I simplified it as best I could:
Energy required to accelerate a 2800 lb car to 100 MPH
1270 kg @ 44.7 m/s
Total kinetic energy = ½ MV^2 = 0.5*1270 * 44.7^2 =
1268787.15 joules
Rotational kinetic energy of a spinning cylinder
E-rotational = ½ I w^2 (w is angular velocity in radians/sec I is moment of inertia )
30 lbs driveshaft = 13.6 kg 3" dia (1.5” radius = 3.81 cm = .0381 m) =
I = .0381 * .0381 * 13.6 = .01974 kg.m^2
Stock wheel = 1964.44 mm circumference
@ 100 MPH our wheels are spinning at
@ 44.7 m/s => 44.7/1.964 = 22.75 RPS @ wheels
Final drive ratio = 4.1:1
Driveshaft RPS = 22.75 * 4.1 = 93.275 RPS = 5596 RPM = 35160.7 radians/min = 586 radians/sec
Rotational energy of the spinning driveshaft = ½ I w^2 = 0.5 * .01974 * 586^2 =
3389.32 joules
Energy stored in moving car:
1268787.15 joules
Energy stored in spinning driveshaft:
3389.32 joules
Total energy (
NOT COUNTING the energy stored in the
REST of the rotating mass) =
1272176.47 joules
For a whopping ratio of
375/1 = .0027 or .27% of the energy in the system.
SO while the engine has stored up a total of 1272176.47 joules in the system, only 3389.32 joules are in the spinning up of the driveshaft.
BRZ gets ~ 16.2 seconds 0-100 mph
That’s 1272176.47 joules / 16.2 seconds or 78529.4 joules/second
OR ~105.36 hp Which for back of napkin calcs is in the ballpark enough (holy shit!).
If you had no energy stored in the spinning of the driveshaft at all: total system energy would be 1268787.15 joules at 100 MPH
If the car delivered the same 78529.4 joules/second it would achieve 1268787.15 joules in 16.156 seconds
So I guess you were all right!
If you reduce the 30 lbs driveshaft to 0 pounds (removing the energy stored in it entirely!) and do otherwise equal runs 0-100 MPH you WILL notice a
44 millisecond difference in a 16.2 second run! Congratulations you are superhuman!
And I was being generous by leaving things out such as the REST of the rotating mass so the actual difference of removing the shaft would be even less.
I never said that it wouldnt make a difference I said that a driver wouldnt be able to
TELL the difference.
Shall I repeat the calcs for a 4 pound reduction of the drive pulley?
Which coincidentally would be MORE than the driveshaft since IT spins to ~6000 RPM
And @
Shutter beat me to it: