Chassis, Body Construction and Frame Alignment
Chassis, Body Construction and Frame Alignment
This domain tests body construction types, structural measurement, pulling and sectioning rules. The exam expects you to distinguish cosmetic alignment from structural correction.
Core rules
- Body-on-frame vehicles use a separate ladder frame; unibody vehicles use welded sheet-metal structures with crush zones and controlled deformation.
- Before pulling, measure control points and diagonals against manufacturer dimensions. A level floor, correct tyre pressures and suspension condition are prerequisites.
- Pull in the direction of the original impact using a frame machine and approved clamps. Pull slowly, release tension and re-measure.
- Heat only where the manufacturer permits; HSS and UHSS may lose strength if heated above specified limits.
- Sectioning must follow manufacturer locations, weld types and reinforcement. Do not section in a crush zone or at a stress concentration unless approved.
- After structural repair, check suspension, wheel alignment and ADAS sensors.
Worked example
A vehicle shows a 12 mm diagonal difference at the front control points. Reasoning: verify the floor and tyre pressures, re-measure, then anchor the vehicle and pull the short diagonal. Re-measure after each pull; stop at the specified tolerance. Straightening by eye or pulling before measurement risks over-pulling and hidden damage.
Traps
Pulling before measuring; heating high-strength steel; sectioning at a crush zone; ignoring suspension damage; assuming door gaps prove the frame is straight.
Key points
Measure control points and diagonals before pulling. Pull in the impact direction and re-measure after each pull. Heat only within manufacturer limits for HSS/UHSS. Section only at approved locations with correct weld types. Recheck suspension, alignment and ADAS after structural repair.