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I tend to agree with insite. There are numerous torsional forces in any internal combustion engine, the pulsing of the engine upon firing of each cylinder is easily heard as a car idles ...and the mechanics are the same at higher rpms, but not as noticeable to our ears. The absolute need of a mass damper to avoid breaking a crank seems pretty remote, but I guess it is possible - depending on what the crank is made of..
I would expect this would have everything to do with crank manufacturing techniques and engineering. There will always be resonant frequencies that every crank wants to vibrate at, and this is true in the torsional direction too. How compliant the crank is in the torsional direction is what matters, as also at what frequency the crank wants to recoil at when a torque load is released.as speed increases the the rotational forces will certainly cross that frequency and the crank will be excited. A billet or even a forged crank will by it's nature have a higher natural frequency than a sintered metal crank. They are also much stronger because of the way they were manufactured.
Personally I am not a fan of sintered metal cranks, I believe they are more likely to fail in torsional loading scenarios.
Are the cranks that have failed, all been sintered metal cranks?
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