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Old 05-17-2020, 12:12 PM
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Join Date: Jun 2011
Last Online: 10-18-2020 02:29 PM
Location: Paraguay
Posts: 35
Default CRANK BALANCING

Some people will tell you it is impossible to balance the 2 stroke engine for little vibration but Iíve 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 ďa little vibrationĒ? Itís little enough that your hands and forearms donít get fatigued just from the vibration.
In one DirtRider magazine issue Rick Johnson was talking about his 1986 works CR250. He said: ĒFor example, the magic Honda always had was the cranks. There was something about those cranks- I canít 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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Old 07-10-2020, 09:56 PM
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Join Date: Jun 2011
Last Online: 10-18-2020 02:29 PM
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Look at this graphic which shows two test cases on the left and what their balance would look like on the right if they were balanced using the balance factor. I can tell you from riding my bikes that what the graph says and what I experience are nearly exactly the same. The AX100 graph has 100% balance (where it crosses the zero line) a bit higher than I prefer now but that was back when I had the program set to balance it that way. At top RPM anything within 50 of the zero line is not felt hardly at all on a small engine. I think the main flaw using a balance factor is that there is no way to scale it for top RPM. I'm sure I could figure that all out but I'm not going to because I've already expended a lot of brain power getting this calculator to work just right. Any way the results here speak for themselves. Using balance factor is as hit and miss as throwing a dart trying to hit the bullseye. It's just no good.
http://www.dragonfly75.com/moto/imag...ancefactor.png

Last edited by jaguar; 07-10-2020 at 10:01 PM.
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