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Is a balanced single-cylinder crankshaft a myth?

2 stroke motorized bicycle engine kits need careful installation and setup, find out how from our professionals here!


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Old 06-29-2012, 06:11 PM
Mozenrath Mozenrath is offline
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Question Is a balanced single-cylinder crankshaft a myth?

I know I've asked questions about crankshaft balancing in the past, but now that I've ordered a knew crank for my engine, I plan on actually attempting to make it balanced.

I've looked through this forum, and the information on balancing is quite vague. There are no visual instructions or really instructions at all for how to do it with our china girls. In fact, I'm not entirely sure anyone in this community has actually done it.

Here's a quote I found in a thread about balancing a 2-stroke engine:
http://www.radiocontrolzone.com/arch.../t-182527.html
Quote:
A single cylinder engine will never be balanced. Balancing the crank on the engine does nothing for it, because there is nothing to oppose the force from the piston. On any real engine there are multiple pistons whose force is used to counteract the force that any one piston will make on the engine. In a single cylinder engine there is nothing to counteract the force. A counter weight will not work because the force that the piston places on the crank from combustion and compression cycles are never the same. Point being the force on the crank is constantly changing because the speed of the piston is consantly changing as it go's through the engines stroke. In a multi-cylinder engine there are other pistons going through the same phenomenon to counteract. But in a single cylinder engine there is nothing. To have the engine truley be balanced you would need a counter weight that is constantly changing its effectiveness on the crank to match the pistons g-force on the crank. But thats impossible in these engines.
Is there any validity to this? Does this mean that I can't balance my crankshaft for less vibration?

Also, I keep reading on the internet about weights, counter weighs, etc. Where on these crankshafts would one add weight? Does one simply drill out material in the right places to transfer weight?
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Old 06-29-2012, 06:35 PM
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Default Re: Is a balanced single-cylinder crankshaft a myth?

there are a few threads about this,as well as truing the crank.i feel that truing a crank would make it run smoother and also give the bearings greater longevity.
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Old 06-29-2012, 07:22 PM
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Default Re: Is a balanced single-cylinder crankshaft a myth?

You can never perfectly balance a single cylinder, but you can decide what RPM will be smoothest.

It might be a challange to balance for some exact RPM, but it's pretty easy to tell if it's way off, and make an improvement.
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Old 06-29-2012, 07:29 PM
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Default Re: Is a balanced single-cylinder crankshaft a myth?

For the most part you'll be wanting to remove weight. Usually from around the crank pin. Here's a crankshaft I did balance work on with pretty good success. It's one of the kind with the bolt on flyweights. I removed about 40 grams from this one.

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Old 06-29-2012, 07:46 PM
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Default Re: Is a balanced single-cylinder crankshaft a myth?

Cool explanation of balance in a single cylinder engine:

http://modelenginenews.org/etw/etw_bal/p2.html
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Old 06-29-2012, 08:52 PM
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Default Re: Is a balanced single-cylinder crankshaft a myth?

The quote the OP posted is absolutely correct. There is no way to effectively 'balance' a single cylinder engine to make it run as smooth as one with multiple cylinders/pistons. You are fighting physical laws that can't be changed. Yes, what Biknut said is true. You can balance for a certain RPM but in reality who rides, or operates and engine at one specific RPM? It might be okay if you're talking about a pump or a weedeater but that's not how we use our engines. Our speed varies often and so does engine RPM.

In fact what we feel is not all vibration from an unbalanced condition but as the text said, you feel the power pulses from the piston being pushed and pulled and pushed again, the rapid directional changes in the moving mass or crank and piston. Those you can't 'balance out'. It's the nature of the beast.

That said, yes there are probably crankshafts in the Chinese 2 strokes that could benefit from a little work but no matter what you do a single cylinder, especially a 2 stroke single cylinder engine is never going to run 'smooth'. Want 'smooth'? Go electric.

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Old 06-29-2012, 09:35 PM
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Default Re: Is a balanced single-cylinder crankshaft a myth?

Someone told me or I read it sometime that only an engine divisible by 6 cylinders can have a truly balanced crank. Good info though here
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Old 06-29-2012, 10:11 PM
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Default Re: Is a balanced single-cylinder crankshaft a myth?

Quote:
Originally Posted by Rocky_Motor View Post
Someone told me or I read it sometime that only an engine divisible by 6 cylinders can have a truly balanced crank. Good info though here
I heard 90 drgree V8s were good too.
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Old 06-30-2012, 01:37 AM
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Default Re: Is a balanced single-cylinder crankshaft a myth?

Some of the 80 motors I've bought seemed to be balanced for about 10-20 mph. They were very heavy on te crank pin side of the flywheel. They vibrated a lot after 25 mph. That works for a lot of people. I like them to run smoother in the 20-30 range. At lower speeds it's still not too bad, because at lower rpms you get a little different kind of vibration that's not as bad as high rpm vibration. That's what I think anyway.
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Old 06-30-2012, 08:10 PM
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Default Re: Is a balanced single-cylinder crankshaft a myth?

The balanced HT engine, that is a myth.
Vibrations are there and will be there. The question is, how strong they affect us.

In my opinion, it is a myth that the desired engine speed has a lot of influence whether one chooses a high balance factor or a low one, IMHO.
All single-cylinder, which I drove, which ran smoothly at high speeds did so well at medium speeds.

Here is a picture how single-cylinders become gentle.

Multipaul
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