George at Santa Paula Airport is right. Flywheels can go up into lots of
pieces. Sometimes just into a few which can make them quite heavy.
Cast iron and steel go up into few pieces.
Wound fiber flywheels go up into many pieces in general I have heard. I
didn't check it for myself.

The following video shows a Mazda Rotary flywheel desintegration
http://www.youtube.com/watch?v=L2pz1PkhAo8

Randolph

Well racers typically wind the Mazda engine to 11,000 to 12,000 RPM because
the Mazda rotary is able to do that. That is providing you don't miss a shift.
If that happens you are looking at 14,000 or more RPM.
That year car's engine had a red line of 6500 RPM as I had an RX4
station wagon. Perhaps the racer was dumb enough to use the stock
cast iron flywheel designed for 6500 RPM. This is a free country,
thank God, and therefor it is a self cleaning oven :)

Composite flywheels, as far as I know, come apart in harmless fibers much like
a mop or broom as I recall.

Paul Lamar