The Act of Crank Balancing

Discussion in '2-Stroke Engines' started by StrontiumEthics, Apr 30, 2012.

  1. StrontiumEthics

    StrontiumEthics Motored Bikes Sponsor

    Hey guys, ive been doing a lot of reading about crank balancing these HT engine. It seems as thought most people use between a 53%-55% balancing factor which is fine. I'm trying to get up to about 8500-9000 rpms without vibrating things apart. I have an extra crank that I don't need to practice on from a 66cc engine, and im sure that any sort of balancing would do these engines justice.

    I know that the 48cc crank was used and because of the heavier top end the engine vibrates like a mofo at higher rpms. In general for the 66cc do I begin with adding weight or removing weight to the crank?

    As for the Crank balancing factor, is it that I add 53%-55% to the lower end of the crank (flywheel)?

    I will be working on a crank balancing device that I can use to measure the weight of the crank.
     

  2. Fabian

    Fabian Well-Known Member

    From what i understand the subject of engine balancing has been covered in depth, maybe many times over.
    Try using the search function for crank balancing, otherwise Google is your friend.

    I've never gone so far as crank balancing because my application has the engine operating in the midrange speeds, between 2,500 and 4,500 rpm.

    Unfortunately single cylinder engines cannot be balanced for a broad rpm spread, but only in a narrow rpm range; vibration progressively getting worse outside of the chosen rpm sweet spot.

    If it's set for reasonable balance at 9,000 then it will vibrate appallingly at 4000.
    The only real solution (aside from a balance shaft) is to add a second cylinder at 90 degrees to the existing cylinder centre line.
     
  3. jaguar

    jaguar Well-Known Member

    Before going to the extreme trouble of drilling on the crank wheels why don't you just try to lighten up the wrist pin? It worked for me. Obviously these things aren't too far out of whack for such a small change to make a big difference. And drilling a bigger hole in the wrist pin doesn't weaken it too much because a few horsepower engine without excessive compression doesn't require much strength there.
     
  4. StrontiumEthics

    StrontiumEthics Motored Bikes Sponsor

    Drilling a hole in the wrist pin might be a bit risky with the high compression setup that I have.
     
  5. Purple Haze

    Purple Haze Active Member

    I think Jag is on to something, KB pistons does this routinely on their racing pistons, and they have waaaay more stress put on them than our little motors are capable of. My trick is to match the weight of the piston assembly with the 49cc assembly. Works for me.
     
  6. StrontiumEthics

    StrontiumEthics Motored Bikes Sponsor

    100% I actually tried drilling out that wrist pin yesterday. No joke that stuff is some HARDDD ****. I couldn't get through it with my industrial drill press. Im in the process of figuring this out

    Purple Haze what do you mean by " match the weight of the piston assembly with the 49cc assembly" How is that possible the assembly is much heavier than the piston no?
     
  7. Fabian

    Fabian Well-Known Member

    You will need a "full cobalt" drill bit to drill through hardened material.
    Can you remove some of the weight from the piston skirt?

    Much easier than lightening the wrist pin.
     
  8. Purple Haze

    Purple Haze Active Member

    The assembly consists of the piston, rings, pin, bearing, and clips. The heaviest by far is the pin. Concentrate most of your effort in lightening there, you can get real close. The piston itself doesn't weigh much, but can be lightened some. Any balancing on these engines is a compromise, as Fabian states so well. As for weakening the the pin, it could be case-hardened for peace of mind, though I don't think it necessary. As long as you can get close to the 49cc weight, it'll really help smooth out the vibrations.
     
  9. Purple Haze

    Purple Haze Active Member

    Yes, the best place to start is on the skirts. These pistons are waaay too long, but have to be because of the port design. I try to trim the bottom of the skirt where it doesn't affect any port timing. Don't worry about equalizing side to side weight, it has little to no effect on balance. It's the total weight (moving up and down that matters. Don't remove any material where it will compromise the strength of the piston in the pin area.
     
  10. jaguar

    jaguar Well-Known Member

    Removal of weight from the piston is only for fine tuning because aluminum is so light. Drill out the wrist pin with a 7.5mm drill bit. If there's still some vibration then you can try to drill a couple holes in the upper part of the connecting rod.
     
  11. StrontiumEthics

    StrontiumEthics Motored Bikes Sponsor

    I sucessfully ported my engine yesterday and took it for a test run today. I got her up to 8500rpms. The low end was a bit slower but I will try to compensate byy increasing the header of my expansion chamber because I have it turned for high rpms right now. After about 5500 rpms I believe the power band kicks in and then I'm off straight up to 8500rpms smooth with a 40t sprocket. I calculated it to be about 40 mph or so. At 8500 rpms the vibrations are pretty bad. I'm going to get man hands on a cobalt bit today if I can to lighten things up.

    Jag I like your idea about the connection rod. Are you sure it shouldny weaken things to much?
     
