2. Occupant kinematics in far-side impacts

In a far-side impact, the struck side of the vehicle is opposite the occupant's seating position. The occupant therefore receives no direct structural push from the struck door or B-pillar; instead, the vehicle body is accelerated laterally away from the occupant, and the occupant's own inertia carries the body toward the struck side. This indirect loading path is what makes far-side kinematics fundamentally different from near-side kinematics, and it is the reason far-side countermeasure design cannot simply mirror near-side practice.

Throughout this chapter, lateral direction is defined in the vehicle coordinate system: +Y points toward the struck side (the side of the impact), and −Y points toward the non-struck side. Longitudinal +X is forward. All descriptions of occupant motion, belt routing, and interior contact refer to this convention unless a different reference frame is explicitly stated.

Motion phases of the far-side occupant

Far-side occupant motion is usefully decomposed into four overlapping phases. The boundaries are not sharp, but each phase has a distinct dominant mechanism and a distinct set of observable quantities.

Phase 1 — Vehicle response and occupant lag. The struck structure deforms and the occupant compartment acquires lateral velocity and yaw. Because the occupant is coupled to the vehicle only through the seat, belt, and friction, the occupant lags the compartment. During this phase the relative lateral displacement between occupant and seat grows, and the belt begins to load. Key observables are compartment lateral acceleration, yaw rate, and the onset time of belt force.

Phase 2 — Restraint engagement and pelvis–torso coupling. The lap belt and buckle pretensioner load the pelvis, while the shoulder belt loads the inboard shoulder. In far-side loading the shoulder belt's geometry is unfavorable: it runs from the outboard upper anchor diagonally across the chest to the inboard buckle, so its lateral force component pulls the torso toward the struck side rather than restraining it. The result is a characteristic torso rotation about the occupant's vertical axis and a lateral excursion of the upper body that can exceed the pelvis excursion.

Phase 3 — Head and neck excursion. The head, coupled to the torso through the neck, continues laterally after the torso has begun to decelerate. Head lateral excursion relative to the compartment is the single most consequential kinematic quantity in far-side impact, because it determines whether the head reaches the far-side interior or the far-side occupant. Neck lateral bending and axial rotation combine, and the timing of peak head excursion relative to peak torso excursion governs the severity of any subsequent contact.

Phase 4 — Interior interaction and rebound. The occupant either contacts the far-side interior (door trim, B-pillar, console, or the far-side occupant) or is arrested by the restraint system alone. Contact produces a secondary load path and a change in the occupant's rotational direction; rebound then carries the occupant back toward the original seating position, where a second, usually milder, interaction with the belt or seat may occur.

The figure above is a schematic, not measured data. It illustrates the characteristic ordering that appears consistently in far-side sled and vehicle tests: pelvis excursion saturates first, torso excursion continues and peaks later, and head excursion peaks last and reaches the largest magnitude. The dashed line represents the lateral distance at which the head or upper torso would reach the far-side interior; where the head curve crosses it, contact occurs.

Restraint coupling in the far-side case

The three-point belt is the primary coupling element, and its behavior in far-side loading is dominated by two geometric facts. First, the outboard upper anchor is on the side away from the occupant's motion, so the shoulder belt's lateral component is small and, in the early phase, can even be directed toward the struck side. Second, the buckle is inboard, so the lap belt's inboard segment crosses the pelvis at an angle that permits the pelvis to slide laterally along the belt rather than being firmly held.

This produces the well-known belt slip and torso rotation phenomenon. In practice, engineers observe it as a rapid increase in the angle between the shoulder belt and the torso centerline, together with a lateral migration of the lap belt across the pelvis. Countermeasures that address it include:

• Buckle pretensioning and lap-belt pretensioning, which remove slack before the occupant has developed significant lateral velocity, reducing total excursion.
• Lap-belt geometry changes such as a more outboard buckle or an added lap-belt anchor point, which increase the lateral restraint component on the pelvis.
• Shoulder-belt guide or anchor repositioning, which changes the direction of the shoulder-belt force vector and can reduce torso rotation.
• Seat-integrated restraints, including side bolsters and, in some designs, a far-side airbag mounted in the seat back or console.

Coupling is not limited to the belt. The seat cushion and backrest friction, the seat pan angle, and the presence of a center console all influence how much of the occupant's lateral momentum is transferred to the vehicle before the belt loads. A high-friction seat surface delays belt engagement but increases the load transmitted through the seat; a low-friction surface does the opposite. This trade-off is one reason far-side restraint tuning is iterative rather than analytic.

Interaction with the vehicle interior

When the head or torso reaches the far-side interior, the contact location and the local stiffness of the interior determine the secondary load path. The most common contact points are the far-side door trim upper bolster, the B-pillar trim, the center console, and — in vehicles with a far-side occupant — the other occupant's body or restraint. Each has different implications:

Because the head is the last structure to peak and the most compliant part of the chain, interior contact timing relative to peak head excursion is the critical design variable. If contact occurs before peak excursion, the interior absorbs energy and reduces peak head displacement but introduces a concentrated load; if contact occurs after peak excursion, the head has already reached its maximum displacement and the interior only affects rebound. Moving the contact earlier is generally beneficial for displacement but requires careful management of head acceleration and rotation.

Practical observation and verification

In a sled or vehicle test, the kinematic phases are verified from high-speed video and from instrumentation. A typical instrumentation set includes a head center-of-gravity accelerometer triad, a chest accelerometer, a pelvis accelerometer, belt load cells at the retractor and buckle, and a seat pan load cell. The following checks are useful when reviewing a far-side test:

1. Phase timing: confirm that belt force rises before significant head lateral displacement. Late belt rise indicates slack or pretensioner timing issues.
2. Excursion ordering: verify pelvis peak precedes torso peak, which precedes head peak. A reversed or simultaneous ordering suggests an unintended contact or a restraint routing problem.
3. Belt slip: track the lap-belt position relative to the pelvis marker; large migration indicates the pelvis is sliding rather than being restrained.
4. Contact evidence: look for trim deformation, paint transfer, or head-form contact marks, and correlate their timing with the head excursion trace.

The practical takeaway is that far-side occupant kinematics is a chain of coupled events: vehicle response sets the relative velocity, the belt determines how much of that velocity is transferred to the occupant and in what direction, and the interior determines what happens when the remaining excursion is spent. Countermeasure selection must therefore be evaluated against the full chain — phase timing, excursion ordering, belt slip, and contact location — rather than against a single peak value.