The way I'm seeing it they're saying spreading the flywheel across, nearer the bearings introduces the forced coupling effect. A central flywheel shouldn't allow this to generate however likely creates more tendency to whip as pointed out elsewhere in this thread. The article does state dynamic balancing addresses the forced coupling effect though.
Commando Crankshaft Porn
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Yep RennieK that agree's with my physics training {bfd eh] so ideally we'd just have to balance a single plane disc/plate but d/t the practical lengths involved and asymmetry we must spend more balance both ends to get smoother. I've experienced both un dynamic and factory balanced cranks to conclude its fairly subtle difference to an isolastic pilot but the engine parts attached to crank sure benefit.
acotrel
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Burgs, I agree with what you've said 'Phil Irvine's various books are full of the possible combinations/solutions, but at the end of the day single cylinder and vertical twin cylinder engines can only be balanced to provide a satisfactory amount of balance at a certain rev range, unless you have something to counterbalance such as balance shafts or insulation such as the isolastic solution of the Commando range.'
When balance shafts are used to reduce vibration at high revs, and the crank has been balanced to reduce vibration at low revs, as it is in a standard commando , the stresses still exist between the balance shaft bearings and the crankshaft bearings. The motor might feel smoother, however the bearings and crankcase and the coupling between the balance shafts and the crank must cop a beating. I know balance shafts have been used in a lot of Japanese bikes, however I think If I was racing one, I would remove the balance shafts and rebalance the crank for high revs. I know Rohan will probably disagree, he seems to have the opinion that crankshaft balance is about rider comfort. My feeling is that the use of balance shafts is only a cosmetic effect and rider comfort is not the primary consideration. The common failure in standard commandos which have been over-revved is the crack in the crankcase wall though the through the main bearing housing. I know many racers buy the aftermarket cases with the thicker walls, however I believe the failure should not happen even with the standard cases. It will be interesting to see what happens if the 961 is ever raced in anger, as I believe it has balance shafts.
When balance shafts are used to reduce vibration at high revs, and the crank has been balanced to reduce vibration at low revs, as it is in a standard commando , the stresses still exist between the balance shaft bearings and the crankshaft bearings. The motor might feel smoother, however the bearings and crankcase and the coupling between the balance shafts and the crank must cop a beating. I know balance shafts have been used in a lot of Japanese bikes, however I think If I was racing one, I would remove the balance shafts and rebalance the crank for high revs. I know Rohan will probably disagree, he seems to have the opinion that crankshaft balance is about rider comfort. My feeling is that the use of balance shafts is only a cosmetic effect and rider comfort is not the primary consideration. The common failure in standard commandos which have been over-revved is the crack in the crankcase wall though the through the main bearing housing. I know many racers buy the aftermarket cases with the thicker walls, however I believe the failure should not happen even with the standard cases. It will be interesting to see what happens if the 961 is ever raced in anger, as I believe it has balance shafts.
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acotrel said:
"Cosmetic effect"?
Care to expound on this "feeling" a bit more here? If not got rider comfort than then what would so many manufacturers go through this trouble.
acotrel
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With a normal 360 degree parallel twin motor , however you balance the crank it must be out of balance either at low revs or high revs. If it is a racer like mine, it has a high balance factor which moves the imbalance lower down the rev range. At those revs the loads created by the imbalance are much less, and not as destructive as if the imbalance occurs at high revs. When you fit balance shafts to remove vibration, they do not change what the crankshaft is doing. If it is still out of balance at high revs it is still beating the bearings and crankcases, even though you might not feel the vibration because of the balance shafts. The balance shafts simply provide forces in the opposite direction which cancel the thump from the crankshaft, how can they change the loads on the bearings ? As I said they are essentially cosmetic. I think you will find the 961 will still have criticality aspects in relation to main bearings and their housings. I suggest the major objective in fitting the balance shafts is rider comfort, not mechanical reliability. If the 961 has a crankshaft which is balanced for high revs, and the balance shafts remove the rocking effect at low speeds, that would be great. I cannot imagine how the balance shafts could be set up to do that, and still run at the higher speeds.
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comnoz said:
It's to do with, for example, frequency of rotating masses (rpm) which is variable and natural frequency of assemblies, which is fixed. When the frequency of the rotating mass equals the natural frequency of the assembly it creates resonance (vibration) hence race bikes have balance factors so frequency at high rpm is outside of the assembly (motorcycles) natural frequency. Of course on our engines there is reciprocating engine parts to consider, but these too create a frequency.
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comnoz said:
Jim,
My reply was in answer to your question "....how can an engine be in balance at one rpm but out at another". I only understand some of the fundamental principles of transmitted vibration and do not claim to understand all the theories and maths involved in engine balancing so can't answer your second question. However, resonance does not get excited by a frequency - it is the product of a frequency acting on a mass when the exciting frequency is close to or equaling the natural frequency of the mass. Resonance is not 'in' the frame.
