2. Occupant kinematics in side pole and oblique impacts

In a side pole impact, the vehicle is struck at a narrow, localized contact patch, typically a pole or tree of 250–380 mm diameter, at a relatively high closing speed. Because the pole does not deform the full door structure the way a flat barrier does, intrusion is concentrated and the occupant compartment is loaded asymmetrically. In an oblique impact, the principal force vector is rotated relative to the vehicle's longitudinal axis, so the occupant experiences combined lateral and longitudinal acceleration. Both configurations produce kinematics that differ fundamentally from the pure lateral MDB (moving deformable barrier) test, and both are disproportionately represented in serious real-world injury outcomes. This chapter examines how the occupant moves, how restraints interact, and which injury mechanisms dominate.

Reference Frame and Sign Convention

All descriptions in this chapter use the vehicle-fixed coordinate system: +X forward (longitudinal), +Y toward the right side of the vehicle, +Z upward. The struck side is the near side; the opposite side is the far side. For a left-hand-drive vehicle struck on the driver's door, the driver is the near-side occupant and the front passenger is the far-side occupant. Occupant motion is described relative to the vehicle interior, not the ground, because restraint performance is governed by relative displacement within the compartment.

Near-Side versus Far-Side Kinematics

The near-side occupant is loaded early: the door trim, armrest, and side airbag contact the pelvis and thorax within the first 20–30 ms, and the occupant is accelerated laterally toward the vehicle centerline. The far-side occupant has no direct structure between the body and the impact; instead, the far-side occupant continues in the original direction of travel (inertial motion) while the vehicle accelerates away, producing a large relative displacement across the cabin. This is the defining feature of far-side loading: the occupant "crosses" the interior and interacts with the center console, the opposite door, or the far-side restraint system.

Two parameters dominate the far-side response. The first is the lateral Δv imparted to the vehicle; a pole impact at 32 km/h can produce 20–28 km/h of lateral velocity change over 40–70 ms. The second is the occupant-to-vehicle phase lag, which determines whether the far-side occupant reaches the far door before or after the vehicle's peak lateral acceleration. A short-duration, high-magnitude pulse (typical of pole impacts) leaves the occupant still moving laterally when the vehicle has already begun to rebound, increasing the relative excursion.

Oblique Impact: The Combined Loading Vector

In an oblique impact, the barrier or pole is oriented at an angle to the vehicle axis, commonly 15°–30° in regulatory and consumer-test configurations (for example, the NHTSA oblique test uses a 35% overlap, 56 km/h, 15° angled MDB). The occupant's inertia vector is correspondingly rotated, so the thorax and pelvis are loaded in a combined lateral-plus-longitudinal direction. The practical consequence is that the occupant slides forward and laterally at the same time, engaging the seat belt's shoulder and lap portions asymmetrically. The lap belt may load one iliac crest more than the other, and the shoulder belt may slip off the shoulder or load the neck rather than the clavicle. This coupling is the reason oblique impacts produce a distinct injury signature: unilateral pelvic fractures, rib fractures concentrated on one side, and a higher incidence of head contact with the A-pillar or far-side interior than in pure lateral tests.

Far-side and oblique occupant loading paths in a side pole impact Top view of a vehicle struck by a pole on the left side. The near-side occupant is loaded early by the door and side airbag; the far-side occupant travels across the cabin toward the far door and center console. An inset shows the oblique loading vector resolved into lateral and longitudinal components. Top View: Side Pole Impact — Near-Side vs. Far-Side Loading Paths Vehicle (top view) POLE localized contact NEAR driver door + side airbag early load (20–30 ms) FAR passenger inertial excursion across cabin (relative to vehicle) center console far door / B-pillar late contact Oblique Loading Vector (15°–30° from vehicle axis) +X +Y resultant lateral component longitudinal component θ Legend near-side early load far-side inertial path late far-side contact

Restraint Coupling: How Systems Interact

Restraint coupling refers to the interaction between restraint subsystems that were each tuned for a different loading direction. In a side pole impact, the side airbag and curtain airbag are the primary near-side restraints, while the seat belt and far-side airbag (where fitted) govern the far-side occupant. The coupling problem arises because the far-side occupant's belt geometry is optimized for frontal loading: the shoulder belt routes over the outboard shoulder, and the buckle stalk is on the inboard side. Under lateral acceleration, the occupant's torso rotates and the shoulder belt can migrate toward the neck or slip off the shoulder entirely, a condition known as belt derailment. Once derailed, the upper torso is unrestrained laterally and the head becomes the leading contact point.

