Smoothing Out The Engine

Discussion in 'General Questions' started by cosworth, Apr 11, 2013.

  1. cosworth

    cosworth Member

    Has anyone tried balancing one of these cheap motors?
     

  2. geebt48cc

    geebt48cc Member

    Well, the best thing I've done to help, is put a better wrist pin bearing in. The better bearings are lighter, and help quite a bit.

    Glen Look here======POCKETBIKEPARTS.COM======Piston Wrist Pin - 10mm x 36mm for 44mm pistons
     
  3. jaguar

    jaguar Well-Known Member

    If you put in a lighter wrist pin and use a Jaguar CDI you will be all set. If you drill some holes in the flywheels to more correctly counterbalance the piston/wrist-pin then it still will vibrate at top rpm because that is mostly caused by the standard ignition firing too early. Click on my signature link and read more on the subject.
     
  4. geebt48cc

    geebt48cc Member

    Yes, I'm using the Jag CDI as well with set up. IT DOES HELP CONSIDERING WHERE THESE ENGINE COME FROM, and compairing them from stock..................
     
  5. cosworth

    cosworth Member

    Wow, what a wealth of information you've provided. Lots of reading to do with much thanks.
     
  6. Fabian

    Fabian Well-Known Member

    The Jaguar CDI helps to make the engine "run" smoother, but you cannot balance a single cylinder engine unless using a balance shaft.

    You can only reduce the level of vibration at a specific rpm zone by altering the balance factor percentage - anything outside of that rpm zone will have the engine vibrate just as bad as before, if not worse than the original balance factor.
     
  7. south_MS

    south_MS New Member

    How would you balance the flywheel? They are dynamically balanced (sometimes called spin balanced) at the factory, much like your tires are balanced while they are being spun. I wouldn't touch the factory balancing because it is not a simple hole drilling. Although I don't know if any of these Chinese motors are balanced at the factory at all. I personally wouldn't drill any holes anywhere.
     
  8. jaguar

    jaguar Well-Known Member

    Fabian, it might be that the reason balancing is rpm range dependant is that this info is always in reference to race engines with expansion chambers that increase the dynamic cylinder pressure at the top rpm range. That pressure is one of the things that have to be counterbalanced since the piston is basically trying to push the engine "up" when rising and pushing against it. So a downward force from the flywheel is needed, and had by lightening the flywheel close to the connecting rod pivot point.
    But most people here use the standard muffler that has no compression increasing effect at top rpm. Therefore these engines should be able to have very good balancing (which was not accomplished at the factory).
     
  9. Fabian

    Fabian Well-Known Member

    A single cylinder piston engine cannot be mechanically balanced - altering the balance factor simply shifts the minimum perceived vibration either up or down the rpm range and that can be proven with maths.
    Combustion related vibration (sometimes referred to as secondary balance) is minimal compared to mechanical imbalance.

    The only way a single cylinder engine can be balanced is with the use of balance shafts.
    Unfortunately maths has been said to be the language of God, and no one messes with god when it comes to maths.

    and here is the maths, once again:

     
    Last edited by a moderator: Dec 15, 2015
  10. jaguar

    jaguar Well-Known Member

    These arent spin balanced at the factory, I guarantee you.
    The size of the flywheel holes should be a percentage of the weight of the top half of the connecting rod plus piston/rings weight plus wrist pin weight.
    You can increase the hole size or you can use a lighter wrist pin. Both affect balancing in a positive way on this engine. But there is no getting around the need for a 2 stroke CDI to replace the 4 stroke one that comes with it because it has too advanced an ignition timing at high rpm.

    Fabian, your non-understandable video (because of teachers heavy accent) doesnt prove what you are saying. But anyway your statement of 2 strokes not being able to be balanced is too broad. You need to give details and specify.

    My position is that an engine w/o an expansion chamber can be balanced enough so that the handlebars don't vibrate excessively which is tolerable for bike riders seeing how they adjust themselves to so many discomforts anyway. The standard Grubee engine vibrates at an untolerable level. It can be brought closer towards perfection, enough to make it a pleasant experience to ride.
     
