In a message dated 4/21/02 1:56:04 AM Central Daylight Time,
kennedy@computer.org writes:
<< Well, my previous queries haven't gotten many answers,
but as my pappy always used to say, "try, try again". ;^)
Hope you all don't mind... >>
I was hoping someone like Byron would have stepped up to answer that one, but
I will take a stab at it with this letter.
<< I was looking at the friction circle of a number of my
runs... and noted that its more of a friction triangle.
I asked about the pointiness at the bottom in a previous
note... this note is about the flatness at the top. >>
I remember some of your previous question. The "fullness" of the friction
circle shows how well, you (AND/OR) your car can load the tires in any
direction. It is possible the car can't fill in the transition between
braking and cornering. But in most cases, with very smooth inputs, most cars
will match their average cornering with the total G's of braking and
cornering combined, and the driver is not using the whole area. The course
can also make it hard to use all of the area. This is where it can be very
helpful to have different drivers take your car out while you log them with
Geez. If just one driver is able to fill in your friction circle to a nice
apple, then you know the car and surface can do it, and you can work on your
inputs. But if noone can get the car to fill out the transition area, then
maybe the car does need work, Geez alone might not answer this one.
Look for clues though at the ends of fast sections entering sweeping turns.
If you just have a cross for the friction circle, you are doing all of your
braking in a straight line, and then turning while coasting. I was almost
this bad 4 years ago. Your triangle means you are combining turning and
braking to a point, but you could be braking more later into the curve. For
me, it was a matter of trusting that the car would stick in those situations,
and Both my wife and I filled out the circle quite a bit in just a couple
weekends with Geez. The useage went way up, and our times came down. Even
with good brakes on our 95 Celica ST, we never did reach the limit of braking
the car could do. We would see very high peak braking G's, almost 1 G, but
would only sustain about .65 G on a good day. I never did figure out why
though. We no longer own that car, and are re learning our current car. The
"total" G's shows how well your are using the limits. If you can pull 1.3 G's
in a curve, and 1 G under braking, the total G's should be able to hold 1.15
G's while you are braking and turning in.
<< While my car does 1.0x g's laterally and braking, it
can only pull about 0.45g's accelerating. And it seems
it can pull that many g's even when the lateral g's are
near max. In a way that makes sense... acceleration isn't
traction limited, but power limited. But then again, when
turning hard, I should be scrubbing away notable acceleration. >>
.45 G's of forward acceleration takes alot of power. The meak 95 Celica ST
would only spike to .5 G's and sustain less than .3 G's in acceleration once
we were over 20 mph. In that car, we could run full acceleration even while
turning at 1G and the car would just start to push a little. It barely had
enough power to make the inside front tire sqeak. Our current car on the
other hand can hit .6 G's at the torque peak of second gear. This is enough
to spin tires with only a slight curve. If you had unlimited power available,
you should be able to make the top of the friction circle fill in similar to
the bottom. Obviously, brakes have more "power" than the engine.
<< Thinking to my driving, I certainly don't go to full throttle
until I have it near-straight. So, unless from moderate to
full throttle I only manage a few hundredths of g's, something
doesn't seem right. Could it be that my gCube is artificially
limited in the forward direction? (its reading wrong?) >>
If you calibrate it before your run, it should be fine.
<< How many g's should a 400hp 3000# car be pulling, approximately? >>
Fun with math.
Let's change this to torque at the wheels. ASSUME you are geared to make 400
hp at 60 mph, and your tires are exactly 24 inches tall. The tire has to turn
at a surface speed of 88 feet per second. (5280 feet in one minute) and the
tire is about 75.4 inches in circumference (24 inches x pi) for a tire RPM of
840 rpm.
HP = (RPM x TORQUE) / 5151
or (5151 x HP) / RPM = torque
Plug in Hp of 400 and RPM of 840 and we get a torque at the tire of 2453 lb
ft of torque. Sonce we chose a tire that is 12 inches in radius, the force
pushing the car forward is the same as the torque. So you have a peak of 2453
lb's pushing 3000 pounds. That is nearly .82 G's if all 400 hp were at the
tires, and there was no drag or friction in the whole thing. Let's do the
same math backwards. At what ground speed are you reaching .45 G of
acceleration? Let's try it for the same 60 mph. We already know the tire is 1
foot in radius. So we know it is turning at 840 rpm still. .45 G's of 3000 #
is 1350 pounds of force pushing the car. On the 24 inch tire it comes out to
1350 lb ft at 840 rpm.
That works back to only 220 hp actually making it to the ground.
<< Just how flat are your friction circles across the top? >>
On the 95 Celica, it was FLAT dead straight across from about .8G's left to
.8G's right. We could hold that car just about flat out in a slalom. On the
turbo beast we are driving now, the top is starting to get a point up like
your triangle of braking, but in power. I am still messing with suspension to
keep the inside rear tire plabnted, as it lifts and spins in the air way too
easy. The factory limited slip unit is just too weak.
Gary M.
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