3. Occupant kinematics in far-side crashes
A far-side crash is any impact in which the principal force acts on the side of the vehicle opposite the occupant. For a belted driver struck on the right side, or a belted right-front passenger struck on the left, the restraint system no longer has a door, B-pillar, or side airbag between the occupant and the impact. The occupant therefore becomes a free body that translates laterally across the cabin, and the center console, the far-side interior trim, and — in vehicles so equipped — the center airbag become the primary interaction surfaces. Understanding this motion path is the foundation for judging whether a center airbag actually earns its mass and cost.
Why the far-side load case is kinematically distinct
In a near-side impact, the struck-side door and side airbag load the occupant early and close to the impact point, so the occupant's lateral excursion is short and the restraint force is applied through the pelvis, thorax, and shoulder in a fairly predictable sequence. In a far-side impact, the occupant first moves with the vehicle's lateral acceleration before the belt and seat react, then rebounds and translates toward the struck side. The lap-and-shoulder belt geometry is asymmetric: the shoulder belt anchors on the outboard side, so during far-side motion the belt tends to slide off the inboard shoulder and the torso rotates about a vertical axis. This produces a characteristic combination of lateral translation, torso yaw, and head lag that is difficult to control with a conventional three-point belt alone.
The practical consequence is that the head is the last structure to be restrained and the first to contact the far-side interior. Head excursion toward the struck side, head rotation relative to the torso, and the timing of head contact against the center console or far-side trim define the injury risk that a center airbag is intended to reduce.
Biomechanical response sequence
During the loading phase, the pelvis is held by the lap belt and seat cushion friction while the upper torso rotates and translates inboard. The shoulder belt loads the outboard clavicle and rib cage, producing a diagonal belt path across the chest. As the torso yaws, the head — restrained only by the neck — lags behind the torso and then whips laterally. In a far-side event the head commonly travels 250–400 mm laterally relative to the seat centerline before contacting interior structure, depending on impact severity, belt geometry, and occupant size. That excursion is the design target for center airbag coverage.
Two injury mechanisms dominate. First, direct head-to-interior contact produces focal skull fracture and, at higher severity, brain injury from linear and rotational acceleration. Second, the neck experiences combined lateral bending, axial rotation, and tension as the head decelerates against the interior while the torso continues moving — a loading mode poorly represented by frontal or near-side test protocols. The center airbag's job is to intercept the head before interior contact, distribute the load over a larger area, and reduce the rotational component by cushioning the head rather than stopping it abruptly.
Dummy response and what the signals tell you
Far-side evaluation is typically performed with a Hybrid III 50th percentile male or, increasingly, the THOR-50M, seated in the far-side position with a standard three-point belt. Instrumentation of interest includes head accelerations (three-axis), upper-neck load cells (Fx, Fy, Fz, Mx, My, Mz), chest deflection, and pelvis acceleration. The dummy's response should be read as a sequence rather than a single peak:
- Early phase (0–40 ms): belt loading and torso yaw; head acceleration is low but neck lateral bending moment begins to build.
- Mid phase (40–90 ms): peak head lateral excursion and, if no center airbag is present, head contact with the console or far-side trim; head resultant acceleration and neck moment peak here.
- Late phase (90–150 ms): rebound and secondary contact; chest deflection and pelvis response dominate.
When a center airbag deploys, the expected signature is a reduction in peak head lateral excursion, a later and lower-amplitude head acceleration peak, and a reduced neck lateral bending moment — provided the bag is positioned to intercept the head path rather than the shoulder. A bag that is too low or too far forward will load the shoulder and chest, leaving the head unprotected and sometimes increasing torso rotation.
Injury criteria and how they map to center airbag performance
No single regulated criterion covers far-side loading, so evaluation combines several established metrics. The table below summarizes the criteria most often used and what each one reveals about center airbag effectiveness.
| Criterion | Signal / measurement | Typical threshold (reference) | What it indicates for a center airbag |
|---|---|---|---|
| HIC36 / HIC15 | Head resultant acceleration | HIC36 ≤ 1000; HIC15 ≤ 700 | Whether head contact is cushioned; sensitive to bag stiffness and coverage |
| Head excursion | Lateral displacement of head CG relative to seat centerline | Project-specific; often ≤ 300 mm target | Direct measure of whether the bag intercepts the head before interior contact |
| Neck lateral bending moment (My) | Upper-neck load cell | Project-specific; commonly ≤ 57 N·m reference | Reveals whether the bag loads the head or the shoulder; high My suggests head lag |
| Neck axial tension (Fz) | Upper-neck load cell | Project-specific; commonly ≤ 3.3 kN reference | Indicates head–torso separation during rebound; bag should not push the head away from the torso |
| Chest deflection | Rib deflection (IR-TRACC or chest band) | ≤ 63 mm reference (HIII) | Detects over-loading from a bag positioned too low or too stiff |
These thresholds are reference values drawn from established frontal and side protocols; they are not a substitute for the OEM's own far-side acceptance criteria, which are usually set per program and per occupant size. The important engineering point is that the criteria are read together. A center airbag that lowers HIC but raises neck My has likely shifted the load path from the head to the neck without solving the underlying kinematics.
Reading a far-side run: a worked example
Consider a 50 km/h moving deformable barrier impact at 90° to a belted 50th percentile driver, with and without a center airbag. In the baseline run, the head reaches 340 mm lateral excursion at 72 ms and contacts the console, producing a head resultant acceleration peak of 145 g and HIC36 of 980. Neck My peaks at 61 N·m at 68 ms. With the center airbag, head excursion drops to 265 mm, the head acceleration peak falls to 92 g, HIC36 falls to 610, and neck My falls to 44 N·m. Chest deflection is essentially unchanged, confirming the bag is loading the head rather than the torso.
These numbers are illustrative and must be replaced with actual sled or vehicle data for any real program. The diagnostic logic, however, is transferable: compare excursion first (did the bag intercept the head?), then acceleration and HIC (was the interception soft enough?), then neck moments (did the load path stay in the head rather than migrating to the neck?), and finally chest deflection (did the bag avoid over-loading the torso?).
Practical implications for center airbag design
Three design levers follow directly from the kinematics. First, coverage height and width must match the head excursion envelope for the occupant sizes in scope; a bag sized only for the 50th percentile will under-cover small occupants, whose heads travel farther relative to the seat. Second, inflation timing must place a fully inflated cushion in the head path before the head arrives — typically 40–70 ms after T0 in a 50 km/h event — which constrains the inflator and module packaging. Third, cushion pressure and venting determine whether the head is decelerated gradually or stopped abruptly; a bag that is too stiff will lower excursion but raise HIC and neck moment, trading one injury mode for another.
Evaluation should therefore report excursion, HIC, neck moments, and chest deflection as a set, across at least two occupant sizes and both far-side seating positions. A center airbag that improves all four metrics without degrading chest loading is a genuine performance gain; one that improves excursion alone is not yet proven.