1. Chapter 20 Measuring Vehicle Damage

2. Objectives
Explain how impact forces are transmitted through frame and unibody construction vehicles
Describe how to visually determine the extent of impact damage
List the various types and variations of body measuring tools
Analyze damage by measuring body dimensions

3. Objectives (continued)
Explain the importance of the datum plane and centerline concepts as related to unibody repair
Interpret body dimension information and locate key reference points on a vehicle, using body dimension manuals
Discuss the use of tram bars, self-centering gauges, and strut tower gauges
Diagnose various types of damage, including twist, mash, sag, and side sway

4. Objectives (continued)
Given a damaged vehicle and a body specification manual, locate and measure key points using a tape measure, tram bar, and self-centering gauges
Explain the operation of electronic laser, ultrasonic, and robotic arm measuring systems

5. Introduction
When a vehicle is in a high-speed collision, impact forces can bend the frame
Damage extent must be carefully evaluated
Vehicle measurement involves using specialized tools to measure the location of reference points
These measurements are compared to the dimensions of an undamaged vehicle
By comparing the two, the extent of the damage can be determined

6. Why is Measurement Important?
Severe body or frame damage can change steering or suspension geometry
Measurement gauges are used to check specific frame and body points
They allow you to quickly measure the direction and extent of vehicle damage
Control or reference points are specific locations on the frame or body for making measurements
The reference points must be restored to their factory dimensions

7. Damage Diagnosis
The estimator must assess the severity and extent of the damage and find all parts affected
To avoid missing the effects of damage on unrelated systems, damage should be assessed with the proper tools and equipment
Determine the direction and force of impact, and whether damage is confined to the body
Inspect the parts along the path of impact, measure the major parts and check body height, and check for suspension and body damage

8. Impact Effects
A modern vehicle is designed to withstand the shocks of normal driving
In body-over-frame construction, the passenger area is enclosed with panels of steel attached to a structural frame
In the unibody construction, the metal body panels are welded together to make a unit
Under the force of impact, the frame-type vehicle and the unibody react very differently
Damage assessment and repair also differs

9. Analyzing Collision Forces
The body technician may need to know:
The size, shape, position and speed of the vehicles involved in the collision
Angle and direction of the vehicles at impact
The number of passengers and their positions at the time of impact
Certain types of damage occur in a predictable pattern and sequence
If a driver’s first reaction is to turn away from the danger, the vehicle will take a hit on the side

10. Analyzing Collision Forces (continued)
If the driver’s reaction is to slam on the brakes, the direction of impact would be frontal
Frontal impact could leave a large opening between front upper part of door and roof line
Damage will vary depending on what was struck
If the impact was spread over a larger area (such as a wall) damage will be minimal
The smaller the area, the greater the severity of damage

11. Figure A hard frontal impact causes major or primary damage to the front end. Minor, secondary damage occurs elsewhere from the shock wave flowing through the body

12. Types of Frame Damage Side sway damage Sag damage
Results from side collision impacts
Recognized by a gap at the door on the long side and wrinkles on the short side
Sag damage
Section of the frame is lower than normal
Caused by direct impact from the front or rear
Detected by an irregular gap between the fender and the door

13. Types of Frame Damage (continued)
Enough sag in the frame can be present to prevent body and panel alignment even though wrinkles or kinks are not visible
Mash damage is present when any section of the vehicle is shorter than factory specifications
Usually limited to the forward of the cowl or rearward of the rear window
Indicated by wrinkles and severe distortion in fenders, hood, frame horns or rails

14. Types of Frame Damage (continued)
Diamond damage
One side of the vehicle is moved to rear or front
Causes the frame to be out of square, and is caused by impact off-center
Affects the entire frame, not just the side rails
Visual indications are hood and trunk lid misalignment

15. Types of Frame Damage (continued)
Twist damage
One corner of the vehicle is higher than normal
Can happen when a vehicle hits a curb or median strip at high speed
Most accidents result in a mix of one or more of these damage problems

16. Unibody Vehicle Damage
Unibody is designed to absorb a collision impact
Visualize the point of impact as the tip of a cone
Centerline points in the direction of impact
Depth and spread indicate the direction and area the collision force traveled
Tip of the cone is the primary damage area
Effects of the impact as it travels through the body structure is the secondary damage

17. Unibody Vehicle Damage (continued)
The unibody is designed with crush zones, engineered to collapse in a predetermined way
Effects of the impact shock wave are reduced as it is dissipated by the body structure
Frontal damage results from a head-on collision
In a minor impact, the bumper is pushed back, bending the front side members
If the impact is further increased, the front fender will contact the front door

18. Figure 20-10. These are typical crush zones on a unibody vehicle
Figure These are typical crush zones on a unibody vehicle. They should be checked closely for buckles, bends, paint or sealer cracks and other signs indicating damage.

