Thread: SS Shift Point
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Old 03-16-2009, 10:16 PM   #10
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Originally Posted by zebra View Post
great explanation... for DGthe3!

care to tell us normal guys what that means?!
Heh. I put in a picture so you can see the equation better.

Basically what it means is that if you divide the new gear ratio by the old gear ratio, and multiply that by the ratio of the flywheel torque at the new RPM divided by the flywheel torque at the redline, and get a number greater than one, you need to short shift at the highest RPM where that number would be 1 or lower.

For example. An electric motor has the following torque curve.

and a 2 speed transmission, with first gear being a straight 1:1 gear and the second being a 0.5:1 "overdrive" gear.

At the 10,000 RPM "redline" in first gear, the motor would be producing a net 2 ft*lbs at the wheel. But at 5,000 RPM after the shift, it will be making 3.5 ft * lbs. Meaning there is a point where you could have short shifted for a larger net "power under the curve."

Heres a visual representation of this.

As you can see, the total area under the curve is higher in the short shifted instance than it is when you shifted at the redline - because while the curves are nearly identical, for that split second after you shifted (leading up to when you WOULD have shifted) you have more power.

This happens very rarely with modern transmissions (once you have a lot of gears to play with you can prevent this pretty easily), but it is a theoretical possibilty, especially once you start modding.
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