 Here's some good quotes from http://victorylibrary.com/mopar/crank-bal-c.htm that says the main reasons I have never had any interest in trying to balance a crank using a balance factor. - - - - - - - - - - The "balance factor" is at best a compromise, and partially suppresses vibration at some RPM and power/vacuum levels. Classifying the connecting rod's upper and lower halves as "reciprocating" or "rotating" is not completely accurate. The rod's pin eye does reciprocate, but the rod's absolute upper end (including the material closing the top of the eye), and the rod beam between the pin eye and the crankshaft's rod journal follow different and more complex paths. The rod's big end does rotate, but only the imaginary line marking the contact with the crankshaft's rod journal is "pure" rotation, the rod's big end actually oscillates as well. Mathematical-based formula using only conventional factors will never predict accurately how well a given engine will run, even at a given RPM, because the dynamic forces aren't limited to reciprocating vs. rotating weight. The forces acting on the rod + crank-pin (mass inertia) are not only the reciprocating weight (as listed above), but also the forces present in the cylinder and combustion chamber above the piston. No formula is "correct", some just come closer than others, by the "empirical" method - they've been tried + adjusted by experiment. 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. - - - - - - - - - - - - - What he also lacks saying is that the inertia of the reciprocating parts doesn't have the same % change with RPM change that the centrifugal force of the crank wheels does, so that the balance factor needs to lessen as peak RPM is raised. The balance factor method has always lacked a chart to go by so that you'd know what balance factor to use for any given RPM. Playing around with my spreadsheet I could see that as you almost double the peak RPM the balance factor would have to be 1/2 of what it was. So like that site says the balance factor method depends on trial and error. That means it's an incomplete system which is why I made my spreadsheet. With it you can keep changing the balance hole data till it balances at the desired RPM. It is the only program to date that painstakingly figures out the con rods contribution to the reciprocating and rotational forces, every 15 degrees. It doesn't cheat and use a percentage of the con rod weight for each force. That method is inaccurate. |