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Old 03-16-2010, 09:56 AM   #16
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If they were just trying to balance against a fraction of the piston/rod(not the entire wieght is offset because of pivot/balance points of the stroke) Why not just drill out to remove wieght. The lugs are usually made of tungsten-very heavy. They are there to ADD mass in specific parts of the lobe. Adding mass to a steel crank to offset a aluminium piston is not neccessary to harmonicly balance a crank.
Cranks already have almost no weight where you would need to remove for balancing

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Old 03-16-2010, 10:13 AM   #17
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The "extra weight" I was referring to was the slug. You still remove material to install the slugs yet they can be positioned incrementally +/- of the zero balance point.


The "inertness" of the added weight in a single cylinder engine when placed in different positions on the counter weight will aid or detract in the torque being built by the firing of the cylinder.

If there is no effect why are the slugs placed where they are in the counter balance? They are opposite of the knife edge and right of pin center. Why not just manufacture the crank correctly in the first place?
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Old 03-16-2010, 11:08 AM   #18
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The "extra weight" I was referring to was the slug. You still remove material to install the slugs yet they can be positioned incrementally +/- of the zero balance point.


The "inertness" of the added weight in a single cylinder engine when placed in different positions on the counter weight will aid or detract in the torque being built by the firing of the cylinder.

If there is no effect why are the slugs placed where they are in the counter balance? They are opposite of the knife edge and right of pin center. Why not just manufacture the crank correctly in the first place?
Exactly, the reason the weight is added where where it is is to balance the crank/rod/pin/clip/piston assembly, if not, they would just leave more material around the crank's main journal and have the same overall/total weight.
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Old 03-16-2010, 11:35 AM   #19
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having a crank that is truely straight is more important then balanced
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Old 03-16-2010, 05:39 PM   #20
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having a crank that is truely straight is more important then balanced
Plastic mallet and a 2x4........ ???
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Old 03-16-2010, 06:36 PM   #21
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Default Well Then Whats Up With This?

I have a AxeRossi crank that has 2 tungsten? plugs right of center. And I have a Nova P5? crank that is drilled just left of center. Both cranks are identical in dimentions. I am actually running the P5 in the Axerossi(broke a damn screw off in the end) Why the different methods? I understand the flywheel adds mass but so does the crank. Look at MX bikes they have vitually no flywheel. In fact if run run in enduros with a MX bike most will add a heavy flywheel to smooth power and keep from stalling. More tractor like toque is what is desired. Im not trying to confront anyone I'm just asking.
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Old 03-16-2010, 09:41 PM   #22
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In fact if run run in enduros with a MX bike most will add a heavy flywheel to smooth power and keep from stalling. More tractor like toque is what is desired. Im not trying to confront anyone I'm just asking.

We have the same in R/C. M2C makes a variable weight flywheel.

http://trackstarrc.com/trackstar/ind...mart&Itemid=64
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Old 03-16-2010, 11:01 PM   #23
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Inertia is changed by either using steel or alum flywheel. Has nothing to do with the slugs added to the crank that is for balancing the crank only. When you knife edge the crank or add scoops or whatever you took metal off the crank. To put it back in balance you have to add heavy metal slugs has nothing to do with inertia. If you think you are adding weight to add to inertia you going to have one out of balance beast that will shake an vibrate the whole car apart. Yes you can balance 1 cyl engine it's called weighting the piston/rod assemble then using bob weights (same weight as piston/rod assemble) attach them to the rod jounal and spin the crank to balance. Like in the same manner when you balance your wheels. Except it's hooked to a machine that tell you where exactly where to add or remove metal to balance it.

