by Ciampone (Ciampone) » 05 Feb 2002 14:17
I think Your evaluation is good, to do a better one, You need to know many more things....
And the results will change very little, I assure You, I have done some similar evaluation for trains.
The concept is right : less mass is like more torque : You have more acceleration.
I hope to bore no one with this explanation...
In your count You have no consideration that the wheels are rotating mass!
Reduce weight in rotating mass is much better than reduce the same weight in non-rotating mass, for example with fiberglass fenders or lexan windows.
Think that: why in race engines they search to have the lightest crank as possible?
All the rotating mass have a moment of inertia (lb*ft*ft or kg*m*m), for example, for a point, a little ball of lead (an object very small with mass), his moment is :
his mass for his distance from the center of rotation (distance squared)
For a solid, full, disk with constant thickness
J = M*r*r/2
There is a similar formula in this case :
T(torque)= J(inertia's Moment)* A(Angular Acceleration)
To have a rotating acceleration on a flywheel, alone, with no car, You have to put a torque
To have more acceleration You have to reduce the moment of inertia or increase torque.
In a car, or any other vehicle, there are rotating and non-rotating mass, so the rotating
mass counts 2 times, 1 for total mass, 1 for moment.
To do some counts, You can use the same formula you wrote but You must insert the equivalent mass, that usually is 10% more than the mass of the car.
At this point, I'm sure You say :" And what *#@* is the Equivalent Mass??"
EM = M + J1*(w1*w1)/(v*v) + J2*(w2*w2)/(v*v)+......
w is the rotating speed or angular speed of something that rotate (rad/s)
v is the speed of the car (m/s)
As You see, to do an accurate count You must know the moment of inertia of :
crank, gearbox, driveshaft, axles, wheels (steel and alluminium, with the tyres on)......
And finally you'll discover that.... Your weight reduction is 36 pound on the E.M. is about
1,1 * 36 = 39,4 pound...