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 [2 stroke] Weight or balancing factor

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AuteurMessage
Vortex



Nombre de messages : 41
Localisation : Luxembourg
Date d'inscription : 26/11/2013

MessageSujet: Re: [2 stroke] Weight or balancing factor   Dim 16 Fév 2014 - 23:13

Thank you guys for those interesting details.
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Manuel Rainer



Nombre de messages : 98
Localisation : Italy
Date d'inscription : 30/10/2012

MessageSujet: Re: [2 stroke] Weight or balancing factor   Mar 18 Fév 2014 - 20:30

hi Ian

i have meant the actual crankshaft balance weights i don't know why i have written counterbalancing weight Mx.
after reading the article from tony foale there is all clear to my, one thing that i dont know how to do is how i can measure the balancefactor from a balancing shaft.

thanks Manuel
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Ian Harrison



Nombre de messages : 100
Localisation : United Kingdom
Date d'inscription : 28/08/2012

MessageSujet: Re: [2 stroke] Weight or balancing factor   Mer 19 Fév 2014 - 2:17

Manuel Rainer a écrit:
hi Ian

i have meant the actual crankshaft balance weights i don't know why i have written counterbalancing weight Mx.
after reading the article from tony foale there is all clear to my, one thing that i dont know how to do is how i can measure the balancefactor from a balancing shaft.

thanks Manuel

Hi Manuel

A balance shaft doesn't have a balance factor in the sense that a crankshaft does as there is no reciprocating weight to balance against (which is what the balance factor tells us).

I think you are asking "How do I calculate the required shape and size of the balance shaft counterbalance weight(s)"?

For me and our engine in particular,  the design and calculation was carried out by Paul, our design engineer. However, I think I can have a stab at the design theory.

So if your engine has a total reciprocating weight of (say) 250 grams (200 grams piston plus 50 grams upper rod) and a stroke of 60mm. Firstly you need to ensure a 50% balance factor for the crankshaft (in this case a counterbalance weight Mx = (250/2) - 50 = 75 grams would indicate that this is correct.

You then want to ensure that your balance shaft has a centre of mass equivalent to 125 grams (50% of the reciprocating weight) at a distance of 60mm from the counterbalance spindle axis. The shapes and sizes to achieve this can be many and varied. More weight at a smaller radius, less weight at a bigger radius, twin weights, offset weighted drive gear, etc.

The balance shaft is then geared so that the centre of mass of the balance shaft is in phase with the crankshaft (i.e. at the bottom at TDC) and rotates in the opposite direction.

In this way the counterbalance shaft balances out the remainder of the reciprocating weight (125 grams) and cancels out the rotational forces generated by the crankshaft.

Here are images of our twin weight balance shaft assembly which also uses an offset weighted drive gear.

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Best Regards

Ian Very Happy
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Manuel Rainer



Nombre de messages : 98
Localisation : Italy
Date d'inscription : 30/10/2012

MessageSujet: Re: [2 stroke] Weight or balancing factor   Mer 19 Fév 2014 - 18:12

Ian Harrison a écrit:

I think you are asking "How do I calculate the required shape and size of the balance shaft counterbalance weight(s)"?
yes that's what i mean. i haven't it asked right.
Ian Harrison a écrit:

You then want to ensure that your balance shaft has a centre of mass equivalent to 125 grams (50% of the reciprocating weight) at a distance of 60mm from the counterbalance spindle axis.
You have to take the 50% for 1 part because you have 2 counterbalancing weight on the shaft. right? if a engine has 1 it has to be 100%.
Ian Harrison a écrit:


Here are images of our twin weight balance shaft assembly which also uses an offset weighted drive gear.

[Vous devez être inscrit et connecté pour voir ce lien]

[Vous devez être inscrit et connecté pour voir ce lien]

Best Regards

Ian Very Happy
very beautiful parts

in your case if you have the offset weighted drive gear it reduces the counterbalance weight of the other parts on the balancing shaft a bit?

lot of thanks Manuel
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Ian Harrison



Nombre de messages : 100
Localisation : United Kingdom
Date d'inscription : 28/08/2012

MessageSujet: Re: [2 stroke] Weight or balancing factor   Mer 19 Fév 2014 - 23:10

Manuel Rainer a écrit:
You have to take the 50% for 1 part because you have 2 counterbalancing weight on the shaft. right? if a engine has 1 it has to be 100%.

