These engines have a 270 crank. The 270 crank configuration is supposed to be the optimal set up for a twin in terms of limiting the vibration.
So... why do they need a balance shaft...?
If the cranks were properly dynamically balanced, could the balanced shaft be removed?
I have no specific plans to do so, I’m simply hypothesising.
Nigel,
The conservation of crankshaft momentum on the 270 crank design does help reduce vibration, but there are 2 other problems specially for a vertical twin.
First, the distance between bore centers on a vertical twin will produce a rocking couple on a 270 degree engine, or any crankshaft layout other than 360 degrees.
On other 270 degree designs, like Ducati 90 degree V twins, the distance between bore centers is minimal with the rods right next to one another on a since crank pin.
This reduces the rock couple significantly virtually eliminating the effect at normal engine speeds.
The other issue for primary balance is the effect of unbalanced crank flywheel mass.
We all know that the crank flywheel mass balances the reciprocating mass of the piston/wrist pins/rings/rod as they move up and down in the bore.
However, the flywheel mass is not balanced by anything when the flywheel moves forward or backwards as the crank rotates.
This acceleration of the flywheel mass produces a harmonic that pulses forward and backwards in a vertical plane.
Left unchecked the harmonic produces serious vibration. The balancer shaft is designed with lobes that counteract the unbalanced mass of each flywheel.
The balancer shaft rotates in the opposite direction from the crank, and the lobes are timed to exactly (or closely) counteract the mass of the opposing flywheel mass.