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two stroke vibration

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Ok i just read the new dirtrider and rick johnson is the guest editor and he is talking about his 86 works cr250 in an article. He cant give the details even though they don't even race 2 stroke anymore but he says his bike had no vibration because of the crank. I come from a four stoke and thinking about buying a 250 smoker but just rode a service honda cr500 and the vibration killed me in the 50 mile loop i rode it on. Does anyone have clue what they do to accomplish that?

thanks

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"No vibration" was a clear exaggeration. Single cylinder reciprocating engines are fundamentally impossible to completely balance. The best that can be done is to get them close, which is more easily done with a balancer shaft. Without one, the best shot you have is to get it smoothed out within a certain RPM range reasonably.

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ok i understand that i guess i said it wrong and i quote from the magazine. "For example, the magic honda always had was the cranks. There was something about those cranks-I cant tell you what they did to them-but when you got on and rode there was virtually no vibration to your hands."

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you can adjust the balance factor so the bulk of the vibration goes up and down. or fore and aft, or any angle between. Between might be inline with frame mounts, or in the case of the factory honda a few different cranks were built and riders gave input which is usually the common way things are done. The could have also run a balance shaft in the case. You can reach a point where strengthening the frame to handle vibration induced failure (cracks) adds more weight than a balance shaft. This was huge hurdle in twostoke road racing (which in honda case, spilled over into factory mx).

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The problem faced with single cylinder engines is twofold. First, you have a rotating mass that has to be balanced. This by itself would be easy except for the second part I'll get to in a minute. The rotating mass is simply the whole crankshaft, and the weight of the rod's big end. An imbalance here will produce a primary vibration matching the RPM of the crank.

The real problem is the inertia of the piston. Assume the cylinder is vertical for the sake of this discussion. As the piston is accelerated up the bore, it produces a downward reaction equal to its mass. As it is brought to a stop and accelerated downward, there is an upward reactionary force created. This results in a secondary vibration on a vertical axis. A balancer can be employed to counter this, but because a balancer shaft rotates rather than reciprocates, it creates not only a vertical vibration to counter the piston, but a horizontal one that isn't offset by anything. To make matters worse, the force created by the piston reversing at the top is stronger than that created by reversing at the bottom because of the effect of the angularity of the connecting rod.

What ends up needing to be done is for a compromise to be struck between balancing rotating and reciprocating masses so that a reasonable result is achieved. Without a balancer shaft, the best that can be hoped for is to get the engine fairly well smoothed out in the RPM range where you will most often use by balancing to a percentage of the reciprocating weight, rather than attempting to balance the rotating assembly. We used to do this with all the big British singles. It was just a matter of where you wanted it to shake.

Inline fours with 180 degree cranks are among the most intrinsically balanced engines, because each of the above forces have an opposing force at work. But even they have some imbalance related to the TDC/BDC mismatch. V8's, V12's, and horizontally opposed engines are next.

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It is worthwhile to state the case for 90 degree V twins where, again for discussions' sake, you have a piston moving up and down in the verticle plane, and one moving left and right in the horizontal plane. Thus, you have the opportunity to balance both up and down motion and left and right motion with flywheel weights. Ducati actually made a single cylinder engine with a "fake piston" balance mechanism in the verticle plane that supposedly worked quite well. I don't have much technical detail on this but, there's a photo of a cutaway of this engine on this page on ADVRider: http://www.advrider.com/forums/showthread.php?t=80568&page=13 :busted:

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To make matters worse, the force created by the piston reversing at the top is stronger than that created by reversing at the bottom because of the effect of the angularity of the connecting rod.

But even they have some imbalance related to the TDC/BDC mismatch

Awesome explaination, thanks!:busted: This, in particular, is something I hadn't considered.

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The problem faced with single cylinder engines is twofold. First, you have a rotating mass that has to be balanced. This by itself would be easy except for the second part I'll get to in a minute. The rotating mass is simply the whole crankshaft, and the weight of the rod's big end. An imbalance here will produce a primary vibration matching the RPM of the crank.

The real problem is the inertia of the piston. Assume the cylinder is vertical for the sake of this discussion. As the piston is accelerated up the bore, it produces a downward reaction equal to its mass. As it is brought to a stop and accelerated downward, there is an upward reactionary force created. This results in a secondary vibration on a vertical axis. A balancer can be employed to counter this, but because a balancer shaft rotates rather than reciprocates, it creates not only a vertical vibration to counter the piston, but a horizontal one that isn't offset by anything. To make matters worse, the force created by the piston reversing at the top is stronger than that created by reversing at the bottom because of the effect of the angularity of the connecting rod.

What ends up needing to be done is for a compromise to be struck between balancing rotating and reciprocating masses so that a reasonable result is achieved. Without a balancer shaft, the best that can be hoped for is to get the engine fairly well smoothed out in the RPM range where you will most often use by balancing to a percentage of the reciprocating weight, rather than attempting to balance the rotating assembly. We used to do this with all the big British singles. It was just a matter of where you wanted it to shake.

Inline fours with 180 degree cranks are among the most intrinsically balanced engines, because each of the above forces have an opposing force at work. But even they have some imbalance related to the TDC/BDC mismatch. V8's, V12's, and horizontally opposed engines are next.