19. Unibody Vehicle Damage (continued)
If the shock is great enough, the front fender apron and front body pillar will be bent
If a frontal impact is received at an angle, lateral as well as vertical bending occurs
If impact to the vehicle rear is small, the rear bumper, back panel, trunk lid will be deformed
If the impact is severe enough, the quarter panels will collapse to the base of the roof panel
Side damage will cause the door, front section and center body pillar to deform

20. Unibody Vehicle Damage (continued)
When the central area of the front fender receives an impact, the front wheel is pushed in
If severe, the suspension parts are damaged and the front wheelbase may be changed
Top impacts result from falling objects or rollover
This type of damage involves the roof panel, roof side rail, quarter panels, and windows
When a vehicle has rolled over, the extent of the damage can be determined by the deformation around the windows and doors

21. Dimensional References
A datum line or datum plane is an imaginary flat surface parallel to the underbody of the vehicle
Plane from which all vertical dimensions are taken
Center plane (centerline) divides vehicle into two halves
Body center marks are stamped onto the sheet metal in both the upper and lower body areas
All lateral dimensions are measured from the center, and most vehicles should be symmetrical

22. Dimensional References (continued)
Think of the vehicle as a rectangle divided into three zero plane sections: front, center and rear
Symmetrical means that the dimensions on the right side are equal to the dimensions on the left
If the vehicle is asymmetrical, these dimensions are not the same
In such cases, use gauges that can be adjusted to compensate for the asymmetry

23. Vehicle Measuring Basics
In unibody construction each section should be checked for diagonal squareness
Length and width should also be compared and the center section should be used as a base
You must start with at least three dimensions you know are undamaged
Reference points refer to the part of panel locations used to give unibody dimensions
The distance between reference points can be measured with a tram bar or a tape measure

24. Types of Measuring Equipment
Measuring equipment found in collision repair shops can be divided into four basic systems:
Gauge measuring systems
Universal measuring systems
Dedicated fixture systems
Electronic measuring systems
Gauge measuring systems use sliding metal rods and adjustable pointers with ruled scales

25. Figure Here the tram bar tip is touching the reference point on the damaged vehicle’s radiator support. It is not centered in the hole so the unibody must be straightened.

26. Types of Measuring Equipment (continued)
Tram bars are used for measurement and self-centering gauges check for misalignment
Tracking gauge checks alignment of front and rear wheels
Tram bar is a measuring rod with two adjustable pointers attached to it
Best areas to select for tram bar measurements are attachment points for suspension and mechanical parts

27. Types of Measuring Equipment (continued)
Point-to-point measurement means the shortest distance between two points
Datum measurement refers to the distance between points measured from the datum line
Upper body damage can be determined by using a tram bar and steel measuring tape, with the same procedure as for underbody damage
When checking front end dimensions, measure the attachment points for suspension and mechanical parts

28. Types of Measuring Equipment (continued)
Each dimension should be checked from two additional reference points, with at least one diagonal measurement
The longer the dimension, the more accurate the measurement
Body side structure damage can be evaluated by the fit and operation of the doors

29. Types of Measuring Equipment (continued)
Diagonal line measurement compares dimensions across an opening, or between four reference points
Should be used when body data dimensions is missing, or the vehicle was damaged in a rollover
Not adequate when both sides of the vehicle are damaged, or the frame is twisted
Damage to the rear body can be analyzed by the fit and operation of the deck lid
Measure rear body together with the underbody
Check dimensions from two or more references

30. Self-Centering Gauges
Self-centering gauges show alignment by projecting points on the vehicle’s structure into the technician’s line of sight
Establish the vehicle centerline and datum plane
To use self-centering gauges begin by hanging two gauges near the center of the vehicle
Hang self-centering gauges from two places where there is no visible damage, and two more where there is obvious damage

