Is a lightened flywheel overrated? (I think so)

[Chris in CT]

OK,OK I was going to try to stay out of this, but I just can't help myself! I'd like you to think about the word, "damping" and throw the balance weight out the window for a moment. That 63 # flywheel was put there to "damp" the impulses from the cylinders as they fired. The 63# flywheel also allowed the driver to "set it and forget it" as far as the timing advance lever was concerned, since at the lower compression of the stock engine the revolving flywheel would easily overcome timing inconsistencies.
What happens with the so-called "counterbalanced" crankshaft is that each of the slings (counterweights) "damps" the combustion impulses right under the subject cylinders, rather than at the end of a long, twisty, spindly piece of forged steel. Once you have effected that "equalization" of combustion impulses, it is no longer necessary to have 63# of cast iron at one end of the engine - in fact, it is counter-productive because it will continue to cause the crankshaft to twist with each impulse where with a lightened flywheel that twist will be proportionately reduced.

The only way to actually prove this to our scientifically-minded brethren is to perform a Fourier Analysis on the rotating assembly, a complicated process requiring a lot of time and equipment.

Oh, incidently, have those of you who have removed 20# or so from your flywheel noticed that the engine is much more sensitive to the position of the ignition advance lever than it was when the flywheel was full weight? I have. So much so that I am tempted to go out and get a centrufigal dizzy! When I go for a drive, it keeps me quite busy trimming the thing. Happy Motoring, Guys!
Chris

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www.burlingtoncrankshaft.com

[BRENT in 10-uh-C]

Originally Posted by Chris in CT

OK,OK I was going to try to stay out of this, but I just can't help myself! I'd like you to think about the word, "damping" and throw the balance weight out the window for a moment. That 63 # flywheel was put there to "damp" the impulses from the cylinders as they fired. The 63# flywheel also allowed the driver to "set it and forget it" as far as the timing advance lever was concerned, since at the lower compression of the stock engine the revolving flywheel would easily overcome timing inconsistencies.
What happens with the so-called "counterbalanced" crankshaft is that each of the slings (counterweights) "damps" the combustion impulses right under the subject cylinders, rather than at the end of a long, twisty, spindly piece of forged steel. Once you have effected that "equalization" of combustion impulses, it is no longer necessary to have 63# of cast iron at one end of the engine - in fact, it is counter-productive because it will continue to cause the crankshaft to twist with each impulse where with a lightened flywheel that twist will be proportionately reduced.

The only way to actually prove this to our scientifically-minded brethren is to perform a Fourier Analysis on the rotating assembly, a complicated process requiring a lot of time and equipment.

Oh, incidently, have those of you who have removed 20# or so from your flywheel noticed that the engine is much more sensitive to the position of the ignition advance lever than it was when the flywheel was full weight? I have. So much so that I am tempted to go out and get a centrufigal dizzy! When I go for a drive, it keeps me quite busy trimming the thing. Happy Motoring, Guys!
Chris

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www.burlingtoncrankshaft.com

Yeah, me too Pete!

Hey Christopher, I'm glad you jumped in!! Thanx!! So using a Burlington crank as the baseline, what do you feel is the optimum weight for the flywheel/PP assembly on your crank using a 5.5/5.9 head and a Stipe cam that makes good torque in the low range??

[MikeK]

He He, now I'm loving it too. I used to make engineering students pee their pants by chalking "Fourier" on the board. Applied to physical materials harmonic analysis problems, you'll need to decide what transforms are applicable. I vote for using eigenfunctions as delineators. LINK

Sometimes in engineering you hit the theoretical wall, and just need to do hard physical operational testing if you want answers. In lieu of setting it all up on a test stand (or cleaning up all the puke after the above lecture) I offer the following alternative calculator. I believe this will give the answers Brent seeks.

[goodcar]

Originally Posted by MikeK

He He, now I'm loving it too. I used to make engineering students pee their pants by chalking "Fourier" on the board. Applied to physical materials harmonic analysis problems, you'll need to decide what transforms are applicable. I vote for using eigenfunctions as delineators. LINK

Sometimes in engineering you hit the theoretical wall, and just need to do hard physical operational testing if you want answers. In lieu of setting it all up on a test stand (or cleaning up all the puke after the above lecture) I offer the following alternative calculator. I believe this will give the answers Brent seeks.

Very amusing, my kind of teacher. Had one applied calculus class in my electronics tech school education. Got really good grades but never had a clue as far as really understanding it. Now almost 72 but back then had a good memory and knew if I got a certain type of problem I knew what the answer should look like. Had I gone on to more advanced math I would have flunked. Nice pictures and explanations Mike, you're obviously very smart but I really appreciate your humor and cut the crap conclusion. Drive and enjoy the Model A. There are numerous aspects to this hobby, something for everyone.

