Understanding Off-Tracking Physics
Off-tracking is the geometric phenomenon where the rear axle of a trailer follows a path inside the arc taken by the tractor's front axle during a turn. The amount of off-tracking is directly proportional to the wheelbase of the trailer (the horizontal distance from the kingpin to the rear axle centerline) and the severity of the turn. For a simple 90-degree turn, the off-tracking can be calculated using the Pythagorean theorem: if the front of the tractor travels along a radius R, the rear of the trailer travels along a radius equal to the square root of (R squared minus wheelbase squared). For a 53-foot semi-trailer with a typical 41-foot wheelbase, making a 90-degree turn with a front wheel radius of 45 feet, the rear axle follows a radius of approximately 18.4 feet — an off-tracking value of 26.6 feet. This means the trailer rear swings approximately 26.6 feet inside the path of the front wheels. This is why combination vehicles must start right turns from near the center line, allowing the trailer to clear the curb. Off-tracking increases with trailer length, decreases with larger turn radii, and is additive for each additional trailer in a multiple-trailer combination.