3. Occupant kinematics in side impact

Side impact is fundamentally different from frontal impact, and that difference drives every design decision in side airbag and curtain airbag calibration. In a frontal crash, the vehicle has a long crumple zone ahead of the occupant, and the restraint system has roughly 80–120 ms to act before the occupant loads the belt and airbag. In a side impact, the striking vehicle or pole is often only 300–500 mm from the occupant's torso, and the struck vehicle's door structure deforms inward within 20–40 ms of contact. The occupant is loaded laterally through the door trim, armrest, and B-pillar almost immediately. The airbag must therefore be fully inflated and in position before the occupant's thorax reaches the door interior — a timing problem measured in single-digit milliseconds.

Occupant Kinematics in a Lateral Collision

Consider a nearside occupant (the occupant on the struck side) in a 50 km/h MDB (moving deformable barrier) side impact, the configuration used in UN R95 and FMVSS 214. The barrier contacts the door at t = 0. The door outer panel begins to crush inward, and the door inner trim starts moving toward the occupant at roughly 6–10 m/s. The occupant, restrained only by the seat and the door, initially stays nearly stationary in inertial space — the vehicle moves laterally under the occupant. Relative motion between the occupant's pelvis and thorax develops because the pelvis is coupled to the seat cushion and the thorax is free to rotate and translate. This produces a characteristic lateral bending and rotation of the torso, with the shoulder lagging the pelvis by 10–20 ms.

The critical geometric event is the occupant-to-door gap closure. In a typical sedan, the free gap between the occupant's thorax and the door trim is 150–250 mm. At a door intrusion velocity of 8 m/s, that gap closes in about 20–30 ms. If the side airbag has not deployed into that gap by then, the thorax loads directly against rigid door structure, and rib deflection rises sharply. This is why side airbag timing is dominated by time-to-gap-closure rather than by occupant ride-down, which is the governing logic in frontal restraint design.

Injury Criteria Relevant to Side Impact

Side impact injury assessment uses a different criteria set from frontal impact. The most relevant are:

• Rib deflection (D_max) — measured in millimeters by rib deflection gauges on the ES-2re or SID-IIs dummy. FMVSS 214 sets an upper rib deflection limit of 44 mm for the ES-2re; Euro NCAP uses a similar threshold. Rib deflection is the primary thoracic injury predictor in side impact because lateral loading produces rib fractures and flail chest at lower energy than frontal loading.
• Viscous Criterion (VC) — the product of chest compression velocity and compression, expressed in m/s. VC captures soft-tissue injury risk that pure deflection misses, particularly for the liver and spleen. A common limit is 1.0 m/s for the ES-2re thorax.
• HIC (Head Injury Criterion) — still used, but in side impact the head is usually loaded by the curtain airbag, the B-pillar, or the striking vehicle's hood. HIC₃₆ limits of 1000 (FMVSS 208 style) are applied, but the more relevant side-specific metric is often head excursion and head-to-interior contact, because the curtain airbag's job is to prevent the head from striking the B-pillar or glazing.
• Pelvis force and acetabular load — measured at the iliac crest; side airbags with pelvis coverage reduce pelvic fracture risk, which is a common AIS 2–3 injury in near-side crashes.
• Neck lateral bending moment — relevant when the head is arrested by the curtain while the torso continues laterally, producing a lateral neck moment.

Timing Windows for Side Airbag Intervention

The side airbag timing window is bounded on both sides. Too late means the occupant has already loaded the door — the airbag fires into a closed gap and provides little benefit, or worse, the deploying bag pushes the occupant's arm or thorax against the door. Too early means the bag is at peak pressure while the occupant is still moving toward it, which can increase loading rather than absorb it, and in out-of-position (OOP) scenarios can cause injury from the deployment itself.

The practical timing chain for a side airbag is:

1. Crash sensing — side pressure sensors in the door or B-pillar detect the pressure wave from door deformation. Detection time is typically 5–12 ms after t₀, depending on sensor placement and threshold.
2. Algorithm decision — the SDM (sensing and diagnostic module) confirms a deployment event. This adds 2–5 ms.
3. Squib firing and inflator initiation — 1–2 ms electrical delay plus inflator rise time.
4. Bag inflation — a typical side thorax airbag reaches 80–90% of full inflation in 12–20 ms from squib fire; a curtain airbag takes 20–30 ms because of its larger volume.

Summing these, a side thorax airbag typically reaches working pressure at 20–35 ms after t₀. The gap-closure estimate above (20–30 ms) shows why the margin is thin: the bag must be in position at or before the moment the thorax arrives. Calibration therefore targets the bag's peak pressure timing to coincide with the occupant's maximum lateral velocity, not with t₀.

Calibration Implications

Two calibration levers dominate side airbag timing. The first is sensor threshold and algorithm aggressiveness: lowering the pressure-sensor threshold fires earlier but increases the risk of deployment in non-crash events (door slam, low-speed parking contact). The second is inflator output and bag volume: a higher-output inflator reaches working pressure faster but raises OOP injury risk and cost. In practice, calibration engineers sweep the sensor threshold and inflator output against a matrix of crash pulses (MDB 50 km/h, pole 32 km/h, oblique 90° pole) and verify that the bag's peak pressure lands within the gap-closure window for each pulse.

A useful verification step is to instrument the door inner panel with a displacement transducer and the dummy thorax with a deflection gauge, then plot both against time on the same axis. If the thorax deflection begins to rise before the bag reaches 50% pressure, the calibration is late and the sensor threshold or inflator should be adjusted. If the bag reaches full pressure more than 10 ms before the thorax arrives, the calibration is early and OOP risk should be re-checked with the OOP test matrix (5th percentile female, 50th percentile male, 95th percentile male, and child dummy positions).

Summary of the Timing Logic

The side impact problem reduces to a race between two clocks: the occupant-to-door gap closing in 20–30 ms, and the airbag reaching working pressure in 20–35 ms. The calibration target is to make the bag's peak pressure coincide with the occupant's arrival at the door, with enough margin to cover pulse-to-pulse variation but not so much margin that OOP risk grows. Injury criteria — rib deflection, VC, HIC, and pelvis load — are the scorecard that tells you whether that timing was achieved, and they must be read together with the timing traces rather than in isolation.