Car Impact — Occupant & Cervical-Spine Loads (Frontal Rigid Barrier)

A belted occupant “rides down” the crash as the front structure crushes, so the severity of the deceleration depends on how far the occupant is brought to rest, which combines crumple distance and restraint stretch. This tool takes a frontal impact into a rigid barrier at constant average deceleration and reports the whole-body deceleration in g, the force on the body, and the loads the neck must carry to decelerate the head as it pitches forward: axial force, shear force and flexion moment at the base of the skull. These are compared with the FMVSS 208 neck-injury criterion, combined into the neck-injury index Nij, where a value of 1 or more indicates a significant risk of injury. The inputs that drive it are the impact speed and the vehicle (its mass and effective ride-down distance). This is a simplified rigid-body engineering estimate for a belted 50th-percentile male — not a medical prediction; real outcomes depend on pulse shape, airbag timing and intrusion.

Deceleration is the average g-level the occupant sees, and Body Force is the corresponding whole-body inertial force. Neck Axial, Neck Shear and Neck Moment are the head-supporting loads, and the bars compare each with its injury threshold, with the red marker at the limit. The Nij index combines the axial and moment loads; a value below 1 is within the criterion and a value of 1 or more means the simplified estimate predicts a significant injury risk. Severity rises very steeply with speed (it grows with the square of speed), so the most effective way to recover margin is a lower impact speed; the next lever is a vehicle with a longer ride-down distance, which spreads the same energy over more travel. The Nij-versus-speed chart shows the speed at which the selected vehicle crosses the limit, which is usually more informative than a single point.

The model assumes a constant average deceleration, so it ignores the real shape of the crash pulse, its peak and its timing, and it does not model the seat belt, load limiter, airbag, seat, steering column or intrusion into the cabin. The head and neck are a single lumped mass and a simple pitch model for one 50th-percentile belted occupant, not a dummy with measured responses, and only the compression-flexion combination of the neck criterion is evaluated. Vehicle masses and ride-down distances are representative values rather than test data, and only a full-width rigid barrier is considered; offset, oblique, side and rear impacts are not covered. Use it to see how speed and vehicle stiffness drive the loading; a regulatory or design assessment needs crash tests or a detailed multibody and finite-element simulation.