Vehicle Body Construction, Materials and Fasteners
Construction types
- Body-on-frame: a separate ladder chassis with the body on rubber isolators; common on pickups and older 4x4s.
- Monocoque/unibody: welded box sections carry all loads; most passenger cars. Crumple zones absorb energy while the occupant cell stays rigid.
- Spaceframe: tubular frame with bonded or bolted panels.
Materials
Mild steel is formable and weldable. High-strength and ultra-high-strength steels, including boron (martensitic) grades, appear in pillars, sills and bumper beams. Aluminium is used for bonnets, doors and some full structures; plastics such as PP, ABS, PA and PC for bumpers and trims; composites for some panels. Each material has its own repair rule, and the OEM procedure overrides workshop habit.
Fasteners and joining
Spot welds, MIG welds, structural adhesive, blind rivets, rivet nuts, clinch joints, bolts, plastic clips and screws. Torque-to-yield bolts must be replaced, not reused. Isolate steel from aluminium to prevent galvanic corrosion. Restore sealer, weld-through primer, cavity wax and anti-corrosion coating after every repair.
Worked example
A B-pillar on a late-model car is boron steel, and the OEM says no heat and no straightening. The correct repair is replacement at the specified sectioning points with the specified rivets and adhesive. Heating above roughly 600 °C destroys the microstructure that gives the pillar its strength, so a straightened pillar fails in the next crash.
Traps
- Heating ultra-high-strength steel to shrink a dent.
- Welding a mild steel patch into a structural section.
- Reusing torque-to-yield bolts or self-locking nuts.
- Bolting aluminium to steel without an isolator.
- Leaving weld seams without primer and sealer.