No Manuel, It dosen't matter if the balance shaft has 1 weight or 10!! It has to counterbalance the remaining 50% of the reciprocating weight that isn't balanced by the crankshaft.

Manuel Rainer a écrit:
very beautiful parts

in your case if you have the offset weighted drive gear it reduces the counterbalance weight of the other parts on the balancing shaft a bit?

Yes that's right, Manuel and thank you for the compliment.

Best Regards

Ian
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Simo_SRT



Nombre de messages : 1
Localisation : Italia
Date d'inscription : 29/11/2012

MessageSujet: Re: [2 stroke] Weight or balancing factor   Ven 21 Fév 2014 - 14:58

Vortex a écrit:
Hy guys,
I need your advice concerning a 125 cc single cylinder crankshaft without conterbalancing.

i changed the standard crank from

Total weight crankshaft : 2705 gr.
Conrod:             106mm
Rod/Stroke ratio: 1,947
Weight nacked conrod: 117,20 gr.
Conrod part alternativ: 40,80 gr.
Crankpin+2silver+cage: 149,90 gr.
Conterbalancing weight Mx: 100,44 gr.
Weight piston complet: 174,90 gr.
Balancing factor calculated: 65,5%
to
Total weight crankshaft : 2345 gr.
Conrod:                     106 mm
Weight nacked conrod: 117,20 gr.
Conrod part alternativ: 40,80 gr.
Crankpin+2silver+cage: 131,10 gr. (crankpin with hole 6mm)
Conterbalancing weight Mx: 101,20  gr.
Weight piston complet:        161,00 gr.
Balancing factor calculated: 70,9 %

In order to reach this balancing factor i had to place the tungsten very outside of the crank, so the inertia is higher with less weight.

Now the engine works so much better from 9500-13000 rev/min, it revs up much quicker, the braked dyno marks + 2,5 hp in that range
but it seems that the engine has to much problems to "pass" over to higher revs, like against a wall and loosing more than 10 hp in the upper range 13300-14500 rev/min.

To much inertia or to less weight, what do you think?

Thanks a lot.

Hi Vortex,

In my humble opinion is not properly correct to consider a balancing percentage of the alternative forces >50% because of is not correct from a physical point of view. In fact, you can balance an alternative force with the crankshaft (the rotates!) since you can decompose this alternative force into 2 rotating forces (50% + 50%), the former rotating with the shaft and the latter rotating in the opposite direction.
As a consequence, if you don't use counterbalancing, you are able to equilibrate just the component that has the same direction of the shaft, that is the 50% of the 1st order inertial forces due to your alternative masses.
Indeed, if from your calculation you have 70%, from a practical view you have something similar to (50-20=30%) but with a different behavior of your system due to the different inertia.
Are you sure that with your balancing scheme you have still equilibrated the 100% of the rotating forces?

Sorry for my english because certainly my message contains a relevant number of errors...but I thind (and hope) that the meaning is quite clear.

Greetings from Italy

Simone

PS: I have noticed that probably more or less the same thing is just written by other users
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Ian Harrison



Nombre de messages : 100
Localisation : United Kingdom
Date d'inscription : 28/08/2012

MessageSujet: Re: [2 stroke] Weight or balancing factor   Dim 23 Fév 2014 - 21:18

Hi Simone

In spite of some problems with the English, I think that I agree with what you are saying and many thanks for your contribution. A lot of this is also covered in the Tony Foale articles.

Some useful information and downloads here, particulary the last one (for me): [Vous devez être inscrit et connecté pour voir ce lien]

What would be really interesting would be if you could provide a solution based on the theory for Vortex by answering his initial question? That is were, I believe, most complicated theory falls down. It explains everything but it often doesn't really provide definitive practical help (that works!)

I suppose that's what keeps us all interested!!

I guess what I'm saying is that The World, and 2-stroke engines in particular, are too complicated to be explained and successfully designed by theory alone. So the various simulation packages rely on feedback from practical experience to modify the SIM in order that it can be more accurate in the future.

