Originally posted by Andy
Would I be correct to say that if there was a counter weight on the crankshaft above, the primary balance would be perfect and the blue arrow would go away?
Yes Andy, you would be 100% correct.
This is possible for an engine with the cylinders at 90 degrees.
Take a look at the V60 engine (Aprilia) and see what happens. It's the opposite than the V90 really. The primary forces can't be completely balanced as the graph is an elipsis, but the secondary can be easily balanced!
It's the result of the function of sin and cos of these angles.
Aprilia opted for the V60 in order to create a more compact engine, but it's not easy to balance this engine.
It has two counterbalance shafts, one countershaft that turns in the opposite direction than the crankshaft, and one smaller one which is in the rear cylinder head and balances the moment of the first countershaft!
Their plan was to remove the 2nd one from their racing bikes, I don't know if the did it.