dynamic crankshaft balancing

Discussion in 'Motorized Bicycle General Discussion' started by metalliatic, May 22, 2010.

  1. metalliatic

    metalliatic New Member

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    has anyone had thier crank dynamicly balanced at a machine shop? on a balancing machine at high rpms? or does every one just do a static balance at home?
     
  2. foureasy

    foureasy New Member

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    i have balanced and trued cranks that will rotate in excess of 11000 rpms. pm if you are interested
     
  3. metalliatic

    metalliatic New Member

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    do you do a static or dynamic balance?
     
  4. foureasy

    foureasy New Member

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    static. and i am the ONLY one who sells any kind of performance crank
     
  5. metalliatic

    metalliatic New Member

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    how do you know how many rpm its balanced at if you dont spin it?
     
  6. taddthewadd

    taddthewadd New Member

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    and how much do you charge?
     
  7. foureasy

    foureasy New Member

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    i do spin it...... in my 8hp test motor....... to 9800rpms
    guaranteed to be as smooth as silk from 3500-11000
    fully loaded with bearings and delivered to your door for $100

    [​IMG]
     
  8. taddthewadd

    taddthewadd New Member

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    That seems fair to a person who can't do it himself. I will order one someday hopefully not too far off.
     
  9. metalliatic

    metalliatic New Member

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    how much of a differance does it make? more power? higher top speed? higher revs?
     
  10. GoFastBicycles

    GoFastBicycles New Member

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    I would say higher RPM's means more power, more power might mean more speed.

    Adam
     
  11. taddthewadd

    taddthewadd New Member

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    I would imagine a smoother running engine. I haven't taken out a crankshaft but I assume by the looks of it it is the heaviest rotating or moving part of the engine so it would probably make a big difference in making the engine run smoother.

    foureasy, How far are they out of balance on average from the factory? Also when you run a stock unbalanced crank on the electric motor at speed does it vibrate real bad?
     
  12. foureasy

    foureasy New Member

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    you know that vibration near top speed where it feels like the motor is gonna jump off the bike and run along next to you? that no longer happens.
    vibrations like that not only shake everything loose and break stuff, but it also costs you horsepower.
    its kinda hard to explain, but crankshafts in a single cylinder motor, (or a v twin with both rods on the same journal) can never be balanced. anyone that says otherwise is wrong. all you can do is move the "sweet spot" higher or lower in the rpm range. the sweet spot on a stock crank is about 1000-5000 rpm, and the sweet spot on my "balanced" crank is 3500-11000 rpm. you will notice a definite shake below the sweet spot, but it will not break bolts or cause any parts to fatigue since they are low frequency vibrations.
    this is not a boost bottle situation where any performance increase is a placebo effect. you WILL have a very smooth running engine above 3500 rpm, and a balanced and trued crank is a must for any performance motor that sees sustained high rpms.

    @tadd
    all of my testing has been done in my 8hp bike, not on the bench.
    one of the laborors at my day job, Carl, finally put one of my balanced cranks in his stock motor a couple days ago, after it sat in his toolbox for more than a month. he was pissed that he didnt do it sooner, (he waited until he junked the cylinder that was on there to split the cases. he rides his bike 5-10 miles a day and says he will never go back, he can cruise full throttle without his hands going to sleep.
    all its gonna take is a couple of you guys to post some results
     
  13. taddthewadd

    taddthewadd New Member

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    I am saving for it as we speak. BTW 8 hp! What have you done to get that much hp and has it been reliable compared to a stock motor. I know in general the more mods the less reliable but curious how yours is doing. Also, what size sprocket are you running and how fast does your bike go?
     
  14. metalliatic

    metalliatic New Member

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    thats what ive been looking for is someone that has done it and let me know what a differance it makes. my local machine shop said they would do it for $100 and i could have it back the next day so i think i will go that way when im ready. they have a machine that dynamicly balances the crank with in 1 gram of being true. i didnt want to pay $100 to balance a crank because i only paid $90 for a brand new complete 68.5cc engine($75+$15 shipping). but from what ive learned so far $100 is a good price.
     
  15. foureasy

    foureasy New Member

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    @tadd
    it eats a piston, (i use the stock chinese piston) about every 100mi. im about to start performing some tricks on a new piston now. 41t sprocket and the pipe will not let it rev over 9800rpm which is about 39mph. 30-40 takes less than a second. any faster and ill start getting tickets and or die.

    @metal
    hmm.... mine comes with bearings and mine has been tested. ask them what balance factor they use. remember what i said about single cylinder cranks.



    its not how fast you go... its how you go fast
    actually, its how fast you get there
     
  16. metalliatic

    metalliatic New Member

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    i started building a bike for top speed, but now im working to make it quick instead of fast. but i would like to be able to break 40mph with out a long run up to it. i think my shift kit makes it harder to get the high top speed. its good for low speed take offs and getting up to 30mph. 6th gear boggs the motor down and the i start loosing speed. does best in 5th gear for top speed.
     
  17. metalliatic

    metalliatic New Member

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    i took my crank to the machine shop, now he says it will cost a lot more. he did not know its a pressed together crank. he will have to take it apart, put it back together with the bobwieght, balance it, take it back apart and put it back together with the rod. each time its taken apart or put together it has to be pressed and trued. so now thats out.
     
  18. taddthewadd

    taddthewadd New Member

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    metalliatic, why not get one from foureasy. Seems like a no brainer.
     
  19. metalliatic

    metalliatic New Member

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    thats what im going to do now
     
    #19 metalliatic, May 25, 2010
    Last edited: May 25, 2010
  20. emdude

    emdude New Member

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    Hey foureasy that crank you offer sounds like a sweet deal. And for a what your asking I'm interested but have to ask a few questions:

    Is the crank you offer for the standard 66 CC (what they call the 80 CC engine)?

    Assuming a yes on the above I would imagine your crank is made is made to work with the stock piston. No mods or cuts on the piston and stock wrist pin. I ordered a spare piston some time ago for my engine from someone and it clearly came with a wrist pin for a smaller piston?!?!? Aside from slopping side to side it would have weighed less had I used it. Maybe the question to ask here is what combined piston an wrist pin weight is your crank made for?

    I live in a an area with lots of hills. It's killer when you have to stop at one and restart. I've been thinking of going to a 50 tooth rear and making up top end speed loss with higher engine rpm, the built in "vibration rpm limiter" of the stock engine setting the current limit with 44 teeth in the rear at around 34 mph. your crank might get me some of the way there.

    Regards

    emdude
     

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