How to balance a Crankshaft???

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nightcruiser

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I have a 2010 Grubee SkyHawk 66cc kit, got her running well this summer, I am pretty happy with my setup.
I can tell when riding, specially at high RPM, that things could be balance better in the motor. I came across a couple guys selling fine tuned motors the other day, the one thing they touted that I haven't done is balancing the crankshaft. I have heard people say these 66cc motors have a crankshaft that is balanced for a 48cc top end, and that is why the 66cc motors run rougher than the 48cc motors.
So I am wondering what all is involved in balancing a crankshaft? I have done some reading on the internet, nothing specific to these single cylinder china girl motors though. I understand that it has to do with adding or taking away mass from the flywheel in the appropriate places, but I have no idea how to determine where to add/remove the weight to achieve balance....
If there are any experts out there on crankshaft balancing that are willing to take the time to explain I would really appreciate it. Even better would be some kind of web guide or youtube video that covers this topic.
The temp here unfortunately is only about 40 degrees today, I am afraid riding weather is over until next spring. In the meanwhile I am gonna rip the bike apart and rebuild it, I would like to try and balance the crankshaft while I am at it if at all possible....
Thanks in advance to anyone who has advise to offer me....
 
Venice Motor Bikes

Venice Motor Bikes

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Mar 20, 2008
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I'm not knocking other vendors, but I'd be carefull of people saying that they sell 'balanced' engines... (Because there aren't any reputable vendors (that I've ever heard of) that sell balanced engines).
 
biknut

biknut

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Venice Motor Bikes said:
I'm not knocking other vendors, but I'd be carefull of people saying that they sell 'balanced' engines... (Because there aren't any reputable vendors (that I've ever heard of) that sell balanced engines).
Click to expand...
The only difference between the vendors I've used was in the quality of their service, and customer service. If we're talking about stock china girl parts, there hasn't actually been any difference in the parts. Sometimes they're good, sometimes they're crap.

I've come to the conclusion that to build one good motor with stock parts, I'll probably need to buy about 4 motors. Key word here is stock parts. If I had a source for good crankshafts, I could cut that number down to 2 motors lol.
 
Goat Herder

Goat Herder

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We are getting closer to understanding this.. If you by some great grand 0L luck you... have a balanced crank then don't fix it:)
 
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biknut

biknut

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Goat Herder said:
We are getting closer to understanding this.. If you by some great grand 0L luck you... have a balanced crank then don't fix it:)
Click to expand...
Actually I think you're right. I believe that the crankshafts in china girl motors are the weakest link in the motor. Many of the other parts in the motor can be poorly finished, and the motor will still be ok, or easy to fix if it's assembled well enough, but not the crankshafts. If the crankshaft is bad, then it's a bad motor every time.

So I say if you start with a good balanced and true crankshaft, you won't need to fix it, much, probably lol.
 
Goat Herder

Goat Herder

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The other biggie is the precariously hanging resonating kit exhaust pipe. If the pipe was to be secured very tightly to the bike frame. ''if done right'' Then there is added support for the jug. Think Uni-body structure for strength and integrity..

These jugs are not very secure to the engine block at all to begin with..

The stock pipes rattle like a resonating tuning fork and IMO cause a ton of harm.. The jugs need dowel guides etc that simply are not there.

I have seen smooth china cranks and I have seen very rough ones. It's easy to tell tho.
 
biknut

biknut

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Sep 28, 2010
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Goat Herder said:
The other biggie is the precariously hanging resonating kit exhaust pipe. If the pipe was to be secured very tightly to the bike frame. ''if done right'' Then there is added support for the jug. Think Uni-body structure for strength and integrity..

These jugs are not very secure to the engine block at all to begin with..

The stock pipes rattle like a resonating tuning fork and IMO cause a ton of harm.. The jugs need dowel guides etc that simply are not there.

I have seen smooth china cranks and I have seen very rough ones. It's easy to tell tho.
Click to expand...
Yeah the stock exhaust is very problematic for sure, but that's not too difficult to deal with.