I had early 347 stroke kit it was turned down 351W crank to fit a 302 it took 7 slugs of heavy metal (mallory 50 bucks a slug 1x1 round) to balance it out. Or is someone going to tell it was done for inertia because every machine shop and engine builder will laugh at you. I'm a engine builder and mechanic
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Old 03-17-2010, 03:17 AM   #24
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Inertia is changed by either using steel or alum flywheel. Has nothing to do with the slugs added to the crank that is for balancing the crank only. When you knife edge the crank or add scoops or whatever you took metal off the crank. To put it back in balance you have to add heavy metal slugs has nothing to do with inertia. If you think you are adding weight to add to inertia you going to have one out of balance beast that will shake an vibrate the whole car apart. Yes you can balance 1 cyl engine it's called weighting the piston/rod assemble then using bob weights (same weight as piston/rod assemble) attach them to the rod jounal and spin the crank to balance. Like in the same manner when you balance your wheels. Except it's hooked to a machine that tell you where exactly where to add or remove metal to balance it. In other words I call it compressing forces and unlike other timings it is a reciprical timing and a rotational timing, 2 separate timings.

I had early 347 stroke kit it was turned down 351W crank to fit a 302 it took 7 slugs of heavy metal (mallory 50 bucks a slug 1x1 round) to balance it out. Or is someone going to tell it was done for inertia because every machine shop and engine builder will laugh at you. I'm a engine builder and mechanic
Your talking about a 4 stroke full size car engine. 2 stroke have a entirely different approach. Depending where you place the tungsten slugs depend on what power curve you want. Completely different from a tire. Tires don't have to compensate for reciprocating mass only rotating mass. When I build a drag engine it is completely different the off road or oval or street drags, even boat engines have there own setup on how the weight effects them, even aircraft is much different then these other typs of engines. Each engine has many different mods on them just depending on what type of driving you do. the primary force is equivalent to the component along the line of stroke of the centrifugal force due to an equal rotating mass with the crank and at crank radius. Consequently, in the case of a single-cylinder engine, the primary reciprocating force could be balanced by a rotating mass on the other side of the crank pin. However this would introduce an unbalanced component of the centrifugal force of magnitude perpendicular to the line of stroke. A compromise solution (partial balance) is usually applied at the factory, the inertia force being reduced to a minimum when a variable percent of the reciprocating mass is balanced.

The secondary force is similarly equivalent to the component of the centrifugal force of mass M at radius r/4n rotating at being coincident with the crank at inner dead-centre. Inertia begins at 0 center and increases exponentially with speed works it's way outward. depending on the amount of weight and where it's located at determines who fast the transference of that weight is located trough out the cycle. I call it compressed timing but is easier to call it crank timing balance that the mass of people can understand better. A v8 has opposing energy forces and is a different balance altogether.

Last edited by Rick B; 03-17-2010 at 04:34 AM.
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Old 03-17-2010, 11:21 AM   #25
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Your talking about a 4 stroke full size car engine. 2 stroke have a entirely different approach. Depending where you place the tungsten slugs depend on what power curve you want. Completely different from a tire. Tires don't have to compensate for reciprocating mass only rotating mass. When I build a drag engine it is completely different the off road or oval or street drags, even boat engines have there own setup on how the weight effects them, even aircraft is much different then these other typs of engines. Each engine has many different mods on them just depending on what type of driving you do. the primary force is equivalent to the component along the line of stroke of the centrifugal force due to an equal rotating mass with the crank and at crank radius. Consequently, in the case of a single-cylinder engine, the primary reciprocating force could be balanced by a rotating mass on the other side of the crank pin. However this would introduce an unbalanced component of the centrifugal force of magnitude perpendicular to the line of stroke. A compromise solution (partial balance) is usually applied at the factory, the inertia force being reduced to a minimum when a variable percent of the reciprocating mass is balanced.

The secondary force is similarly equivalent to the component of the centrifugal force of mass M at radius r/4n rotating at being coincident with the crank at inner dead-centre. Inertia begins at 0 center and increases exponentially with speed works it's way outward. depending on the amount of weight and where it's located at determines who fast the transference of that weight is located trough out the cycle. I call it compressed timing but is easier to call it crank timing balance that the mass of people can understand better. A v8 has opposing energy forces and is a different balance altogether.
You way off but believe what you want too. What you are saying 2 strokes are not balanced only 4 strokes are. You are so far off from that. Inertia thing you are talking about is more done by the flywheel not the crankshaft. You can't lighted or add metal to the crank without changing the mass weight of the piston/rod assemble. They have to be balanced to each other.