Three coupling effects deserve specific attention:

  • Belt-to-airbag timing conflict. If the far-side airbag deploys while the occupant is still moving laterally, the airbag can push the occupant back toward the near side, increasing excursion and potentially causing the head to strike the near-side interior on rebound.
  • Pretensioner and load-limiter interaction. A pyrotechnic pretensioner that fires on lateral acceleration will pull the occupant inboard, which is beneficial for the near-side occupant but can worsen the far-side occupant's position by removing slack that would otherwise allow controlled ride-down.
  • Seat structure and recliner loading. In oblique impacts, the seat back is loaded in combined bending and torsion; recliner failure allows the occupant to rotate rearward, changing the belt fit and moving the head out of the airbag's effective zone.
When reviewing far-side kinematics in a sled or full-vehicle test, track the occupant's head center-of-gravity trajectory in the vehicle frame at 5 ms intervals. A far-side head excursion exceeding roughly 300 mm laterally (measured from the seated H-point) is a practical indicator that the restraint system is not controlling the far-side path, and that head-to-far-interior contact is likely.

Injury Mechanisms

The injury mechanisms in these configurations are direction-specific. For the near-side occupant, the dominant mechanisms are pelvic ring fracture from door intrusion, thoracic rib fracture from side airbag and door trim loading, and lateral head impact against the B-pillar or window frame. For the far-side occupant, the mechanisms shift: contralateral pelvic fracture from lap-belt loading on the inboard iliac crest, cervical and upper thoracic injury from shoulder-belt loading of the neck when the belt derails, and head injury from contact with the center console, the far door, or the far-side B-pillar. In oblique impacts, the combined vector produces unilateral rib fractures on the loaded side and a higher likelihood of abdominal injury from the lap belt sliding up over the iliac crest.

A useful way to reason about severity is the loading-path continuity concept: an injury is most likely where the restraint or structure loads the body through a path that is not aligned with the body's load-tolerant axis. The pelvis tolerates lateral loading well through the iliac wings but poorly through the pubic symphysis; the thorax tolerates distributed lateral loading but poorly tolerates concentrated loading near the sternum or the lower rib cage. Far-side and oblique configurations are dangerous precisely because they route load through these less-tolerant paths.

Practical Analysis Workflow

When analyzing a side pole or oblique event, the following sequence keeps the kinematics and injury assessment grounded in observable data:

  1. Establish the vehicle pulse. Extract the lateral and longitudinal acceleration at the struck-side sill and at the far-side sill. The difference between these two signals quantifies the yaw and lateral deformation the occupant compartment experiences.
  2. Compute the occupant's relative trajectory. Integrate the vehicle pulse twice to obtain vehicle displacement, then subtract from the occupant's free-flight displacement (assuming the occupant remains at initial velocity until first contact). The result is the relative excursion used to judge restraint engagement timing.
  3. Identify first contact and restraint engagement. Using high-speed video or CAE animation, mark the frame at which the near-side airbag, far-side airbag, belt, and interior surfaces first contact the occupant. Compare these timings against the restraint firing times.
  4. Map contact to body region. For each contact event, record the body region loaded and the approximate load direction. Flag any contact that loads the neck, the lower rib cage, or the pubic symphysis.
  5. Correlate with injury outcome. Compare the contact map against the AIS-coded injuries from the crash or the dummy response channels (rib deflection, pelvic force, head acceleration). A mismatch between a high-magnitude contact and no injury, or a low-magnitude contact and severe injury, usually indicates a load-path concentration that warrants closer review.
Do not assume that a far-side occupant is protected simply because the near-side airbag and curtain deployed. In pole impacts, the far-side occupant's excursion is often larger than the near-side occupant's, and the far-side restraint (belt plus far-side airbag) may not engage until 60–80 ms after impact—well after the head has already begun moving toward the far interior. Timing, not just presence, determines outcome.

Key Takeaways for Analysis

Far-side and oblique loading paths are governed by the phase relationship between vehicle deceleration and occupant inertia, not by the presence of a single restraint. The near-side occupant is loaded early and locally; the far-side occupant is loaded late and across a longer path. Restraint coupling—belt routing, pretensioner timing, airbag firing—can either control or amplify that path. Injury mechanisms follow the load path: pelvic ring and thoracic injury on the near side, cervical, contralateral pelvic, and head injury on the far side, with oblique vectors adding unilateral rib and abdominal involvement. The analysis workflow above converts these concepts into a repeatable procedure that ties pulse data, contact timing, and injury outcome together.

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