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 This is a flywheel scatter shield made by Titan Motorsports for a Mark IV Supra.  It's required according to the NHRA rulebook for any car running quicker than 11.49 in the quarter mile.

My hypothetical flywheel is made out of a basic steel alloy with a density of 7.7 Mg (mega-gram) per meter cubed.  It has a diameter of 300mm (just under 12″ for you non-SI unit folks) and a thickness of 10mm (just under half an inch).  The resulting mass is 5.44kg making for a pretty light flywheel being right at 12lbs; for reference, a stock 350Z flywheel weighs 28lbs and has a 22lb clutch attached to it.  So you can see, my hypothetical flywheel is pretty light.

 These are the equations for calculating the mass and rotational inertia for a disk (i.e. flywheel) and hollow cylinder (i.e. driveshaft).  We need to calculate the inertia around the X-axis.  Rotational kinetic energy is equal to half the inertia times rotational speed squared.  I snagged these from my old engineering text book, “Fundamentals of Machine Component Design” by Juvinall and Marshek.

Spinning the flywheel to a speed of 9000rpms results in a rotational kinetic energy of 27182J, or about the equivalent of 0.0002 gallons of gas.  Not much you say?  Consider that a Prius gets about 50mpg, that's enough fuel to propel the Prius for 55ft.  However, the engine of the Prius is roughly, maybe, somewhere around 35% efficient.  So there's really enough energy in the flywheel to move the Prius for half a football field.  I don't know about you, but I'd be tired after pushing a car that far.  Also, keep in mind this is my hypothetical lightweight flywheel.  The combo of the 350Z flywheel and clutch would have about four times the energy due to the extra mass.

I do have a story about a flywheel failure used in a non-automotive application.  I was recently in an engineering training course and one of the guys there had done some work related to engines used on trains and boats.  He'd heard of a story of a flywheel coming loose on a boat with it promptly shooting through the bottom of the hull.  Needless to say, the boat sank.

Onto another car part for comparison, we'll calculate the rotational kinetic energy in a driveshaft.  The assumptions are that it's made of 6061-T6 aluminum, 3.5″ outer diameter, 0.125″ wall thickness, and a 56″ length.  This results in a mass of 3.35kg.  Spinning it to 6850rpms gives a rotational energy of about 117J, or a heck of a lot less than the flywheel and that's probably why drive shaft loops look a lot less beefy than flywheel scatter shields.

 This is an example of a driveshaft loop from BMR suspension.  If the driveshaft fails at any point along its length, the loop keeps the shaft from flopping around and destroying everything around in the proximity.

The last item we'll examine is a turbocharger.  If you spin a turbo to a speed beyond its design criteria, the compressor and turbine wheels can literally explode.  The centrifugal forces become so great that they cause the wheels to fly apart; not too many things in this world can handle being spun up to 300,000rpms.

 The aftermath of a compressor wheel burst test by Garrett by Honeywell.  There are no holes on the perimeter and nothing makes it past the compressor housing.