MM-5

Hi all

I would like to get more information and speculate about the S38B38 crankshaft vibration damper. Is it "tuned" to reduce torsional vibration at some specific rpm or rpm range? I found this universal information about vibration dampers from wikipedia:

Tuned absorber type of "dampers" often referred to as a harmonic dampers or harmonic balancers (even though it technically does not dampen or balance the crankshaft). This damper uses a spring element (often rubber in automobile engines) and an inertia ring that is typically tuned to the first torsional natural frequency of the crankshaft. This type of damper reduces the vibration at specific engine speeds when an excitation torque excites the first natural frequency of the crankshaft, but not at other speeds. This type of damper is analogous to the tuned mass dampers used in skyscrapers to reduce the building motion during an earthquake.

Is S38B38 vibration damper this type? If it´s tuned for some specific rpm or rpm range, what happens when flywheel and clutch mass is reduced very much?

From original dual mass flywheel and original clutch assembly combined weight 20,37 kg to custom made flywheel 4 kg and racing clutch 3,5 kg combined weight 7,5 kg. There is 12,87 kg less weight in the end of the crankshaft with these parts. The manufacturer has put very much weight in the end of the crankshaft and the design was done to use with this original vibration damper, that weight´s 4,54 kg and belt pulley is 880 grams, so combined weight 5,42 kg in front of the crankshaft.

What happens to the designed vibration reduction properties when there is lightweight parts in the end of the crankshaft and much lighter engine internals (complete piston -150 gram less/piston than original, connection rods -100 gram less/connecting rod than original) and all with very well balancing.

What happens to vibration dampers ability to dampen torsional vibration in this kind of engine? Is it capable to reduce torsional vibration at higher rpm / rpm range, or is it´s factory designed rpm range still functioning? In that lightweight internal engine the rev range is going to rise as high as 8000 rpm. Is this possible with the original vibration damper or must it be changed to some other type vibration damper like fluid damper?

Those M1 racing engines in late 1970´s and early 1980´s had 9200 rpm rev limit. Did they use original vibration damper in that engine? Or could it be possible to take the whole vibration damber away or use much lighter one, because inline 6 engine design when very well balanced has very little vibration compared to inline 4 engines. Crankshaft lenght may be the reason that it must be used.

Please give me your thoughts about this interested subject.


Best Regards, Janne.

MM-5

Here is a good link to Dinan where is explained why it is essential to use vibration damper:

Dinan BMW - The Dangers of Power Pulleys and Understanding the Harmonic Damper

But I have still the question, how can original vibration damper control torsional vibrations at higher engine speeds than original 7200 rpm? As how I understand that writing in the link it is more important to have original weight vibration damper when flywheel and clutch are much lighter:

"This motion is complicated in the case of a crankshaft because the amplitude of the vibration varies along the shaft. The crankshaft will experience torsional vibrations of the greatest amplitude at the point furthest from the flywheel or load."

It would be also very intressent to know what are the so called critical rpm areas of this crankshaft S38B38. How to get these are another story . Maybe BMW Motorsport knows?

Best Regards, Janne.

Eku

Moi Janne

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