4. Occupant restraint system requirements

In a DVP&R for crashworthiness, the occupant restraint system is where regulatory compliance, customer-facing safety performance, and supplier interfaces converge. A restraint requirement is only useful in a DVP&R if it names the physical item, the performance it must deliver, the test or analysis that proves it, and the acceptance criterion. This section translates the restraint system into verifiable requirements across five subsystems: seat belts, airbags, seats, the steering column, and child restraint anchorages (CRAs).

Seat belts: from anchor strength to dynamic occupant protection

Seat belt requirements split into structural, dynamic, and functional categories. Structural requirements cover anchor point strength and belt anchorage location: FMVSS 210 in the US and ECE R14 in Europe define static pull tests on anchorage points, with typical loads around 13.3 kN per anchorage in prescribed directions. The DVP&R entry should specify the anchorage, load direction, fixture, and pass criterion (no release, no failure of the anchorage or surrounding structure beyond defined limits).

Dynamic requirements are proven through sled tests. A frontal sled pulse (for example, a 50 km/h, 28 g pulse approximating FMVSS 208 or ECE R16 conditions) is run with instrumented Hybrid III or THOR dummies, and the requirement is expressed as occupant injury criteria: HIC15, chest deflection, chest acceleration (3 ms clip), and femur loads. A representative requirement might read: "Driver position, 50th percentile male Hybrid III, frontal sled at 50 km/h: HIC15 ≤ 700, chest deflection ≤ 63 mm, femur axial load ≤ 10 kN." These thresholds are examples consistent with regulatory limits; the DVP&R must cite the actual regulation or internal target.

Functional requirements address the retractor and buckle: spool-out force, retraction force, locking behavior under deceleration, buckle release force (typically 40–80 N), and durability cycling (thousands of extend/retract cycles). These are often verified on bench fixtures rather than full vehicles, which makes them cheap, early, and repeatable — good candidates for the first DVP&R test entries.

Airbags: deployment logic, inflation performance, and out-of-position protection

Airbag requirements must capture both the crash sensing/deployment decision and the bag's physical performance. The deployment requirement defines the sensing threshold and time-to-fire: for a frontal impact, the airbag must deploy within a window (commonly 15–30 ms after the triggering event) so that the bag is inflated before occupant contact. The DVP&R entry should specify the crash mode, impact speed, and the required deployment state (deploy / no-deploy), because a no-deploy requirement at low speed is just as important as a deploy requirement at high speed.

Inflation performance is verified with static deployment tests (tank tests or open-air deployments) measuring pressure-time and bag fill time, and with out-of-position (OOP) tests using child dummies or small female dummies positioned close to the module. OOP requirements are typically expressed as injury criteria under FMVSS 208 or ECE R94 static deployment conditions. A practical DVP&R entry: "Passenger airbag, 3-year-old dummy in OOP position 1, static deployment at 25 °C: HIC ≤ 1000, no neck injury above threshold."

Environmental robustness matters: airbag inflators are tested across temperature extremes (e.g., −35 °C to +85 °C) and after aging, because propellant burn rate and bag fabric stiffness change with temperature. The DVP&R should include at least one cold and one hot deployment to confirm the bag still fills within the timing window.

Seats: structure, head restraint, and occupant retention

Seat requirements cover structural integrity, head restraint geometry and performance, and seat belt integration. Structural tests include seat anchorage strength (FMVSS 207/208, ECE R14/R17) and seat back moment tests. Head restraints are verified against FMVSS 202a or ECE R17: geometry (height and backset) plus a dynamic whiplash test with a BioRID dummy, with criteria such as NIC (neck injury criterion) below a defined threshold.

Because seats carry belt anchorages and often airbag modules, the DVP&R must also verify the seat as an assembly: belt routing, buckle position, and the interaction between seat travel and belt geometry. A useful requirement is a seat-position sweep: at full-forward, mid, and full-rear positions, confirm the belt does not rub on trim, the buckle is reachable, and the geometry stays within the design envelope.

Steering column: energy absorption and driver protection

The steering column is a controlled energy-absorbing element. Requirements include column collapse force and stroke (the column must deform axially within a force band, e.g., 2–6 kN, over a defined stroke), column tilt/telescope retention, and the steering wheel's interaction with the driver airbag. In a frontal crash, the column must absorb energy without transmitting excessive force to the driver's chest. The DVP&R entry should specify the collapse test (quasi-static axial compression or dynamic sled) and the force-displacement acceptance band.

Column requirements also include the steering wheel rim and hub strength, and the airbag module retention: the module must remain attached during deployment. A deployment test with the column at its extreme tilt positions confirms the bag does not interfere with the wheel or column shroud.

Child restraint anchorages: geometry, strength, and usability

CRAs are governed by FMVSS 225 and ECE R14/R16 (ISOFIX). Requirements include anchor location and geometry (the ISOFIX anchor bars at a defined spacing and height), static strength (a pull test, typically around 8–10 kN per anchorage pair depending on the standard), and the presence of a top tether anchorage with its own strength requirement. The DVP&R should also capture usability requirements: the anchorage must be findable and engageable with a child seat connector without tools, and the top tether must be routable without interference.

A practical verification is a fit check with representative child seats: install a rear-facing infant seat and a forward-facing convertible seat at each seating position, confirm connector engagement force is within the specified range, and confirm the top tether routes cleanly. This is a low-cost, high-value test that catches packaging problems early.

Building the DVP&R entries: a worked example

Consider a frontal driver restraint requirement. The DVP&R row should be structured so that a reviewer can trace it end to end. A representative entry:

Requirement ID:  RS-FR-012
Subsystem:      Driver seat belt + steering column + driver airbag
Requirement:    In a 50 km/h frontal barrier impact, the driver restraint
                system shall limit occupant injury to HIC15 <= 700,
                chest deflection <= 63 mm, and femur load <= 10 kN.
Verification:   Full-vehicle frontal barrier test, Hybrid III 50th male,
                driver position, 3 repeats.
Acceptance:     All injury criteria met in all 3 tests; airbag deployed
                within 15-30 ms; column collapse within 2-6 kN band.
Evidence:       Test report TR-208-xxx, high-speed video, sensor plots.
Status:         Planned

Note that the requirement is written as a performance outcome, not a design description. The verification method names the test type, dummy, and repeat count. The acceptance criterion is numeric and traceable to a regulation or internal target. This structure is what makes a DVP&R auditable and useful during design reviews.

Common failure modes and how to catch them early

Restraint DVP&R programs frequently stumble on a few recurring issues. First, interface gaps: the seat supplier verifies belt routing, the belt supplier verifies retractor performance, but no one verifies the combination at the vehicle level. The seat-position sweep test closes this gap. Second, temperature effects on airbag timing: a bag that fills in 22 ms at 25 °C may fill in 30 ms at −35 °C, pushing it outside the timing window. Cold and hot deployments catch this. Third, OOP performance: a bag tuned for the 50th percentile occupant may injure a child dummy positioned close to the module. OOP tests must be scheduled early, because they often drive inflator or venting changes. Fourth, CRA usability: an anchorage that passes the static pull test but is buried behind trim will fail the fit check and generate customer complaints. The child-seat fit check is cheap and should be done at the clay model stage.

Finally, keep the DVP&R entries aligned with the program's build phases. Bench-level tests (buckle release, retractor durability, CRA fit check) belong in the early prototype phase; sled tests follow once the body structure is representative; full-vehicle barrier tests come last. Sequencing the entries this way means failures surface when design changes are still inexpensive, which is the entire point of a well-constructed DVP&R.