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gofour
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(8/14/02 1:45:13 am) |
Track bar
answer
There are many types of track bar configurations in use and
different mounting methods. I will be describing the most common
configuration used for oval track racing.
The track bar is
also known as the panhard bar. The purpose of this is to locate the
rear end along the centerline of the chassis. It is attached to the
rear end housing on one end usually near the center of the rear axle
at the differential ( sometimes on the axle housing itself depending
on the type of car: short track, intermediate, or superspeedway) and
is attached to a mounting bracket on the chassis just inside of a
rear wheel (most often the right rear). We will assume a single
panhard bar mounted to the RIGHT rear for this example.
The
mounting bracket used to have a series of vertical holes drilled in
it so that the bar could be unbolted and moved up or down as
required then rebolted. The mounting bracket is still this way in
many racing series. The Cup cars now have the bracket fabricated
such that the bar can be adjusted up or down using a long
through-bolt similar to the jacking bolt used to adjust wedge. They
use a wrench through the rear window as well.
The length of
the track bar (or panhard bar) is also adjustable. Each end of the
bar has a threaded hiem joint that can be screwed in or out to
either lengthen or shorten the bar. Teams have different approaches
to the initial set-up of the track bar. One common approach is to
set the length of the track bar such that the the rear end is
centered in the chassis when the bar is in the middle of the
adjustable chassis bracket. The bracket is often mounted to the
chassis in such a way that the track bar is perfectly horizontal
(level) at the lowest setting. Therefore, the distance between the
center of the rear end and the chassis mount would be shortened as
the bar is moved upward and the distance would be lengthened as the
bar is moved downward.
Adjusting the track bar end UPWARD
moves the rear end (and therefore both rear tires - because they are
connected by a common axle housing) TO THE RIGHT.
Adjusting
it DOWNWARD moves the rear end (and both rear tires) TO THE
LEFT.
So whats the point you ask?
Lets start by
understanding the term track as it relates to the car. When both the
front and rear tires are lined up perfectly parallel (down the same
centerline from front to rear) it is said that the rear wheels are
"following the front wheels". This is called "tracking straight".
When the rear wheels are shifted to the right or left of the front
wheels it is said that the rear wheels are "dog tracking" (based on
a dogs tendency to shift its front and rear legs in opposite
directions when changing directions on the run).
I am not
fond of the term track bar because it is misleading. Many folks
become confused when talking about the track bar because of the dog
tracking example. Often people think that shifting the rear end to
the right or left will make the car turn to one side or the other
when rolling forward in a straight line. This is not the case. When
the rear end is positioned squarely in the chassis (trailing arms
are the same length -more on these another time - and the rear end
is perpendicular to the centerline) the car will still roll straight
despite the rear axle (and both rear tires) being shifted to the
right or left.
Therefore, I will refer to the "track bar"
using its proper name panhard bar during the rest of this
explanation. Panhard (old time automotive inventor) invented this
diagonal link to laterally position the rear axle in his
cars.
Now that we understand that adjusting the panhard bar
shifts the rear end right or left it is easy to imagine that this
will have an effect on the cars handling. We need to understand one
more concept first. This is the concept of roll center. Roll center
is simply the point at which the sprung weight of the car rolls from
side to side. This is also one of the most misunderstood concepts in
chassis set-up. I am not going to go into the full
physics/engineering version here. It is a rather complicated concept
that includes lateral forces, rotational forces, center of gravity
location, raising and lowering of roll centers and effective roll
couple, etc...
Soooo... I am going to explain it in a way
that you can imagine for practical application. Pretend that a car
has had its tires removed and is only supported by two jackstands.
One in the center of the front of the car and one in the center of
the rear of the car. These two jack stands would represent the roll
center of the car. It would be at this point that the weight would
balance. The car could easily be rocked from side to side (weight
would roll from one side to the other). Lets concentrate on the rear
of the car. If the jackstand were moved to the right then more of
the cars weight would be placed on the left side of the jackstand.
If the jackstand were moved to the left then more of the cars weight
would be shifted to the right side of the jackstand.
O.K. now
lets put the tires back on.
We remember that a car in a turn
transfers weight from the inside of the car to the outside of the
car due to centrifugal force. On an oval track that means more
weight moves from the left side to the right side in a turn. Thus,
when the rear end is moved to the right more weight is moved to the
left of the centerline. This allows more weight to transfer during
cornering (more body roll). The opposite is true if the rear end is
shifted to the left more then weight is moved to the right of the
centerline and less weight is available for weight transfer to the
right side (less body roll) during cornering.
In the swaybar
discussion we determined that the more the body rolls the more the
right side spring is compressed and the more the left side spring
expands. The more the left side expands the less force is placed on
the tire, the smaller the contact patch, the less traction is
available and the looser the car becomes.
Conversely, the
lower the body roll the more traction is maintained by both tires
and the tighter the car becomes.
The crew chief sees it this
way:
Raising the track bar makes the car looser
(oversteer)
Lowering the track bar makes the car tighter
(understeer)
Enough for now - enjoy.
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ZNAKOMI
Administrator
Posts: 1579
(8/14/02 5:11:18 pm)
 |
Re: Track bar
answer
That's really interesting, moving it up puts a greater percentage
of weight on the left side, but in a corner you'll actually end up
with less on the left side because that greater percentage is now
available to be thrown right. Who woulda thought!
Hey gofour,
maybe you could bundle these posts together and sell 'em to
nascar.com. I learned more in a few posts than I've learned in 10+
years of watching it on TV. I think a lot of fans would love to read
this stuff. You explain these things extremely well.
Thanks
for taking the time!
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