The cause of the spun crank...

[M4_Johnnys]

So I've been thinking about this lately and realized a couple of things and wanted to get a few opinions.

If the timing gear system was completely frictionless, it wouldn't matter how much torque your engine makes, there would never be an opposite torque on the other side of the timing hub holding it in place for it to spin. The hub only needs to hold enough torque to turn the cams. If the timing system was frictionless and had no inertia the hub could be made out of tissue paper and still hold it and you could still launch your car all day long.

So let's say the cams require 50Nm to turn, when the hub degrades to a point where it can no longer hold 50Nm it will spin. It doesn't matter if your engine makes 300Nm of torque or 1200Nm as the hub will only see the amount of torque that is opposite it.

So it comes down to:
1) Either there are design/manufacturing faults in the hub where it degrades over time when the rest of system is working perfectly fine and eventually lets go. Could be the metallurgy is out of spec of the hubs from supplier etc...

2) There is something amiss in the cam system putting undue opposite force on the hub wearing it down over time, maybe the tensioner is too tight, or the viscosity of the oil too high (wrong oil or not warmed up). Could even be an issue with a cam bearing but this would probably be found pretty quickly on rebuilds.

3) (probably not likely this one) The engine has slight detonation which causes the cylinder pressure to push back on the valves at times it's not supposed to, causing a bit of jerk (derivative of acceleration) to the cam and hence back to the hub acting a bit like an impact wrench breaking it lose.

I've got no one around to bounce these thoughts of so you guys are the unlucky fellas!

[MR RIZK]

You also have the vibration damper and the load created by the two belts.

[NYG]

The problem with all of this speculation is that we don't have a reliable and consistent sample of crank hub failures. Just a bunch of random people on forums giving you half of the story on what seems to be mostly, but not exclusive to, failures that occur on M-DCT running much higher horsepower than the car was designed for. That being said, it isn't as common as some people lead you to believe and the cause isn't as defined as some people lead you to believe. Until BMW makes an official statement, which I haven't seen, this would suggest that this isn't a problem that aftermarket companies have convinced you that it is. Money talks, if warrantied engines were having these damages regularly, BMW would have done something major by now.

I say all this as a forensic/structural engineer. I'm not a MechE but there's way too many variables and inconsistent information being fed (or lack there of) to really come to a conclusion. Component failures of dynamic systems with such a small and inconsistent samples are a lot harder to diagnose than by just analyzing the failure mechanism of the part. It wouldn't cost BMW that much money to turn this into a non-issue.

[M4_Johnnys]

You are correct NYG, this is all 100% speculation but I'm not referring to any failures that have happened, I'm just trying to analyze the system from first principals. Whether it happens all the time or it's only ever happened once it's not relevant as I just wanted to have a discussion.

I spent some time as a powertrain quality engineer in an engine plant for GM (Only 1 year many many moons ago) my job was to diagnose these things using Six Sigma principals (GM called it Red X). Whenever engines have failed and gone to a dealer they would be returned back to us for analysis, I would assume the same thing happens with BMW at least with engines in Europe.

[NYG]

Ah, well then even with a year of experience, you're way more qualified on this topic than I am.

Not to change the topic but given that you're in AUS, did you work with Holdens a lot? Love those cars.

[M4_Johnnys]

Not saying I'm an expert, just not a total newbie!

Yup you got it, I worked for holden for a bit before moving to Europe to work for the big boys.

I was working in the V6 engine plant, was our first direct injection motor at the time. our V8s were all LS motors so they were shipped directly to the car assembly plant in South Australia, the plant I was located at was in Vic with the rest of engineering. So sad the Australian automotive scene is gone!

[Kooch]

Originally Posted by M4_Johnnys

You are correct NYG, this is all 100% speculation but I'm not referring to any failures that have happened, I'm just trying to analyze the system from first principals. Whether it happens all the time or it's only ever happened once it's not relevant as I just wanted to have a discussion.

I spent some time as a powertrain quality engineer in an engine plant for GM (Only 1 year many many moons ago) my job was to diagnose these things using Six Sigma principals (GM called it Red X). Whenever engines have failed and gone to a dealer they would be returned back to us for analysis, I would assume the same thing happens with BMW at least with engines in Europe.

I've been thinking the same. To spin the hub you basically need the crank to turn and something to "hold back" the timing chain. Only link I can see to higher power output is the rate of rpm changes of the engine. Which in theory the opposing force would be inertia of the valve train and associated components. So higher power would mean faster rate of change of rpm when applied to some point that the crank wants to accelerate faster than the valve train. Just my thoughts.

Edit: meant to quote your first post...

[SYT_Shadow]

Originally Posted by M4_Johnnys

So let's say the cams require 50Nm to turn, when the hub degrades to a point where it can no longer hold 50Nm it will spin. It doesn't matter if your engine makes 300Nm of torque or 1200Nm as the hub will only see the amount of torque that is opposite it.

Let's start with the basic equation of physics and fun stuff:

Force = mass * acceleration

The mass of the cams/rotating assembly is defined and constant. Let's say it's 8 kilos

It's the same if the car is accelerating or decelerating, so for example a DCT shift usually is deceleration but this doesn't change the math.

