Crank Balancing, the right way

jaguar

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#1
Some people will tell you it is impossible to balance the 2 stroke engine for little vibration but I’ve done it successfully and so have others. Most engines are fairly well balanced until the owner installs a non-standard piston or changes the compression ratio. What is “a little vibration”? It’s little enough that your hands and forearms don’t get fatigued just from the vibration.
In one DirtRider magazine issue Rick Johnson was talking about his 1986 works CR250. He said: ”For example, the magic Honda always had was the cranks. There was something about those cranks- I can’t tell you what they did to them but when you got on and rode there was virtually no vibration to your hands."

The Balance Factor Method of Crank Balancing
A quote from Crankshaft Balance Factors sums up my thoughts about the "balance factor" method.
"The bottom line is that the physics and mathematics involved in how the engine operates are far too complex to make a formula-based balance factor any more than a reasonable compromise."

A truly comprehensive crank balance calculator would be more than one simple formula and would take into consideration all the forces on each part every 15 degrees of crank rotation which includes the centrifugal force and inertial force. It would include the con rod by separating it into 4 centers of mass and doing the calculations for each center.

Essential to know is that crank balancing is dependent on max RPM because as RPM increases the centrifugal and inertial forces dont increment at the same rate. So a good calculator should show a graph of crank imbalance throughout the usable RPM range so you can decide on how to change it. You would be able to choose to balance it perfectly only at a certain high RPM, or make a compromise of balance by sacrificing some high RPM balance for better mid range balance which may be needed for trail or street bikes.

Of course all the parts need to be measured so there is a reasonably accurate assumption of each parts weight. That requires splitting the cases. I did that on my Suzuki 100 and then later on I used a heavier piston and needed a balance refinement and so instead of splitting the cases to drill side holes in the cranks I opted to just drill holes in the crank wheels from above, with the cylinder removed. You can read my whole journey of discovery at How to Balance the Engine for less Vibration and More Top RPM
 
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#2
For those who don't know, a single cylinder engine has reciprocating mass, in the piston+rings, pin and rod.

We could add an equivalent balance weight at 180 degrees to the crankpin. A 100% balance factor.

But what happens is that weight has little to oppose it, when the crankpin is at the 90 and 270 degree positions. Leading to an engine that has no vertical vibration, but massive horizontal vibration. This is no better than an engine without a crankshaft balance weight.

The 53% balance factor (53% of the reciprocating mass) is an attempt at matching real world vertical vibration and horizontal vibration. It's just a starting point. Real world data will "adjust" the counterweight mass and position slightly from the starting point.

I have an ACES 20-20 and an ACES Cobra, helicopter balancing setup. It includes sophisticated sensors and a calibrated output that provides a real time depiction of the balance situation. It's not unusual for single cylinder engines to have more vertical vibration than horizontal. Due to insufficient counterweights.

Anyone who tells you that a single cylinder engine can be balanced to something better than equal levels of vibration up/down and fore/aft is incorrect. A well configured, non counterbalanced single will always have vibration commensurate with it's reciprocating mass. It can never be perfectly smooth.

Unless.... the single cylinder uses 2 counter-rotating crankshafts.
 
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jaguar

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#3
yes a normal 2 stroke can never be perfectly balanced and the near-perfect balance is unnoticeable vibration with small engines and sometimes annoying vibration on large engines (which is why everything at 300cc or larger should have a counter balance shaft)
 
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Pete Payne

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#4
Good stuff guys!
I rebuild and true a lot of cranks - probably two a week. I just true them up good. I have only played with the math for balancing a crank (deep math there-makes my brain hurt) . I have measured and weighed stock cranks but never actually added or took off weight of the crank myself.
 
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#5
Most regrettable tatoo trends, Plent O' Fish dating sites, hmmmm, Geico savings, hmm, Oops, Yeah crank balancing is essential to a properly running engine, next time I split my cases on my CR's I shall have my crank taken care of...