The amount of flywheel mass affects the rate at which the engine increases or decreases RPM.
It's similar to a heavy car and a light car with the same power.
The heavier car will not accelerate as quickly as the lighter car, nor slow down as quickly.
A heavy flywheel can slow down the amount of acceleration out of a corner or from a stop or during gearshifts.
A light flywheel allows rapid acceleration of the bike, but if too light, the engine can accelerate so quickly that the tire can lose traction and spin up.
Nowadays, F1, MotoGP, and WSBK use traction control to control that, so they can use small amounts of flywheel mass for quick engine acceleration.
The flywheel and clutch diameter on a F1 car is smaller than most bikes (about 6").
For downshifts, a heavy flywheel will resist allowing the engine to match speed with the rear wheel, and might cause the rear tire to lose traction.
These days, slipper clutches moderate that effect.
The same can be said for shift changes as the engine resists matching RPMs with the next higher or lower gear.
When I changed out my steel clutch plates for aluminum Hyperplates, I immediately felt a difference in shifting. Far smoother and easier due to less mass in the clutch assembly.
Singles and twins use heavier flywheels than multi-cylinder bikes since their rotating components have more mass (pistons, cranks, etc.). The extra flywheel mass smooths out the power pulses.
Multi-cylinder engines have lighter components and the power pulses are smaller and occur more often than singles or twins.
They can use lighter flywheels.
As an example, I have an RC51 and an FZ1.
Both are 1000cc engines, but I have to downshift 2 gears to get the same engine braking effect I get just by rolling off the throttle on my RC51.
There's less rotating mass on the I-4 than my V-2.
Last edited by SubSailor; 06-09-2011 at 10:18 AM.