Low-Speed Off-Tracking (Swept Path)
How far a turning truck's rear axle tracks inside the front axle's path: OT = R - sqrt(R^2 - sum(L^2)), R the turn radius and L each unit's wheelbase (tractor wheelbase + trailer kingpin-to-axle, summed in quadrature). A single unit with a 20 ft wheelbase on a 50 ft radius off-tracks 4.2 ft. The low-speed check for whether a rig holds its lane on a turn or intersection. High-speed off-tracking (rear swings out) and swept-path width are separate; the design vehicle and agency govern.
Formula and source
OT = R - sqrt(R^2 - sum(L_i^2)); effective wheelbase = sqrt(sum(L_i^2)). R the turn radius, L_i each unit's wheelbase (tractor wheelbase + trailer kingpin-to-rear-axle).
AASHTO Green Book (Policy on Geometric Design of Highways and Streets) low-speed off-tracking; standard swept-path geometry.
Audience
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