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When a passenger vehicle is involved in a collision, quite often, the damage extends beyond what is visible to the naked eye. Underneath the crumpled sheet metal can be a more serious problem; namely, a damaged unibody or frame. If this has occurred, it will be very difficult to re-align the vehicle. In addition, resulting suspension damage will affect the ride and handling characteristics. The net result is an unsafe vehicle.
On the surface, the repair goal for a collision-damaged vehicle is for (1) all the body panels to fit properly with the correct panel-to-panel gap, (2) the doors, trunk lid/tailgate and hood to open and close properly, (3) the windows to function correctly and (4) the paint color to match across the damaged area to the undamaged area. In other words, the vehicle should look as new.
But, there is more to a collision repair than meets the eye. The vehicle has to perform on the road too. This means the car tracks straight, rides comfortably and turns correctly when steered. In order to accomplish this, the unibody and sub-frame must be repaired, if damaged. It is on these surfaces that the suspension components are mounted. If the surfaces are damaged, it will be impossible to correctly align the vehicle and achieve maximum vehicle performance.
In order to correctly repair structural damage, it’s important to understand the critical angles under the sheet metal. There are four critical angles: (1) steering axis inclination, (2) camber, (3) caster and (4) toe.
The steering axis inclination, SAI, is the angle between a true vertical line and a line drawn through the center of the upper ball joint, or strut (depending on the suspension design) and the lower ball joint. The vertical line starts at the center of the tire at the road contact point (Figure 1). SAI is the critical angle used to determine if the structural dimensions are correct.
If the SAI is on the low side of the spec, look for the problem on the lower sections of the suspension; conversely, if the angle is on the high side, look at the upper sections. Be aware if the SAI is greater on one side of the vehicle compared to the other side. If this is the case, check the position of the engine cradle or crossmember.
Camber is the next angle to be checked. It is the vertical tilt of the wheel (inward or outward) as viewed from the front of the vehicle. This tilt is measured from the same vertical line used to measure SAI (Figure 1).
Camber and SAI added together gives you the Included Angle (IA) (Figure 1). IA can determine if they are bent or incorrectly adjusted parts between the upper and lower pivot points of the suspension.
It should be mentioned that many technicians often use the camber measurement as the reference point for the other remaining measurements. They may believe (incorrectly) that camber must be set to specs even if it affects other alignment angles. One alignment angle should never be adjusted out-of-spec to make another angle within spec.
Caster is the vertical tilt of the steering knuckle (forward or rearward) in reference to the position of the lower ball joint and upper ball joint, or strut. It is viewed from the side of the vehicle (Figure 2). Caster is measured in degrees relative to a true vertical center line. Although caster does not affect tire wear, it does affect the stability of the vehicle. As a result of a collision, the steering knuckle can be damaged, changing the position of the knuckle and/or ball joints. If there is an imbalance of caster from one side to the other, the vehicle will drift to the side with the least amount of positive caster.
Toe is the inward or outward angle of the wheels as viewed from above the vehicle (Figure 3). It is measured in degrees or inches. Toe has a profound effect on tire wear. If toe is properly adjusted, it allows the wheels to travel in the same direction. If toe is not properly adjusted, however, the misaligned wheels will scuff the tires sideways, causing rapid tire wear.
Notes on Unibody and Sub-frame Repair
With the use of advanced steels on late model vehicles, it is important to know when to use heat — and when not to use heat — when straightening a unibody and/or sub-frame section. FCA US LLC engineering’s position on the use of heat during collision repair is as follows:
Any body panel or frame component damaged which is to be repaired and reused, must be repaired using the cold straightening method. No heat may be used during the straightening process.
During rough straightening prior to panel replacement, damaged panels or frame components may be heated to assist in body/frame realignment. The application of heat must be constrained to the parts which will be replaced and not allowed to affect any other components.
This no heat recommendation is due to the extensive use of high strength and advanced high strength steels in FCA US LLC products. High-strength materials can be substantially and negatively affected from heat input which will not be obviously known to the repairer or consumer.
Ignoring these recommendations may lead to serious compromises in the ability to protect occupants in a future collision event, reduce the engineered qualities and attributes, or decrease the durability and reliability of the vehicle. Failure to follow these instructions may result in serious or fatal injury.
The amount of high strength-low alloy steel (HSLA) used on a particular vehicle depends on the make, model and year. In an effort to reduce confusion over the large number of steel grades in use, and the repairability and weldability concerns involved with each, FCA US LLC has instituted new nomenclature which is applicable to material call-outs and Body In White (BIW) exploded views released for use in the repair industry.
High Strength (HS) steels are identified with the labels and colors HS (green) and Very High Strength (VHS) (red). On the exploded views, these labels are used as the call-outs and the components are shown in the appropriate color. These exploded views can be found online at TechAuthority.com. Enter high strength steel, or similar phrase, in the Search box on the home page. The document titled, Standardized Steel Identification, will be displayed for the particular vehicle being repaired.