Do You Measure Up?

Accurate measuring is an important element in precision collision repair.

In order to repair a vehicle that has sustained collision damage, accurate measuring is essential. This will ensure that the replacement panels, as well as the movable pieces, will fit in the same manner as when the vehicle was built. In other words, after the unibody and subframe have been straightened, the new fender has to fit correctly, the doors must operate smoothly and the hood must open and close without a hitch.

The required repair work can only begin when all of the major manufacturing control points in the damaged area are returned to the original specifications. This is accomplished by body shop techs who, (1) measures accurately, (2) measures often and (3) rechecks all the measurements.

Understanding Body Damage

As any good body man knows, collision damage to a unibody vehicle involves much more than can be seen with the naked eye (Note: since the majority of passenger vehicles are of unibody construction, we will limit our discussion to that design). The cone concept best describes the nature of the damage to a unibody vehicle.

You must understand that a unibody vehicle (made entirely from pieces of thin steel welded together) is designed to absorb energy from a collision. When a unibody vehicle is involved in a collision, it collapses as it absorbs the energy of the impact. Not only is the energy absorbed at the point of impact, this energy spreads through an ever increasing area of the unibody until the energy is completely dissipated.

Imagine a cone at the point of impact. The centerline of that cone is the direction of the impact while the width of the cone represents the area through which the energy will travel. The damage at the point of impact is the primary damage and the resultant damage to the rest of the vehicle is the secondary damage.

For example, imagine a collision in which a car is hit broadside on the driver side. The primary damage will be the crushed door and door frame, while the secondary damage can be the roof that has shifted to the side of the impact due to inertia. Such damage might not be readily apparent to the naked eye, but when the vehicle is measured, it will be discovered. That’s one of the basic arguments for the need for accurate measurement—you can’t see all that is wrong.

Many vehicles have crush zones that collapse, or crush, upon impact. These zones are designed to collapse in a predetermined manner to protect occupants and to localize the damage. This will minimize secondary damage. These zones are located in the front and the rear of the vehicle.

Visual and Physical Inspection

Before you start measuring the damaged vehicle, it’s always a good idea to make a good visual inspection of the vehicle. Stand back and take a good look. Try to estimate the size and scope of the damage. Also, try to see if there is any twisting, bending or slanting of the vehicle.

Checking door, trunk and hood alignment can also be done at this time. Simply try to open each door, checking its action and alignment. It’s a good idea to check the clearances on one side of the car and compare those numbers to the other side. Finally, check the operation of the hood and the trunk lid. After you’ve finished the inspection, you can start measuring.

Measuring the Vehicle

In order to accurately measure the damaged vehicle, you will need the body dimension charts. In Chrysler service manuals, these charts can be found in Sections 13, titled Frame & Bumpers and Section 23, titled Body. In these sections, you will find all the charts you will need to accurately measure the vehicle in question. Figure 1 shows a typical dimension chart for a windshield opening.

When repairing a unibody vehicle, the center section should be used as the base when reading structural alignment. All measurement and alignment readings should be taken relative to this center section. Begin your measurements in the center section, or the closest undamaged area to the center section. Remember, in order to accurately measure a vehicle, you must start with at least three reference points that you know are right.

The dimensional references used when measuring a damaged vehicle are the datum line and the center line. Very simply, the datum line is an imaginary flat surface parallel to the underbody of the vehicle at some fixed distance from the underbody. It is from this plane that all vertical dimensions are taken. The center line is that line which divides the vehicle into equal halves.

There are several different types of measuring systems available today. Some of this equipment is simple, while some of it is complex. The gauge measuring system is the most basic of all, but the other systems are built upon this system with varying degrees of complexity.

Basically, there are five styles of measuring equipment. In addition to the gauge system, there is the universal, dedicated fixture, laser and sonic. Obviously, the laser and sonic measuring systems are the most current and the most expensive. The volume of work done in your shop, along with your budget, will determine which system is best for you.

Let’s take a look at the gauge measuring system in order to review some important points about measuring. Most of these concepts apply to the other styles of measurement. In this system there are three basic measuring tools: (1) the tram gauge, (2) the centering gauge and (3) the tape measure (which we should include because it is used for simple measurements).

The tram gauge, shown in Figure 2, is a scaled rod used for measuring. Basically, it is a ridged tape measure with pointers. The pointers allow measurements between reference points to be taken from the center of holes or bolts. Digital versions of the tram gauge are also available.

The tram gauge measures one dimension at a time. This dimension is recorded and must be cross-checked from additional control points, with one being a diagonal measurement. This can be a time consuming process. On the other hand, a mechanical measurement system, with several mechanical pointers attached to form a precision measurement bridge, provides instantaneous readings. The pointers align with the reference points, or the pointers do not align.

The second basic measuring tool is the centering gauge (see Figure 3). This gauge does not measure a specific dimension; rather, it is used to determine alignment, or misalignment. A typical centering gauge has a hanger rod on each end that is used to suspend the gauge from the underside of the vehicle. A center pin, or target, is used to view the centerline of the damaged vehicle.

Figure 4 illustrates checking for sag using centering gauges. Several center gauges are suspended from the underside of the unibody along the length of the vehicle. By looking down the centerline of the vehicle through the target, you can determine if the body has sagged (something not easily done by sight without the gauges).

The third basic measuring tool is the simple tape measure. It is difficult to use a standard tape measure for center of hole-to-center of hole measurements like a tram gauge, but simple length reference measurements can be made. If you want to use the tape measure to check hole-to-hole dimensions, machine the tip to a narrow point to increase the accuracy of your measurements.