Off-tracking is what happens when a vehicle turns and its rear wheels follow a shorter, tighter path than its front wheels. Every vehicle does this to some degree, but the effect becomes dramatic with longer vehicles like semi-trucks, buses, and vehicles towing trailers. The longer the distance between the front and rear axles, the more the rear wheels “cut” to the inside of a turn.
How Off-Tracking Works
When you steer a vehicle, you’re directing the front wheels. The rear wheels can’t steer on their own, so they get pulled along behind, tracing a path closer to the center of the turn. Picture pushing a shopping cart around a corner: the back wheels always swing inside the arc the front wheels take. This gap between the front and rear wheel paths is the off-tracking distance.
Two main factors control how much off-tracking occurs. First, the distance between the front axle and the rear axle (the wheelbase). A longer wheelbase means greater off-tracking. On an 18-wheeler, the distance from the kingpin (where the trailer connects to the tractor) to the rear axle can be 40 feet or more, which creates a substantial difference between the front and rear wheel paths. Second, the sharpness of the turn matters. Tighter turns with a smaller radius produce more off-tracking, while gentle, sweeping curves reduce it significantly.
This is why you’ll sometimes see truck drivers swing wide to the left before making a right turn. They need to create enough room so their trailer’s rear wheels don’t hop the curb, clip a sign, or roll over a sidewalk.
Low-Speed vs. High-Speed Off-Tracking
Off-tracking behaves differently depending on how fast a vehicle is moving. At low speeds, like turning through an intersection, the rear wheels track inward, cutting closer to the inside of the curve than the front wheels. This is the type most people picture, and it’s the reason intersections need wide curb radii to accommodate large trucks.
At highway speeds, something different happens. The rear of a long vehicle can actually swing outward, beyond the path of the front wheels. This high-speed off-tracking is less intuitive but creates its own set of risks, particularly when trucks change lanes or navigate freeway ramps. The trailer’s rear end drifts toward adjacent lanes, which can catch other drivers off guard.
Why It Matters for Safety
Off-tracking creates a blind-spot hazard that’s responsible for a specific type of collision sometimes called a “squeeze” crash. When a truck turns right at an intersection, the cab clears the corner with room to spare, but the trailer’s rear wheels sweep inward across the space where a cyclist, pedestrian, or smaller vehicle might be positioned. A car sitting just to the right of a turning truck can get caught between the trailer and the curb with no escape route.
Large trucks may also off-track into adjacent lanes or onto shoulders during curves on highways. Because the rear wheels don’t follow the same line as the front, the trailer can encroach into neighboring lanes even when the driver is holding steady in their own lane. This is more pronounced on narrower roads and tighter curves, and it’s one reason highway curves are sometimes built wider than straight sections.
How Roads Are Designed Around It
Highway engineers design intersections and curves with off-tracking in mind. The standard design vehicle for U.S. roads is a tractor-trailer combination with a 53-foot trailer, and intersection geometry has to accommodate its turning path. This influences the radius of curbs at intersections, the width of lanes on curves, and how much extra pavement is added on the inside of highway bends.
On curved roads, engineers add extra lane width, called curve widening, specifically to account for the space that off-tracking consumes. The Federal Highway Administration’s design guidelines detail how much additional width is needed based on the curve’s radius and the design vehicle’s wheelbase. Without this widening, trucks would routinely cross lane markings on curves, creating head-on collision risks on two-lane roads.
Intersection corners in urban areas are often built with larger turning radii than passenger cars alone would require. Some intersections use mountable curbs or truck aprons (those slightly raised concrete pads you sometimes see at the inside corner of a turn) so that a truck’s rear wheels can track over them without the truck needing to swing into oncoming traffic lanes.
Calculating Off-Tracking Distance
Engineers use a straightforward relationship to estimate off-tracking: it’s the difference between the turning radius of the front axle and the turning radius of the rearmost axle. For vehicles with multiple trailer segments, you add up the contributions from each wheelbase in the combination. A single-unit truck with a 20-foot wheelbase turning on a 50-foot radius will off-track less than a tractor-trailer combination with 45 feet between the kingpin and rear axle making the same turn.
The math gets more complex with multi-trailer combinations because each articulation point (each hitch connecting one unit to the next) adds its own off-tracking contribution. Doubles and triples, the tandem trailer configurations legal in some states, produce compounding off-tracking that can require significantly more space than a single 53-foot trailer.
Technologies That Reduce Off-Tracking
Steerable rear axles on trailers are one of the most effective mechanical solutions. Instead of the trailer’s rear wheels being fixed, they pivot during turns, actively following a path closer to the front wheels. This can dramatically reduce how much the rear end cuts inward on tight turns, which is especially useful for long trailers navigating city streets.
On the electronic side, active rear-wheel steering systems use sensors and control algorithms to adjust wheel angles in real time. Some systems steer the rear wheels in the same direction as the front wheels during high-speed maneuvers, which improves stability and reduces the outward drift that causes high-speed off-tracking. Research from SAE International has shown that model-based control systems can successfully prevent jackknifing during sudden lane changes by managing the off-tracking behavior through the tractor’s rear-wheel steering.
For everyday drivers, the practical takeaway is simpler: give trucks extra room at intersections. If you’re stopped next to a semi that’s turning, the space between you and the trailer can disappear fast as those rear wheels sweep inward. Staying well behind the truck’s rear axle, rather than pulling up alongside it, is the safest position.