You must be thinking about a boxer here? The inline four is not known as smooth engine, and without a balancer assembly of some kind most of the 4 cyls do have vibration issues. With more than 4 cylinders the engine becomes naturally easier to balance due to cylinder arrangement. The inline 6 for example is much smoother because of this, and the in line 8 is better yet. Am remembering the once popular GM 2.5L 4 cylinder and the "balancer box" they were using on it to smooth it out in the early 90's, which was at the end of its run.

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Download the engine balance factor analysis tool from here under free software http://www.tonyfoale.com/ or direct link http://www.tonyfoale.com/progs/BF.exe

It's not very useful, but lets you understand the principles of a single cylinder engine vibrations and balancing without a counter balancer. Basically the only thing you can do is to distribute one big force around into continuous smaller one. Also it lets you understand how the rod ratio affects balance.

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Inline 4s are not balanced. They have primary balance in that the piston weights an crank-throws are balanced, but because of the difference in piston acceleration at the bottom of the stroke vs. top, causes a secondary imbalance. Flat 6,8,12. Inline 6,8 and V12,16 have perfect primary and secondary balance.

Issue gets worse the larger and faster the engine is. Go find an old Ford Ranger with the 2.3/2.5 Pinto engines. They vibrate a ton compared to modern engines.

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cr250 are nothing to complain about. the vibration is damn minimal.

the cr500 you rode, yes. they vibe alot.

but after a month on the 500 you would just get used to it anyhow.

ballancing a crank on a old steel bore cr250 is pointless. because everytime you rebore, your new os piston will have a different weight, therefore your crank will be out of ballance again.

i ride a 500 and just put up with the vibes. when your going fast, you dont notice them.

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straight 5 and 9 cylinder engines have much better balanced cranks than straight engines with an even number of cylinders or even a v-8. generally only see these in large medium speed diesel engines though.

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This all makes sense to me, but the bigger question for me is, why does the two smoker vibrate more (or seem to vibrate more) than a four-stroker? If it's because of the balancer shaft on the 4, then again, why don't they use a balancer shaft on the two smoker? I think this would be a huge advantage! Am I missing something? Maniac

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This all makes sense to me, but the bigger question for me is, why does the two smoker vibrate more (or seem to vibrate more) than a four-stroker? If it's because of the balancer shaft on the 4, then again, why don't they use a balancer shaft on the two smoker? I think this would be a huge advantage! Am I missing something? Maniac

2strokes are racing machines. not ment for pleasantries.

to add a ballancer would sap power and add weight and its just not needed.

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This all makes sense to me, but the bigger question for me is, why does the two smoker vibrate more (or seem to vibrate more) than a four-stroker? If it's because of the balancer shaft on the 4, then again, why don't they use a balancer shaft on the two smoker? I think this would be a huge advantage! Am I missing something? Maniac

I suspect because of the speed, weight of the piston, and the short connecting rod in modern 4-strokes, without a balance shaft they'd vibrate the whole bike to pieces very quickly.

So a balance shaft is pretty much required on a modern 4-stroke, but it happens to cancel out the vibrations good enough to the point where they're less than an balance-shaft-less 2 stroke.

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I suspect because of the speed, weight of the piston, and the short connecting rod in modern 4-strokes, without a balance shaft they'd vibrate the whole bike to pieces very quickly.

So a balance shaft is pretty much required on a modern 4-stroke, but it happens to cancel out the vibrations good enough to the point where they're less than an balance-shaft-less 2 stroke.

i actually thought the yz450 2010 has no balancing shaft!

i could be wrong.

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The problem faced with single cylinder engines is twofold. First, you have a rotating mass that has to be balanced. This by itself would be easy except for the second part I'll get to in a minute. The rotating mass is simply the whole crankshaft, and the weight of the rod's big end. An imbalance here will produce a primary vibration matching the RPM of the crank.

The real problem is the inertia of the piston. Assume the cylinder is vertical for the sake of this discussion. As the piston is accelerated up the bore, it produces a downward reaction equal to its mass. As it is brought to a stop and accelerated downward, there is an upward reactionary force created. This results in a secondary vibration on a vertical axis. A balancer can be employed to counter this, but because a balancer shaft rotates rather than reciprocates, it creates not only a vertical vibration to counter the piston, but a horizontal one that isn't offset by anything. To make matters worse, the force created by the piston reversing at the top is stronger than that created by reversing at the bottom because of the effect of the angularity of the connecting rod.

What ends up needing to be done is for a compromise to be struck between balancing rotating and reciprocating masses so that a reasonable result is achieved. Without a balancer shaft, the best that can be hoped for is to get the engine fairly well smoothed out in the RPM range where you will most often use by balancing to a percentage of the reciprocating weight, rather than attempting to balance the rotating assembly. We used to do this with all the big British singles. It was just a matter of where you wanted it to shake.

Inline fours with 180 degree cranks are among the most intrinsically balanced engines, because each of the above forces have an opposing force at work. But even they have some imbalance related to the TDC/BDC mismatch. V8's, V12's, and horizontally opposed engines are next.

You didn't mention the second-order vibration caused by the rocking motion of the rod?

And I've always understood that a 90* V12 and a 120* inline six are the most closely balanced engine designs for both primary and secondary vibrations, is this not correct?

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