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Can physics and math save you money? I think it might this time.
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
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