Monocoque body

Almost every Indian hatchback, sedan and crossover, from the Alto to the Creta and the XUV700, is a monocoque (also called unibody). There is no separate frame underneath. Hundreds of pressed steel panels are spot-welded into a single shell, and that shell carries every load: the weight of the engine, the forces from the suspension, the twisting on a rough road and the crash energy in an accident.

Strength comes from shape, not thickness. The sills along the door bottoms, the pillars, the roof rails, the floor cross-members and the front and rear longitudinal members are all box sections, thin steel folded into closed tubes that are very stiff for their weight. In the engine bay a subframe, bolted to the shell, carries the engine and the lower suspension arms.

• Panels are stamped from sheets of steel of different grades; the safety cell uses high-strength steel, the crumple zones softer grades.
• Robots weld them into a bare shell, the body-in-white, which is then painted and fitted out.
• In a crash the front or rear members fold in a planned way, absorbing energy over a longer distance.
• The passenger cell around the people stays rigid.
• Suspension, engine and subframes bolt to reinforced mounting points in the shell.

The result is lighter and stiffer than a body sitting on a frame, which helps fuel economy, handling and crash scores alike; it also allows a flat, low floor. The trade-offs are that heavy off-road abuse or a bad crash can bend the shell itself, which is harder to repair than swapping a frame section, and that a monocoque cannot easily take a different body on the same underpinnings. That is why pickups and the toughest SUVs still use a ladder frame.