Hey Ron,
I've attempted to answer this a couple of times, and keep
getting dragged off into other tasks, so my apologies for
the late reply.
Well, heck, you've got me a little bit stumped. I'm going
to hope that it's a problem with the G-Cube, because hey, we
can certainly replace that easily enough. However in
another case a user told me about something similar, but
when the G-Cube was put into another car the problem
completely disappeared, indicating it wasn't the G-Cube.
Here's the case for why it probably isn't the G-Cube...
When you do your calibration (the one where you put your
G-Cube on each side in turn) what you are doing is setting
adjustment levels for how the G-Cube perceives 1g in each
direction. Once that calibration is done (and especially
when it's repeated with very similar results) we know that
at 1g the G-Cube is extremely precise and repeatable.
There is a tiny "moment arm" inside the G-Cube that deflects
as g's are applied to the G-Cube, and that flexing is
precisely measured and reported back to the recording
computer as a duty cycle. We then convert that number to a
g number by applying what we learned in the calibration
process.
The design of the moment arm is such that it's extremely
consistent and linear across the entire rated g range,
(which by the way is from -2g to +2g). The spec sheets on
the accelerometer package show deviations beyond linear of
only negligible amounts (in the 1%-2% range), with the bulk
of this deviation in the low side of the scale, say at 0 +-
0.25g. So in these instances the deviation should be below
what we even measure. At 1g we are calibrated to the exact
number, so that should be our most accurate point. The
typical values that we reach out on the course in the
lateral direction also happen to be right around our test
value of 1g, so our accuracy is really maximized in the
precise area where we need it most.
All of this leads me to conclude that it would be *extremely
rare* for such a large deviation, the 0.15g you indicate in
your post, to be in the G-Cube, especially so close to the
test point of 1g.
So can I imagine any scenario where the G-Cube could be
causing this? Well, yeah, but it's pretty unlikely. If
there were a flaw in the moment arm that caused it to
essentially hinge at a precise g point, that g level being
somewhere over 1.00g, but below about 1.25 g, we could get a
reading like you are describing from the G-Cube. I wish we
had a good repeatable test to measure in the 1.00 - 1.50g
range, but we don't. If we did that would prove or disprove
our case one way or the other. Without that, all I can say
is the scenario I described above the for the problem to
occur is one that I made up, and not something we've ever
seen before.
So, let's talk about the car a little bit.
First, don't worry about where the G-Cube sits in relation
to the CG of the car, unless you feel that you must have
readings from that location. The car does get slightly
different readings at different locations, but not enough so
to be worrisome. The accuracy isn't effected by the
location of the G-Cube, btw, it's just that you get the
readings as they occur in that particular location. Not a
big deal.
What is a big deal is the alignment to straight ahead.
Let's assume the alignment is off by 5 degrees, pointing
slightly left instead of straight forward, and compare two
turns, left and right, in which we are pulling 1.20 g's
laterally, while simultaneously accelerating at 0.30g.
(This is a total g of 1.24 -- within a reasonable range for
the test car.) In both the left and right turns, the
lateral g's are lowered (but just a little) by the fact that
the lateral force isn't running perfectly down the lateral
pole. Instead of the lateral force being at 90degrees, it's
at 85 degrees on the left side, and 95 degrees on the right
side. The difference this makes is negligible. Instead of
1.2g's, we'll only see 1.195g. (sin(85) * 1.20). So in
fact after rounding, there's no measurable error there. But
wait! We have 0.30 of acceleration there that is also going
to spill over into our lateral readings. In the case of
the left hand turn we have 0.30 * sin(5) that will be added
to our lateral g's. This brings us back up to 1.23 g's.
Conversely, the right hand g's go down by the same amount,
down to about 1.17g's.
So after a little math, even a relatively large alignment
error of 5 degrees only produces a spread of 0.06g's. We'd
need 10 degrees of error to produce a 0.10g spread. This is
a lot of error. So again, I'm not convinced this is it.