If resonance (vibration) is not being produced then the exciting frequency is not equal to the natural frequency of the assembly and there would be no vibration transmitted through the main bearings at that specific rpm.
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The engine always vibrates as its out of balance, the frame does not vibrate when the engine is not running but will transmit some vibration when the engine is running. The resonance is when the frames natural frequency is matched by the frequency of the vibration of the engine and so the vibration is amplified by the frame.
comnoz said:
Crankshaft harmonics will cause a balanced engine to vibrate and even break, aithough it will still be balanced, hence the need for harmonic balancers.
So from your own statement there is no need to spin the crank at high speed because the read out should be the same at 500rpm as it is at 2500rpm, but at the higher speeds you may be getting flex or harmonics up setting your readout. The S&W balancer used to spin the cranks over slowly based on a weight and length calculation, if spun at high speed it was a disappointing waste of time chasing false imbalance around the crank.
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kommando said:
I agree with the engine always vibrating, but at varying amplitudes and frequency dependent on rpm. Can't see how the frame amplifies the vibration, but it will resonate. On a Commando vibration is only transmitted when the engine frequencies are close to or equal the iso rubbers natural frequency, typically between 2,000 - 3,000rpm. Outside of this rev range everything separated by the rubbers from the engine assembly is relatively smooth.
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Yep Comnoz showed us a smooth spining static balanced crank that about UFO's as rpm increased. Ya can only talk in generalities of principles w/o any hard figures or process to follow as besides the internal imbalances magnitude increasing with rpm and the various bowing and springing along a long shaft that pales when the compression, subtracting and adding to piston mass on balance factor it also jump rope shapes crank for off center spinning in parts of the spin cycle. Trial and error is only way to fine tune then measure to report objectively after a subjective test ride on where to compromise next. To any some flavor to this the scary looking fast rpm comnoz ran up for us, is barely into the normal threshold of isolastion rpm with much more spin drama just lolly gagging with hwy traffic and whew wee when passing.
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What I am saying is a 360 degree twin engine always produces forces from imbalance. Those forces always increase with engine RPM regardless of balance factor. Those forces are seen by the main bearings and cases.
Balance factor can only determine which direction the peak forces are fed into it's mounting. If they are fed in at the wrong frequency and plane then the frame may resonate which will amplify the vibration as felt by the rider and maybe other motorcycle parts which may break and fall off.
I have trouble calling an engine which produces forces from imbalance "in balance" .
I would have to say the balance has been optimized for it's chassis or environment. Maybe that is just the engineer in me.
And yes I realize there is no need to spin a crank more than slowly to find it's balance. Spinning it any faster is only for demonstrating how much it vibrates to those watching. Jim
Balance factor can only determine which direction the peak forces are fed into it's mounting. If they are fed in at the wrong frequency and plane then the frame may resonate which will amplify the vibration as felt by the rider and maybe other motorcycle parts which may break and fall off.
I have trouble calling an engine which produces forces from imbalance "in balance" .
I would have to say the balance has been optimized for it's chassis or environment. Maybe that is just the engineer in me.
And yes I realize there is no need to spin a crank more than slowly to find it's balance. Spinning it any faster is only for demonstrating how much it vibrates to those watching. Jim
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IRRC 750's @ 6000 rpm apply ~7 tons between the bearings and goes way up fast after that. So technically correct, can't balance a turd just pick an imbalance compromise amount to balance to. Easy enough to experiment 'balancing' smooth at lower rpm then see if stays calm at higher rpm, then balance at higher and see if calmer or not lower down. Dynamic balancing ain't same thing as setting Balance Factor. We know mid 50's % BF gives easiest average forces on bearings so anything above or below that is for frame or pilot reasons and may be a non issue on engine internals.
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You may find this pertinant, the "whirl mode" http://people.cs.clemson.edu/~steve/Spiro/electra2.htmlAl-otment said:
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@acotrel: I see what you are visualizing. The term cosmetic you used threw me off - perhaps a better term would be comfort. Bearing forces and crank/engine case loads will increase due to the imbalance when using balance shafts - the analog would be mounting an engine solid to an immovable object versus floating in a frame (isolastic or solid mount).
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I grew up about a mile from Miami International AP so got to see evolution up close. In and out of sync props and engines were interesting to hear all day and night. Commando engine cases have a whirl resistant weakness that runs from crank hole to upper rear case bolt hole. I found this quote interesting as Lord's Mount are the two studs separated by rubber. The isolastics had to be cut down in half 3'x's to get isolation just under 2000, as both Norton tester Frank Damp and hobot have experienced for delightful lack of sensation above idle's pleasant throbs. Mufflers, oil tanks and headsteady are examples of Lord's Mount not quite up to the task forever.
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