Frame, Chassis and Structural Damage Assessment
Frame, Chassis and Structural Damage Assessment
The exam tests how to recognise structural damage, measure it and decide whether to repair, replace or reject a vehicle.
Core rules
- Unibody and body-on-frame vehicles use different measuring systems: datum points, control points, centreline and symmetrical dimensions.
- Damage types include mash, sag, sway, diamond and twist; each shows a different pattern of gap and height change.
- Measure before pulling, during pulling and after pulling; a frame machine or laser/computer system records the control dimensions.
- Straightening uses anchored pulls in the opposite direction of impact; heat is restricted on high-strength steel and forbidden on some grades.
- Structural panels, rails and pillars must be replaced or sectioned only at OEM-approved locations.
- A vehicle with damaged suspension mounts, firewall or passenger cell may be a total loss even if it looks repairable.
Worked example
A front impact has closed the door gap on one side. Reasoning: the door gap change suggests body sway or mash, so measure the underbody control points before pulling. If the strut tower is displaced, pull the rail on the frame machine while monitoring the dimensions, then re-measure and check the door gap.
Traps
Pulling without measuring, heating structural steel, hiding damage under filler, and missing a bent suspension mounting point. Candidates also forget to check the opposite side for sway.