[Chris in CT]

Originally Posted by BRENT in 10-uh-C

Yeah, me too Pete!

Hey Christopher, I'm glad you jumped in!! Thanx!! So using a Burlington crank as the baseline, what do you feel is the optimum weight for the flywheel/PP assembly on your crank using a 5.5/5.9 head and a Stipe cam that makes good torque in the low range??

Hi Brent,
This is getting pretty funny. Eigen functions, huh? That Ouija board is looking pretty good right now. Again, if we were to perform a Fourier Analysis on the rotating assembly, we could come up with an ideal weight for the flywheel/pressure plate assembly. My guess, based entirely on Ouija board consultations would be somewhere between 30 and 36 lbs as an ideal weight for an I-4 of 200 cubic inches and three mainbearings. My own flywheel is 40lbs stand-alone, but the next time I have the engine apart I think I'll try to get it to 36 w/ pressure plate. I have a single channel Fast Fourier Analyser, but you really need a two channel unit to do the necessary calculations... By the way, is your Ouija board single or dual channel?

Happy Thanksgiving to all cranks everywhere - balanced and unbalanced!

[Rowdy]

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Now another thought. If a person chooses to lighten a flywheel some will depend on where the weight is removed the most, outboard or inboard next to the crank flange. Just some thoughts about inertia in this whole debate. Rod

[Jim/GA]

Originally Posted by Rowdy

Now another thought. If a person chooses to lighten a flywheel some will depend on where the weight is removed the most, outboard or inboard next to the crank flange. Just some thoughts about inertia in this whole debate. Rod

I agree. See post #35.

[Jim Huseby]

I have more layman's questions concerning the energy the flywheel must store and release. At firing impulse on a stock A, the crankshaft flexes torsionally from the energy imparted at the journal and must then "snap back" (and vibrate somewhat torsionally). When it snaps back (primary return pulse), it tries to impart more spin to the flywheel and it also tries to push the rod and piston backwards. Am I right so far? This un-even rotation of the rotating mass (it was already uneven because of uneven rod angularity during each rotation, pressure cycling, etc.) results in another character in the vibration signature. What is the duration of the primary pulse/return pulse? The point in crankshaft angularity when the return pulse will occur increases as rpm rises and will not only change the vibration signature, but will also change the magnitude and timing of the pulses in relationship to the crank's natural frequency. (Harmonics implied.) One of many new thoughts to me, as already stated in this thread, is that the counterweight's mass performs another important function in addition to "counterweighting". (You can't completely counterweight a mass that is part unevenly-reciprocating and part rotating with a purely rotating mass.) That other important function is to reduce crank flex.
It seems to me that the vibration signature would be different for any point it could be measured along the length of the crankshaft. This thread has moved my "theoretical wall" either past my present view or I am against the wall.

[Pete]

Originally Posted by Jim Huseby

I have more layman's questions concerning the energy the flywheel must store and release. At firing impulse on a stock A, the crankshaft flexes torsionally from the energy imparted at the journal and must then "snap back" (and vibrate somewhat torsionally). When it snaps back (primary return pulse), it tries to impart more spin to the flywheel and it also tries to push the rod and piston backwards. Am I right so far? This un-even rotation of the rotating mass (it was already uneven because of uneven rod angularity during each rotation, pressure cycling, etc.) results in another character in the vibration signature. What is the duration of the primary pulse/return pulse? The point in crankshaft angularity when the return pulse will occur increases as rpm rises and will not only change the vibration signature, but will also change the magnitude and timing of the pulses in relationship to the crank's natural frequency. (Harmonics implied.) One of many new thoughts to me, as already stated in this thread, is that the counterweight's mass performs another important function in addition to "counterweighting". (You can't completely counterweight a mass that is part unevenly-reciprocating and part rotating with a purely rotating mass.) That other important function is to reduce crank flex.
It seems to me that the vibration signature would be different for any point it could be measured along the length of the crankshaft. This thread has moved my "theoretical wall" either past my present view or I am against the wall.

A good point Jim. If you take the reverse impulse moment and apply Chlomondely's grillage coefficient you will see that the forward strake force is cancelled and the engine will run like a top.

[MikeK]

Originally Posted by Chris in CT

. . . I have a single channel Fast Fourier Analyser, but you really need a two channel unit to do the necessary calculations... By the way, is your Ouija board single or dual channel?

Single channel:

Dual channel:


If you have a single channel fast fourrier transform (FFT) device you could still do independent acceleration analysis at different axis points and get a very good data set for good 'ol pencil and paper 3-axis graphing. It would take a while (forget 'fast').

Of course, there will always be 968 1/2 variables unaccounted for in any specific engine build, so perhaps Brent's "Come one, come all.." statement is valid. A multi channel Ouija may be the way to go.