Byron,
I agree with your assesment, and I agree with the old G-Analyst suggested
procedure. The only problem is that we are shifting in about 100 ms in our
FSAE cars, and the G-Analyst records data every 100 ms; therefore, we don't
ever see a drop-out during the shift. In a street car, you might see it,
but I haven't studied street car graphs.
-Woods
At 10:06 AM 5/9/00 -0700, you wrote:
>Hey Dr. Bob,
>
>Your point is well taken that the actual acceleration comes
>from what happens after the trans ratios and final drive are
>taken into effect. Even if torque at the flywheel drops off
>at higher RPM, the multiplication through the gears often
>makes the lower gear/higher RPM a better choice. Hence it's
>often better to stay in the lower gear longer. Just where
>this benefit begins to die off is more complicated to
>calculate.
>
>The old G.Analyst literature gave a procedure for finding
>proper shift points using the longitudinal strip charts.
>
>Their method was to record several full-throttle test runs
>with shift points at various RPMs. For instance, redline,
>redline-200, redline-400, etc. Once this is done, you can
>then review the Accel Chart on each run, and compare the
>acceleration g's just prior to the shift to those just after
>the shift.
>
>If the accel g's prior to the shift are higher than those
>after the shift, you would like to continue using that gear
>longer (if you can), so you should move your shift point up,
>if possible. If the accel g's prior to the shift are lower
>than those after the shift, you stayed in the gear too long,
>and you should shift sooner.
>
>The G.Analyst folks didn't specifically address the spike of
>accel that the clutch release after the shift produces, but
>I would assume you'd look for the first plateau after the
>drop-increase of the shift.
>
>Do you agree with this assessment as a final litmus test for
>finding proper shift rpm? It seems intuitive enough.
>
>With all of this said, for the majority of autocrossers out
>there, the only shift we have to worry about is the 1-2
>shift, and the vast majority of vehicles will benefit more
>from the gear than the torque, meaning that you should shift
>just before you hit rev limiter (or blow the mechanical
>fuse, whichever occurs first).
>
>--Byron
>
>"Dr. Bob Woods" wrote:
>>
>> 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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