But go ahead add more mass to the crank watch it destroy itself it's your engine.
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Old 03-17-2010, 11:34 AM   #26
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Default Single Cylinder Engine Balance

This article is pretty straight forward and helped me understand what is involved with balancing a single cylinder two stroke engine. You all have good input on the subject. Very entertaining thread...

Single cylinder engines
A single cylinder engine produces three main vibrations. In describing them we will assume that the cylinder is vertical.

Firstly, in an engine with no balancing counterweights, there would be an enormous vibration produced by the change in momentum of the piston, gudgeon pin, connecting rod and crankshaft once every revolution. Nearly all single-cylinder crankshafts incorporate balancing weights to reduce this.

While these weights can balance the crankshaft completely, they cannot completely balance the motion of the piston, for two reasons. The first reason is that the balancing weights have horizontal motion as well as vertical motion, so balancing the purely vertical motion of the piston by a crankshaft weight adds a horizontal vibration. The second reason is that, considering now the vertical motion only, the smaller piston end of the connecting rod (little end) is closer to the larger crankshaft end (big end) of the connecting rod in mid-stroke than it is at the top or bottom of the stroke, because of the connecting rod's angle. So during the 180° rotation from mid-stroke through top-dead-center and back to mid-stroke the minor contribution to the piston's up/down movement from the connecting rod's change of angle has the same direction as the major contribution to the piston's up/down movement from the up/down movement of the crank pin. By contrast, during the 180° rotation from mid-stroke through bottom-dead-center and back to mid-stroke the minor contribution to the piston's up/down movement from the connecting rod's change of angle has the opposite direction of the major contribution to the piston's up/down movement from the up/down movement of the crank pin. The piston therefore travels faster in the top half of the cylinder than it does in the bottom half, while the motion of the crankshaft weights is sinusoidal. The vertical motion of the piston is therefore not quite the same as that of the balancing weight, so they can't be made to cancel out completely.

Secondly, there is a vibration produced by the change in speed and therefore kinetic energy of the piston. The crankshaft will tend to slow down as the piston speeds up and absorbs energy, and to speed up again as the piston gives up energy in slowing down at the top and bottom of the stroke. This vibration has twice the frequency of the first vibration, and absorbing it is one function of the flywheel.

Thirdly, there is a vibration produced by the fact that the engine is only producing power during the power stroke. In a four-stroke engine this vibration will have half the frequency of the first vibration, as the cylinder fires once every two revolutions. In a two-stroke engine, it will have the same frequency as the first vibration. This vibration is also absorbed by the flywheel.
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Old 03-17-2010, 11:37 AM   #27
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You way off but believe what you want too. What you are saying 2 strokes are not balanced only 4 strokes are. You are so far off from that. Inertia thing you are talking about is more done by the flywheel not the crankshaft. You can't lighted or add metal to the crank without changing the mass weight of the piston/rod assemble. They have to be balanced to each other.

But go ahead add more mass to the crank watch it destroy itself it's your engine.
I agree here. Also an automotive mechanic. Logic is logic.
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Old 03-17-2010, 12:55 PM   #28
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Thanks for posting that mike !!

Ends some confusion that was going on here

These engines are so simple yet so complex aren't they ?
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Old 03-17-2010, 01:30 PM   #29
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I think we are talking about 2 different things here so I'll leave you with your thoughts.
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Old 03-17-2010, 02:02 PM   #30
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Rick, He's a four stroke, multi cylinder guy. It took me a while to get schooled on why these little nitro engines work the way they do and the effects things like balance have.

pdmustgt, I mean no disrespect. But I have to ask a question. Do you balance a Top Fuel engine the same way you do one that burns Methanol or Gasoline?
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