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Measuring Basics

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.

By Tony Passwater
3/1/1998

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 total system.

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.

Measuring Lingo

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

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

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 accessories.

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 centered.

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 gauge scale.

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 important.

Centerline/Datum Gauges

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, height (Z).

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 view."

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 device.

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 control point.

The theory behind these gauges uses symmetry to its fullest.

Dimension Guides

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 symmetrical).

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 point.

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 (one-dimensional tool).

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 dimensions.

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 measurements.

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 view.

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 Successfully

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 time.

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 "simple" ones.

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 (Tony.Passwater@aeii.net).

Straight Talk
A shop owner advises that to make frame straightening profitable again, quit charging by the hour and use a dollar figure for the job.

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 than that.'

"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 to buy.

"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."

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