SchmidtMotorWorks wrote:While the single may be "inheritly unbalanced", you absolutely can reduce the vibration at higher rpm's by increasing the reciprocating % factored into the bobweight.
Nope absolutely not, changing the bobweight only changes the direction of the vibration. The physics formula for this have been well known and unchanged for centuries. If you tell me the weight of the componets and the bobweight(s), I can tell you exactly what the diection and magnatude of the vibration will be. Now I can model it in CAD and know the loads and stresses on the parts.
I even have a CAD parametric crankshaft that has a program I wrote that gives instructions to the designer suggesting multiple choice changes that can be made to balance the crank until it is within a prescribed balance tolerance.
When you balance them in real life, the biggest errors come from the detailer that sands off the maching scallops.
Maybe you are only considering some of the current single cylinder engines with very clean shaped counterweights. We really have not experienced demand to balance these engines. In this case the only thing we would affect would be altering the reciprocating component.
These engines do have uniform counterweights. At some point the only required correction would be for inconsistency in the density of the steel.
Yet, if you did consider an old Briggs and Stration crank, you would not be so quick to dismiss the need for correction on these cranks. Maybe you never balanced one of these in real life? They are not even close.
It is my opinion, that it would be very difficult to model all of the dynamic loads that act upon the single cylinder crank while under load at rpm.
But if you simply attached a vibration pickup to an actual engine while running you could measure the phasing and magnitude of the vibration.
Then, the ideal counterweight and/or correction could be determined with trial weights and vector addition.
I personally have not had the opportunity to do this, (except for experience comparing the vibration, running these engines on my dyno). I expect the results would prove a reduction in vibration by reducing the reciprocating weight and at the same time, increasing the reciprocating bobweight %.
For many decades, the recommended reciprocating percent has been 65 to 81% on single cylinder Briggs or Tecumseh race engines, depending on rpm.
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