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Squaring
a race car is key to proper race car set up. Pointing all 4 wheels in the same direction
allows for all of your adjustments to perform in a consistent and predictable
fashion.
Let's spend a minute getting on the same page in reference
to squaring a race car. For the sake of clarity this article targets Late Model
style cars, but the recommendations apply to many race car types. For the
purposes of this article we are assuming that the housing is perfectly straight
and your frame is in factory condition.
If you identify your goal before starting it helps in the
end result. Common knowledge states that the rear end must be square. However,
the question at hand is square to what? Referring to the drawing
below will help keep things clear as you read the recommendations.
To have a rear end installed correctly we must square the
housing to something. But square to what? The common answer is that we need to
be square to the frame rails. If your frame rails are perfectly straight then a
good result can be gained from utilizing the frame rails as a reference point.
The reality is that frame rails are not straight. Race car
frame rails are made from mild steel that is simply pulled from a rack. The
steel is not that straight to begin with and welding helps to distort it
further. My opinion is that the frame rails are hard to rely on consistently.
Really our goal is to "square" the rear end
housing to the front end versus the frame rails. The steel is along for the
ride. The front suspension pivot points are the true reference points. By
embracing this concept the frame can have some normal bends and bows and you
can still be assured a square rear end. My recommendations involve squaring to
the front pivot points as this will provide the most consistency.
In addition, we want the rear end housing to be located
correctly from side to side in the car. For best performance lining up the
right side tire seems to be best. To set the rear end location correctly the
front end adjustments must be set up correctly and be in race ready condition.
The right side tires carry the highest forces so keeping the
right side tires in line with each other creates the most stability and
maximizes left side weight. Lining up the right side tires is my
recommendation.
However, your car builder may support a different left to
right location so you need to insure that you follow car builder's
recommendations so that the needs of the rear suspension links are met. For
this article we will assume that the car builder laid out the suspension with
the right side tires lined up. You will want to pay special attention to this
detail as it is a higher priority to have the trailing arm brackets and top
link hardware to be located in the proper location. For example, you want the
top link to be perpendicular to the rear end housing and parallel to the car.
Trailing arms must also be running forward at the correct car builder design
angle. These design elements must be considered so be sure to consult your car
builder about his design parameters for the Left/Right location for the rear
end housing.
Ok, now that we are on the same page, let's review:
The front end is set and is race ready. The car is set at ride height by using
solid links in place of the shocks or it is sitting up on stands at race ready
ride height with air pressure and stagger set properly. Your rear end housing
has been checked and it is not bent and it is ready for installation. We also
need to insure that the panhard bar is set to the correct height and that the
trailing arm and top link angles have been set properly.
 Step
1: Setting Up The Reference String
Set
up a string on the right side of the car and extend the string well past the
front and rear tires. Attach the string between two simple jack stands. Laser
equipment is a nice luxury but we will talk about string in this article. I
have found that it is best to set the string up at a height off the ground
equal to the bottom of the frame rail.
Now that your string is set parallel to the car turn the
right front wheel until it is parallel to the string. Record the distance from
the string to the right front tire. Take notice that you are taking in the
effects of camber by setting the string at frame height versus spindle height
helping you to align the rear contact patch with the front contact patch.
Negative camber at the right front moves the contact patch
to the right. We set our string even with the frame rail so the effects of
negative camber are considered as compared to setting the string at spindle
height. Setting the string at ride height allows the rear contact patch to more
closely line up with the front contact patch which is offset to the right due
to the effects of negative camber.
Step
2: Aligning The Right Side Tires
Now
we want to begin lining up the right side tires. Match the distance from the
right rear tire to the string so that the distance is equal to the distance
from the right front tire to our reference string. Quickly check at the right
rear tire that your rear end square is at least in the neighborhood. If the
string is parallel to the frame then the front and rear sidewalls of the right
rear tire should be parallel to the string as well.
If the rear end measures differently from the sidewalls of
the right rear tire to the string then average the difference to locate the
rear end. For example, your measurement to the string at the right front tire is
6". We want the same at the rear but when checking we end up with
5-15/16" and 6-1/16". As the average of 5-15/16" and
6-1/16" equals our 6" dimension at the right front tire we can be
content with the lateral location and we are now ready to set the housing
square to the front pivot points. Leave your string in place as you will want
to verify the lateral location setting the square. Adjusting the trailing arms
can have an effect on the lateral location. You will need to double check the
lateral location after completing the squaring step.
