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

The best part of that video is you see the flywheel go vertical and then
it lands on the track, gets a little traction and zooms into the grass
and then gets lots of traction and takes off like a bat out of hell.
Watch for it.

Rob Woods