    Last edited: Apr 14, 2013
  11. cosworth

    cosworth Member

    Once again a wealth of very useful information. Thanks very much for your time.
    Cosworth
     
  12. Fabian

    Fabian Well-Known Member

    I don't need to give any details because all the detail is in the maths.
    Since when does maths written on a whiteboard become unintelligible due to a lecturers accent. A complete nonsensical argument on your behalf - i expected better from you.
    Yes he does have a heavy accent, but i didn't have any issues understanding him, although i had to listen carefully and everything he talks about is written up on the whiteboard.

    The maths still stands - a single cylinder engine cannot be balanced, unless using balance shafts.
     
  13. jaguar

    jaguar Well-Known Member

    The non-linear forces in a 2 stroke make it impossible to balance it perfectly. But a moderately good balance is possible.

    If all engine engineers believed that it cant be balanced then they would put forth no effort to do so. But the fact that they do a fairly good job of balancing them totally destroys your all-or-nothing point of view.
    I agree that it cant be perfectly balanced.
    I disagree with the viewpoint that it cant be balanced.

    I have owned at least 7 different street and dirt bikes and not a one of them had anywhere near the imbalance that these Grubee engines have. Surely that was done on purpose to limit top rpm. No one can be that incompetent, can they?
     
  14. Fabian

    Fabian Well-Known Member

    a good portion of the reason why these engines feel so unbalanced is the lack of mass that surrounds them, which in most cases only adds up to around 10 kilos (for the bike). Considering the engine weighs around 10 kilos, you already have issues, given that there isn't enough frame weight to dampen out vibration; not helping the situation is most aluminium frames are made from thin wall material, allowing the engine to effectively bounce around on the frame tubes.

    This simple fact is why rubberised engine mounts make little if any difference to vibration transmitted through a bicycle frame and that concept has been worked to death.
    If looking for the the best option to quell engine vibration, the solution is a heavy wall steel frame, with lead shot inserted into the handle bar ends.
     
  15. Fabian

    Fabian Well-Known Member

    I too have owned more than a few single cylinder dirt bikes and can say that they all had unacceptable levels of engine vibration - but when the manufacture only gives you the option of a single cylinder engine, you only have the option of purchasing a dirt bike with a single cylinder engine.
     
  16. Fabian

    Fabian Well-Known Member

    That's a bit like saying a woman can be half pregnant. Either a woman is pregnant or she isn't - there is no half way option of only being "a little bit pregnant", no matter how much you try and alter the viewing perspective.
    It's a bad analogy with respect to a motorized bicycle, but still a valid analogy.

    The best results you can achieve are to alter the balance factor to minimise vibration at the chosen rpm zone of most frequent engine operation - anything outside of that rpm zone will have the engine giving ever escalating vibration, until the rpm drops to a point that inertia forces significantly fall away, which wouldn't be far from idle rpm.
     
  17. Fabian

    Fabian Well-Known Member

    You just can't argue with the maths !!!
     
  18. jaguar

    jaguar Well-Known Member

    Math is just a tool. You can pick and choose aspects of the whole truth to present to reinforce your "imbalanced" point of view. Math shows that a bumblebee can't fly. get my point? Let me restate my opinion:
    It can't be perfectly balanced.
    It can be moderately balanced.
    The Grubee engine isn't even moderately balanced.
     
  19. Fabian

    Fabian Well-Known Member

    I place my trust in maths, and the maths says that a single cylinder engine cannot be balanced, except to minimise vibration at a chosen rpm zone by the adjusting the balance factor.
    I suggest you watch the video again, and if you don't understand the maths the first time, just watch the video a second or third time.

    Doesn't matter which way you look at things, the crankshaft becomes a massive out of balance force when at 90 degrees to the cylinder centre line, because there isn't a piston and connecting rod to balance out the counterweight force.

    That's the reason why a 90 degree V-twin is a logical solution for perfect primary balance.
     
  20. Fabian

    Fabian Well-Known Member

    Since when can't bumble bees fly - there is a bumble bee spammer on here that flies from one motorized bicycle forum to the other.

    I rest my case
     
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