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Comment |
gofour
Unregistered
User
(8/9/02 9:58:11 pm) |
You have the shock theory correct. The idea that
weight transfer is occurring at the incorrect time or at the
incorrect rate. Shocks are the most important device used in
controlling weight transfer. The term shock is a poor choice. Shocks
were originally designed as a damper. The springs would continue to
"bounce" compress and rebound if something was not in place to stop
this. The shock places resistance to both compression and rebound in
order to control this. To simplify the explanation imagine a piston
in a sealed cylinder with fluid on top and underneath (not exactly
correct but good enough for this discussion). Now imagine two valves
in the piston. One valve is designed to allow fluid to pass through
the top of the piston when it is compressed upwards in the tube and
another valve that allows fluid to pass through the bottom of the
piston when it is pulled downward in the tube. Changing the size of
the holes in the valves (larger or smaller) changes the rate at
which the fluid can flow through. The faster the fluid flows the
easier it is for the shock to compress or rebound. Teams constantly
test different combinations on an apparatus called a shock dyno
which measures resistance to compression and rebound. Rarely is the
compression and rebound set the same on a shock. Lots of folks refer
to how stiff a given shock is. What they are really referring to is
how much resistance the shock provides against compression. Most
people are not aware of the rebound component. In the extremes a
shock can also be used as more than a damper with a little
creativity). A shock can be valved such that it does not resist
compression very much but greatly resists rebound. This is the type
used at a big track like Daytona where the teams want the car to be
pushed down by the aero downforce placed on the car. They will use
soft springs and a shock that compresses easily but is difficult to
make rebound (extend). Here the shock helps keep the car down as it
passes through the air. Other tracks it is important to have a high
resistance to compression in order to keep the tire pushed down on
the track for maximum traction and a low resistance to rebound to
return the shock to this state as fast as possible. These are not
the normal uses however. Shocks are used to control the RATE of
weight transfer.
Now lets imagine the shocks and springs as
a pair. The spring as you know is produced of different gage wire.
The larger the diameter of the wire the more difficult it is to
compress the spring. A 600 pound spring means that it takes 600
pounds of force (weight) to compress the spring 1 inch. The rating
(in pounds) of a spring is important because this is the factor that
determines how much weight it takes to compress the spring which
equals the amount of weight it takes to lower the car that amount.
If you remember this thread began with a discussion of how raising
or lowering a corner of the car changed the weight on that corner of
the car. Therefore the spring determines how much weight can move to
a certain corner. The shock determines how fast that weight can move
onto that corner and how fast the weight can move off that corner.
(most people do not think about weight moving off a corner).
In short you are correct. If the team felt like the weight
was moving too QUICKLY or too SLOWLY they would change the shock
setting first. If they felt too MUCH weight was moving they would
change the springs. (or the sway bar - which I'll explain if you
ask)
You asked about spring selection and tire wear. Your
question was something along the lines of does tire wear determine
the spring used. The answer is "kinda". Usually tire wear is a
result of something other than the spring selection on a given
corner. Springs do not really "press" the tire down on the track.
They simply determine how much weight can be placed on a given
corner of a car. Remember for every force there is an equal and
opposite force. Lets think about this as it relates to the racecar.
The car weighs 3400 lbs. Therefore the car can only push down on the
track 3400 lbs worth of force. The track is pushing back on the
bottom of the tires 3400 lbs. This would equate to 850 lbs per tire
if everything was distributed evenly. Now if we keep in mind that
the normal cross weight on a race car is 51 to 55 percent then we
understand that no two tires across the corners of the car are
carrying more than +or- 5% of their shared load. A car could be set
up where the right rear carried as much as 80% of this shared load
but not likely. Even so this tire would only see an additional 255
lbs of load. Now keep in mind that rear weight % (48 to 51%) must be
maintained and that the Nascar rule book does not allow and a total
left side weight % greater than 54%. These factors dictate what you
can do such that is unlikely a team could conceive of a weight
distribution that satisfied cross weight, rear/front weight, and
left side max %, and still severely overweight one corner of the
racecar.
The spring selection does still effect tire wear
however. If a spring setup allows too much weight to transfer to
that corner under motion in a turn then the amount of weight placed
on that tire could indeed wear it out faster. Shocks play a roll
here as well. If a shock is used that allows weight to transfer to a
tire too quickly then it may cause it to momentarily overcome the
traction limit of the tire which would cause the tire to slide
across the track surface and induce wear as well.
Springs and
shocks are usually chosen as a result of the drivers input and are
changed to achieve a certain feel for the driver. Tire wear while
important is usually a secondary consideration. Tire temperature is
more important. Today's racing tires do not produce optimum grip
until they reach 200 degrees F. They also begin to become damaged if
run much more than 20 to 30 degrees above this temperature for
extended periods. The teams measure tire temperatures as a method to
adjust camber (which I'll explain if anyone wants) and to determine
what the car is doing. Tire temperatures tell much about what the
car is doing (pushing, loose, etc) For example if the right front is
much hotter than the other tires the car is obviously pushing, if
the right rear is much hotter then the car is loose. This is how
baseline spring choices are usually made for a given
track.
The problem comes in when the driver reports a
condition that is contrary to what the tire temperatures indicate.
This could be for any one of a number of reasons and is where the
ever famous "communication" between driver and crew comes in.
Sometimes track conditions like oil on the track gives the driver a
loose feeling but tire temperatures may be perfect. Sometimes the
brake bias may cause a slip angle problem for such a brief period
that the driver feels something that the crew can not see on
temperature readings. Often the body roll on a car may give a driver
the feeling something is happening that really is not. (Many "feel"
a sensation of a push that is not there). I can not tell you how
many times a driver has gotten the car "perfect" to his feel only to
achieve a slow speed. Other times a driver may not like the feel but
turn fast laps. I concede this is not normally the case but it does
happen. Crew chiefs have their hands full. Unless they are in sync
with a driver it can be maddening for them. Crew chiefs are often
wrong also. Tires can be too hot for an instant in a turn and then
cool on a straightaway to represent something false.
