3. Injury criteria and assessment tools for far-side occupants

Far-side occupant protection differs from near-side protection in one fundamental way: the occupant is not retained by a door or a B-pillar, so the body travels across the cabin and interacts with the console, the far-side interior, and the far-side restraint. Consequently, the injury mechanisms that dominate far-side loading are not the same as those used to tune a near-side frontal airbag. Head excursion across the centerline, lateral neck bending, contralateral chest deflection, and abdominal loading through the console are the governing concerns. This chapter defines the metrics used to quantify those concerns, explains how each is computed from dummy or human-body-model output, and describes how the metrics are applied in a development loop.

Head Metrics: HIC, HIC36, and the Rotational Extensions

The Head Injury Criterion remains the baseline head metric in essentially every regulated and consumer far-side assessment. It is computed from the resultant translational acceleration at the head center of gravity:

HIC = max over [t1, t2] of  ( t2 - t1 ) * [ (1/(t2-t1)) * ∫ a_res(t) dt ]^2.5

where a_res(t) is expressed in g, and (t2 - t1) is limited to 36 ms (HIC36)
or 15 ms (HIC15) depending on the assessment protocol.

In practice the integral is evaluated on the filtered acceleration signal — typically SAE J211 Channel Frequency Class 1000 for the head — and the window search is performed over the whole event. A HIC36 of 1000 is the classical threshold; many consumer protocols and internal targets use HIC15 ≤ 700 as a more demanding development target because it penalizes short, sharp pulses that a 36 ms window can dilute.

HIC alone is insufficient for far-side because the dominant head path is lateral and often oblique, producing substantial rotation. Two supplementary metrics are therefore used. Brain Injury Criterion (BrIC) is based on the maximum angular velocity about each anatomical axis, normalized by critical values (approximately 66.3, 56.5, and 42.4 rad/s for the x, y, and z axes in the standard formulation), with BrIC = (ωx/ωxc)² + (ωy/ωyc)² + (ωz/ωzc)². A BrIC of 1.0 corresponds to a 50% risk of a diffuse axonal injury–type outcome. Head Injury Power (HIP) integrates both linear and angular power terms and is useful when the head strikes a compliant or edge-like surface such as the console or far-side trim.

For far-side development, the practical reading is: track HIC15 for focal contact, BrIC for the rotational component induced by the lateral neck moment, and the head excursion envelope (maximum lateral Y-displacement of the head CG relative to the seat centerline) as a kinematic gate. A design that satisfies HIC but exceeds the excursion envelope will usually fail the far-side interior interaction later in the event.

Neck Metrics: Nij, Nkm, and Lateral Bending Moment

The neck in far-side loading is loaded primarily in lateral bending and axial compression, with a superimposed axial rotation as the head rolls. The standard Nij criterion combines axial force and sagittal bending moment:

Nij = Fz / Fzc  +  My / Myc

with tension/compression and flexion/extension critical values
(e.g. Fzc = 6.8 kN tension, 6.16 kN compression; Myc = 310 N·m
flexion, 135 N·m extension for a mid-size male dummy).

Nij is a sagittal-plane criterion and therefore does not capture the far-side mechanism well on its own. The Nkm criterion extends the same normalization concept to the lateral plane by combining the lateral shear force and lateral bending moment, and it is the more relevant neck metric for far-side. In parallel, the peak lateral bending moment at the occipital condyle is monitored directly, with internal development targets commonly set in the 40–60 N·m range for the upper neck, depending on occupant size and the assessment protocol.

Two practical cautions apply. First, neck signals are highly sensitive to head–neck joint stiffness in the dummy or HBM, so metrics must be compared only within the same model version. Second, because the far-side head often contacts the far-side occupant or interior, the neck moment peak may occur after the head contact rather than during the initial inertial phase; the time history, not just the peak, should be reviewed.

Chest Metrics: Deflection, VC, and the Contralateral Problem

Chest injury in far-side loading arises from two distinct sources: belt loading through the inboard shoulder belt and direct contact with the console or far-side occupant. The primary metric is chest deflection, measured as the maximum compression of the rib cage, usually reported as an absolute value (mm) and as a percentage of the half-chest depth. For a mid-size male dummy, a deflection of 63 mm is the classical 50% AIS ≥ 3 threshold; development targets are typically set well below that, in the 35–45 mm range, to preserve margin.

