Todd,
Just a comment on your observation about shift time.
Realize that engine torque, and hence the measured acceleration, increases
to a certain speed, peaks, and then decreases. Whenever you pass the peak
torque (on your way to peak power), you will see a decrease in acceleration
g's. Then when you shift, you might be below the peak torque rpm again.
The point is this, you will see dips in the acceleration g's; but, that
time is not the time necessary to shift.
We have a wonderful combined hand clutch and shifter on our Formula SAE
cars (you can download an in-car video from one of our cars on our website
<fsae.uta.edu>. We measure our shifts at about 100 ms to 150 ms (for
reference Formula 1 hydraulic shifters are in the 30 to 50 ms range). On
our acceleration g's plot, we too see 500 to 700 ms duration of decreased
acceleration, but we know that this is not the shift time, it is the
decrease in engine torque.
Now, before someone thinks that engine torque alone relates to
acceleration, and that you should shift to keep your engine at peak torque,
let me state the following. The car is accelerated by the torque on the
rear axle, not the engine. Granted, when you are in a given gear, the peak
acceleration will come at peak torque. However, if you were to downshift
at the engine speed corresponding to peak torque, you would have the engine
operate at a higher rpm, and you would get more torque multiplication at
the rear wheels. With more torque at the rear wheels due to being in a
lower gear (even though you are at a higher engine speed and decreased
torque from the peak torque), you will get more vehicle acceleration. You
actually want to keep the engine near peak engine power, not peak engine
torque. Some folks say operating at peak engine torque gives maximum
acceleration; it does not, maximum engine power yields maximum acceleration.
-Dr. Bob Woods
Faculty Advisor, UTA FSAE
>On a side note, I've always measured "shifting time" from the point when
>the acceleration starts to drop (i.e. off the gas) until the
>accel curve's slope becomes positive (i.e. on the gas) no? The .7 that
>you have noted seems to be from when the accel starts to dip, until when
>it is maxed again. If I understand this correctly, the .7 would
>represent the total accel time lost due to shifting, but not the
>physical time it actually took to shift, no?
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