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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