3. Seat Belt Routing Verification for Booster Seat
Why Belt Routing Is a Measurable Test Variable
In a frontal sled or crash test with a high-back booster seat, the three-point belt is not simply "worn" — it is a controlled load path. The lap section must transfer restraint force into the iliac crests of the pelvis, and the shoulder section must cross the clavicle and sternum while staying clear of the neck. When the belt is routed outside the booster's belt guides, or when the guides are misused, the geometry shifts: the lap belt rides up onto the abdomen (raising submarining risk), or the shoulder belt migrates laterally toward the neck (raising neck loading and reducing chest engagement). For a 6-year-old dummy, the pelvis is small and the abdominal region is compliant, so a 20–30 mm lap-belt migration can be the difference between pelvic loading and soft-tissue loading.
This case treats routing verification as a static, pre-test measurement task. You will set up a Hybrid III 6-year-old (HIII-6C) in a high-back booster on a vehicle seat, route the belt through the designated guides, and then quantify lap and shoulder belt position against anatomical landmarks. The output is a pass/fail record with measured coordinates, not a visual impression.
Test Objects, Reference Frame, and Instrumentation
Define the coordinate system before measuring, because every number below depends on it. Use the vehicle coordinate system: X positive forward, Y positive to the dummy's left, Z positive upward, origin at the vehicle seat reference point projected to the floor. All belt positions are reported as (X, Y, Z) in millimetres relative to this origin. If you instead use a dummy-local frame, state it explicitly and do not mix frames in one table.
- Dummy: HIII-6C, seated upright, back against the booster backrest, knees bent over the booster's front edge, head level.
- Booster: high-back booster with two lap-belt guide slots (left/right) and one shoulder-belt guide (typically a clip or slot on the headrest side). Record the manufacturer's specified belt path from the label.
- Belt: standard three-point retractor belt, buckle stalk on the dummy's right (or as installed), tongue latched, no slack beyond the retractor's normal take-up.
- Measurement tools: a three-axis digitizer or a belt-position gauge (a thin flexible strip with a marked centreline), plus a steel rule for vertical offsets. A 3D coordinate measuring arm is preferred; a photographic method with a calibrated scale bar is acceptable if you document the camera position.
- Landmarks: iliac crest (left and right), anterior superior iliac spine (ASIS), clavicle midpoint, sternum notch, and neck base (lateral). Mark them with small adhesive targets before digitizing.
Routing Procedure and Measurement Points
- Seat the dummy. Place the HIII-6C in the booster, push the pelvis rearward until the buttocks contact the booster seat pan and the back contacts the backrest. Confirm the dummy is not leaning and that the booster itself is compressed into the vehicle seat cushion (a booster that rocks will change belt geometry).
- Route the lap belt. Pass the lap section through the booster's lap-belt guide slots on both sides, then latch the tongue. The belt must lie flat, not twisted, and must sit low across the pelvis — the belt centreline should cross the iliac crest region, not the abdomen. Pull the shoulder section to take up slack, then let the retractor settle.
- Route the shoulder belt. Pass the shoulder section through the booster's shoulder-belt guide (or under the clip, per the label). The belt should cross the clavicle midpoint and descend toward the buckle side, staying at least 25 mm lateral to the neck base. If the guide is on the headrest, verify the belt does not contact the headrest shell.
- Digitize the belt centreline. Record at least three points per belt section: lap belt at the left guide exit, at the ASIS crossing, and at the buckle; shoulder belt at the guide exit, at the clavicle crossing, and at the sternum. Also digitize the landmark targets.
- Compute offsets. For the lap belt, compute the vertical distance from the belt centreline to the iliac crest target (positive = belt above crest). For the shoulder belt, compute the lateral distance from the belt centreline to the neck base (positive = belt outboard of neck).
Acceptance Criteria and Failure Symptoms
Use a written acceptance band rather than a single number, because belt geometry varies with booster design and vehicle seat contour. A defensible starting band for a 6-year-old in a high-back booster is: lap belt vertical offset 0 to +15 mm above the iliac crest (never below the crest, never more than 15 mm above); shoulder belt lateral offset ≥ 25 mm from the neck base; belt not twisted; belt fully seated in each guide; no belt contact with the booster's rigid shell edges. Record the band and its source (manufacturer label, regulation, or lab procedure) in the test record.
| Check | Measurement | Pass condition | Typical failure symptom |
|---|---|---|---|
| Lap belt height | Vertical offset, belt centreline to iliac crest | 0 to +15 mm | Belt above +15 mm → abdominal loading, submarining risk |
| Lap belt lateral | Belt seated in both guide slots | Fully seated, no twist | Belt outside guide → rides onto abdomen or slips off pelvis |
| Shoulder belt lateral | Distance from neck base | ≥ 25 mm outboard | Belt within 25 mm → neck loading, poor clavicle engagement |
| Shoulder belt path | Contact with clavicle midpoint | Crosses clavicle, not upper arm | Belt on upper arm → belt slips off shoulder in crash |
| Guide engagement | Visual + tug check | Belt retained under 50 N pull | Belt pops out of clip → loss of routing in dynamic event |
Reading the Routing Diagram
The diagram below shows the lateral (side) view of the dummy in the booster with the belt path and the two critical offsets. The lap belt is shown crossing the iliac crest with a vertical offset dlap; the shoulder belt is shown crossing the clavicle with a lateral offset dsh measured to the neck base. The green band marks the acceptable lap-belt zone; the red zone marks the submarining-prone region below the crest and the neck-loading region inboard of the shoulder-belt limit.
Verification Record and Re-Check Logic
Complete the routing check as a record, not a memory. For each measurement, log the raw coordinates, the computed offset, the acceptance band, and the pass/fail result. If any offset is marginal (within 5 mm of a limit), repeat the seating and re-digitize once; if the marginal result persists, document it and flag the configuration for the dynamic test so the post-test belt marks can be compared against the static routing. This is the practical link between static routing verification and the injury outcomes the test is meant to measure: a lap belt logged at +18 mm above the crest predicts abdominal loading, and a shoulder belt logged at 20 mm from the neck predicts neck loading — both should be visible in the post-test belt-mark pattern and in the dummy's load-cell channels.
- Pass: all five checks in the table pass, and no marginal offsets remain after one re-seat.
- Conditional: one marginal offset persists; record it, proceed, and add a post-test belt-mark comparison to the test report.
- Fail: any offset outside the band, or any belt not seated in its guide; re-route and re-measure before the test, and do not proceed with a failed routing.
Keep the digitized landmark coordinates with the test file. They let a reviewer reconstruct the belt path without re-running the setup, and they make the difference between "the belt looked fine" and a defensible, repeatable routing verification.