Smaller motors, less mass, easier to rev high. Kawasaki bike has little torque compared to horse power.
Balanced? Extremey balanced. That doesn't "make" high rpm as much as it keeps the motor from tearing itself apart AT high rpm.
BUT you've touched on one of the more fascinating aspects of high rpm. How in the heck can the valves keep up at extreme rpm? Through out automotive history THAT has been the rpm limiting factor. And for MOST cars today, remains the MOST limiting factor to rpm.
Picture this: The lobe on the cam pushes down on the valve, which then opens. Cam turns, lobe moves away form the big part to the little part and the spring on the valve pushes it back toward the lobe of the cam and closes the valve. Cam pushes, valve spring pushes back, they stay happily married, valve up valve down.
High rpm:
The cam pushes on the valve SO fast it "throws" the valve open with such violence the poor valve spring can't push back fast enough. The valve has been accellerated open SO quick the mass/weight of the valve want it to keep going, right through the top of the piston if you would! The spring is TRYING to push the valve back, but it cannot. For a very brief moment in time, the valve is NOT in touch with the cam, which has moved on with it's life. There is the valve, hanging open waiting for the kiss of death from the top of the piston, which is being shoved up it's hole by the brutal and unforgiving cranky shaft.
This is "valve float" (all though it's beginning to sound a LOT like my divorce years ago, I was the poor valve, ).
So extreme valve SPRING PRESSURE is required to to keep the valve in it's place. And the lobe of the cam has to be shaped "just right" so it's not SO violent as to abuse the "poor valve". The cranky shaft don't care about all that, it's going to shove the piston around with no thought about the poor valve.
Ernie
(who now needs a drink, as I contemplate how my life is like a valve and lawyers are a cranky shaft).
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