The Viscous Criterion captures rate-dependent soft-tissue injury and is computed as the product of chest compression velocity and instantaneous compression:

VC = max over t of [ dC(t)/dt * C(t) ]

where C(t) = deflection(t) / half-chest-depth, and dC/dt is
evaluated on the CFC 180 filtered deflection signal.
A VC of 1.0 m/s corresponds approximately to a 25% risk of
AIS >= 3 soft-organ injury.

Far-side adds a complication that near-side does not: the deflection is frequently contralateral to the belt, produced by the far-side interior rather than by the restraint. A dummy chest deflection of 40 mm driven by a console contact has a different injury interpretation than the same 40 mm driven by belt compression, because the loaded area and the underlying organ are different. For this reason, far-side chest assessment should report deflection together with the contact location and the loading path, and HBMs are increasingly used to resolve regional rib strain rather than a single global deflection number.

Abdomen Metrics: Compression, Penetration, and Belt Geometry

Abdominal injury in far-side is dominated by belt loading — particularly the inboard lap and shoulder belt as the occupant slides laterally — and by console contact. The principal metrics are abdominal compression (absolute and normalized to the abdominal depth) and penetration of the loading surface into the abdomen. The classical threshold for AIS ≥ 3 abdominal injury is a compression of approximately 39% of the abdominal depth, or an absolute compression of about 88 mm for a mid-size male; development targets are usually set at 25–30% to retain margin.

Because the abdomen is highly rate-sensitive and the far-side belt path is oblique, the belt–abdomen interaction geometry matters as much as the peak value. Two practical checks are used: the lap-belt angle relative to the iliac crest (a belt that rides above the crest concentrates load in the abdomen), and the time at which abdominal compression peaks relative to chest compression — a late abdominal peak often indicates the occupant has submarined or slid under the belt, which is a common far-side failure mode.

How the Metrics Fit Together in a Development Loop

The four metric families are not independent. A design change that reduces head excursion — for example, a more aggressive far-side airbag — may increase neck lateral moment and chest deflection by coupling the head to the torso earlier. The assessment therefore needs to be read as a vector, not as four separate pass/fail gates. The figure below shows the loading path and the metric that observes each segment of it.

The diagram makes the coupling explicit: the head excursion arrow passes through the neck, so any restraint that arrests the head also loads the neck; the chest and abdomen are loaded by the same belt path and the same console contact, so a belt routing change that helps the chest can hurt the abdomen. In a development program this is handled by evaluating the metric vector at each design iteration and rejecting changes that trade one region's margin for another's failure.

Practical Assessment Workflow and a Worked Example

A reproducible far-side assessment follows a fixed sequence. First, define the coordinate convention: the vehicle x-axis forward, y-axis to the occupant's left, z-axis up, with the head excursion measured as the lateral displacement of the head CG relative to the seat centerline in the y-direction. Second, filter each signal per SAE J211 with the channel-appropriate CFC (1000 for head acceleration, 600 for neck force and moment, 180 for chest deflection and abdominal compression). Third, compute each metric over the full event window, not a truncated one, and record both the peak value and the time of peak.

As an illustrative example (values are representative of a mid-size male dummy in a 50 km/h far-side sled, not measured data): head resultant acceleration peaks at 78 g with a 12 ms pulse, giving HIC15 ≈ 520; peak angular velocity about the y-axis is 41 rad/s, giving BrIC ≈ 0.62; upper-neck lateral bending moment peaks at 48 N·m at 96 ms; chest deflection peaks at 38 mm at 88 ms with VC = 0.71 m/s; abdominal compression peaks at 27% of depth at 104 ms. The late abdominal peak relative to the chest peak is the diagnostic signal: it suggests the occupant is sliding under the lap belt, and the corrective action is a lap-belt anchor or geometry change rather than more airbag output.

The metrics described here form the quantitative backbone of far-side development. They are necessary but not sufficient: the pass/fail decision must also account for the loading path, the contact location, and the timing relationships that the numbers alone do not reveal. The next stage of the program applies these metrics inside a structured design-of-experiments loop, which is where the coupling between head excursion, neck moment, and chest deflection becomes the central optimization problem.