Quote:
Originally Posted by xxxJohnBoyxxx  Here is the deal. Every pound of rotating mass you remove is the same as removing 15 pounds of car weight. So you remove 3 pounds of driveshaft and since it is a rotating part it is like removing 45 pounds of car weight (Rotational mass R x 15= lighter car without removing weight. Lots of small things add up and make a big difference |
In the case of a flywheel, I believe that formula applies nicely. Similarly with tires and wheels.
However, in those cases the mass is quite far from the axis of rotation, and although you are absolutely correct in your statement that the acceleration will be slightly quicker, since the mass (in question) is so close to the axis of rotation, you would use a different formula.
On-axis it is precisely "1 times" (1X) of course, and as you move away from the axis of rotation, it increases (probably logarithmically). It takes less torque to get weight close to the axis of rotation moving that weight that is far from the axis of rotation. (a longer lever) This frees up torque to get the car moving.
Since the drive shaft is ~4" as opposed to ~12" (for flywheel) or ~18" for a tire/wheel you can envision how this would apply. Based on your formula, and assuming linear as opposed to log curve (15" fits well between flywheel and tire diameters), I would say it is more like ~4X, not 15X weight reduction. The down side is once it is moving (finished accelerating), it is still just dead weight. The formula applies to acceleration; deceleration is a strict 1:1 factor, unless this weight is contributing to the continued motion of the car (such as a flywheel [if left in gear] or heavy wheels) in which case it also affects deceleration. Overall weight (regardless of location) also applies to cornering, but like dynamic weight, higher weight has a greater impact than lower.
I therefore stick to my suggestion because unless you are watching every ounce in the car (rotating or not- which is not a bad idea mind you) you may be better served to spend money elsewhere, such as rotors, wheels or a flywheel to get the better return on investment. Or gut the interior for a greater weight loss at no $$ cost, and loose the weight higher up too.
I see the price is similar, and if it is in fact lighter, I say go for it! It can't hurt as long as the durability is comparable.
