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Re: WHY WOULD U REDUCE CRANK COUNTER WEIGHTS
[Re: bwdst6]
#1160356
01/19/12 01:57 AM
01/19/12 01:57 AM
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Joined: Jul 2010
Posts: 2,091
Delray beach, Florida
Performance Only
top fuel
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top fuel
Joined: Jul 2010
Posts: 2,091
Delray beach, Florida
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Quote:
Quote:
We're just saying that the distribution of that mass away from the centerline will affect how quickly the rotating assembly can be accelerated.
Yeah, I know. And That's incorrect. If the moment of inertia is the same than rotational acceleration is the same. This is just high school physics.
If you want to keep the mass closer to the centerline you'll need to add more of it to keep balance.
you would really need to understand how engines are balanced to understand why it's not as simple as you think.
here's a little more info compliments of the internet.
Moment of inertia is the term used to measure or quantify the amount of mass located at an object's extremities. For example if all the mass of an object was located in a small compact size (like a lead ball) its moment of inertia would be small compared to the same amount of mass shaped into a dumbell. Because a dumbell has most of its mass located farther from its center. But there is a "qualification" here. Moment of inertia is calculated relative to a hypothetical spin axis. Once you choose the spin axis then you calculate the moment of inertia by multiplying the mass times its distance to the spin axis squared; I = MR^2
So in the example between the sphere and the dumbell the moment of inertia of the dumbell would be significantly larger relative to a spin axis perpendicular to the dumbell length. If you instead choose your spin axis to lie thru the dumbell parallel to its length then its moment of inertia,relative to this axis, would be much smaller because the mass would be located closer to that axis. And ,in fact, it might even be smaller then the moment of inertia of a sphere about an axis thru its center.
A way to store kinetic energy is with a spinning object. In calculating how much energy would be stored in a spinning object the energy is proportional to the moment of inertia. So all other things being equal you would choose a spin axis and object shape that gave you the largest moment of inertia for maximum stored energy.
Last edited by Performance Only; 01/19/12 02:04 AM.
machine shop owner and engine builder
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Re: WHY WOULD U REDUCE CRANK COUNTER WEIGHTS
[Re: bwdst6]
#1160361
01/19/12 06:13 AM
01/19/12 06:13 AM
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Joined: Jul 2009
Posts: 413
Norway (old world)
Oyvind Mopar
mopar
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mopar
Joined: Jul 2009
Posts: 413
Norway (old world)
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Quote:
Quote:
We're just saying that the distribution of that mass away from the centerline will affect how quickly the rotating assembly can be accelerated.
Yeah, I know. And That's incorrect. If the moment of inertia is the same than rotational acceleration is the same. This is just high school physics.
If you want to keep the mass closer to the centerline you'll need to add more of it to keep balance.
Yes, I would do it your way, and it is also proven (refer to Cab Burge below) in the racing crankshaft where they put the weight in the periferi (mallory metal on pendulum throws), even undercut the throws to take away dead weight. Turning down the throws on another engine is going in the opposite direction, It is all about balancing forces most efficiently, and get the total weight as low as possible.
Another way to do it is like Frod and GeM do, to save an ounce of cheap material (to gain profit) by putting the mass externally to the damper and flexplate. Increases stress (if you can really say these engines are stressed!) but saves on total weight. Most efficient way to reduce the crankthrow for total weight saving would be to cut the sides from them, and keep the mass opposite of the crankpin where it generates most of the counterforces for balancing the rod/piston assy.
My 
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Re: WHY WOULD U REDUCE CRANK COUNTER WEIGHTS
[Re: Oyvind Mopar]
#1160362
01/19/12 07:57 AM
01/19/12 07:57 AM
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Joined: Jan 2003
Posts: 3,575
The Netherlands
BigBlockMopar
master
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master
Joined: Jan 2003
Posts: 3,575
The Netherlands
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Quote:
If you want to keep the mass closer to the centerline you'll need to add more of it to keep balance.
Technically/Mathematicly, that line is correct.
But not in real life.
Because, going from stock to a performance engine, balancing a (stock or new) crank never means 'adding' weight somewhere.
If you need to do that, you've screwed up earlier and now need to fix your mistake.
Race parts are pretty much always lighter than factory parts.
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Re: WHY WOULD U REDUCE CRANK COUNTER WEIGHTS
[Re: SCATPACK 1]
#1160363
01/19/12 10:23 AM
01/19/12 10:23 AM
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Joined: Nov 2004
Posts: 3,544
Syracuse,NY
CompWedgeEngines
master
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master
Joined: Nov 2004
Posts: 3,544
Syracuse,NY
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I'm just curious for the sake of being curious here...lol...I know some engineers, I know some very well " entrenched" shops who do some pretty technical builds shall we say,I know some NASCAR guys, I know some everyday guys who build a lot, I know some manufacters that BUILD and engineer these cranks, and I have my own 27 years experience and so forth and my current engine shop. My simple question is here, how many of the guys responding own, operate, or HAVE operated a crank balancer? Not just watched one run, not watched their crank get balanced, but actually DID it? We have 27 something responses, lets see how it shakes out. How many guys have built over 100 engines, over say 35 engines, and how many maybe 5 or less engines? My opinion is there is a TREMENDOUS mix of theory and reality here. What SHOULD work, what COULD work, and what DOES work.My point being, both sides have valid points, but I think in REALITY, in an engine, under load, accelerating down a track, things arent always as assumed. I know some things ( that I am not going to get into a debate on...lol) thaty absolutely DO work, in a stock eliminator car, that most of you would say is wrong.It goes against much of the theory, but in a practical mans thinking, makes sense.It revolves around the 4 speed components, and involves some of what is being discussed here. No finger pointing, just some things that popped up in my mind while reading this. I have my own curiosity is all.....carry on Hey, its cold and wintertime, I'm just thinking. 
RIP Monte Smith
Your work is a reflection of yourself, autograph it with quality.
WD for Diamond Pistons,Sidewinder cylinder heads, Wiseco, K1 rods and cranks,BAM lifters, Morel lifters, Molnar Technologies, Harland Sharp, Pro Gear, Cometic, King Engine Bearings and many others.
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Re: WHY WOULD U REDUCE CRANK COUNTER WEIGHTS
[Re: CompWedgeEngines]
#1160364
01/19/12 11:03 AM
01/19/12 11:03 AM
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Joined: Jan 2003
Posts: 2,129
Vermont
TrWaters
top fuel
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top fuel
Joined: Jan 2003
Posts: 2,129
Vermont
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I am in the 5 or less category.  To answer the OP question, I would say the least technical answers would be 1: for clearance issues, and 2: to aid in the balancing procedure.  After that I would guess to modify the effects on acceleration.  But then again, I am not an engine builder, nor do I claim to be.
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Re: WHY WOULD U REDUCE CRANK COUNTER WEIGHTS
[Re: CompWedgeEngines]
#1160365
01/19/12 11:06 AM
01/19/12 11:06 AM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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I have actually balanced 4 cranks(my own stuff) on a balancer(using the shops balancer)before that they (shop) did the balancing and I've built a FEW engines 
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Not all "theorys" are accurate, especially when it comes to racing 
