4. Case 04: Shoulder Belt Force–Time Trace Interpretation from a Sled Test

Why the Force–Time Trace Is a Kinematic Record

A shoulder-belt load cell in a frontal sled test does not measure "belt quality." It measures the reaction force the belt applies to the occupant through the D-ring and retractor path, and that force is the product of restraint geometry, occupant motion, and belt-payload interaction. Two belts with identical hardware can produce very different traces because the occupant's chest moves differently. The diagnostic skill is therefore to read the trace as a sequence of kinematic events: slack take-up, engagement, ride-down, peak loading, and unloading. Hardware (webbing stiffness, retractor lock, pretensioner, load limiter) shapes the trace, but it never explains it alone.

Anatomy of a Frontal Sled Shoulder-Belt Trace

A typical 50 km/h frontal sled pulse with a 3-point belt and a 50th-percentile Hybrid III produces a shoulder-belt force trace with four recognizable regions. The boundaries are not fixed times; they are defined by the occupant's motion relative to the belt.

• Phase A — Slack take-up (0 to ~15 ms): near-zero force. The occupant has not yet moved enough to remove the initial webbing slack and retractor spool free-play. Any force here is noise or pretensioner firing, not restraint loading.
• Phase B — Engagement and ramp (≈15 to ≈55 ms): force rises as the chest advances into the belt. The slope reflects effective belt–thorax stiffness and the rate at which the chest is decelerated. A steep ramp with an early knee often indicates the belt is loading a stiff structure (clavicle/sternum) before the rib cage distributes load.
• Phase C — Ride-down and peak (≈55 to ≈90 ms): the force plateaus near its maximum while the chest decelerates with the vehicle. Peak force timing relative to the sled pulse peak is the key kinematic marker: a peak that arrives well after the vehicle deceleration peak means the occupant is still moving forward into the belt.
• Phase D — Unloading (≈90 ms onward): force falls as the occupant rebounds and the retractor takes up webbing. A slow, rounded decay suggests controlled energy return; a sharp drop or a secondary spike suggests belt slip, retractor lock release, or a load-limiter payout event.

Worked Interpretation: A Representative Trace

The following table is a teaching example built from a plausible 50 km/h frontal sled run (Hybrid III 50th, 3-point belt, pretensioner fired at 12 ms, no load limiter). Values are illustrative, not measured data, and are used to demonstrate the interpretation logic.

Three features carry the diagnostic weight. First, the pretensioner step at 12 ms is a hardware event, not a kinematic one — it should be identified and excluded from the loading-phase analysis. Second, the peak at 68 ms lags the vehicle pulse peak by roughly 23 ms, which is consistent with the chest continuing to advance into the belt after the vehicle has begun to decelerate. Third, the unloading from 85 to 140 ms is gradual, with no secondary spike, indicating the retractor locked and released cleanly and no load-limiter payout occurred.

Relating Trace Features to Occupant Kinematics

The trace is only diagnostic when each feature is mapped to a motion. The SVG below shows this mapping: the sled pulse, the shoulder-belt force trace, and the occupant chest displacement are aligned on a common time axis so that phase boundaries and peak timing can be read across all three.

The alignment exposes the central diagnostic relationship: the belt force peaks at ≈68 ms, but the chest reaches maximum forward excursion at ≈120 ms. The belt is already unloading while the chest is still moving forward — the occupant is being decelerated by the belt and then by the retractor take-up, not by a single rigid restraint. If the belt peak had coincided with the chest excursion peak, the interpretation would shift toward a stiffer, more direct load path and a higher risk of concentrated thoracic loading.

Separating Hardware Effects from Kinematic Effects

Before attributing any trace feature to occupant motion, rule out the hardware explanations. The table below lists the common confounders and the trace signature each produces.

Practical Reading Procedure

1. Overlay the shoulder-belt force trace on the sled pulse and the chest deflection channel on a common time axis. Do not read the force trace in isolation.
2. Mark the pretensioner step and exclude it from phase analysis.
3. Identify the four phase boundaries from the chest displacement curve, not from fixed times: slack ends when displacement begins, engagement ends when the force ramp flattens, ride-down ends when force begins to fall, unloading ends when force returns to residual.
4. Record peak force, peak timing, and the lag between vehicle pulse peak and belt force peak. A lag greater than ~15 ms indicates the occupant is still advancing into the belt at peak load.
5. Characterize the unloading slope and check for secondary spikes. A spike that coincides with chest displacement reversal is kinematic; one that does not is hardware.
6. Cross-check every feature against the hardware/kinematic table before writing the diagnosis.

Deliverable: Trace Interpretation Record

The following record captures the minimum information needed to defend a trace interpretation in a diagnostic report. It is filled with the teaching example above.

The record is deliberately short: it forces the analyst to commit to phase boundaries, peak timing, and an attribution for each feature. A trace interpretation that cannot fill these fields is not yet a diagnosis — it is a description of a curve.