Chapter 20 Measuring Vehicle Damage

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

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

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

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

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

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

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

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

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

Figure 20-2. 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

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

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

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

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

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

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

Figure 20-10. These are typical crush zones on a unibody vehicle Figure 20-10. 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.

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

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

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

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

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

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

Figure 20-15. 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.

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

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

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

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

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

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

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

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

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

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

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

Figure 20-25. Note various fixture types Figure 20-25. 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.

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

Figure 20-34. Study the basic parts of a laser measuring system Figure 20-34. 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.

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

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

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

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