The crankshaft on the other hand is a different story. I think even if, let's say you get a perfect one, the hollow crank pin flexes at high speed. Probably even more if the balance isn't very good. The reason I think that, is because I can flex it a little bit between my fingers. It's probably not as bad with a 50. When you add flex to not very true crank, you end up with bearing problems, funny noises, and worn out blocks. A good crank would avoid all that.
 
BigE

BigE

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Nov 5, 2011
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I found this guide on balancing a Chinese 2 stroke motor.

1) Take the complete weight of the connecting rod big end and the big end bearing - 40.3 grams + 7.1 grams = 47.4 grams
If your engine has thrust washers, add the thrust washer weight to "step 1"

2) Take the complete weight of the connecting rod small end and the total weight of the piston and rings, piston pin, small end bearing and clips - totals 105.5 grams

3) Multiply the figure 105.5, by your desired balance factor. For arguements sake, i'll use a common balance factor figure of 55% > 105.5 x 0.55 = 58.025

4) Add the total weight of "step 1" to the balance factor figure of "step 3" - 47.4 + 58.025 = 105.425

The required bob weight to balance the crankshaft to a balance factor of 55% will be 105.4 grams

5) Assemble the crankshaft by lightly pressing the two halves and crankshaft pin in place, only so it will hold together and you can true the crankshaft on a set of V-blocks for the balancing proceedure.

6) Place the trued crankshaft ends or shafts on parallel knife edged surfaces or use two reasonably sized drill bits that do not have any scratches or deep grooves - you want a perfectly smooth and horizontal surface for the crankshaft to be able to rotate on.

7) Make some sort of hook like device to hang off the crankshaft pin that weighs exactly the same as the figure you've calculated in "step 4"

8) You want to be able to rotate the crankshaft in any position without the crankpin falling downwards.
Generally it seems to be accepted that a slightly heavy counterweight is desirable (the side opposite the crankpin).
If the crankpin falls downwards, you can either remove metal from the crankpin end, or add heavy metal (like brass or malory metal) to the counterweight end, till you get the crankshaft to balance.

9) Once the crankshaft has been balanced, take it apart and install the big end bearing and connecting rod (thrust washers also if the crankshaft used them) and press the crankshaft together and true the crankshaft.

10) Take your crankshaft and put it in a plastic bag and place it it the freezer. After about an hour, take your crankshaft bearings (for the left and right side shafts) and remove the dust seals if they are installed.
Now drop the bearings into a pan filled with hot oil.
The bearings will expand in the oil and the crankshaft will shrink ever so slightly.
Take the crankshaft out of the freezer and prop it up so the shafts are vertical.
With a pair of smooth faced plyers, take a heated bearing out of the oil and carefully drop it on the shaft, don't delay, do the other side as well.
The bearing should just drop straight on, no pressing or force should be needed.
Make sure you have pressed the bearings into contact with the face of the crankshaft before the bearings cool down and the crankshaft heats up.
If they are not seated squarely on the shaft after everything has equalised in temperature, you will have no hope of bashing them flat and straight - you'll only damage the shaft and the bearings.

11) Before assembling the crankshaft in the case halves, along with the clutch shaft, place the assembled crankshaft with it's crankshaft bearings in the freezer and use heat to expand the cases allowing for an easy fit of the bearings into their respective recievers.
Assembly should take place without too much force being applied on the crankshaft.
If you need heaps of pressure to get the case halves to close up, you'll press the crankshaft out of "true" and the engine will vibrate, regardless that it has been balanced.

12) It's best to install the crankshaft seals after the cases are clamped together.

Here is a simpler meathod but not as accurate.

By placing the crank in a stand that will allow it to turn freely.
V-Blocks with 2 bearings on each side which the crank spins on.
We could follow the method of balancing a grinding wheel.

You would need some round magnets to use for weights place them on the crank to find the correct position of the weights.

The round magnets 2 for each flywheel (4 total) should weigh more than the total weight of the unbalance. " piston conn rod pins bearings ect.

Once you find the proper placement to achieve balance you will need to drill out the crank flywheels and place weights in there = to the weight of the magnets + the weight of the removed material.

This would be the simpler non mathematical less accurate method of balancing.

I take NO CREDIT for this information I just found it.
 
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