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The more user dependent the measuring system, the greater the chances of error. For this reason, so-called "simp,tapes and gauges – aren’t simple to use properly.
Measuring … We’ve talked about it since
the change to unitized construction, and we struggle with it when
our "trained technician" leaves or when the wheel alignment
guy calls. Why? Because it’s a vital part of the repair process.
Knowing how measuring devices work and how manuals reference their
points is as important as knowing how your estimating systems
calculate labor operations (outside in or inside out) and what’s
included or not (p-pages).
The first rule to remember about measuring
is this: The more user dependent the system is, the more complicated
the usage becomes. You may be saying to yourself, "Wait,
my measuring tape is really simple to use." But, in reality,
it would be the most complicated system to choose if it were your
The second rule to remember about measuring
is that the more user dependent the system is, the more inconsistent
the usage becomes. Let’s face it, we’re humans, and mistakes are
very possible when judgments are made at 3 p.m. on Friday.
It’s important to note that this article is
part one of a two-part series on measuring theory. In this article,
we’re going to examine four measuring tools: measuring tapes,
tram bars, centerline/datum gauges and dimension guides. I’ll
explain their theory of operation, as well as the complications
of verification and consistency of usage. In the next article,
"Advanced Measuring," we’ll look at universal mechanical
(including laser) and computerized measuring systems.
Today’s vehicles have improved dramatically
since the 1980s due to improved manufacturing processes that hold
incredibly close "build tolerances." Build tolerances,
which were once a common topic of discussion, are quickly becoming
a minor consideration, since many vehicles these days are built
with true 1-3 mm build variation; today’s manufacturing methods
can be credited with this accomplishment, and they’ll continue
to get better. It’s been said by techs, "I’d prefer to work
on actual 0 mm build tolerance vehicles – that way, if the measuring
says it’s off, then I know there really is damage." This,
however, isn’t just wishful thinking; it could very well happen
in the not-so-distant future!
Though build tolerances aren’t as much of
a concern as they used to be, understanding that vehicles are
built three dimensionally is important.
The "XYZ" coordinates refer to length
(X), width (Y) and height (Z) of each "control point."
A control point is a point the manufacturer uses as a "control"
or check for accuracy. These may be places on which special jigs
are mounted or where laser systems verify accuracy. A "reference
point" is a point that’s been verified as reliable for measuring
and gauging purposes and may not be part of the engineering blueprints.
Each measuring system has its own limitations
or "resolution." The best way to describe resolution
is to compare it to reading a small sign on a building from across
the street. Even with poor eyesight, we see the shape of the sign
and the word, "Warning." With good eyesight, we can
read further, "Beware of Dog." But, because there’s
still smaller print that we can’t read, we have two choices: Use
more accurate tools, such as binoculars, or continue to use the
same tools – in this case, our eyes – past their intended usage;
for example, walking up to the sign and reading, "If you
can read this sign, our attack dog is within range." Oops!
Measuring tapes have been used by all of us
to determine damage and validate repairs. But, since it’s the
most user-dependent tool used today for measuring, it’s the most
inconsistent. For years now, when teaching seminars, I’ve given
four attendees four measuring tapes and had them individually
measure four points on a vehicle and write down their measurements.
I then summarize and write the dimensions on an overhead transparency
(keeping identities secret).
In all cases, we never get the same measurements.
Also in all cases, the variations are greater than 3 mm – the
generally accepted repair tolerance. I’ve also had the same people
take the same readings a few days later and not get the same measurements
they got the first time. The conclusion? How can you rely on a
system that cannot produce repeatable readings?
The proper operation of a measuring tape includes
basic items that are very difficult to accomplish consistently,
such as making sure there are no bends in the tape, reading the
scales while viewing at only 90 degrees, placing the tape end
in the same location of the reference point every time, reading
the same location on the other reference point every time and
ensuring the tape clip doesn’t have excessive play or binding
in the reference point. Sounds pretty difficult to ensure consistency
for resolution of 3 mm or less, doesn’t it?
How do you validate the repairs as a manager
or owner? If you have a problem with the wheel alignment, how
do you recheck? The answer is, you can’t easily!
Tram bars/gauges are tools that remove some
of the variables of a measuring tape. Since the proper usage is
to preset the pointers at the proper dimension and gauge the vehicle
(not measure), setting the gauge to the proper dimension is more
consistent than attempting to measure the reference points. Proper
usage, however, may include two technicians working together,
which can cause miscommunication – leading to inconsistent conclusions.
Most inconsistencies of tram-gauges result
from improper care and a lack of understanding. For example, tram
gauges with built-in scales can become very inaccurate if the
gauge isn’t kept from damage. Leading manufacturers have, for
years, placed tram gauges on their pulling accessory boards. Why?
