Aasco flywheel is here.

[Jake Raby]

Insite,
we see that with EVERY LWFW that we balance, yours was no different.

Sorry about not having time to talk to you about it.. we were a little tied up with a car that was stolen for over a week and just showed up.. I had to Commandeer a Mini Cooper driven by a Priest to chase the Boxster down!

Long story :-)

[Cloudsurfer]

Originally Posted by insite

You're assuming the aasco unit is balanced. I had mine done this week. The clutch / fw combo were 17g out of balance. 15g of that was the flywheel. That can break a crankshaft.

07proto - I use a sprung spec stage 1

That's interesting. When I had my AASCO flywheel/ SPEC pressure plate balanced to my crank, the pressure plate was off 2 grams, the flywheel was off 3 grams. That's more in line with the quality standards that I expect from top notch companies like AASCO or SPEC.

[AndyA6]

Originally Posted by Jake Raby

Insite,
we see that with EVERY LWFW that we balance, yours was no different.

Sorry about not having time to talk to you about it.. we were a little tied up with a car that was stolen for over a week and just showed up.. I had to Commandeer a Mini Cooper driven by a Priest to chase the Boxster down!

Long story :-)

Please DO TELL!

Sounds worthwhile

[thstone]

Originally Posted by insite

How many g's do you think are exerted outward by centripetal force at the edge of a flywheel turning 7300 rpm? 100? 1000?

Try ELEVEN THOUSAND THREE HUNDRED. 11,300g at my redline. That means 17grams becomes 425 lb. Over time, that wobble can absolutely fatigue the crank. It's not forged, it's sintered.

My point is that the crank won't fatigue because the force (your calculation of 425 lbs) is far below the fatigue load. This situation isn't like bending a paper clip back and forth until it breaks. This load level is more like me trying to bend a street light post. No matter how long I push on it (months, years, decades), it ain't never gonna break.

This is why owners can drive around with a failing DMFW for thousands of miles (which produces 100x the force of your example LWFW) and everything is still ok.

[insite]

Originally Posted by thstone

My point is that the crank won't fatigue because the force (your calculation of 425 lbs) is far below the fatigue load. This situation isn't like bending a paper clip back and forth until it breaks. This load level is more like me trying to bend a street light post. No matter how long I push on it (months, years, decades), it ain't never gonna break.

This is why owners can drive around with a failing DMFW for thousands of miles (which produces 100x the force of your example LWFW) and everything is still ok.

Sorry, you have no idea what you are talking about

[insite]

Originally Posted by Cloudsurfer

That's interesting. When I had my AASCO flywheel/ SPEC pressure plate balanced to my crank, the pressure plate was off 2 grams, the flywheel was off 3 grams. That's more in line with the quality standards that I expect from top notch companies like AASCO or SPEC.

I have the printouts from the machine. I'll post them when I have a minute to scan them. I also took some pics of the fw with the material removed. I could feel / hear it over 4k rpm. The only reason I pulled the tranny was to balance.

Jake had another good point: the knock sensor can pick this up & dial back the timing.

[Cloudsurfer]

Originally Posted by insite

Sorry, you have no idea what you are talking about

Totally Agree.

[Cloudsurfer]

Originally Posted by insite

I have the printouts from the machine. I'll post them when I have a minute to scan them. I also took some pics of the fw with the material removed. I could feel / hear it over 4k rpm. The only reason I pulled the tranny was to balance.

Jake had another good point: the knock sensor can pick this up & dial back the timing.

Never said I didn't believe ya Just sharing what my results were when I balanced. It would seem very plausible that the knock sensors could pick up this up if there was enough of it too.

[thstone]

Originally Posted by insite

Sorry, you have no idea what you are talking about

Ok, I'll agree and maybe I have it all wrong. Can you please enrich my understanding:

If I recall correctly, most metals have a stress below which they can withstand an infinite number of stress cycles (me pushing on a light post). Above this level, the life disminishes in proportion to the stress and number of stress cycles.

So, the question is: How does the load you calculated (425 lbs) compare to the fatigue stress of sintered steel?

[insite]

you're thinking about this the wrong way. earlier, you said that harmonics were a red herring. they are not; there are force multipliers to consider that make that 425lb load increase by MANY multiples.

one is simple leverage; the flywheel actually sits several inches away from the first bearing inside the case. you have the flywheel flange, some seal surface, some chain sprockets, a little more dead space, and FINALLY a bearing. the 425lb bending moment is actually multiplied several-fold at the point of the first main bearing.

second: harmonics.

consider this: are the pressure waves from my voice strong enough to break a glass? hell no. i can yell at a glass all day long and nothing will happen. however, if ella fitzgerald hits the natural frequency of that glass, it will shatter in a hurry. it's not the force of her voice that does it; it's the fact that she puts the glass into an excited harmonic state that eventually causes enough amplitude to fracture the glass.

if you ever did the glass breaking experiment in physics, you noticed a few things:

1. it WORKS
2. you have to turn up the sound a little bit; if it's too quiet, it doesn't work
3. when you get it loud enough, the glass breaks almost IMMEDIATELY

so to get an object into an excited harmonic state, you need two things:

1. the proper frequency
2. AMPLITUDE: you need enough initial force per cycle to start the runaway harmonic exitation



now on to the engine. there are several forces occuring at once. let's simplify and break them into three planes and a twisting moment:

x plane is axial: along the crankshaft and level with the ground
y plane is axial: along the crankshaft and perpendicular to the ground
z plane is radial: like the flywheel
twisting moment obviously twists the crankshaft


i'm breaking this into multiple posts......post 1 complete.

[insite]

ok, continue. now let's consider our three planes.

in the x plane our pistons are moving. we have a flat-6 motor with three pistons on each side. normally with an inline 3, we'd have a rocking couple as a result of the firing order & odd number of cylinders (the forces would attempt to 'rock' the crank fore & aft). this is precisely balanced by the other bank of cylinders, so a flat 6 is a very well balanced engine to begin with. as a result, heavy counterweights are not necessary.

as for the y plane? since there are no counterweights, things are pretty stable.

combine these factors along with the fact that we have seven main bearings, & things are looking nicely balanced & rigid insofar as the engine configuration is concerned.

now we get into torque. every time a piston fires, it pushes down on a rod journal. this force actually deflects (bends) the crankshaft a little bit, & the crank snaps back. this occurs at the natural frequency of the crankshaft. since the crankshaft is only spinning one way, it isn't possible to counterbalance this force. (FYI, some engines use balance shafts, but they only serve to reduce vibration, NOT do un-do this twisting moment). in engines where the natural frequency of the crankshaft is similar to the frequency of the 1st, 2nd or 3rd order harmonic created at any engine speed between idle and redline, a harmonic balancer is used. it is tuned to the natural frequency of the crank, and the elastomer core of the balancer is designed to absorb this frequency. this prevents an 'excited harmonic state' from occuring at the NF of the crank.

part 2 done, more to come.....

[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: