Here we are talking about two differing effects.
Rotational inertia is scalar - that is it has no directional vector , the larger the mass or the bigger the effective radius then the inertia
number gets larger.
Balance forces are a vector , that is a force created by the eccentric mass has a directional value about the center of rotation.
Thus whenver the balance factor is increased , by more mass added opposite the crank pin , then this added mass will increase the
rotaional inertia that has to be accelerated.
But , here is the kicker - reducing rotational inertia of a 2T crank can be counterproductive.
The lower value of inertia allows the crank to accelerate faster away from TDC due to the gas pressure of combustion.
This faster acceleration results in faster velocity thru BDC.
This effectively reduces the STA of the transfers, and in practice the effect is seen as a lack of overev power.
I first discovered this on a TM125MX , when I took off the ignition rotor and replaced it with just a timing disk.
The engine , that previously spun to well over 13,000 easily, now would not go past 12,000.
Later I discoverd that VHM and Honda both produced higher inertia cranks to be used with no flywheel when converting to total loss.
In our discussion of balance shafts the added inertia may in fact produce better performance due to the above effect.
But - the added inertia may also overcome any added porformace simply due to the extra power absorbed trying to accelerate the added mass.
How much inertia is too much ? - look at an Aprilia crank with many inserts of tungsten close to the outside of the crank discs.
That may be the optimum balance between acceleration rate - Vs overev power.
I think it was Doc Fleck at Queens that first documented this behavour - noting that a heavyer dyno flywheel appeared to make the engine
being tested to create more top end Hp by slowing down the " in cycle speed variation " behavour.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
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