Gear lightening

[Bill Edgeworth]

The math is not all that simple buy you can work out exactly how any particular part can/will affect the acceleration of your car. Hear is a couple of links briefly explaining some of the math involved;
http://hyperphysics.phy-astr.gsu.edu/Hbase/rke.html

http://www.ac.wwu.edu/~vawter/Physic...FormsofME.html

Where it gets complicated is that to get a truly accurate picture of how a part will affect the acceleration of your car (to accurately calculate the energy or work required to accelerate the part to a certain RPM) you don’t need to know its weight you need to know the moment of inertia. Also what I talk about below is only part of the bigger picture you have the whole car to accelerate not just the rotational speed of the part.

To put it all into laymen’s terms not all light parts are created equal. I will use light wheels as an example; two different 10 pound wheels can take significantly different amounts of energy to accelerate through the ¼ mile;
Brand X with a heavier center and super light outer rim will take much less energy to accelerate than brand Y with a super light center and heavier outer rim.
Again keeping it simple and not getting into center of mass and moment of inertia calculations;
Speed = 2*pi*r and kinetic energy = ½ *velocity^2

So comparing the kinetic energy of two identical weights but one twice as far out from the center as the other both rotating at the same RPM the one twice as far out will have 4 times the energy or will have taken 4 times the energy to get it up to that speed.

So the benefit of lightening rotating parts diminishes the closer you get to the rotational center.
Gun drilling axles; almost no benefit, only unsprung weight
Aluminum spool is less effective than the same weight savings by turning the outside of the ring gear.
AND all light wheels are not created equally.
Most people see ET improvement with light wheels because there is such a significant savings in rotational weight compared to any other thing you can do.
Also what this all shows is that rotational weight savings prior to the rear end is very important. Taking rotational weight out of your transmission and engine is far more beneficial as it has to wind up to 5 or 6 times the RPM of your rear tires.
I think the Yak touched on this in another thread talking about the two different diameter clutches and flywheels. Just because two parts weigh the same does not mean they accelerate the same.