It’s very important that tram gauges are not left "leaning"
against the wall. It’s also very important that they’re supported
properly when not in use. I’ve had the opportunity to see many
tram gauges – being stored on their shadow board – become unusable
due to the bending that takes place from not being properly supported
during storage or from being used to hold a few chains or hook
If you use a tram gauge with a built-in scale,
extend your gauge to 5-6 feet and sight down the bar – it should
be straight! If not, the scale doesn’t accurately measure the
position of the pointers.
Also, the pointers themselves must extend
out 90 degrees from the tram bar. If the pointers have play within
the retainer, they must be centered. Many gauges may use a plastic
washer/retainer cap designed to keep the pointers centered so,
if this washer is lost or damaged, the pointers no longer stay
To check the accuracy of a tram gauge, measure
the distance between the tips of the pointers when the gauge is
collapsed to its "storing size" (see Figure 1A). The
reading taken should match the gauge’s scale reading. Then, extend
the gauge to approximately 7-8 feet, and measure the distance
between the tips of the pointers again. They should match the
It’s important for pointer heights to be set
equal if "point to point" measurements are being used
and the scale on the tram gauge is being used. Note: A point-to-point
measurement is simply a straight line between each control or
reference point; this means it can be taken with a measuring tape
as long as there aren’t obstructions (tram gauges can be used
to maneuver around obstacles). Keep in mind, the pointers can
be set at any convenient height(s) unequally, as long as they’re
set by measuring the true distance between the tips of the pointers
rather than reading the gauge scale; this process would still
produce a "point to point" measurement (see Figure 1B).
As mentioned earlier, the proper method for
using a tram gauge is to set the gauge to the correct dimension
and then gauge the vehicle’s reference points. Making sure you
have the proper placement of each pointer without binding is also
Centerline/datum gauges are normally used
in conjunction with the measuring tape and tram gauge, since measuring
tapes and tram gauges only reference two dimensions: length (X)
and width (Y). Centerline/datum gauges bring in the third dimension,
These gauges use dimension manuals to identify
their mounting locations and height settings. The "side view"
of the vehicle’s dimension page is used to locate height settings
of each gauge and the mounting holes. If the vehicle has "asymmetrical"
control points, the width differences are identified in the "bottom
The heights used are referenced from the "datum
plane/line," the imaginary plane/line under the vehicle that
engineers used to reference height changes in the control points.
Since this plane/line is just a reference point related to the
other points, it can be moved equally to better suit the user’s
needs – this can become evident when dimension manuals from another
brand of equipment are used because many equipment companies adjust
the datum height to better work with their specific measuring
The width measurements are normally referenced
to the "centerline/plane." This imaginary line/plane
runs through the center of the vehicle, dividing the vehicle into
symmetric halves. The distance between two control or reference
points left to right can then be calculated by adding the distances
of each to the centerline/plane.
Even though I mentioned asymmetrical control
points, there are no "asymmetrical vehicles." All vehicles
are symmetrical in nature; the left front door is a mirror image
of the right front door (except for Pacers. Remember them?), the
front fenders, frame rails, rockers, etc.; all are basically symmetrical.
There are, however, a limited number of points on some vehicles
that, for many reasons, aren’t symmetrical – but this doesn’t
make the whole vehicle asymmetrical. Without symmetry, the vehicle
not only would be very difficult to build, but also to sell.
As the name of the system implies, centerline/datum
gauges do not measure damage; they gauge damage. After the gauges
are mounted properly, the technician uses the combination of the
centerline pins and the "levelness" of the datum bars
to determine width and height problems.
Using these gauges properly is very important
to their accuracy. There have been many articles and opinions
regarding how these gauges should be used and, unfortunately,
they just add to the inconsistency of the equipment. To begin
to understand how the system works, some basics need to be reviewed:
It requires two "base gauges" to
establish the datum plane and set the "sights" for the
centering pins. These two base gauges are commonly established
in the front and rear torque box areas. However, this can be a
problem if damage has carried into these areas.
In addition, if a full frame is being diagnosed,
it’s critical to check the center section of the vehicle for squareness
(diamond) first. Otherwise, this condition will not be detected
by the gauging (in the past, centerline gauges may have included
a "diamond indicator attachment"). Base gauges should
be inspected carefully to ensure their control points are correct
and the heights of each gauge bar are level to each other; this
establishes the foundation of the vehicle.
Before attaching additional gauges, keep in
mind a few important points: The dimension guide measurements
are taken when the vehicle weight is properly distributed. And,
because placing the vehicle on pinchweld clamps could cause a
change in dimensions from "overhang weight," the vehicle
should be checked to ensure that the weight of the front or rear
sections hasn’t caused a change in dimensions while suspended.
