Crank Hub Failure

JohnDaviz

Lieutenant
Jan 6, 2019
863
577
0
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335i E92 DCT
So there are different threads where here and there ppl talk about the crank hub failure and basically only fixes are proposed.

But can somebody explain to me how this actually happens? Is it on engines with hard cut rev limiter? I struggle to understand when and why this exactly happens.
 
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rac

Sergeant
Nov 14, 2016
341
204
0
Australia
Ride
135i ST
vibration and 100's of cycles of rpm changes loosening the bolt and working against the faces preventing movement.
sudden changes in rpm would accelerate it, at a guess, problem might be less common among autos.
 
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RuskiRacer

Captain
Jul 17, 2019
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757
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2009 e93 335i
Its definitely more common for dcts and manuals thats why its more of an issue on the s55 motors. Same design concept , so since the s55 makes a bunch more torque there more common to spin them. When I spun mine I was at about 7500rpm with 4 guys in my e93 and it sounded awful. Mine probably be used as a 1 piece hub now since it welded all the friction disks to each other essentially
 
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bakerdou

Private
May 13, 2021
42
19
0
The factory crank hub bites with the crank end basically by a friction disc design. It holds tightly most of the time.
When the RPM changes, there will be shear force applied to the disc. The scale of the force is correlated to parameters like engine load, RPM, torque, etc.

So in relative short terms, if there is huge variation of engine rpm under full load, say a downshift when the engine is accelerating full boost, the hub will see the most disadvantageous situation. The shear force may exceed the holding capacity for the friction discs, causing the separation of hub&crank, hence the faulty valves.

In long term situation, the factor of fatigue seems to be more of the concern. If the car is frequently tracked, the friction disc may wear, and eventually lose grab like old clutches.

But I have seen no experiment with all the variables strictly controlled so above is more or less my speculation. The only solution for now seems to be swap out the factory hub with pinned/one piece ones, as long as no friction discs exist for this part.
 
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