Step
3: Setting Up The Reference Straight Edge
Many
people use masking tape, plumb bobs, and chalk lines to square the housing in
the car. Experience has shown that the amount of transfer error reading thick
chalk lines as well as trying to make a mark exactly at the point of the plumb
bob creates too much error. These transfer errors lead to inconsistency and the
work is tedious. Further, each time you adjust the rear end you have to repeat
the plumb bob transfer down to the floor. I believe this method just takes too
long, is frustrating, and is less accurate.
I prefer to go about it a little differently. Remember we
have determined that we want to square to the front pivot points. Ok, now here
is where you need to spend some initial time to insure repeatability and
accuracy. Find a perfect straight edge that spans the length of the rear end
housing.
Next clamp the straight edge to the bottom of the frame
about half way between the front and rear axles. The forward / aft location of
your reference straight edge is not critical. Somewhere around center is good,
but again it is not critical. When clamping the reference straight edge use a
carpenters square and clamp it exactly 90 degrees to the straightest frame
rail. Even though we are going to insure that we are square to the front pivot
points the frame rail does need to be considered. We want the rear end to be
square to the frame rails and we will fine tune to the front pivot points to
eliminate any irregularities in the frame rail. In general the goal is to have
the frame rails, front pivot points and rear end housing in perfect square.
Now that our reference straight edge is clamped in place and
is 90 degrees to the straightest frame rail, we are ready to fine tune the
location of the reference frame rail. Remember we want to eliminate variance in
the frame rails and square to the front pivot points. Hook your tape measure on
the straight edge and measure forward to the front inner pivots. Be sure that
your tape is running straight forward for accuracy. Fine tune your straight
edge until is is exactly parallel to the front inner pivots. Take you time and
locate the straight edge until it is dead square.
Before continuing it is a good idea to mark the location of
the straight edge with a scribe or a peen mark on the bottom of the frame.
These marks will allow you to clamp your straight edge in place in the future
in a minimum amount of time. Should you need to verify that your rear end is
square at the track, you will find the straight edge and indelible marks is a
very time saving luxury.
Step
4: Squaring The Rear End
Hang
two strings off the rear end at the outer most points. You can use two plumb
bobs but a heavy nut tied to a string will work just as well. For optimum
accuracy remove the rear hubs and hang the string off the machined bearing
surface of the rear hubs. However, on a quick change rear end the machined axle
tubes are quite accurate.
Now that your strings are hanging down you can quickly hook
a tape on the reference straight edge and measure back to the strings. Be sure
that your tape measure runs straight back. Adjust the housing as needed. After
an adjustment you can quickly check the square. With the plumb bob and tape
method you would need to transfer all the marks back down to the ground which
takes a long time. The reference straight edge concept allows you to make the
adjustment and measure directly to the hanging string. You can read the exact
measurement very easily.
Because it is so easy to check the square be sure to keep
adjusting until the housing is right on. Spend the time and get it right. With
the plumb and tape method many racers would call it "close enough"
due to the hassle factor of transferring all of the points down onto the tape
on the floor. Spend ample time and get it right.
You will find that the reference straight edge method is
much faster and more accurate. Further, you can carry the straight edge with
you to the track. The plumb and tape method requires a level and flat floor.
You would have a hard time at the track with plumb bobs and tape on asphalt.
The reference straight edge is an easy deal if you have done a good job of
marking and recording the reference points.
Step
5: Verify The Lateral Location
Now
that you have the rear end square you need to verify that the left to right
location still lines up with the right front. If it is still ok left to right
then you are ready to go. If the housing no longer lines up with the right
front you will need to readjust the lateral location. If you move the housing
laterally you will need to repeat the squaring process. It is common to have to
go back and forth between the left/right adjustment and the square adjustment
as they both have an effect on each other.
Step
6: Record Your Reference Marks
You
have spent plenty of time and you have verified that your rear end is dead
square. You have already marked the location of the reference straight edge and
can rely on those points for quick reference. Next, it is wise to record a
reference point for the left right location. On an under slung style late model
you can utilize a good adjustable carpenter square. Place the movable piece of
the square on the right rear brake rotor and slide the rule over until it
touches the under slung frame rail. Write down the reference number in a safe
place. If you have something other than an under slung style frame, simply use
a tape measure from the right rear brake rotor to a reliable reference point on
the frame. Recording this quick reference dimension will prove invaluable when
at the track and really saves time when you need it most.
You will find that having the ability to square to these
reference points will save you a lot of time. Obviously if your frame gets hit
you will need to verify and adjust your reference points. But, if you damage
your rear end housing at the track, you can utilize these reference points very
quickly. You will find that the reference points come in handy when making
panhard adjustments as well.
written by: Jeff Butcher of Longacre Racing Products, Inc
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