Offtracking: Physics and Practical Impact
Offtracking refers to the difference in path between the front axle of a vehicle combination and the rear axle(s) during a turn. When a tractor-trailer turns, the trailer wheels follow a shorter arc than the tractor's front wheels — the trailer cuts the corner. The amount of offtracking depends on the trailer length, the wheelbase, and the degree of the turn. Offtracking calculation (simplified): for a 53-foot trailer with a wheelbase of approximately 40 feet (kingpin to rear axle group), in a 90-degree turn with the front wheels at the outside edge of the lane, the rear trailer axles may cut inside the turn path by 4-8 feet. This is the 'swept path' width — the total width of road surface used by the vehicle combination. At low speed (turning): offtracking is maximum — the physics are purely geometric. A 53-foot trailer making a right turn from a two-lane road may have its rear axles cut 5-8 feet inside the turn — potentially hitting curbs, signs, or parked vehicles. At high speed: forward motion reduces apparent offtracking because the trailer doesn't rotate as sharply during lane changes (though high-speed lane changes create different dynamics through rearward amplification). Practical impact: right turns at intersections and dock approaches require the driver to swing wide (track out) before turning — leaving space for the trailer to cut back in. Experienced drivers develop an intuitive sense of their specific vehicle's offtracking geometry.