Is a balanced single-cylinder crankshaft a myth?

Discussion in '2 Stroke Bicycle Engines & Kits' started by Mozenrath, Jun 29, 2012.

  1. Mozenrath

    Mozenrath New Member

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    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/archive/index.php/t-182527.html
    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?
     
  2. tooljunkie

    tooljunkie Member

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    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.
     
  3. biknut

    biknut Active Member

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    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.
     
  4. biknut

    biknut Active Member

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    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.

    [​IMG]
     
  5. BlueWhizzer

    BlueWhizzer New Member

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  6. 2door

    2door Moderator
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    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.

    Tom
     
  7. Rocky_Motor

    Rocky_Motor New Member

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    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
     
  8. biknut

    biknut Active Member

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    I heard 90 drgree V8s were good too.
     
  9. biknut

    biknut Active Member

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    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.
     
  10. multipaul

    multipaul New Member

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    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
     

    Attached Files:

  11. biknut

    biknut Active Member

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    Well you certainly could be right about that. I'm no authority, that's for sure. But this is what I've noticed from examining a couple of different motors I bought from the Rico Group on eBay. Both were close to the same balance factor out of the box. I estimated it to be something like 15 or 20%.

    The first motor I ran as was until it failed at 550 miles. That motor ran pretty smooth till about 20 mph. At 30 it vibrated bad. 35 and up was terrible.

    The replacement motor was from the same factory. The crank was identical. I calculated I needed to remove about 50 grams from around the crank pin area to get a 58% balance factor. I gave up at about 40 grams. I think the balance factor ended up between 40-50%. Anyway, this motor was a lot smoother at high speeds, but wasn't too bad at low speed.

    I'm under the impression that if I had removed the other 10 or 12 grams it would been even smoother on the top.

    Some of the new cranks I've checked were closer to about 40% balance factor already. All of them were 80s. Because of this discrepancy I feel compelled to check a new motor's balance. It's like 15 or 20%, I'll probably change it. If it's already 40 or 45% I might leave it alone.

    What you're saying is valid, because it seems like at the speeds most people can get up to, anything from 25 to 60% balance factor would probably be acceptable. Most motors probably fall in that range.
     
  12. multipaul

    multipaul New Member

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    Hallo Biknut,

    I am no authority as well. Not on these HT engines. What is valid and works fine, can only be estimated. It is depending on various factors including the kind of frame and the frame material. Additional heavy masses reduce high frequent vibrations. Rubber reduces low frequent vibrations in general.

    We often find values about 57% to 60% balace factor for the reciprocating mass. That reduces the forces on the bearings and the peaks of the forces that reach the frame. 3 peaks of 500N each crank rotation can feel smoother than 1 peak of 1100N. But it must not. So it is possible that one or two of the 500 N peaks are absorbed by the frame and are no longer noticeable.

    In another thread I want to explain how to reach the intended balance factor correctly and how to determine the existing one. With the math formula about that subject, the static balancing of a HT crankshaft.

    Multipaul
     

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