Crank Balancing revisited

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reading several posts from our new expert...nothing he says has made any sense :giggle:

(i cant say ive ever seen a tyre balancer that uses bubble glasses. they spin the wheel up and use some fancy circuitry to figure out the imbalances. just like turbines. shafts. and everything else that rotates and gets balanced.if you want true, dynamic balance, at least.)

iunno... my nsr150 leaves my hands tingling after three minutes, it has a balance shaft...

the hyodung 650 vtwin left my hands numb after half an hour or so...

the 48cc HT, with welded mounts and left stock standard... nah. that never needed any balance, i found the chain caused more vibration than the engine. could ride it all day except for the sore rear end after several hours... there needs to be bead seats for bicycles!
 
HS, dont make empty headed assumptions such as "it comes from the factory that way so it must be right".
My KDX200 was "factory" out-of-balance so much that I had to put shotgun pellets inside the handlebars so I could ride it 4 hours on the trails.
Fabian, I am waiting for your experiential definition of balanced, not a theoretical one.
 
Fabian, I am waiting for your experiential definition of balanced, not a theoretical one.

I place all of my trust in the maths, and the maths say that a single cylinder engine cannot be balanced unless it has dual balance shafts. Other than that, you can only adjust the balance factor to minimise vibration in the rpm zone where it is used most, which in my case is 3,500 - 3,800 rpm.

As an experiment, i ordered a "balanced engine" from Rock Solid Engines, that was "balanced" to 7,500 rpm.
The engine only spent 30 seconds in my bike because at the rpm zone where i mostly operate the engine, it vibrated so bad that the fillings rattled out of my teeth. Needless to say, i went back to a stock standard engine, and have done so ever since. I don't even bother to clean up the cylinder ports anymore.

These days, the only modifications to my engine are purely external, being the installation of a Rock Solid Engines reed valve intake and the RSE Walbro style carburettor and the CR Machine Manufacturing billet cylinder head and the Jaguar CDI and light modification to the muffler.
 
by-product is imbalance produced by the entire ASSEMBLY attached to the single cylinder crankshaft it can't be removed only countered at the opposing end the math it this instance is not a variable it's a given so 'm not sure where you get the numbers ..
 
by-product is imbalance produced by the entire ASSEMBLY attached to the single cylinder crankshaft it can't be removed only countered at the opposing end the math it this instance is not a variable it's a given so 'm not sure where you get the numbers ..[ g force at 12,000 rpm is closer to the actual measurement you discussing will outweigh the crankshaft
 
Fabian has his limited experience which contains useful data but keep in mind that his experience with two strokes is just with his motorized bicycle. I speak from experience of having owned about 10 different 2 strokes starting in 1975 with my beloved Penton.
When someone has very limited experience then of course they can mostly only talk theory.
Unfortunately most people owning a motorized bicycle are first time owners of a two stroke and they don't know any better to know that their ride can be much smoother. Fortunately Fabian relieved much of the vibration in his by installing the Jaguar CDI which corrects the too-advanced ignition that contributes to vibration.
 
I have owned many 2-stroke bikes over the years; incorporating a large single cylinder 500cc Kawasaki KX 500 (which vibrated like a b.a.s.t.a.r.d ) as well as the old 350cc Yamaha RZ 350 parallel twin cylinder, which did have reduced vibration, partially due to having a 180 degree crankshaft configuration and partly due to it's rubber mounting system.

The smaller capacity 125cc single cylinder engine dirt bikes, like the Honda CR 125 vibrated my hands numb, just as easily as did the larger capacity single cylinder engines.
 
excess vibration is a sign of crank imbalance. some vibration on dirt bikes is acceptable but if it keeps you from being able to ride long periods of time then it's not.
 
well, im going to take the advice of the guy that uses RPM in his calculations, and is aware that the g-forces follow a square law, so that RPM is a critical part of the calculations, to not worry about the engine itself but concentrate on the chassis...

