hub and that's great. But that doesn't mean much, because they may not have spun even without the CBC - we can't know the answer for any particular car.

[FrankMstein]

Originally Posted by m3lifecrisis

FrankMstein so after investigating this subject for so long where do you currently stand? Is it (statistically) a non-issue which only happens to drivers with bad habbits (cold booting the car, constant kick-downs, lugging the engine etc.) or just pure luck as in: bad part batches.

I didn't get your point about this considered a US-issue to Europe? Is there really less numbers in Europe or you were being ironic in the sense that people in Europe don't want to recognize the issue?

Also, posted this elsewhere but isn't CBC mechanically pointless? You are securing the weak link (bolt) to something which isn't fixed (the hub) and can still move as long as the bolt starts to back out. Like securing something by adding extra attachment but... to itself! I'm probably missing something as it can't be that obvious/dumb?

Where I stand from a mechanical engineering stand point is that is IS an issue that BMW under designed for all normal uses. There is a chance that some of the friction washers have issues but over a long period of time, probably not the washers fault.

I believe there are possibly a difference in the use between the US vs Europe. There aren't nearly the same failure rate which points directly at human use. This is based on the available data I have seen and heard. Does Europe drive any less hard, not likely at all. Do they warm up the car more before, possibly? Race teams don't have this issue that I have heard. Bimmerworld replaced theirs on GTMore (they sell that part mind you) as a preemptive fix because it IS an issue. If they didn't replace it would it fail. IMO it is way less likely since they don't drive like they are playing their PS5.

The way the car is used is key. I am on the CBC is trash thread (as I believe @wheela is)and I believe it could/should work but they are being implemented wrong and with sub-par screws HOWEVER the broken washer is the failure mode NOT the backing off of a bolt. The washer fails and the bolt is then lose. I don't remove these but some I am sure were not tight to spec (at CEL issue) and loosened which is why the CBC CAN work but not if it splits. Were these not tight to spec at the factory, highly unlikely, highly.

I have elected to buy a warranty that will cover it and everything else for the same price. I drive the car like I don't have a warranty and at the track I drive like I don't have insurance. That may be foreign to some but it's my car. I will not use kickdown (mine is DCT) and I recommend people not drop three gears in one shift or do a money shift in a manual. This is my stance and I am taking all input...

[medphysdave]

Originally Posted by FrankMstein

Where I stand from a mechanical engineering stand point is that is IS an issue that BMW under designed for all normal uses. There is a chance that some of the friction washers have issues but over a long period of time, probably not the washers fault.

I believe there are possibly a difference in the use between the US vs Europe. There aren't nearly the same failure rate which points directly at human use. This is based on the available data I have seen and heard. Does Europe drive any less hard, not likely at all. Do they warm up the car more before, possibly? Race teams don't have this issue that I have heard. Bimmerworld replaced theirs on GTMore (they sell that part mind you) as a preemptive fix because it IS an issue. If they didn't replace it would it fail. IMO it is way less likely since they don't drive like they are playing their PS5.

The way the car is used is key. I am on the CBC is trash thread (as I believe @wheela is)and I believe it could/should work but they are being implemented wrong and with sub-par screws HOWEVER the broken washer is the failure mode NOT the backing off of a bolt. The washer fails and the bolt is then lose. I don't remove these but some I am sure were not tight to spec (at CEL issue) and loosened which is why the CBC CAN work but not if it splits. Were these not tight to spec at the factory, highly unlikely, highly.

I have elected to buy a warranty that will cover it and everything else for the same price. I drive the car like I don't have a warranty and at the track I drive like I don't have insurance. That may be foreign to some but it's my car. I will not use kickdown (mine is DCT) and I recommend people not drop three gears in one shift or do a money shift in a manual. This is my stance and I am taking all input...

I'm impressed by the investigational work on this and the focus on the damaged washers as the underlying cause. It would make sense. I don't plan to update my crank hub. I also haven't looked to see how easily one could periodically check these washers. Also noticed you're from Charlotte. Me too! Thanks for the deep dive into this.

[FrankMstein]

Originally Posted by medphysdave

I'm impressed by the investigational work on this and the focus on the damaged washers as the underlying cause. It would make sense. I don't plan to update my crank hub. I also haven't looked to see how easily one could periodically check these washers. Also noticed you're from Charlotte. Me too! Thanks for the deep dive into this.

Thanks medphysdave If you ever sell your wheels, look me up!

[medphysdave]

Originally Posted by FrankMstein

Thanks medphysdave If you ever sell your wheels, look me up!

No problem. Love those gold wheels. I've got a set of CSR wheels going on for track days. 10.5" square 295/18 should be fun on a 2 series:P

[DetailDevil]

Hey guys,
New to the forum and based in UK.

I have just purchased a May 2016 F82 M4 Coupe (DCT) Mint car, FBMWSH and been extremely well cared for, 2 previous owners and covered 90k miles

I'm aware of the ongoing debate re the Crank Hub, and guessing if this was to have failed on my car, it would have done so by now? The car is totally stock.

I'm considering installing a BM3 stage one map. Not just for the additional power, but reading many posts and reports, it improves the driveability of the car and also has many many features that can be tweaked, burbbleetc, cold start delete etc etc etc and so on.

$64,000 question - Would adding thsi s/w and a stg1 map to the car, togehter with the GTS s/w for the DCT box increase the risk of the crank hub failing?

Any help, thoughts, comments, advice will be greatly appreciated

Cheers
Louie

[williammm]

Would like to check if any of you guy had a crankhub upgraded and after putting back everything the vibration damper wobble? Had replaced another new damper and it's still wobbling. Which lead me to wonder if the crankhub is having issues.

[FrankMstein]

Originally Posted by DetailDevil

Hey guys,
New to the forum and based in UK.

I have just purchased a May 2016 F82 M4 Coupe (DCT) Mint car, FBMWSH and been extremely well cared for, 2 previous owners and covered 90k miles

I'm aware of the ongoing debate re the Crank Hub, and guessing if this was to have failed on my car, it would have done so by now? The car is totally stock.

I'm considering installing a BM3 stage one map. Not just for the additional power, but reading many posts and reports, it improves the driveability of the car and also has many many features that can be tweaked, burbbleetc, cold start delete etc etc etc and so on.

$64,000 question - Would adding thsi s/w and a stg1 map to the car, togehter with the GTS s/w for the DCT box increase the risk of the crank hub failing?

Any help, thoughts, comments, advice will be greatly appreciated

Cheers
Louie

No, its all user error.

[FrankMstein]

Someone is telling me that there were "signs" the CH was impending failure. Then were given these pics and implied the shop said the CH did this w/o any errors.