Road
courses are the toughest. An oval track provides consistency in that
over a number of laps the tire temps will reflect what is happening
on the track. The road course does not always provide that benefit.
Imagine the following: The right rear runs hot through the tight
left hand turns, the left rear runs hot through the tight right hand
turns, the right front runs hot through long left hand turns, and
the left front runs hot through the through the long right hand
turns. The tire that was running hot in a turn would cool through
the other turns. The driver would report that the car was terrible
in every turn but the crew chief would read tire temperatures that
showed the car was perfect. Maddening. Road course cars are usually
set up with little split between the right side and left side
springs. Split is the difference between the spring rates. If a
front left side spring of 600 lbs is used then something close to a
600 lb spring is likely to be used in the front right side. (Oval
track cars used (pre-aero) to use a great deal of split - the right
front was always much stiffer than the left in order to resist the
resultant vector I described in my answer to 4the4). The road course
split is not exactly zero. Most teams try to not only prepare for
both right and left hand turns but also concentrate on improving a
specific turn that they are either having trouble with or a turn
leading onto a straightaway to increase their top speed. Road course
shock packages are usually baseline very similar on the right and
left side as well. Brakes are constantly experimented with also to
help control the rate of weight transfer. Smaller brakes or larger
brakes are used when driver brake bias adjustment is not enough to
solve the problem at hand. Sway bars are critical because they
control weight transfer from right to left as well. I have seen
teams that were struggling to keep up change to the right sway bar
combination and go directly to the front. Anyway if you want to work
your butt off try setting up a heavy car for a road course -
especially with a driver that is not an experienced road course
racer and a crew chief that sees a particular course once a year.
The benefit is that a road course experience will always be a
learning experience about dynamic state mechanics.
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ZNAKOMI
Administrator
Posts: 1563
(8/10/02 12:41:05 pm)
 |
Re: A very long
answer to ZNAKOMI's question
Thanks gofour,
I sat down with my morning coffee to this VERY
interesting post. You are explaining this stuff in a way we can
understand, and not only confirming many of the "ideas" I'd
developed over years of watching, but bring up many new facts (such
as NASCAR's specifics on weight distribution), that I had no idea
existed.
I only got a minute now, but I wanted to give ya a
proper "thanks" for taking the time to write all this out for
us...I'll be back later with other comments I'm sure!
THANKS
for the GREAT posts!
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ZNAKOMI
Administrator
Posts: 1568
(8/11/02 8:06:53 am)
|
Re: A very long
answer to ZNAKOMI's question
Ah...now thayt I am aware of the NASCAR cross weight requirements
it's easy to see that they can't just "put anything under there",
and that left side/ right side spring values are predetirmened to a
narrow window. I guess with this in mind, then the crossweight spec.
along with the fixed weight of the car and downforce expected at the
track, set's the spring values to a narrow range that can be used to
meet the height specification, (and set the height of the body so
it's low, but not dragging). (setting this part always amazed me
that they get these cars to hug down the perfect amount at
speed).
So, I take it with the overall values set, they must mess
with the right front and right rear springs most often for the final
layer of spring tuning, (that's a guess).
The shock stuff seems
to be the most complicated...as in if you want to transfer weight to
the rear to get off a corner, the right front shock must also be
ready to give up the weight fast enough for the transfer, (or the
rear shock will have no weight to take on even if it's set to
compress easily).
I understand about camber as a final setting to
get the contact patch right after all the other settings are
made...so that leaves us with a car sitting on 8 springs (the coil
springs and the air pressure springs)....but I know that's not the
whole story. it's my guess that we're pretty much left with the sway
and track bars, (of course braking rate and accelleration are big
here, but they're straightforward). Yes, I'd LOVE to ask about the
action of these components on weight transfer. So I will: What's the
action of the sway and track bar components?
The other rquestion
I have is in the case of a car set up for an extreme flat track (say
Indy), what are typical weights on the 4 tires of a car at rest?
And, has anyone tried to fathom what the weights are in a corner at
speed (downforce and centrifugal force coming into
play).
Thanks for this great lesson gofour!
4 TO THE
FRONT!
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gofour
Unregistered
User
(8/11/02 12:42:23
pm) |
ZNAKOMI
ZNAKOMI,
I've got a full day but I will get to your
questions. They are good ones. You seem like crew chief material to
me.
Your response is absolutely correct.
I do want
to clear one thing up however. Nascar does not mandate front/rear
percentage, or crossweight. They do mandate left side weight. The
way they do this is by requiring a car to have at least 1600 lbs of
right side weight. The cross weight and rear weight percentages I
gave you are typical of what is required to make a car handle. Your
point is still valid - the window for change is smaller than most
realize.
I am not sure if you are pulling my leg or what but
I have never run into someone that could get a handle on all of this
so quickly. I wonder if you didn't know all this already?
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ZNAKOMI
Administrator
Posts: 1570
(8/11/02 8:45:52 pm)
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Re:
ZNAKOMI
lol, thanks gofour...actually, I think a LOT of readers are getting
it for the first time. Take your time on the comeback, we ain't goin
anywhere.
Answer B, no, but I was always the one on the
bottom carring the piano up the stairs!
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corndogio
Registered User
Posts: 177
(8/11/02 8:55:29 pm) |
Outlaw!
Be nice Outlaw. MMM may have problems right now but this is no
reason to start another war.
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MMM
Fans