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balance a 2 stroke motor

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#1
has anyone had the rotating assembly of a 2 stroke motor balanced before? Id love to get rid of some of the vibration.. if so, how far will I have to tear it apart to get this done?
 

DWreck

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#2
I have never done this but you would have to split the cases and tear it all the way down. I have always dreamed of buying a new bike and having the motor balancing and blueprinted before the bike has ever been ridden. This takes money and I seem to be a little short on it right now.
 
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#3
it sounds like it could end up being more trouble/expensive than worth while. does anyone know if it would even help much with vibration if I could get it done?
 
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#4
Keith, I have an ACES helicopter balancing set at work. A few years ago I used it to balance my '94 Husky 360WXC 2 stroke single. I was able to install 2 pickups (accelerometers) and measure IPS in X and Y axis. I added screws and bolts into threaded holes in the flywheel to change the balance. What I found is a 53% overbalance (balance factor) gave the best results.

However, I was not able to make the engine smooth. I regularly read that a single can be balanced to smooth at a given RPM, (in my case 7K would be nice). This is absolutly false! What can be done with a single is to reduce the vibration in one plane (for insatnce inline with the cylinder) in exchange for more vibration in another plane (for instance fore-aft)

Of course, there is a proper balance for any engine, if for some reason your engine is not properly balanced (quite rare) then a big gain is possible.

Keep in mind that engine and chassis mass are involved in what you feel, and how you choose to balance the engine.

Chris
 
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#5
I did not have to disassemble anything other than the flywheel cover to balance the engine with the helicopter equipment. Sure was fun riding around the airport with wires taped all over my bike.

I came up with a solution using the ACES and removed a small amount of material on the crank halfs during my overhaul a year later.

I could not tell any difference between the bolt in the flywheel and my rebalanced crank. Of course, the weight change was minor.

Chris
 
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#6
Keith.

Have never balanced a 2-stroke crank but, have balanced many 4-stroke cranks. A crank is a crank.

You cannot put a crank into perfect balance. Reason being is you are dealing with two completly different forces. Rotating weight and reciprocating weight.

The rotating weight being the crank, flywheels, and the bottom half of the connecting rod, (all the parts that go round and round). The reciprocating weight is the piston, rings, wrist pin and the top half of the connecting rod, (all the parts that go up and down). The flywheel does not matter since it is a balanced member on its own.

As cujet pointed out, you must use a balance factor. You must determine what RPM you want the engine to run its smoothest. You will still have vibration below that RPM and above that RPM but, when the engine reaches its working RPM it will smooth out.

If you only had a rotating mass, then you could balance the engine perfectly at all RPM's. But, the recriprocating mass, (the up and down weight) is the one that throws a wrench into the works. You have to use a formula for this and a percentage factor.

Inorder to balance any engine, you have to completly dissasemble the crankshaft, rod, etc. and weigh each part, rod, piston, rings, circlips, piston pin, etc. Once you determine the proper balance, you have to either grind or drill into your crankshaft flywheels to put it into proper balance.

If your interested in balancing your engine let me know, I'll PM you the formula.

Also, if you are having problems with excessive vibration, I would first check a few things like loose engine mount bolts or a cracked mount. Most engines come balanced pretty well from the factory. If your crank assembly has ever been rebuilt using an aftermarket rod kit or you are using an aftermarket piston, your balance could be out a little.

Just my $ .02

Ol'89r
 
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#7
While it is true that most engines are balanced internally, it can be done externally. What I did was a version of external balance. Many automobile engines used to come externally balanced also. Not common today, but in the '60s and '70s it was quite common. I chose to use this method initally, as it was very easy and could be done in a few hours. As I mentioned previously, I cannot feel the difference between my externally balanced setup and my conventionally balanced crank.

If you think about it for a bit, counterbalancers are an added on form of balancing also. They could be external to the engine and affixed to it.

Chris
 
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#8
Originally posted by cujet


If you think about it for a bit, counterbalancers are an added on form of balancing also. They could be external to the engine and affixed to it.

Chris
....And that's why they use counterbalancing -- It's the only way to counter that reciprocating mass.

Now the Harley Chicks really like it !
 

DEANSFASTWAY

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#9
Ever see married with children where Al Bundy crashes the Harley and Peg falls in love with the EVO.??
 
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#10
I just wanted to thank you guys for all the great information. Very informative and I appreciate it. I think i better give up though, this whole process is way over my head.

I wonder if anyone has tried any kind of rubber bushings on the motor mounts or anything? I may also have to look into rubber bar mounts.
 
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#11


Inorder to balance any engine, you have to completly dissasemble the crankshaft, rod, etc. and weigh each part, rod, piston, rings, circlips, piston pin, etc. Once you determine the proper balance, you have to either grind or drill into your crankshaft flywheels to put it into proper balance.
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That's only if you don't have all of the fancy helicopter stuff like cujet. :confused: Chris. Using that system, how do you determind the proper balance factor for a particular engine? That sounds like a pretty neat system. :thumb:

Keith. Yes, several manufactures have used rubber mounts on their engine mounts. Way back in the early 70's, Norton had a rubber mounted engine. I think they called it the Isolastic mounting system or somthing to that effect.

There is also a bar damper that fits inside the bar and is supposed to take vibration out of the handlebars. Never tried it though.
 
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#12
OL, in my case The balance factor was measured after dissassembly. That was the only way to know. As far as determining the correct balance factor. I used 2 accelerometers (X and Y axis) and measured accereration in each plane. I added weight to the flywheel in specific locations. Once I got similar numbers in X and Y plane I was done. By the way, this also felt the best.

I ended up removing a little material from the crankshaft counterweight after disassembly. So, initially the engine had too much overbalance for my situation.

Keep in mind that I am running much higher RPM than stock.

I did use flywheel weight for about a year with perfect results.

Chris
 

jaguar

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#13
I know this is an old post but I want to add to it.
Yes a balanced 2 stroke is smooth only around its resonant RPM which should probably be around its peak power RPM.
Yes you can use a fancy trial and error method but I prefer to measure/weigh all the parts and enter them into an inexpensive crank balance calculator to see its current balance/imbalance all thru the RPM range and then make virtual changes until the graph is best. Then implement those changes on the crank. To read more go to How to Balance the Engine for less Vibration and More Top RPM