2. Occupant kinematics in frontal, side, and oblique impacts

Occupant kinematics describes how the human body moves relative to the vehicle interior during a crash pulse. Because restraint systems can only manage energy that is transferred through them, the timing and direction of occupant motion determine which structures load the body, and therefore which injuries are likely. This chapter establishes the kinematic patterns for three impact directions, links each pattern to specific injury mechanisms, and explains where a center airbag (far-side airbag) changes the outcome.

Coordinate Convention Used Throughout

All directions in this chapter follow the ISO 8855 vehicle axis system: +X forward (direction of travel), +Y to the driver's left, +Z upward. Yaw is positive counter-clockwise about +Z. Occupant motion is described relative to the vehicle body, not the ground. This convention is kept consistent in the figure below.

Frontal Impact: Longitudinal Loading and the "In-Line" Occupant

In a full-frontal or offset frontal impact, the vehicle decelerates rapidly while the unbelted occupant continues forward at the pre-crash velocity. The restraint system must arrest this relative motion within roughly 60–120 ms. The dominant kinematic chain is:

1. 0–30 ms: pelvis loads the seat belt (lap portion); the torso begins to rotate forward about the hip.
2. 30–70 ms: chest loads the shoulder belt; the head pitches forward and downward, and the neck experiences combined compression and flexion.
3. 70–120 ms: the head contacts the airbag, which has already deployed; the airbag vents gas to absorb residual energy.

Injury mechanisms in this direction are well characterized: femur and pelvis fractures from lap-belt loading, rib fractures and thoracic organ injury from shoulder-belt and airbag loading, and diffuse axonal injury or skull fracture from head contact with the steering wheel or A-pillar when the airbag is out of position. The key design variable is belt geometry: a lap belt anchored too high rides over the iliac crest and loads the abdomen, while a shoulder belt anchored too low allows excessive torso rotation.

Side Impact: Lateral Loading and the "Out-of-Position" Occupant

Side impacts are the most challenging for restraint design because the distance between the occupant and the intruding door is only 100–250 mm. There is no crush space to manage energy. The kinematic pattern is fundamentally different from frontal:

• The struck-side occupant's pelvis and thorax are loaded laterally by the door trim and armrest.
• The head moves toward the struck side and may contact the B-pillar or window glass.
• The far-side occupant (the person on the opposite side of the impact) is thrown toward the struck side by the vehicle's lateral acceleration, then rebounds.

Injury mechanisms include pelvic ring fractures, lateral rib fractures with pulmonary contusion, liver or spleen laceration, and traumatic brain injury from head contact with the struck-side interior. The far-side occupant is at particular risk because the struck-side occupant and the center console become secondary contact surfaces.

Oblique Impact: The Combined Loading Problem

Oblique impacts (typically 15°–45° from the longitudinal axis) produce a coupled kinematic response: the occupant moves forward and laterally. This is the most difficult case for restraint timing because:

• The shoulder belt may slip off the shoulder (belt migration) as the torso rotates.
• The head can move diagonally, missing the frontal airbag and contacting the A-pillar or side window.
• The far-side occupant can be ejected toward the center or the opposite door.

Injury mechanisms combine frontal and lateral patterns: asymmetric rib loading, neck lateral bending, and head rotation that produces shear strain in the brain. The oblique case is where center airbags provide the greatest incremental benefit.

Where the Center Airbag Fits

A center airbag is mounted between the front seats, typically in the inboard side of the driver or passenger seat back, or in the center console. Its purpose is to manage far-side occupant motion — the occupant on the side opposite the impact. In a side or oblique impact, the far-side occupant's torso and head accelerate toward the struck side. Without a center airbag, the far-side occupant can:

• Collide with the struck-side occupant (occupant-to-occupant loading).
• Strike the center console or instrument panel.
• Be partially ejected through the opposite window in a rollover.

The center airbag deploys in 20–40 ms and provides a compliant surface that decelerates the far-side occupant's torso and head before they reach the struck-side structures. It also reduces the lateral excursion of the head, lowering the risk of head-to-head contact between occupants.

Kinematic Comparison Across Impact Directions

Practical Example: Far-Side Occupant in a 30° Oblique Impact

Consider a vehicle traveling at 50 km/h (13.9 m/s) that strikes a rigid pole at 30° from the longitudinal axis. The far-side occupant (front passenger, unbelted for this illustrative case) has a mass of 75 kg. The vehicle's lateral deceleration pulse peaks at 25 g at 40 ms. Without a center airbag, the occupant's head travels approximately 180 mm laterally before contacting the center console. With a center airbag deploying at 30 ms, the head contacts the airbag at 55 ms, and the lateral excursion is reduced to approximately 90 mm. The head injury criterion (HIC) is reduced from an estimated 850 to 420 in this simplified example. These values are illustrative and would require validation through sled testing or CAE simulation.

Common Failure Symptoms and Troubleshooting

• Airbag deploys but occupant still contacts console: Deployment timing is too late, or the airbag volume is insufficient. Check the lateral acceleration trigger threshold and the inflator output.
• Airbag deploys too early and pushes occupant: Trigger threshold is too low. Increase the lateral g threshold or add a delay based on occupant position sensing.
• Occupant slides under the airbag: The airbag is mounted too high or the seat belt is not pretensioned. Verify the airbag's vertical position relative to the occupant's H-point.
• Head rotates excessively: The airbag surface is too stiff or the occupant's head is not centered. Consider a softer airbag fabric or a larger coverage area.

Summary of Design Implications

Frontal impacts are managed primarily by seat belts and frontal airbags; the center airbag plays a secondary role in offset frontal cases where the far-side occupant moves toward the center. Side impacts demand rapid lateral restraint, and the center airbag is the primary countermeasure for the far-side occupant. Oblique impacts combine both loading directions and represent the case where the center airbag provides the greatest incremental safety benefit. In all three directions, the design objective is the same: reduce the occupant's relative displacement and decelerate the head and torso before they contact stiff interior structures.