Low-Speed Off-Tracking (Swept Path)
How far a turning truck's rear axle tracks inside the front axle's path.
Example
You enter
- Turn radius R (ft) 50
- Tractor / unit wheelbase (ft) 20
You get
- Off-tracking (rear inside front) 4.17 ft
- Effective wheelbase 20 ft
Checked against published worked examples from: AASHTO Green Book (low-speed off-tracking).
Details, formula, and sources
ScopeOT = 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 or tableOT = 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).
Source and editionAASHTO Green Book (Policy on Geometric Design of Highways and Streets) low-speed off-tracking; standard swept-path geometry.
Free accesstransportation.org for TOC; the geometric off-tracking relation is public. AASHTO design-vehicle tables are licensed.
What governsMath aid for personal verification. The ELD on the truck and the carrier tariff are the legal record. State limits may be lower than federal.
API field namesturn_radius_ft, wheelbase1_ft, off_tracking_ft, effective_wheelbase_ft
- Low-speed geometric steady-state OT = R - sqrt(R^2 - sum(L_i^2)); rear tracks insideAASHTO Green Book
- Effective wheelbase units summed in quadrature; tractor WB + trailer kingpin-to-axleswept-path geometry
- Scope not high-speed off-tracking (rear swings out) or swept-path width (add vehicle width)analysis scope