Light weight flywheel - good idea?

[DavidNJ]

Originally Posted by mylesdw

Go on then, put me out of my misery! What have I done wrong:

We have a 40 lb flywheel 11" in diameter at 800 rpm,

Rotational mass (I)= 0.5 * m * r^2

so

I = 2420 = 0.5 * 40 * 11^2

Energy = 0.5 * I * w^2

so

774400000 = 0.5 * 5280 * 800^2

We also have a 2800 lb car travelling at 5 mph,

to make a comparison we need to convert mph to inches per minute

v = 5 / 60 * 63359 = 5280

Energy = 0.5 * m * v^2

39028528010 = 0.5 * 2800 * 5280^2

My energies are a factor of 50 apart!

Units,units, units...

Your inertia is in lb*in². It is 16.8 lb*ft².

However, your rotational speed should be in radians/second not revolutions/min. That is a factor of 377.

So you energy becomes 1/2 * 16.8 lb*ft² * (800/(60*2*pi) rad/sec)². This equals 1800 lbf-ft.

At 5mph your car is traveling 7.33 ft/sec. So your equation is 1/2 * 2800 lb * (7.33 ft/sec)² which equals 2340 lbf*ft.

The 40# figure is a little light. 45-50# is probably closer.