Aasco flywheel is here.

[insite]

continuing....

it is my personal belief that the M96 doesn't really have issues with torsional harmonics. a lot of people make the case that the dual mass flywheel & its elastomer serve as a harmonic balancer. i don't think they really do. in general, torsional harmonics go out of control at the FRONT of the motor because the driveline serves to damp the other end. i think the DMF was designed to reduce driveline noise. this becomes readily apparent when you swap in a single mass LWFW. even with a sprung clutch, the 5-speed gearbox is NOISY without the DMF. if porsche was worried about torsional harmonics, they would have added a harmonic balancer to the front of the crank.

more in a bit.

[insite]

okay, so far we've determined that x-plane, y-plane and z-plane harmonics are not really an issue for this motor. what does that mean, exactly? this doesn't mean that the harmonics don't exist; some are cancelled, but some DO exist. it simply means that they don't exist with enough AMPLITUDE to cause an excited harmonic state. it's a lot like that glass in physics class: even if the frequency is right, without enough volume, the glass is unaffected.

at certain RPM, harmonics create, in effect, a standing wave throughout the crankshaft. that standing wave exists at the NF, but with amplitude insufficient to feed itself. were it to be excited further by some external force at the same frequency, its amplitude would grow exponentially until it is either effectively damped by the rest of the assembly OR until something breaks.....

more to come

[insite]

so we understand that at certain RPMs, we probably have a controlled standing wave in the crank. what happens if the flywheel / clutch is out of balance about 17g as in my case?

first, there is centrepetal force to consider. at 7300RPM at the edge of a 15" disk, the equivalent static mass is about 425lb. if the first bearing is 3/4" wide and sits 3" from the flywheel flange, the leverage at the center of the bearing means the 425 lbs of force is multiplied by 16. this is now 6800lb.

now what if the standing wave in the crank has a frequency equal to 1, 2 or 3x the engine speed (i.e. 122Hz, 243Hz or 365Hz)? now that controled standing wave has an external stimulus to its amplitude. every 'peak' in that standing wave is pushed up further by 6800lb. at this point, it is possible to create an excited harmonic state. this will further increase that 6800lb force, and that 'wave' will ripple along the entire crank.

so what happens? first is some noticable vibration. beyond that, it's possible that nothing happens. it's also possible that the force at the first bearing begins to hammer it flat. it's also possible that the crystalline structure of the crank provides one or more favored flow paths for the additional stress to follow. if this is the case, cracks will form & eventually the metal will fail.

M96 crankshafts are sintered & hardened. i'm not a metallurgist, but basically, sintered metal parts are manufactured from powder. the powder is heated in a gas layer to a temperature below its melting point. at a certain point, the atoms start to diffuse across the particle boudaries & molecular bonds are formed. it's sort of a compromise between the weakness of casting & the cost of forging. at any rate, issues with sintering have to do with shape & graining. it is possible to have some molecular 'weak spots' that are prone to fatigue & fracture.

the bottom line is this: if the stars align, so to speak, a catastrophic failure will occur. without significant research & data, it isn't possible to know specifically what all the driving factors are. one thing is for sure, though: imbalance at the flywheel can serve to STRONGLY exacerbate conditions that were previously in control. my guess is that the broken M96 cranks w/ the LWFW have more to do with imbalance than they do with removal of the DMF.



to other engineers: i know i left a lot of stuff out; i did that on purpose. this is a rough, brief primer that hopefully non-engineers can understand. also, i may have one or two mistakes as harmonics are not my specific field. i believe the gist, however, to be correct.

[Topless]

Nice! Excellent analysis of mechanical resonance as applied to our Boxster drive trains. You have a patient writing style that is clear and easy to understand Kevin.

For more in depth info on the physics of harmonic resonance you can start here:
http://en.wikipedia.org/wiki/Mechanical_resonance

Or read the writings of Nikola Tesla, who's amazing discoveries in harmonic resonance are still not fully understood.

[JAAY]

I live next door to Tesla's old plant. Really!

[Topless]

Originally Posted by JAAY

I live next door to Tesla's old plant. Really!

Very cool piece of science and engineering history!

[insite]

Originally Posted by Topless

Nice! Excellent analysis of mechanical resonance as applied to our Boxster drive trains. You have a patient writing style that is clear and easy to understand Kevin.

thanks! glad it was coherent.

[thstone]

Thank you very much. I greatly appreciate the time that you put into this response. Let me digest and get back as soon as I can.

[insite]

a few pics of the balanced assembly.

flywheel: