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      03-26-2008, 09:01 PM   #88
enigma
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Drives: E92 M3 and Elise
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Quote:
Originally Posted by footie View Post
I don't think, I know that the jerk has been engineered.
Someone feel free to check the following math.

Assuming a flywheel of ~10kg, 0.15m radius

I = 0.5 * 10 * (0.15) ^ 2 = .1125

E = 0.5 * I * w^2 = 0.05625 * w^2

At 8400 RPM: E = 0.05625 * (8400/60*2*3.14)^2 = 43.48 KJ
At 6000 RPM: E = 0.05625 * (6000/60*2*3.14)^2 = 22.18 KJ

So after shifting we have ~21KJ of energy stored by the cars flywheel (not counting the other moving engine bits) that has to go somewhere.

If the car (1650kg) is going ~44mph in 1st (20m/s) the car currently has:
1650kg * 20^2 = 660KJ of energy.

If the DCT simply disengages the previous gear and engages the next and dumps that stored energy to the rear wheels you now have 660 + 21 = 681Kj

sqrt(681Kj / 1650) = 20.32 or a sudden velocity increase of 0.32m/s or almost 1 mph. Thats the jerk, its not simulated or made up, they just don't try to hide it under the cover of "smooth" like Audi.

You could apply the energy over time, burn it in the clutches, or reduce the engine output to cover it up. None of those help you go faster.

I suspect by the time you add in the other engine components the stored energy is considerably larger. Enough to put it into the 1-2mph range.

BTW: one "G" is 9.8m/s So if they accelerate the car 0.32m/s in 0.1s. that generates a spike of ~0.3g for that brief instant over the normal acceleration.
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