Testing Crank Balance

Discussion in 'Performance Mods' started by jaguar, May 30, 2016.

  1. jaguar

    jaguar Well-Known Member

    The crank of a 2 stroke has radial forces on it throughout the 360 degrees of crank rotation. These forces can be felt at the handlebars. A good crank balance equalizes the vertical forces with the horizontal so that the pattern of forces throughout the whole crank revolution is close to being a circle when graphed radially. If the counter balance of the crank is lacking then the circle becomes oblong in a vertical pattern. If the counter balance is too much then it becomes oblong in a horizontal pattern. Either too vertical or too horizontal have amplitudes greater than the circular pattern of a balanced engine which produces an annoying vibration at the handlebars. So I am theorizing that the resultant pattern can be seen by using a long thin metal connected to the crank cases while it is running. Here is my proposed method of testing:
    Use a short/thin bicycle spoke to press onto the engines left side cases so the spoke is sticking out to the left. Start the engine and hold the clutch in. Press the spoke onto the cases with your left hand and position your body so your eye is inline with the spoke farther to the left and close to it while controlling the throttle with you right hand. Slowly increase RPM as you watch the spoke end. At top RPM it should remain centered. If not then the crank is out of balance. Top RPM is the most important because that is when the forces on the crankshaft are the strongest.
    On my Suzuki 100 the spoke end only leaves the centered position around 3300 RPM. Then it vibrates from about 10:30 to 4:30 o’clock (making the upward direction of the cylinder at 12 o’clock). Below and above that RPM the spoke has only the slightest of vibration, mostly staying centered. This bike has very little vibration at the handlebars which is one thing I like about it.
    If any of you have a bicycle wheel handy I’d like you to remove one spoke (I did it without removing the wheel from the bike.) and give this test a try and then report back here. Tell the engine size, whether or not it has a high compression head or aftermarket CDI, what year it was bought new, and where you bought it. I think the older engines have the most vibration.
    If everyone has a similar experience that it only vibrates at one RPM then that means it is only showing the resonant vibratory frequency of the frame/engine and not the radial forces on the crankshaft.
    [​IMG]
     

  2. jaguar

    jaguar Well-Known Member

    Oh I get it. You all know your engine is unbalanced and you don’t want anything reminding you of that. It’s like in the realm of health, everyone knows they have multiple health problems and don’t want a doctor telling you to change your lifestyle and eating habits, unless of course you’ve reached a critical stage of crisis and you feel there is no other choice.
    And god only knows how excruciatingly hard it is to remove a spoke. yeah, right

    I really don’t understand this attitude “oh it’s not too bad”. From the first day I felt my engine was unbalanced I was determined to fix it. I mean why put up with something that doesn’t need to be put up with?
    Tearing into a 2 stroke to understand how it works and get experience repairing it is a great experience. It is extremely rewarding to take on a project and complete it. I wish I could convince more of you to dig in. Increase your skill/knowledge level and walk away happier with a better functioning engine. It’s a win-win proposition. There’s a first time for everything. When I was 9 years old I fixed my bicycles flat tire without any ones help or anyone showing me how to. When my car and my motorcycles broke I just dove in to figure out the problem/solution. I learned by doing. It’s a great adventure. try it!
     
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  3. Legwon

    Legwon New Member

    wtf is ur problem man??? not everyone is on here 24/7 like you.
    i WAS gonna read the entire post until i seen ur stupid rant, that irritated me so much i just dont give a dam what you gotta say.
     
  4. Nanonevol

    Nanonevol Member

    I'll try that, after I finish my cigarette. Does it have to be a spoke or would a coat hanger wire work? ROTFL!
     
  5. FurryOnTheInside

    FurryOnTheInside Active Member

    I can try it after I get the whole thing built, but that's going to take some time! I will certainly be trying to drill balancing holes from the edge of the crank flywheels to reduce the amount of disassembly and make it easier to tweek the balance (deepening rather than widening the holes).

    "A good crank balance equalizes the vertical forces with the horizontal so that the pattern of forces throughout the whole crank revolution is close to being a circle when graphed radially. If the counter balance of the crank is lacking then the circle becomes oblong in a vertical pattern. If the counter balance is too much then it becomes oblong in a horizontal pattern"

    If balancing a single cylinder engine can only be RPM specific then I would like to have best balance at the cruising (most used) RPM. It is interesting that (I infer) the vibration will be horizontal when RPM is below, and vertical above the RPM that the engine is balanced for. On a dual suspension frame this may mean that the vibration is felt more at the handlebars when below and more at the pedals when above the balanced RPM since the rear suspension allows vertical movement at the pedals. I will have to wait to find out for certain which is more bothersome!
    If my crank is out of true in another plane this could be an impossible task though as I don't think I can get it trued. I wonder if it can be partially remedied by another hole...
     