Generally, this isn’t an issue if the vehicle is raised on its
suspension mounting locations or the majority of the vehicle’s
weight remains on the suspension when mounted in pinchweld clamps.
Another important factor is gauge spacing.
It’s unrealistic to think that you can mount three additional
gauges in the area of the collision (front or back) and not have
viewing problems with today’s downsized vehicles. With all the
gauges there, they become very difficult to see properly. The
best approach is to verify the area farthest from the primary
damage (rear section on a front-end hit) with a couple of gauges
and then remove them; work your way from the farthest damage from
the primary damage toward the point of direct impact. This technique
requires patience and a clear picture of the damage before repairs
begin – the reason why this system is very user dependent.
Many other factors may also lead to inconsistent
gauging, including maintenance of the gauges (not snapped together
properly), color, improper lighting and improper mounting in a
The theory behind these gauges uses symmetry
to its fullest.
Dimension guides are a valuable tool that
have been misunderstood for many years and may provide door, windows,
engine and luggage-compartment opening information. This information
can be valuable when needed, but it can also be obtained, in most
cases, by measuring the undamaged side (since the vehicle is basically
These guides also provide information such
as hole/bolt sizes and underhood, bottom and side views. Most
dimensions are given as a point-to-point measurement, and it’s
explained from where in the reference or control point that the
measurement was taken. In most cases, the rule for point-to-point
measurements is closest edge to closest edge, unless the point
listed is a bolt, nut, stud or nonremovable plug; in those cases,
it’s measured to the center of that specific reference point,
which means that a point-to-point measurement between a frame
hole to a cradle bolt could be closest edge of the hole to the
center of the cradle bolt.
It’s also important to remember that if parts
are removed, the reference point may be given with the part and
bolt in place. And the parts may require removal for the reference
Two other types of measurements also are used
that aren’t point-to-point measurements: bar measurements and
datum length measurements.
Bar measurements aren’t used today with any
current measuring systems, so not much discussion will be given.
They are the distance between two points along the datum plane
(see Figure 1C) and require the use of a tram-gauge set with the
pointers at datum height (or variance) and with the gauge measuring
scale used (not measured tip to tip). These are not the side-view dimensions.
Datum length measurements are listed in the
side view of dimension manuals. This is the true length (X) coordinate
used during manufacturing by the engineers. These dimensions cannot
be used except with 3-D measuring devices, unless converted for
use with a tram gauge (two-dimensional tool) or measuring tape
This datum length measurement in the side
view has been used for years with a tram gauge and has been the
incorrect proof that the vehicle wasn’t built to proper tolerance.
Part of the reason for this may be in how dimension manuals explain
their use. One popular manual explains the following: "All
side-view dimensions are parallel or perpendicular to the centerline
of the vehicle. Side-view lengths are measured center to center."
But what does it mean to be parallel or perpendicular to the centerline?
It means a one- or two-dimension tool, such as a measuring tape
or tram gauge, cannot be used.
Why? If all frames were "straight ladder
types," then all points would be the same height and have
the same centerline distance. Of course, this isn’t the normal
structural design. It’s very unusual to have reference or control
points on the same side with the same datum height and width.
When measuring with a measuring tape, it’s only measuring a straight
line between points and can’t be made parallel to the datum plane
and centerline at the same time, as required by the side-view
With a tram gauge, the pointers can be set
at datum variance to keep the gauge parallel to datum. But, to
keep it parallel to the centerline would require special modifications
(see Figure 1D). This is not your normal tram gauge; it does,
however, look very close to a universal mechanical system. This
example can maintain parallel to centerline and datum at the same
time by the offset in the bar and by the pointers being set at
different heights. The scale in the gauge could be used just as
the scales on the universal measuring bridges are used today,
and the side-view dimensions can be converted to be usable with
a tram gauge and even further converted to give point-to-point
A Greek named Pythagorus proved that the longest
side of a right triangle multiplied by itself (squared) is equal
to the sum of the squares of the other two sides. Sounds complicated
but, with a calculator with a square-root function, it really
is quite easy to convert a side-view measurement for use with
a tram gauge. (Keep in mind that all side-view measurements are
measured center to center – in all cases – unlike point to point.)
As a quick example, imagine the side-view
dimension listed is 1,050 mm. You then must find the difference
the two points are from the centerline by looking at the bottom
view. For this example, imagine it’s 206 mm.
This side-view measurement can be converted
now with the following formula: C2 = (1,050 mm)2 + (206 mm)2.
