Ladder-frame chassis

Look under a Fortuner, a Scorpio-N, a Thar or any truck and you will see the ladder frame: two long rails of pressed steel, usually C-shaped in section, running the length of the vehicle and joined by cross members like the rungs of a ladder. Everything else is bolted to this frame. The engine and gearbox sit between the rails on rubber mounts, the axles hang below it on leaf springs or coil springs, and the body is a separate shell that sits on rubber bushes along the top.

Because the frame does all the structural work, the body can be almost anything: an SUV cabin, a pickup bed, a tipper body, a bus. That is why truck makers sell a bare chassis-cab and let a body builder finish it.

• The rails are rolled or pressed from thick steel and joined by riveted or welded cross members.
• Engine, gearbox, axles and suspension are mounted to brackets on the rails.
• The body is lowered on and bolted down through rubber bushes that absorb vibration.
• Loads from towing, carrying and rough roads pass through the rails, not the body.
• A damaged section of frame or body can be repaired or replaced on its own.

For the driver this means a vehicle that shrugs off overloading, rough tracks and towing, and that is cheap to repair after a knock. The costs are weight, a higher floor (you climb up into the cabin), a body that can shake a little on its mounts, and a frame that bends rather than crumples, so crash protection depends more on the body design. A monocoque is the lighter, more refined alternative used by most cars.