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Always wondered just how much force (torque) was wasted spinning up those heavy Saab flywheels. I took it upon myself to go and find out ...

Newton's Second Law: Rotation

**Net External Torque = moment of intertia X angular acceleration**

I used my C900 non-turbo as an example. It accellerates at .41 g's in 1st gear and at 6000 rpm it's at 32 mph. This gives it rotational accelleration of 27.96 radians/second/second. The turbo Saabs will accelerate a bit quicker in 1st, but not more than say 25% ...

The flywheel on the later model Saab C900s is about 10.25 KG. Moment of inertia (I) for a circular disc =

**1/2 x Mass x Radius^2**...

... I assumed a radius of about 15 cm, or .15 metres, and assumed that the mass is more or less evenly distributed through the flywheel ...

Using this, it's moment of inertia is:

.5 x (10.25) x (.15)^2 =

**.1153kgm^2**

Putting these figures into the original equation gives:

.1153 x 27.96 =

**3.22 NM or about 2.4 lb-ft**of torque.

So why then are alloy flywheels considered "cheating" in some forms of professional racing? I suppose in some forms, 2.4 lb ft could be the difference between winning and losing. Given the cost it's understandable to ban them, otherwise every team would need to spend $500 just for that little "edge" ... but otherwise all it's gonna do is allow you to rev a little quicker in neutral. I can't imagine that novelty being worth $500.

Cheers,

Dubbya