The acceleration is harder to determine. When do the parts see max acceleration? In 1st gear accelerating hard or when the DCT swaps gears in max attack setting?
Regardless, let's assume in a stock car the max acceleration is 10m/s2.

putting this into our formula we're left with:
F=m*a --> F=8*10 --> F=80 Nm

If we tune the car and now can create a max acceleration of 12m/s2, the force is completely different:
F=8*12 --> F=96 Nm

[NYG]

Originally Posted by SYT_Shadow

Let's start with the basic equation of physics and fun stuff:

Force = mass * acceleration

The mass of the cams/rotating assembly is defined and constant. Let's say it's 8 kilos

It's the same if the car is accelerating or decelerating, so for example a DCT shift usually is deceleration but this doesn't change the math.

The acceleration is harder to determine. When do the parts see max acceleration? In 1st gear accelerating hard or when the DCT swaps gears in max attack setting?
Regardless, let's assume in a stock car the max acceleration is 10m/s2.

putting this into our formula we're left with:
F=m*a --> F=8*10 --> F=80 Nm

If we tune the car and now can create a max acceleration of 12m/s2, the force is completely different:
F=8*12 --> F=96 Nm

You need a moment arm to create torque brotha!

Mass * Accel only results in Newtons/LBF

Angular acceleration/velocity is more applicable on the gear.

Once you take into account shear deformation and thermal expansion, it becomes like a 5th order differential equation.

[SYT_Shadow]

Originally Posted by NYG

You need a moment arm to create torque brotha!

Mass * Accel only results in Newtons/LBF

Angular acceleration/velocity is more applicable on the gear.

Once you take into account shear deformation and thermal expansion, it becomes like a 5th order differential equation.

That's why I ignored torque and only spoke of force.
The fastener is actually resisting torque, not force, but as the distance in the fastener is constant you can think of force instead of torque.

By 'think of' I mean understand that OP is incorrect, the amount of force transmitted varies with more acceleration

[behindthen0thing]

i thought about this as well and i dont see how having higher hp would increase the load on the hub unless it was just the rpm changing much faster due to increased power

[M4_Johnnys]

We once had a problem in the engine plant with a pressed fit part, once we looked over the data from the past two years we noticed we only saw it in specific months... Which happened to be summer, turns out the 30 degrees difference was enough to cause an issue in roughly 2% of parts.

Another issue was due to when a big compressor was switched on it caused an issue with the electricity supply and the machine that was doing the head surface would cause an uneven cut (that wasn't even noticeable by eye) and the gasket wouldn't seal properly.

Even though it's fun to speculate we will never know the cause unless BMW tells us and I guarantee you they know the cause.

Only thing we can be sure is there is a defect in a very small percentage of these engines causing catastrophic failure.

[Fuel-It!]

One plausible theory is higher boost is pushing against the valves harder adding more force to the hub while the extra power is increasing it's rate of acceleration. But honestly I think it's just a bunk design. I hope it's fixed for the new M3. Just had Carbahn rebuild our M4 motor due to a spun hub.

[SakhirM4]

Originally Posted by M4_Johnnys

We once had a problem in the engine plant with a pressed fit part, once we looked over the data from the past two years we noticed we only saw it in specific months... Which happened to be summer, turns out the 30 degrees difference was enough to cause an issue in roughly 2% of parts.

Another issue was due to when a big compressor was switched on it caused an issue with the electricity supply and the machine that was doing the head surface would cause an uneven cut (that wasn't even noticeable by eye) and the gasket wouldn't seal properly.

Even though it's fun to speculate we will never know the cause unless BMW tells us and I guarantee you they know the cause.

Only thing we can be sure is there is a defect in a very small percentage of these engines causing catastrophic failure.

In almost all cases of the spun hub, there was no catastrophic engine failure.

[cryptz]

Originally Posted by Kooch

I've been thinking the same. To spin the hub you basically need the crank to turn and something to "hold back" the timing chain. Only link I can see to higher power output is the rate of rpm changes of the engine. Which in theory the opposing force would be inertia of the valve train and associated components. So higher power would mean faster rate of change of rpm when applied to some point that the crank wants to accelerate faster than the valve train. Just my thoughts.

Edit: meant to quote your first post...

wouldnt reving the engine in neutral be a much more drastic rate of rpm increase?

[wsccsw13]

Originally Posted by Fuel-It!

One plausible theory is higher boost is pushing against the valves harder adding more force to the hub while the extra power is increasing it's rate of acceleration. But honestly I think it's just a bunk design. I hope it's fixed for the new M3. Just had Carbahn rebuild our M4 motor due to a spun hub.

B58 and S58 for G series both have a redesigned, keyed hub. BMW learned from this mistake at least.

[///Mangler]

Feedback from the drivetrain aft the crank+ firing pulses+ drag from the ancillaries+ resonant frequency coupling of all inputs......failure.

T

[Racer20]

Originally Posted by ///Mangler

Feedback from the drivetrain aft the crank+ firing pulses+ drag from the ancillaries+ resonant frequency coupling of all inputs......failure.

T

Could that be the reason for the carbon driveshaft change last year? Change the resonant frequency of the system to prevent coupling?

[MR RIZK]

Originally Posted by Racer20

Could that be the reason for the carbon driveshaft change last year? Change the resonant frequency of the system to prevent coupling?

Was for packaging reasons to add emissions related equipment ie: ppf

https://www.bmwblog.com/2017/08/12/c...eplaced-steel/