2. Case 02: Shoulder Belt Path and Contact Patch Mapping on a Seated Occupant
Why the Belt Path Must Be Mapped Before Any Loading Diagnosis
Shoulder-belt loading on the thorax is not a single number — it is the product of a specific routing geometry and the contact patch that geometry produces on a specific occupant. Two vehicles with identical belt hardware can load the chest very differently if the belt leaves the D-ring at a different height, crosses the clavicle at a different lateral offset, or lands on the sternum instead of the parasternal region. Before any force–time trace or injury criterion can be interpreted, the geometric inputs that produced the loading must be documented in a form that later cases can reuse.
This case builds that geometric record for an instrumented seated occupant. The deliverable is a contact-patch map plus a set of measured routing parameters — not a force trace, not an injury prediction. The map becomes the reference geometry against which Case 04's belt force traces and later thorax deflection data are interpreted.
Instrumented Occupant and Measurement Setup
The occupant is a seated anthropomorphic test device (ATD) or a post-mortem human surrogate (PMHS) fitted with a pressure-sensitive contact patch array over the anterior thorax. For a reproducible mapping exercise, the following setup is assumed:
- Occupant: mid-size male ATD, H-point set to the vehicle design position, torso reclined to the seat-back angle specified in the restraint baseline (e.g., 25° from vertical).
- Contact sensor: resistive or capacitive pressure mapping mat, 32 × 32 sensels, 10 mm pitch, covering the sternum and both parasternal regions from clavicle to the lower rib cage. The mat is thin (<2 mm) so it does not itself alter belt routing.
- Belt: standard 48 mm webbing, retractor locked, pretensioner and load limiter disabled or set to their baseline values so the static routing is measured without dynamic take-up.
- Reference frame: a seat-fixed coordinate system with origin at the H-point, +X forward, +Y to the occupant's left, +Z upward. All routing coordinates are reported in this frame so they are comparable across cases.
With the belt routed normally and the occupant settled, the mat records the static contact footprint. This is the geometric baseline; dynamic loading will shift the patch, but the static map defines where the belt can load the thorax.
Reading the Contact Patch Map
The pressure mat output is a 2D field of sensel values. Rather than treating it as a single "peak pressure," the map is read as a set of regions with distinct loading character:
| Region | Location (seat frame) | Typical static pressure | Diagnostic significance |
|---|---|---|---|
| Clavicle crossing | Y ≈ +90 mm, Z ≈ +420 mm | Low–moderate, narrow band | Belt path anchor; misalignment here shifts the whole patch |
| Parasternal band | Y ≈ +40 to +70 mm, Z ≈ +250 to +380 mm | Moderate, elongated | Primary load path onto the rib cage |
| Sternal edge | Y ≈ 0 to +30 mm, Z ≈ +280 to +360 mm | Moderate–high, localized | Concentration risk if belt rides medially |
| Lower rib / costal margin | Y ≈ +30 to +80 mm, Z ≈ +150 to +240 mm | Low, tapering | Indicates belt length and take-up direction |
| Lateral escape zone | Y > +110 mm | Near zero | Belt has slipped off the shoulder — routing fault |
The values above are illustrative for a mid-size male at the stated seat angle; they are not measured results and must be replaced by the actual mat output for the occupant under study. What matters is the pattern: a well-routed belt produces an elongated parasternal band with a tapering lower edge and no lateral escape. A belt that crosses too medially produces a sternal hot spot; one that crosses too laterally produces a narrow clavicle band and a patch that drifts toward the axilla.
Extracting the Geometric Inputs
The map is only useful if it is reduced to a small set of numbers that later cases can consume. From the mat output and the belt path, record the following in the seat frame:
- D-ring height and lateral offset — the anchor point that sets the belt's approach angle. A 20 mm change in D-ring height visibly rotates the parasternal band.
- Clavicle crossing point — the (Y, Z) coordinate where the belt centerline crosses the clavicle line. This is the single most sensitive routing parameter.
- Belt centerline at three thorax stations — upper (Z ≈ +380 mm), mid (Z ≈ +280 mm), lower (Z ≈ +180 mm). Record Y at each; the slope of these three points describes the belt's obliquity.
- Patch centroid and principal axis — the pressure-weighted centroid of the contact patch and its long-axis orientation. This is the geometric summary that a force trace will later be correlated against.
- Patch width at mid-thorax — the Y-extent of the contact band at Z ≈ +280 mm. A narrow band concentrates load; a wide band distributes it.
For the illustrative occupant above, the belt centerline runs from Y ≈ +90 mm at the clavicle to Y ≈ +45 mm at mid-thorax to Y ≈ +55 mm at the lower rib — a slight medial bow that places the patch centroid at roughly Y ≈ +55 mm, Z ≈ +250 mm. These are example values for the stated seat angle and must be re-measured for the actual occupant and vehicle.
Verification and Common Routing Faults
A mapping is only trustworthy if it is repeatable. Re-seat the occupant, re-route the belt, and re-acquire the map; the patch centroid should repeat within about 10 mm and the principal-axis orientation within about 5°. Larger scatter means the belt is not seating consistently — usually because the retractor is not locked or the occupant has shifted.
Faults that invalidate the map: belt riding off the shoulder (patch drifts to the lateral escape zone, near-zero pressure over the sternum); belt crossing the neck (clavicle band disappears, patch shifts medially and upward); mat wrinkling under the belt (spurious high-pressure ridges that are sensor artifacts, not loading). If any of these appear, correct the routing before recording geometry — a map of a mis-routed belt is worse than no map, because it will be used as a false reference.
Practical check: overlay the belt centerline on the pressure map. The highest-pressure sensels should lie within one belt-width of the centerline. Sensels reading high pressure far from the centerline indicate either mat folding or a second contact path (e.g., the lap belt or a clothing fold) that must be excluded from the shoulder-belt patch.
Deliverable: Contact Patch and Routing Record
The output of this case is a single record that later loading-diagnosis cases reference directly. A complete record contains:
| Field | Example value (illustrative) | Unit |
|---|---|---|
| Occupant / ATD type | mid-size male, H-point design position | — |
| Seat-back angle | 25 | deg from vertical |
| D-ring (Y, Z) | +120, +560 | mm |
| Clavicle crossing (Y, Z) | +90, +420 | mm |
| Belt centerline Y at Z = 380 / 280 / 180 | +78 / +45 / +55 | mm |
| Patch centroid (Y, Z) | +55, +250 | mm |
| Principal-axis orientation | 18 | deg from vertical |
| Patch width at mid-thorax | 48 | mm |
| Repeatability (centroid scatter) | ≤ 10 | mm |
With this record in hand, a belt force–time trace can be interpreted against a known geometry: a peak force arriving while the patch centroid sits at Y ≈ +55 mm tells a different story than the same peak with the centroid at Y ≈ +20 mm (sternal concentration) or Y ≈ +100 mm (lateral escape). The map does not predict injury — it fixes the geometric frame in which injury-relevant loading is later read.