Calculus Quiz

[Superman]

This one's for Wayne and the other engineering types. Low mass flywheels and pressure plates. Daily drivers with these units are more difficult to get started from a stop, but the motor does spin more freely. Shifting perhaps becomes smoother because engine speed can be changed more quickly. But I wonder what these low mass parts do to increase useable power. The difference in mass between these lightweight parts versus the stock parts is relatively small, but that weight spins at upwards of a hundred revolutions per second, or more. Getting a 2500-lb car moving from 60 mph to 100 mph costs power. So does getting two pounds spinning from 30 revolutions per second to 100 revs.

Of course, not all the weight is at the outer edge of the flywheel, so in calculating the added useable horsepower we would distribute the reduced weight everywhere within the plane. Of course, the outer edge of a flywheel spinning at 100 rpms is REALLY going fast. So, I think there is some sort of increase in useable horsepower since these few pounds will be missing from the mass that must spin this fast.

Am I making sense? Has anyone calculated the additional power that gets to the wheels because it's not needed to accelerate the flywheel and pressure plate?

[randywebb]

you need to define 'useable power'


power is the rate of energy expenditure, dE/dt

[masraum]

I watched an episode of "Sports Car Revolution" on Speed once, great idea, but horrible execution. They actually dyno cars before and after things like flywheel/clutch changes and brake upgrades to see what difference this stuff makes. They actually saw differences in HP on a dyno which should be an indication that the hp normally used to spin a flywhl is available if some of the mass is removed. As far as calculations, I dunno. I know that you could use either of the following formulas to determine the moment of inertia of a flywheel where m is mass, r is radius, and h is height.

thin disk
1/2mr^2

or to be a bit more exact (but still not quite)

solid circular cylinder
1/12m(3r^2+h^2)

but these don't give you the power required to overcome the inertia, they just give the inertia itself.

[Rob Channell]

if
A = Angular Acceleration (in radians per second squared)
T = Torque in ?? units
I = Moment of Inertia in ?? units

A = T/I

A1 = T1/I1 for original flywheel
A2 = T2/I2 for new fly wheel

Assuming engine torque = same for both flywheels (T1=T2)

Combine equations and substitute for constant Torque
A2/A1 =T2I1/T1I2

A2/A1 = I1/I2
Ratio of new Angular acceleration to old is inversely proportional to the ratio of the new and old flywheel Moments of Inertia

Rough shot. gotta go.

[livi]

Just an casual observation. I believe Porsche put different flywheels in the two versions of the 964 RS. The Touring model got the stock 'heavy/double' item and the Sport model got the lightweight item. Porsche claimed 260 bhp for both.

But maybe thats not what you were talking about here ?

And I could possibly have it wrong in my memory too..

[Bill Verburg]

Absolute steady state hp is not affected but the rate at which revs change is positively affected by a loss of weight of any of the rotating assemblies. The engine builds revs quicker and drops revs quicker. During these accelerations the energy that went into accelerating the heavier assemblies is now available to accelerate the vehicle. The same can be said for wheels, tires, half shafts and brake assemblies.

Some but not all dynos can measure the change.

[dvkk]

Originally posted by Superman
...... Daily drivers with these units are more difficult to get started from a stop,.........

This is a myth.

A car with an ultralight flywheel just takes a different amount of throttle, and a different timing of the gas and clutch pedals. Just like you would expect the pedals to react differently if you switched from your 911 to a Dodge 1 ton diesel pickup.

I used to switch between 2 cars, same make and model. One had a super light flywheel, the other was stock. Everyone says the light flywheel equipped car will have troubles starting off from a stop, but in fact the opposite is true. After being very used to the driveability of the light flywheel car, when starting off from a stop in the heavy flywheel car, I was constantly stalling it. Then I figured out why. The car with the heavy flywheel took longer to build rpms, and I was letting out the clutch pedal too soon. I was used to the light FW's quick accelerating engine.