the charistics of a given crank can be used to predict at what rpm harmonics will cause problems
Yah, but the analysis is really complicated even using ex$pensive software. The point is not to detect some metaphysical, sub-atomic internal property, but to find out when a given piece of metal will ring in sympathy to an external shock. It's just a big, weird-shaped tuning fork (no power or compression forces apply here) and it "rings" at a certain frequency - long and thin makes a deeper tone (lower Hertz frequency) than short and thick (higher). Big bearings raise the tone, long stroke lowers it.
Depending on the firing order, number of cylinders, bank angle etc. there are specific "orders" that will stress the crank, and frequency determines the RPM at which these occur.
Where “O” is the number of the order, and Hz is the frequency
RPM = Hz × 60 ÷ O
Hz = RPM ÷ 60 × O
For an L6, the critical orders are (strongest harmonic first) the 3rd, then 2-1/2th, 2nd, and 6th (the 9th is smaller, 1st is outside the normal RPM range).
I made a stab at how it might be estimated by suspending the surgically-clean, undamaged crankshaft in a soundless environment, and either:
A. striking it smartly with a brass hammer, and using a microphone to record the sound, then reading the oscilloscope trace to get the frequency, or
B. using a tone generator to excite the crank by slowly passing through the suspect period (200-250 Hz for an L6) while intently watching an attached .010" wire for vibration?
No one thought it will work, but they couldn't say why.