  12. jaguar

    jaguar Well-Known Member

    Good work on the porting!
    Like the wrist pin, the connecting rod can handle way more than these little engines can put out. I wouldn't hesitate at all to drill a few holes in it (upper area only).
     
  13. Fabian

    Fabian Well-Known Member

  14. StrontiumEthics

    StrontiumEthics Motored Bikes Sponsor

    Yea.. got the cobalt bits. Only got half way through the pin. The other half was virtually un-openable. Even with a drill-press, and a pneumatic drill, the suck wont budge. I will test it with just a half of the pin gone. I am also in the process of a totally new bottom end remake with better bearings, etc. So I will just drill a hole or to in the upper crank arm, as well as in the flywheel. That should definitely level things out.
     
  15. jaguar

    jaguar Well-Known Member

    If you drill holes in the flywheels then you can always fill them with lead pellets and seal the ends with JBWeld. Test your half pin and see how it goes. Next step could be drilling holes in the connecting rod (just the upper part of it). Taking too much off can leave you with too much vibration just as well but then you'd be scratching your head not knowing if you took off too much or too little. That's why I wouldn't drill the flywheels except as a last recourse.

    I had a machine shop drill out my wrist pin. I didn't hang around to watch and see how they did it but it turned out nice.
     
  16. StrontiumEthics

    StrontiumEthics Motored Bikes Sponsor

    Cool. Also let me point out this fun fact. The stock wrist pin bearing that I had was approximately 6 grams. The replacement bearing that I got from Treatland.tv was 4 grams. So that's 2 grams off. I'm on my way home now from the lab to measure how much reduction in weight is from the half drilled wrist pin. Yea, perhaps I will just wait and see. No sense rushing things if your gonna do a half *** job you know?

    Drilling this wrist pin is an ABSOLUTE challenge. I'm here for about 3 hours trying to take my time and I only got half way... ahh
     
  17. Fabian

    Fabian Well-Known Member

    Drilling hardened steel takes time and burns up cobalt drill bits, but it's the only way to get through such material and accomplish the task, unless you have access to a machine shop with diamond grinding tools.
    If paying for shop time, the boring of the pin will possibly cost more than the engine is worth, so it's back to using your own time and cobalt drill bits.

    The only problem with twist drilling is that it leaves stress risers which form a perfect surface for cracks to propagate.
    A failed wrist pin isn't conducive to engine reliability.
     
  18. StrontiumEthics

    StrontiumEthics Motored Bikes Sponsor

    Ok so I measured how much the pin weighs now. Its about 9.5 grams. Originally it weight around 15.5. So with half the diameter bored out it lost about 6 grams.. Incredible. Its pretty dense material. Now this coupled with a lighter wrist pin makes about 8 grams off the top end. I'm gonna connect it up in a couple hours. I'm really excited about getting rid of the vibrations they really become unbearable after like 6000 rpms. IN addition, the bike feels like it wants to rev higher but the vibrations are stopping it. I reckon it can probably go up somewhere around 9200 rpms. Actually when I rev it without load my tack reads 9200rpms, but the vibrations are insane once again.

    Also on another note. I know its off topic but how can I get new bearings inside a crank case without bashing them in? I read somewhere that I put the crankcase in the freeze and the bearings in hot oil, then it should slip on. What degree of truth is this? Or any other suggestions.
     
  19. Fabian

    Fabian Well-Known Member

    Go to your local "BOC Gasses" or "Supagas" and purchase 2 kilograms (5 pounds) of dry ice pellets, storing them in a small esky.
    Using your assembly method, throw the bearings in dry ice pellets for half an hour, shrinking their size slightly by reducing temperature to around minus 80 degrees Celsius (minus 112 degrees Fahrenheit).
    Whilst the bearings are cooling, heat the cases in the oven to around 120 degrees Celsius (250 degrees Fahrenheit).

    Take one of the cases out of the oven and one of the bearings from the dry ice and simply pop the bearing into the case.

    "Job Done" - No bashing or smashing or swearing or cursing.

    My preference is to freeze the crankshaft in dry ice and heat the bearings, then assemble bearings on crankshaft without any fuss.
    Next job is to freeze the crankshaft and bearing assembly and clutch shaft, whilst separately heating the cases.
    This makes assembly of the bottom end a simple and hassle free process, as everything just fits together nicely without neanderthal force being required.

    Please remember to use protective gloves when handling parts frozen by dry ice as it will burn your skin just as easily as heat will burn your skin.
     
    Last edited: May 4, 2012
  20. Purple Haze

    Purple Haze Active Member

    Instead of drilling the rod, I suggest an old racer trick. Grind the parting line and polish the beam if it's accessable. This removes a considerable amount of weight, and actually makes the rod stronger. The small-block chevy guys did that for years before aftermarket rods became available. Amazing what you can do with a dremel and a few sanding rolls.
     
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