3. Occupant kinematics in rollover

Rollover crashes account for a disproportionately large share of serious and fatal injuries relative to their frequency in the crash population. The reason lies less in the raw energy of the event than in its unusual kinematics: the occupant is subjected to a low-severity but long-duration multi-axis motion, the restraint system is loaded in directions it was not primarily designed for, and the occupant's head and torso frequently move toward the very structure that is deforming. Understanding how the body moves inside the cabin — and how the belt, airbag, and roof interact with that motion — is the foundation for every subsequent design decision in this course.

Phases of Occupant Motion During a Rollover

A rollover is not a single impact but a sequence of overlapping phases. For a typical tripped rollover initiated by a lateral slide into a curb or soft soil, the sequence is: trip and rotation onset, roof-to-ground contact, roof sliding or scrubbing, and rebound and rest. Each phase produces a distinct occupant loading pattern.

During rotation onset, the vehicle develops roll rate and lateral acceleration. The occupant, still belted, is pulled laterally against the belt and begins to move relative to the seat. In the roof-contact phase, vertical acceleration dominates and the occupant's torso is driven downward into the seat cushion while the head continues upward and outward. During scrubbing, the vehicle slides on its roof or side, generating sustained longitudinal and lateral deceleration with the occupant partially inverted. Finally, rebound produces a secondary, lower-magnitude motion that can still cause injury if the occupant has already lost contact with the seat.

The critical insight is that occupant excursion is driven by relative motion between the occupant and the vehicle interior, not by absolute acceleration alone. A restraint system that keeps the occupant coupled to the seat through all phases prevents the head from reaching the roof or the side glass. A system that allows even 100–150 mm of uncontrolled lateral excursion can place the head directly in the roof crush zone.

Injury Mechanisms

Rollover injuries cluster into four mechanisms:

• Head and neck injury from roof contact. When the head strikes the roof rail or roof panel during roof crush, the resulting loading is a combination of compression, lateral bending, and shear. Cervical spine injuries — particularly at C4–C7 — occur when the head is constrained while the torso continues to move, producing a bending moment across the neck.
• Thoracic and abdominal injury from belt loading. The three-point belt concentrates load on the clavicle, sternum, and lower ribs. In rollover, the belt may load the abdomen if the lap portion rides up over the iliac crest, a condition known as "submarining" or "lap belt migration."
• Ejection-related injury. Partial or complete ejection exposes the occupant to direct ground contact, run-over by the vehicle, and impact with external objects. Ejection is the single strongest predictor of fatal outcome in rollover.
• Injury from contact with interior surfaces. Even without ejection, the head, shoulder, and arm can strike the B-pillar, side glass, or seat structure during the lateral excursion phase.

Ejection Pathways

Ejection is not a random event; it follows predictable pathways determined by belt geometry, seat design, glazing, and door retention.

Belt use is the dominant factor. An unbelted occupant is roughly three to four times more likely to be ejected than a belted one in a comparable rollover. However, belt use alone is not sufficient: a belt that is worn with excessive slack, routed under the arm, or positioned too high on the abdomen can still permit partial ejection. This is why modern restraint design focuses on belt fit and geometry as much as on belt presence.

Restraint System Interactions

The restraint system in a rollover must perform three functions simultaneously: keep the occupant coupled to the seat, manage the occupant's energy over a long duration, and prevent the head from reaching the roof or side structure. These functions are in tension with one another.

Seat belt pretensioners and load limiters. A pretensioner removes slack within the first 20–40 ms, pulling the occupant into the seat before the major roll motion develops. A load limiter then allows controlled belt payout — typically 4–6 kN of shoulder belt force — to reduce chest deflection. In rollover, the load limiter must be tuned so that it does not allow so much payout that the occupant's head reaches the roof.

Side curtain airbags. Curtains deploy downward from the roof rail to cover the side glass and often the A- and B-pillars. Their primary rollover function is to prevent head and limb ejection and to provide a cushion between the head and the roof rail. Deployment timing is critical: a curtain that deploys too late (after the head has already passed the plane of the window) provides no benefit.

Seat structure and belt anchorage. The seat back and its recliner mechanism must resist the occupant's inertial load without collapsing rearward. A seat back that yields allows the occupant to move into the rear compartment or to slide under the belt. Belt anchorages on the B-pillar and floor must remain intact under the combined vertical and lateral loads of a rollover.

Visualizing the Kinematic Chain

The diagram below traces the occupant's motion through the four rollover phases and shows how each restraint component acts on that motion. The key relationships to observe are the timing of pretensioner and curtain deployment relative to the occupant's lateral and vertical excursion, and the points at which the belt and seat structure either contain or fail to contain the occupant.

Practical Implications for Restraint Tuning

When tuning a restraint system for rollover performance, engineers typically work with a set of measurable targets rather than subjective judgments. The following example illustrates a representative tuning workflow for a mid-size SUV, using a validated CAE model and a physical sled or rollover test as verification. The values shown are illustrative targets, not measured results from a specific program.

Rollover restraint tuning — example target set (mid-size SUV)

Inputs:
  Vehicle: 2,100 kg curb, 1,680 mm track, 1,720 mm roof height
  Test condition: FMVSS 216a roof crush + SAE J211 rollover sled
  Occupant: Hybrid III 50th percentile male, belt geometry per FMVSS 208

Key parameters to tune:
  pretensioner_fire_time     : 15–25 ms after roll sensor trigger
  load_limiter_force         : 4.0–5.5 kN shoulder belt
  curtain_deploy_time        : < 30 ms after roll sensor trigger
  curtain_coverage           : A-pillar to C-pillar, 1.5 m length
  seat_back_strength         : > 3.5 kN moment at recliner
  lap_belt_angle             : 50–60° from horizontal (iliac crest)

Observable verification criteria:
  - Head excursion < 120 mm lateral from seat centerline
  - No head contact with roof rail or B-pillar
  - Belt force stays within load limiter band
  - No belt slip over iliac crest (lap belt migration < 25 mm)

Typical failure symptoms and troubleshooting:
  - Head contacts roof rail → reduce load limiter payout or advance curtain timing
  - Lap belt migrates over crest → increase lap belt angle or add pretensioner
  - Curtain misses head → check deployment path obstruction by seat or trim
  - Seat back yields → reinforce recliner or add seat-integrated belt

The tuning process is iterative: a change to load limiter force affects head excursion, which in turn affects curtain coverage requirements. The engineer's task is to find a combination that satisfies all criteria simultaneously, not to optimize any single parameter in isolation.

Occupant kinematics in rollover is ultimately a problem of containment under multi-axis, long-duration loading. The injury mechanisms — head and neck loading from roof contact, thoracic and abdominal loading from belt geometry, and ejection through window, door, or roof openings — are all consequences of the occupant moving relative to the vehicle structure. The restraint system's role is to manage that relative motion within a safe envelope. Every subsequent chapter in this course builds on this kinematic foundation: roof strength determines how much the structure deforms, restraint design determines how the occupant interacts with that deformation, and the combination determines the injury outcome.