21.13.
Frame Repairs
Defects in frames and body generally occur due to severe impacts on rough roads and collision with other objects or vehicles. Depending upon the nature of collision, the defects of the following kinds may occur.
• Misalignment in horizontal and/or vertical plane.
• Twisting of main frame and/or sub-frames.
• Buckled main frame and/or sub-frames.
• Bent side members and/or dumb iron.
• Broken or loose gusset plates and rivets.
Whenever the vehicle is subjected to a major collision, the frame alignment must be checked. A visual check generally reveals major misalignment, but in case this fails to indicate the defect, the frame check is conducted as follows (Fig. 21.24).
(a) Wheel Base Check. The front wheels are set in the straight-ahead position and the wheelbase on each side is checked (Fig. 21.24A).
(b) Alignment. To verify parallelism of the rear wheels with each other, a cord or straight edge is held against the rear wheel. Then the front wheel is turned until it is parallel with the cord. The clearance (if any) between the wheel and cord should be the same on both sides (Fig. 21.24B).
(c) Plumb-line Check. A plumb line is dropped from the outside of each fixed shackle of the spring to give eight chalk marks on the floor. The points are joined as shown in Fig. 21.24C. All diagonals should cross the centre line if the frame is correctly aligned. The tolerance for each check depends on the size of frame, but 6 mm is often considered as the maximum.
Frame straightening is a specialized repair and is carried out by using jacks and chains. If the frame has not been heat-treated, it is possible to heat the damaged member to ease the straightening operation.
Cracks can be repaired by welding, and when a reinforcement plate is fitted, it is advisable to taper the end to avoid any sudden change in cross-sectional area.
Fig. 21.24. Chassis checking. A. Wheelbase check. B. Alignment check. C. Plumb-line check.