What the simple act of being able to quantify the balance factor of a crankshaft gives us, is a means to describe what we have found successful and for others to easily understand.

So if we find that the performance of a race machine (and I'm not just talking engine performance) is improved by running a static balance factor of say 69% as opposed to 50%, then it really doesn't matter what the theory might tell us, unless of course it provides a better practical solution or at least an alternative for us to try.

Once we have found a better way, then we need to have a simple way to pass that information on.

Of course you will probably gather that I am one with more practical experience than theorectical training.

Best Regards

Ian Very Happy
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Vortex



Nombre de messages : 41
Localisation : Luxembourg
Date d'inscription : 26/11/2013

MessageSujet: Re: [2 stroke] Weight or balancing factor   Lun 24 Fév 2014 - 0:56

Hi Simone, hi Ian,
I have to agree with you both, and i'm pleased about every contribution to this theme, but i try to understand what happens and why
I never thought before than a different balancefactor or different weight of a crankshaft will give more power to an engine.
I thought that this factors only influence how an engine accelerat or mabe vibre but only displace the range of a given powerband.
For sure there are physical factors in this, and the crankshaft will vibrat in a certain range more or less and this will stress the crankpin and the bearings, than a heavier shaft could assume more energy than a lighter one.
And i know all kind of resonance is bad when it will occur in the wrong moment.
Thank you, again.
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2005bully

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Nombre de messages : 8
Age : 63
Localisation : Midwest USA
Date d'inscription : 07/01/2013

MessageSujet: Re: [2 stroke] Weight or balancing factor   Dim 23 Mar 2014 - 18:49

Information on these forums that is backed with such valuable practical experience is of untold value. Thanks to all for your great lines of input.

I have a  question that hope fully one of you with experience in single cylinder balance factors can answer. When a satisfactory balance factor is found for a particular application, if the displacement of the engine is increased, If the engine is still operating  in the same rpm range will the same balance factor be preferred? I realize rod length and bore to stroke ratios can have an effect, so lets assume that these ratios all remain the same. And limit the changes to stroke, bore dia and the weight of the involved components.      
                                                                                             Kermit Buller
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http://www.buller.net
Frits Overmars

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Nombre de messages : 2131
Age : 70
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: [2 stroke] Weight or balancing factor   Dim 23 Mar 2014 - 19:29

2005bully a écrit:
When a satisfactory balance factor is found for a particular application, if the displacement of the engine is increased, If the engine is still operating  in the same rpm range will the same balance factor be preferred? I realize rod length and bore to stroke ratios can have an effect, so lets assume that these ratios all remain the same. And limit the changes to stroke, bore dia and the weight of the involved components.
If bore/stroke/rod length ratios, rpm range and balance factor all remain the same, the vibration frequency and the ratio between horizontal and vertical vibration forces will also remain the same; only the magnitudes of these forces will increase. In this case it is probable that the balance factor that gave the best results in the original engine, will also be satisfactory for the bigger engine.
But your assumption is not 100% sound, Kermit. If bore/stroke/rod length ratios remain the same, then the mean piston speed at which the engine performs best, will also remain the same and that means lower revs and a lower vibration frequency for the bigger engine. Maybe that doesn't matter, but maybe this altered frequency will bring the frame, or parts of it, into resonance.
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Djamitague

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Nombre de messages : 34
Localisation : Switzerland
Date d'inscription : 18/07/2014

MessageSujet: Re: [2 stroke] Weight or balancing factor   Sam 13 Juin 2015 - 10:37

Ian Harrison a écrit:


What I believe is probably happening with your engine is that you are now hitting a critical vibration at 13,300 that is affecting the carburation very badly and hence engine power, to such an extent that it can't pull through that period. The rider would usually feel that and hear a change in engine note.

Ian Very Happy

Hi Ian,

Thanks for these fantastic contributions that take me (us) out of the dark; experience is unvaluable!

Regarding these vibes that may affect carbs setting, do you expect them to be only effective in upper range (near peak revs), or could also be in the low end area? Considering a 125/150cc range.

I'm running a crankshaft that pulls out a load of vibes until 7000-7500 rpm, and tricky carb setting in that range... absolutely no idea of any factor, only the price I know  Very Happy

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Cheers
Djami
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