"C" will represent the measurement
that allows us to set the tram-gauge length by the gauge scale
and set the pointers at the datum variance as listed in the side
After the math is done: C = 1,070 mm. The
conversion is complete; we now have a "bar measurement."
Another step could be performed to convert
this bar measurement to a point to point, which requires determining
the height difference of each point (datum variance). The example
I’m imagining will convert to a 1,075-mm point-to-point measurement.
Can you figure what the datum variance must have been? (*See answer
at end of article.)
In the past, side-view measurements were mistakenly
used with measuring tapes and tram gauges without success. This
example would demonstrate a 20-25 mm difference! Could this be
why vehicles were thought to be "out of spec"?
Most dimensional manuals begin with vehicle
engineering drawings, physically verified when possible and then
converted to work best with their measuring system(s). These manuals
may not be perfect, but if you run across a dimension that doesn’t
make sense, make sure you’re using it correctly first and then
call the company for assistance before you pull a control or reference
point to a location your experience tells you isn’t right.
Measuring relies on some very basic principles
– no matter what system is used:
- Proper understanding of the dimensions being used.
- Proper use of the equipment being used and its limitations.
- Proper maintenance of the equipment being used.
- Patience to perform the task consistently and accurately every
Consistency will always translate to efficiency and quality. Therefore,
with today’s vehicle designs, it’s critical that all technicians
involved in damage analysis and repairs be well-educated on the
systems being used for measuring – even if the systems are supposedly
Writer Tony Passwater is a long-time industry educator and
consultant who’s been a collision repair facility owner, vocational
educator and I-CAR international instructor; and has taught seminars
across the United States, Korea and China. He can be contacted
at (317) 290-0611, or e-mailed at ([email protected]).
A shop owner advises that to make frame straightening profitable
again, quit charging by the hour and use a dollar figure for the
Q: What are your main challenges when it comes to measuring
and straightening, and how are you trying to overcome them?
A: "The dimensions quoted in the frame manuals are
often based on areas that are inaccessible when the vehicle is
clamped to a frame machine," says Russell Westfall, owner
of Westfall Auto Sales in Cadiz, Ohio. "It’s necessary to
remove too much suspension when using these manuals. The publishers
of these manuals should develop their data with the vehicle set
up for a pull."
Q: Are shops having trouble getting paid for all the time
and work that go into the measuring and straightening processes?
A: "We’ve personally experienced adjusters not wanting
to pay for setup and diagnostic time … yet, they expect an exact
quote for the job. Many adjusters want to ‘cap’ the frame charges
– not that they call it that. They seem to think that eight to
10 hours should repair any damage to any car when the actual job
on some harder-hit vehicles takes as much as 16 hours. [When you]
tell them you need 16 hours, they tell you that ‘you should be
able to do that in half that time’ and that ‘so and so’ would
do it for five or six hours.
"When you hear the name, you know that their equipment consists
of pots or a telephone pole – quite a comparison!"
Q: How do you charge for this type of work?
A: "That’s the problem! We need to get away from hours
and back to the way it was and should be again: a dollar figure
for the job. Then frame straightening will once again become profitable.
If other shops are like our shop, their investment is huge …
equipment, manuals, training. Yet none of that is taken into consideration
by the insurance industry and many of our fellow frame shop owners.
"The rate for frame repair should be based on the savings
to repair [as opposed] to the cost of frame replacement. If a
new frame costs $1,600 and it takes $500 for labor, that’s $2,100.
Most insurance companies want us to cap our frame repairs around
$300 because they say ‘this type of work just isn’t worth anymore
"According to my figures, we just saved the insurance company
$1,800 – and we made about $25 after time and investment. Sorry,
but I think we’re worth a lot more than that. Say we charged 50
percent of just the cost of the frame – $800.00 – then the insurance
company saves $1,300 and we, the frame shop, actually make money.
And we should be looked at as the hero because [the insurance
company’s] actual savings on this repair is 62 percent. Compare
this to their overwhelming drive to use aftermarket parts so they
can save plus or minus 25 percent, and it sounds fair and reasonable.
"If shops don’t rethink their methods of charging for their
services, they’ll be out of business. Technology changes daily,
and it costs a fortune to even attempt to keep up with it. If
you don’t charge properly, where is the money going to come from?
This isn’t just a theory. We’ve implemented it, and it works well
on harder-hit vehicles – without too much negotiating."
Q: Do you have any advice on this type of work?
A: "Buy a measuring system – any system – and just
use it! You can become proficient with any system you can afford
"Don’t assume that any vehicle is a minor tug. If it’s on
the frame machine, treat it as if it’s a hard hit and don’t assume
that there’s not much damage by just looking at it. Verify with
your measuring system and charge for that service – plus, gain
the piece of mind that you know it’s right. No second guessing."