But do be careful and make sure that if you aligning inside
a console for instance, that the console faces perfectly
forward. My Camaro was always difficult to align within the
console because the console was off by about 10 degrees of
straight forward.
One reason the car could be showing more g's one way than
the other is lean. The car leans out of the turns, and we
provide a straight line compensation factor for that lean.
But if the car leans for instance more in left hand turns
than in right we could see this effect on the trace.
However, there's not enough lean in any chassis to make up
0.15g. So I doubt that would be the cause, at least not
entirely. Besides, the most likely difference in lean is
the driver weight being on the down side of the vehicle in
right hand turns, making right turns higher than lefts--the
opposite of your problem.
Next would be the car's actual handling. Since you are
pulling more g's left, this is when the driver is on the
high side. This follows suit with the general theory that
when the weight is on the inside edge the car will handle
better, and on the outside edge the car will handle worse.
But still, 0.15 is a lot.
I can't think of any one thing that could get us a
difference of 0.15g. But are you really sure it's that
much? You said your adjustments always take "more right" to
make the maps look right...how much more? The full scale
only goes to 0.100. If the difference is really 0.100 the
full scale shouldn't be enough. Are you sure that 0.15 is a
fair representation of the amount of the problem? On
average, how much adjustment have you been putting in? If
you find that 0.020 or 0.030 is enough to correct the
problem, I'd re-open the alignment issue. It's the most
likely of the physical explanations.
Next, I'd try to duplicate the error on a different G-Cube,
or in a different car. If you swap G-Cubes with another
user (be sure to re-calibrate!) does he now have the high
left problem, while your problem goes away? Or put a second
G-Cube in the car alongside your G-Cube. Be sure both are
aligned exactly the same, preferably with the edges against
each other to assure that. Then record the runs. There
should be very little difference between the runs. If you
see some sort of evidence of a "hinge" as we discussed
above, that would be a clincher. Or if you could develop a
simple course with a long sustained left followed by a long
sustained right, we could run the course several times the
regular way, then flip the G-Cube over (straight upside
down, but still pointing forward). This swaps the two
lateral poles, and should produce a mirror image map. If
that mirror-image map now shows the problem to the right
instead of the the left, and it does it consistently, that
might also be a good clincher. Again, my HOPE would be that
it's the G-Cube...it's the easiest fix of the possibilities.
In summary, I don't have any sure things. The easiest thing
to do is to just replace the Cube, but I doubt that that's
the actual problem. Call me if you'd like to discuss some
possible tests further. I definitely want to address this
during this off-season time!
--Byron
800-775-9511
Ron Katona wrote:
>
> Hi all, newbie list member question here...
>
> 100% my GEEZ data from this season showed left G forces (peak and
> sustained) about .15 G higher than right Gs. The car subjectively does
> not feel like it turns better to the left. It has coil-overs and has
> been corner balanced, carefully aligned, and weighs only 60 lb more (out
> of 3130) on the left side with the driver in place. Other people have
> driven the car and haven't noticed that it won't turn right (maybe I've
> found the perfect NASCAR setup).
>
> I'm using a G-Cube with a Palm-IIIxe and the latest versions of GEEZ
> downloaded from the website. I've calibrated the G-Cube several times
> using the Palm and the same batteries used for the runs. The G-Cube is
> velcroed placed in a little bin between the front seats and is quite
> snug in there. It's about as close to the CG of the car as I can get it
> and appears to be level and aligned with the centerline of the car.
>
> FWIW, I've seen peaks of up to 1.40 G left, but only about 1.25 right.
> Sustained is about 1.25 Left and 1.10 right. All other data seems normal
> for this car (ESP M3). In order to get my maps to look like the courses,
> I usually have to add "more right" on the calibration window. To me, the
> right turn data seems much more realistic.
>
> .15 G seems significant to me. Could the car really be that much better
> to the left, or is it more likely that the G-Cube is
> misaligned/calibrated? If it's the G-Cube/GEEZ software, what's the most
> likely problem? Any ideas, opinions, advice greatly appreciated.
>
> Ron Katona
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