    Last edited: Jun 1, 2016
  6. jaguar

    jaguar Well-Known Member

    coat hanger wire should work. If the wire movement is too little then use a longer section. I used a short spoke from a 20" wheel and the movement was almost 1/4".
    it's best to balance for top RPM because the forces are the strongest there.
    But actually a good balanced engine gives little vibration throughout the whole RPM range.
     
    Last edited: Jun 1, 2016
    FurryOnTheInside likes this.
  7. Wolfshoes

    Wolfshoes Member

    Perhaps a lazer pen or device would better visualize the motion than a spoke. I see online they can cost as little as $6. I'm going to do the test of crank balance. The test looks like a good sanity test of an engine before moving forward with higher compression.
    Back around 1970 Suzuki would offer what I used to call a "hop up kit" of parts for your 125cc $550 two cycle enduro that in a afternoon of swapping parts would raise the rpms from 7000 to 10000. My bike was bumblebee yellow with a black stripe on the engine and was fast enough without the kit. My buddy with the blue bike, not exceptionally up to speed with do it yourself mods, had as I remember, a vibration free but noisy racing bike before dinner.
     
  8. jaguar

    jaguar Well-Known Member

    I have a laser I can try at night but I doubt it will be as good as a wire. That is because the wire can amplify the movement and make it more obvious to the human eye.
     
  9. 45u

    45u Active Member

    [QUOTE
    If my crank is out of true in another plane this could be an impossible task though as I don't think I can get it trued. I wonder if it can be partially remedied by another hole...[/QUOTE]

    If the crank is out of true then putting more holes in it is not fixing the problem. If you lived near me I would check and true it for nothing.
     
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  10. Frankfort MB's

    Frankfort MB's Well-Known Member

    If it's out of balance then it would cause vibration as much as an out of true crank
     
  11. jaguar

    jaguar Well-Known Member

    If you both ar referring to the crank wheels having additional weight due to manufacturing irregularities then you are right, unless the location of the additional weight is near the crank pin or on the opposite side.
    It best to have it pressed apart and checked on a lathe that it is perfectly round.
    But really that is nit picking.
    I'm sure my old crank was as out of round as any of them but by drilling two 9mm holes it became almost as good as any two stroke I've ever ridden.
    Worry about the big stuff before worrying about the little stuff.
     
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  12. HeadSmess

    HeadSmess Well-Known Member

    shhhh, dont tell him you smoke or he really WILL have a freak out at you! ;)


    does it have to be the spoke off of a 26" wheel or can i use one from a 27" cus i got a whole stack lying around?

    how do i get the spoke out from behind the freewheel, dangnammit?

    answers! i DEMAND EFFING ANSWERS!
     
  13. FurryOnTheInside

    FurryOnTheInside Active Member

    If the crank is out of true then putting more holes in it is not fixing the problem. If you lived near me I would check and true it for nothing.[/QUOTE]
    Out of balance or eccentric with a heavy side that isn't in line with or opposite the big end bearing, is what I meant.. Now I'm not sure what out if true means, wobbly like a bent wheel rim? Thanks for the hypothetical kind offer, if I lived near you I would take up that offer! :)
     
  14. jaguar

    jaguar Well-Known Member

    check out my video about crank balance:
     
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  15. jaguar

    jaguar Well-Known Member

    I just used a laser light to substitute for the spoke and the direction of movement was the same although very small. So I think the spoke is an excellent way to demonstrate the direction of engine movement/vibration because it amplifies the movement.
     
  16. Wolfshoes

    Wolfshoes Member

    Perhaps stick a small curved mirror, like the ones used as a auto rear view mirror add on, to the side of the motor, point the lazer at the mirror and view the reflection.
     
  17. jaguar

    jaguar Well-Known Member

    that would work but you'd have to move the mirror around till its plane of amplification is at the same axis as the laser movement.

    It's so cheap and easy to use a bicycle spoke.
    I'm really disappointed that no one has done the test yet.
     
    FurryOnTheInside likes this.
  18. FurryOnTheInside

    FurryOnTheInside Active Member

    If my engine was rebuilt and running yet I would as I'm interested in any and all comparative tests.
    To make it a standardised test so that we can compare bikes it needs something we all have, and we don't all have a bmx spoke, and those vary in length for flange width and spoke lacing pattern anyway..
    So I wonder if the (measured and possibly cut) inside part of a Biro pen will work? It is shorter but has a bit more flex, still some mass to give it enertia so it should wiggle I think... What do you think?
     
  19. Wolfshoes

    Wolfshoes Member

    In doing the spoke test to a bike, I did run into what is believed to be harmonic frame vibration at throttle midrange. The engine may be in better than average balance out of luck. I'll run the test again.
     
  20. FurryOnTheInside

    FurryOnTheInside Active Member

    We're you sitting on the bike or just holding the handlebar? brakes on fully/not? I'm sure the mass added to the bike matters to the test and to the experience..
     
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