Why does a long truck need so much room at a corner?

A long truck can bend in the middle, but that does not mean every wheel follows the same track around a corner. The powered front vehicle is the tractor. The trailer carries the load behind it. A joint between them allows their directions to differ during a turn.

The tractor pulls on the front of the trailer at that joint. The trailer wheels are farther back. In an ordinary slow forward turn, those wheels roll along a path inside the path of the pulling joint. This inward tracking is called off-tracking.

Think about the trailer as a long object connecting a moving front point to wheels behind it. The front can start changing direction while the rear is still approaching the corner. The trailer gradually swings around, cutting across the inside of the front point’s route. A longer distance between the joint and the wheels generally increases this effect.

The joint therefore solves one problem but leaves another. It allows a long vehicle to change shape around a bend, while the different wheel paths mean the whole vehicle sweeps across a wide area. Some part of a trailer can also swing outward depending on its overhang and the turn.

This is why the empty-looking space beside a turning truck is not necessarily unused space. The front has passed, but the rest of the vehicle is still moving into its path. A corner is a journey for the whole truck, not just its cab.