The big stamped steel sheet under the cabin, stiffened with ribs and crossmembers. It carries the seats and ties the two sides of the body together. In a combustion car a raised tunnel down the middle makes room for the driveshaft and exhaust; in many electric cars the floor is flat, with the battery beneath it.
Vertical posts that hold up the roof, lettered from the front: A beside the windscreen, B between the doors, C behind the rear doors (and D on wagons and SUVs). Sills are the long beams along the bottom of the door openings. Together with the roof they form a stiff ring around the cabin, and they are made of the strongest steel in the car.
A thin roof panel held up by crossmembers (bows) and roof rails along each side. The roof adds a surprising amount of stiffness to the whole body, so cutting a hole for a sunroof, or removing the roof altogether in a convertible, has to be made up for with extra reinforcement elsewhere.
Long beams running along the front and rear of the floor and through the engine bay. The front rails carry the engine and front suspension and join the firewall; the strut towers over the wheel arches stiffen the structure where the suspension loads come in. In a crash they are the parts designed to collapse first, which the Safety section covers.
A door is a thin steel or aluminium shell around an inner frame, holding the window and its mechanism, the lock and a beam against side impacts. Hinges on the front pillar and a check strap guide its swing, a latch and a striker hold it shut, and rubber seals keep out wind noise and water. Frameless doors on coupes trade a little sealing for looks.
The hood (bonnet) and trunk lid (boot lid) are lightweight panels, often aluminium on premium cars, attached by hinges and held up by gas struts. The hood's latch has a second safety catch, and many modern hoods are shaped to give way over hard engine parts, or even pop up on sensors, to cushion a pedestrian. Electric cars add a front trunk where the engine would be.
A bumper is a metal beam behind a plastic cover. The beam is bolted to the ends of the body rails, and the cover is only the cosmetic skin that carries sensors, tow-hook flaps and grilles. Behind it, foam or crush boxes soak up low-speed knocks; how bumpers work in a real crash is explained in Safety.
Plain mild steel has given way to a mix: high-strength steel for most panels, ultra-high-strength boron steel hot-formed for the pillars and sills, aluminium for hoods, doors and sometimes the whole body, and plastic or composites for bumpers and wings. Each saves weight or adds strength where it matters, so a modern body is a patchwork of several grades.
Rust protection and paint
Steel rusts, so bodies are protected in layers: galvanised (zinc-coated) sheet, a phosphate wash, an electro-coat primer dip that reaches every seam, sealant in the joints, then colour and clear lacquer. The underbody gets a stone-chip coating, and wax is sprayed into hollow sections. Thanks to this, many cars carry a long anti-perforation warranty.
Torsional stiffness is how much a body resists twisting; a stiff body lets the suspension do its job and feels solid and quiet. It comes from the shape — closed boxes and triangulated braces — and from how the pieces are joined: thousands of spot welds, laser welds, rivets and structural adhesive. Adhesive spreads load along the whole joint, adding stiffness and damping that spot welds alone cannot.