2. Side impact crash kinematics and injury mechanisms

In a side impact, the struck vehicle has almost no crush space between the door outer panel and the occupant. Unlike frontal crashes, where the engine bay and longitudinal rails provide a long deceleration distance, a side impact delivers energy through a short, stiff load path — typically 150–300 mm of door structure, trim, and occupant clearance. Understanding how the occupant moves, how the barrier and bullet vehicle intrude, and which biomechanical criteria the regulations measure is the foundation for interpreting both FMVSS 214 and UN R135 test results.

Coordinate Conventions Used Throughout This Course

All kinematic descriptions in this course use the vehicle-fixed SAE J211 convention: +X forward (longitudinal), +Y to the occupant's right (lateral), +Z upward (vertical). Accelerations are reported in g and filtered per SAE J211 Channel Frequency Class 60 (CFC 60) for occupant responses and CFC 180 for barrier/vehicle structure unless a regulation specifies otherwise. When a figure or table refers to "inboard" motion, it means motion toward the vehicle centerline; "outboard" means toward the struck side. Keeping this convention consistent is essential because FMVSS 214 and UN R135 report intrusion and occupant excursion in the same axes but with different sign conventions in some published data — always confirm the reference frame before comparing numbers.

Occupant Motion in a Side Impact

When a moving deformable barrier (MDB) strikes the driver side at 90°, the struck door and B-pillar are accelerated inboard within roughly 10–25 ms. The occupant's pelvis and thorax, restrained only by the seat and any side airbag, initially remain near their pre-impact position due to inertia. The result is a relative velocity between the intruding door trim and the occupant's near-side body regions. Three phases are useful to distinguish:

• Contact phase (0–30 ms): Door outer panel deforms, the barrier face crushes, and the door inner panel begins moving inboard. The occupant has not yet been loaded.
• Loading phase (20–60 ms): Door trim, armrest, and any side airbag contact the pelvis, abdomen, and thorax. The occupant is accelerated laterally; the head may rotate and translate toward the struck side or rebound toward the far side depending on restraint geometry.
• Rebound / far-side phase (60–120 ms): The struck-side structures decelerate; the occupant may be thrown back across the cabin, and head-to-far-side-interior contact becomes a secondary injury path.

The critical insight is that injury severity is driven less by peak vehicle acceleration than by the relative displacement and relative velocity between the intruding structure and the occupant's body regions. A door that intrudes 250 mm slowly can be more injurious than one that intrudes 120 mm rapidly, because the occupant has less time to be accelerated away from the intruding surface.

Intrusion and Its Measurement

Intrusion is the inboard displacement of the struck-side interior structure relative to its pre-test position. Both regulations quantify it, but with different emphasis. FMVSS 214 (the dynamic option, S8) requires that the struck-side door not separate from the vehicle, that the door remain closed, and that specific interior components not intrude beyond defined limits into the occupant survival space. UN R135 is more explicit about occupant compartment integrity: it evaluates whether the door opens, whether the door latch releases, and whether the fuel system remains intact, in addition to measuring occupant responses.

In practice, intrusion is measured with string potentiometers or optical displacement sensors mounted between a fixed vehicle reference and the door inner panel at defined heights (typically at H-point height, mid-door, and upper door). A representative test might record:

Sensor: door_inner_pelvis_height (string pot, CFC 60)
t = 0 ms    → 0 mm
t = 25 ms   → 45 mm inboard
t = 45 ms   → 180 mm inboard  (peak)
t = 80 ms   → 165 mm inboard  (residual)
Peak intrusion rate ≈ 6.0 m/s (derivative of displacement)

The peak intrusion rate is often more predictive of injury than peak displacement, because it governs the relative velocity between the door and the occupant. A useful rule of thumb in engineering reviews: if peak intrusion rate exceeds roughly 5–6 m/s at pelvis height, the occupant is likely to be loaded before the side airbag can fully deploy, and thoracic criteria become the limiting factor.

Biomechanical Criteria in FMVSS 214 and UN R135

The two regulations use overlapping but not identical injury criteria. The table below summarizes the criteria most relevant to side impact assessment; values are the regulatory limits, not design targets — engineers typically aim for 15–25% margin below the limit to account for test-to-test variation.

Note that FMVSS 214 uses the ES-2re dummy for the driver and SID-IIs for the rear passenger in the optional dynamic test, while UN R135 specifies the WorldSID 50th percentile male. The dummy choice changes the measured deflection because the WorldSID has a different rib geometry and instrumentation than the ES-2re. This is why a vehicle that passes FMVSS 214 with comfortable margin may still require structural or restraint tuning to pass UN R135 — the criteria are similar but the measurement hardware is not interchangeable.

From Kinematics to Design Decisions

The practical takeaway is that side impact performance is governed by the time history of relative motion between intruding structure and occupant, not by any single peak value. Three design levers follow directly from the kinematics:

1. Reduce intrusion rate by adding door beam stiffness, B-pillar reinforcement, or a rocker-to-rocker load path that transfers barrier energy into the floor and opposite side.
2. Accelerate the occupant earlier with a side airbag and seat-mounted thorax bag that deploys before the door trim reaches the occupant, reducing relative velocity at contact.
3. Manage far-side motion with seat belt pretensioning and, where fitted, far-side airbags, to limit head excursion across the cabin during rebound.

With these kinematic and biomechanical foundations established, subsequent chapters can examine how the barrier, dummy, and instrumentation are configured to capture these quantities reproducibly, and how test results are judged against the regulatory pass/fail thresholds.