I've been messing around with different balance factors for a 277°. The charts show a 360° for comparison. Not knowing what the stock balance factor is, I left it at the "perfect" 64%. There is also a 277° at 64% to get an idea of what it looks like when you re-phase without rebalancing, and the "perfect" 53%. I'll explain the 100% and 85% as we go along.
First we'll look at the up/down vibration.
It's no surprise that the 360° vibrates a lot here. The rephase is lower but more importantly, it only shows a single cycle, which means that the secondary forces are being canceled out. You can eliminate the primary forces with a 100% balance factor. The little ripple that is left is the secondary force which remains because they are not perfectly 90° out of phase. On a true 270° the secondary forces are practically zero.
Next we'll look at the front/back vibration.
Here the "perfect" 277° is about 60% of the 360° and the 100% is a bit higher than the 360°. Setting the balance factor to 85% matches the vibration to to the 360°.
Now for the rocking couple.
Here is where the 360° shines, absolutely zero. The "perfect" rephase actualy has the highest rocking couple at TDC and the lowest at BDC. The 100% balance factor cancels the primary forces so what you see is the secondary forces, a perfect sin wave at twice the frequency of the RPM. You saw that the secondary forces cancel out in the up/down vibration chart, but here they double.
There is another couple to consider as well. Imagine you are looking down at the engine. The two crank throws are not in the same plane, so if the right one is pushing forward and the left one is pushing back the engine will want to "wag". To not confuse this with what we are already calling the rocking couple, I'm going to call it the wagging couple.
Again, the 360° is zero. The piston motion has no effect here so it's simply a matter of greater balance factor equals greater wagging couple.
Based on this, I think I'm going to try an 85% balance factor on my next build. The up/down vibes will be greatly reduced but remain at the primary frequency. The front back vibration will be the same as stock which is cool because I like watching the front wheel wobble at idle. The rocking couple won't be much different than what I have now (re-phase without rebalance).
