2. Rollover mechanics and occupant kinematics
Rollover is one of the most demanding load cases for occupant protection, not because it produces the highest peak deceleration, but because it combines a slow, multi-axis rigid-body motion of the vehicle with a violent, localized interaction between the occupant's head and the ground or the vehicle interior. A curtain airbag (CAB) that performs well in a side impact may still fail in a rollover if its coverage, deployment timing, and pressure retention are not matched to the specific kinematics of the event. This section establishes the physical basis for those requirements before any LS-DYNA model is built.
Rollover Phases and the Resulting Occupant Motion
A typical untripped rollover proceeds through three distinguishable phases. In the trip or ramp phase, lateral tire forces generate a rolling moment about the longitudinal axis; the vehicle rotates from roughly 0° to about 90° of roll angle while still translating forward. In the free-flight or airborne phase, the vehicle separates from the ground, and the occupant, restrained mainly by the seat belt, begins to move relative to the vehicle interior because the belt cannot fully couple the torso to the decelerating body. In the ground-contact phase, the roof rail, A-pillar, or roof surface strikes the ground, producing a short-duration, high-magnitude pulse and, critically, a large relative displacement between the occupant's head and the side glazing aperture.
The key quantity for CAB design is not the vehicle's absolute motion but the relative head-to-vehicle trajectory. During the airborne phase the occupant's head can travel laterally and vertically toward the side window opening. If the head passes beyond the plane of the glazing before the CAB is inflated, the airbag can no longer intercept it. This is why deployment timing is measured in milliseconds and why the airbag must be triggered by a roll sensor (typically a gyroscope measuring roll rate, often combined with a lateral accelerometer) rather than by a simple lateral impact threshold.
Injury Mechanisms in Rollover
Three injury mechanisms dominate rollover statistics and each imposes a different requirement on the restraint system:
- Head and neck injury from ground contact. When the roof rail contacts the ground, the occupant's head may be outside the window opening and strike the ground directly. The resulting head acceleration and neck bending moment are the primary cause of severe head injury and cervical fracture. The CAB must be positioned and pressurized to act as a cushion between the head and the ground, which requires coverage that extends below the beltline and a pressure that does not vent too quickly.
- Partial and complete ejection. Ejection is the single strongest predictor of fatality in rollover crashes. It occurs when the occupant's upper body or whole body passes through the side glazing aperture. The CAB's role is to form a continuous barrier across the aperture, resisting the occupant's outward motion and, in some designs, the glazing itself.
- Head contact with the interior. Even without ejection, the head can strike the B-pillar, roof rail, or the far-side interior. The CAB must therefore cover the full length of the side aperture, not just the front or rear portion.
Curtain Airbag Design Requirements Derived from Kinematics
The kinematic analysis above translates into four concrete design requirements that will later be encoded as LS-DYNA model parameters:
| Requirement | Physical Basis | Typical Design Target (example values) |
|---|---|---|
| Coverage length | Must span the full side aperture from A-pillar to C-pillar to intercept the head at any fore-aft position | ≥ 90% of aperture length; extends 50–100 mm below beltline |
| Deployment timing | Must inflate before the head reaches the glazing plane during the airborne phase | Roll-rate threshold ≈ 1.5–3.0 rad/s; time-to-full-inflation ≈ 30–50 ms |
| Pressure retention | Must remain inflated through the ground-contact pulse, which can last 100–200 ms | Peak pressure ≈ 200–300 kPa; leak-down time constant > 150 ms |
| Anchoring and tethering | Must resist the occupant's outward load without being pushed out of the aperture | Tethers to roof rail and A/B/C-pillar; anchor strength ≥ 3–5 kN per attachment |
These values are illustrative and must be confirmed against the specific vehicle architecture, occupant size, and regulatory or consumer-test protocol (e.g., FMVSS 226 for ejection mitigation, or the rollover portion of a consumer rating program). The important point is that each target is traceable to a kinematic or injury mechanism, not chosen arbitrarily.
From Kinematics to Simulation Inputs
When this chapter's concepts are carried into LS-DYNA, the rollover event is typically modeled in one of two ways. A full-vehicle rollover simulation uses a rigid or deformable ground plane and an initial roll rate and translational velocity applied to the vehicle; the occupant is represented by a Hybrid III or THOR dummy with a validated belt model. A subsystem sled or component test isolates the CAB and a simplified occupant model, applying a prescribed relative velocity and roll angle to reproduce the aperture interaction. The first approach captures the coupling between vehicle deformation and occupant motion; the second is faster and better suited to parameter studies on airbag pressure and venting.
For either approach, the essential inputs are the same: the vehicle's roll-rate time history, the occupant's initial position and belt configuration, the CAB's geometric coverage and folding pattern, and the gas-inflation (mass-flow) curve. The essential outputs are the head's lateral excursion relative to the aperture, the head acceleration, the neck force and moment, and the airbag's internal pressure history. A simulation is considered to have reproduced the physical event when the head excursion and the airbag pressure history both fall within the expected ranges; a common failure symptom is that the head passes the aperture plane before the airbag reaches full inflation, which points to a deployment-timing or coverage problem rather than a pressure problem.
The figure above summarizes the three-phase rollover sequence and the corresponding CAB coverage requirement. The upper row shows how the vehicle's roll motion and the occupant's relative drift develop across the trip, airborne, and ground-contact phases. The lower panel shows the side aperture and the CAB coverage envelope, including the extension below the beltline and the tether anchor locations that resist outward occupant load. Together they illustrate why coverage length, deployment timing, and pressure retention must be treated as coupled requirements rather than independent specifications.