If anyone is going to break a crank, then, it will be me. So far, two years on from a major rebalance (they said the engine tried to jump off the rig is was so out of balance with the new heavier rods) I have done many miles, including going from Carlisle to Goodwood and back at full chat down the motorway. I always drive the Aprilia enthusiastically (that is what it is for) and should the engine get a bit hesitant give it a blast at full revs to clear the plugs. Have been doing this for 2 years and more, so far so good.
One concern some have (jump in please anyone with more information) with the "vibration free method" is that the correction weight is applied or removed at the ends of the crank on the flywheel and front pulley. The thought is this (insert Have I Got News For You style "allegedly") sets up a twisting vibration or resonance in the crank which may have been the cause of a couple of crank failures.
They put the engine as a unit on a rig much like at the tyre fitters. Perhaps think of it as making each end vibrate to counter the internal vibrations so it doesn't jump about on its mountings so much. Money no object people have a modern style counterbalanced crank made to match the rod and piston combo.
Recent problems might also have been machining of the crank beyond limits, or stress raisers, or going beyond rev limits either on a "motorway" basis or through the gears or with missed changes or too much blipping with no load on it or any one or more other causes. In one case hanging a supercharger off the front with its pulsing load could have been a factor along side the increased torque it produced.
Perhaps its as simple as a smooth engine with steel rods gets (relatively) thrashed. Back in the day a week flat out round a banked track day and night to setup records would be considered an extreme test. Now we have the roads to pound along beyond the design limits as a matter of course.
David