31. Self-Centering Gauges (continued)
The gauges are equipped with center pins or sights which remain in the center of the gauge regardless of the width of the horizontal bars
When sighting gauges for parallel, stand in the middle, scanning with both eyes
Readings should be made at the outer edge of the self-centering gauge, not in the middle
The gauges should always be set at the same height or plane
The gauges remain in place during the repair

32. Using the Datum Plane
To read for datum, read across the top of all four gauges to determine if the datum is correct
If all gauges are parallel, the vehicle is on datum
To check for centerline misalignment
The center pin on the No. 2 gauge must be lined up with the centerline on the No. 3 gauge
Then the pins of No. 1 and No. 4 can be read
The strut tower gauge shows misalignment of the strut tower/upper body to the centerline

33. Universal Measuring Systems
Universal measuring systems measure several reference points at the same time
May be mechanical or laser or a combination
When the system is set up properly you can monitor the key points by looking at the pointers
The pointers can be damaged during straightening, so lower them for the pull
Most universal measuring manufacturers have specific dimension charts intended for specific equipment only

34. Mechanical Measuring Systems
A typical mechanical system consists of:
A bridge that runs the length of the vehicle
Sliding arms that mount to the bridge
Pointers that are mounted to the arms
Specific features for vehicle make and model
The pointer may accept special adapters to fit over a bolt head or into a reference point hole
Mechanical measuring systems are designed for each family of body styles

35. Dedicated Bench and Fixture Measuring Systems
In a dedicated bench and fixture system instead of taking measurements, dedicated fixtures are used to check the body and frame alignment
The dedicated bench consists of a strong, flat work surface to which fixtures are attached
Fixtures are thick metal parts that bolt between the vehicle and bench to check alignment
If the fixture fits the vehicle, the underbody, strut towers, etc. are in perfect alignment

36. Dedicated Bench and Fixture Measuring Systems (continued)
Bolt-on fixtures are used when attachment is required to steering or suspension mountings
Pin-type and strut fixtures are used to mate with reference point holes in the underbody
Bench extensions are included where the length of the vehicle requires fixtures to be positioned beyond the bench surface
At least three fixtures should be set up on undamaged areas, then place as many as possible in the damaged area of the vehicle

37. Figure 20-25. Note various fixture types
Figure Note various fixture types. Fixtures mount on the frame rack and fasten to major structural panels on the vehicle. If the bolts or pins on the fixtures do not line up, the vehicle unibody is not aligned and is damaged.

38. Electronic Measuring Systems
Electronic measuring systems use a computer to control the operation of the measuring system
May use a laser scanner and reflective targets, ultrasound, or robotic measuring arms
A laser measuring system uses a beam of light aimed at a target or measuring scale
Target is hung or attached to the vehicle
Laser passes through center of the scale on the target when measuring point is in correct position

39. Figure 20-34. Study the basic parts of a laser measuring system
Figure Study the basic parts of a laser measuring system. Laser directs beam or beams of light under and around the vehicles. Reflectors or beam splitters then show the locations of reference points on the vehicle.

40. Ultrasound Measuring Systems
Ultrasound measuring systems are similar to laser systems, but use sound instead of light
An ultrasonic receiver beam mounts under the vehicle center section
Ultrasound probes are mounted at the reference points
Each probe generates sound waves, and the receiver detects them
The computer calculates the exact location of each probe and reference point being measured

41. Computerized Robotic Arm Measuring Systems
A computerized robotic arm measuring system uses a track-mounted robot arm to measure reference points on the vehicle
Robot must be moved by hand into contact with each reference point
The reading is stored in a computer
Control unit can store data and specifications for up to four vehicle models
Measurements and deviations can be printed

42. Summary Two basic types of automotive construction:
Body-over-frame (BOF) vehicles
Unibody (or monocoque) vehicles
Vehicle damage is broken into five categories: side sway, sag, mash, diamond, twist
Four basic measuring equipment systems: gauge, universal, dedicated, and electronic

43. Summary (continued)
Universal measuring systems have the ability to measure several reference points at the same time, making the job easier and more accurate
Universal measuring systems can be mechanical or electronic, or a combination
Laser measuring system uses a strong beam of light, beam splitters, and targets to measure vehicle damage
Extremely accurate when properly installed and used