There is some misunderstanding here. No way. I have never said such a thing and it has no basis in fact. The closest comment to that is that I said that the out of balance force is proportional to the SQUARE (not square root) of RPM. That is the force not the balance factor. There is no single optimum balance factor for any rpm. The balance factor alters the direction of the maximum force and in any given chassis the way that it vibrates depends on that direction. So the optimum balance factor depends on the chassis to which an engine is fitted. Sometimes, due to resonance of certain chassis members vibration may be much worse at certain RPM and adjusting balance factor may help with that. So the required balance factor is not something that is proportional to this or that parameter. In the absense of very sophisticated dynamic FEA the only way to determine the "optimum" balance factor for a given type of engine in a given frame is by experimentation and testing. This can be very tedious and as long as vibration is not excessive there is no need to go chasing the "optimum", whatever that means.


rather than the advice of this brush turkey that hasnt learnt to keep his mouth shut yet, despite the glaring mistake in his "theory" and "calculations"...

Yes but my theory of crank balancing excludes RPM from the equation completely. If the out of balance force is a certain amount at 6000 rpm then it will be twice that at 12,000 rpm. It's equally out of balance at both those rpm. Finding the right balance has nothing to do with rpm.

this guy only seems to have posted on here to stop someone from taking his knowledge and distorting it til its no longer knowledge but a load of BS...
I wasn't commenting on your "theory of crank balancing", I was merely correcting the misquoted statement about what I said in the mentioned article.
In any case the out of balance force at 12,000rpm will not be double that at 6,000rpm it will be 4 times.


isnt it amazing how in reality, its a SQUARE law, but in jaguars calculations...its just a mere doubling?

hang a weight on a string. it weighs a gram. spin it at an RPM so its producing 1 g of centrifugal force.
now double the speed. the g-force will now be 4! double it again...9 g. doubled again? 16g...

theres nothing worse than chinese whispers :cry: and they dis information they transmit

its a shame...all these noobies come on here, think theyll double their power and turn their bike into a screaming demon by clicking on jags signature link...but anyone with half a brain and previous experience, once having read any of his posts, can do no more but laugh at the erroneous information supplied..

i wonder just how many engines have been destroyed by beginners tearing things down, grinding this, grinding that, to find that they shouldnt have? think theyll make things better but just end up with scrap metal :(
 
Great post HeadSmess

Couldn't have put it better myself.

After messing around with the internals of these engines for some years now, i no longer bother to open up the engine, because with a standard exhaust, it makes no meaningful difference to power. Ironically, my most powerful engine had the worst looking cylinder, both inside and outside, and i was running the lowest compression billet cylinder head that CR Machine Manufacturing produces, being the three hole version, with the third hole filled with a spark plug to further reduce compression.

* The Jaguar CDI has greatly improved the reliability of my engines, but in back to back testing with the standard CDI, it only gives a marginal power increase at best.
** The Rock Solid Engines reed valve intake noticeably improves low and midrange torque but doesn't give any power increase.
*** The CR Machine Manufacturing billet cylinder head gives greatly reduced cylinder head temperatures over the standard cylinder head, but only gives marginal power increase.
**** The Walbro style diaphragm carburettor is a vast improvement over the standard NT carburettor in ease of air/fuel adjustment and general cleanliness, but doesn't give any power increase over the NT when jetted correctly.

I have ridden with someone who (supposedly) knew everything about 2-strokes, with his bike having a modified expansion chamber from a dirt bike attached to his engine.
One thing i can say is that his bike made a he!! of a racket; revving the bejesus out of the engine, and everyone living 5 miles away could hear us coming down the road.
I can say that my bike (with a standard engine, running at low rpm; using torque boosting measures) didn't go any slower than his bike, even with the trailer attached and i ended up towing him home as the thing disintegrated with just about every part vibrating off the bike, not to mention the engine calling it quits when the big end failed from excessive rpm, which was an easily predicted outcome.
 
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