3. Occupant packaging and geometry

Airbag performance is decided long before the first simulation runs. The restraint system can only work with the space the occupant package gives it. Seating position, H-point, steering wheel and instrument panel (IP) layout define where the module can sit, how it must be aimed, how much free volume it has to inflate into, and where the vent can exhaust gas without injuring the occupant or damaging trim. This chapter treats packaging as a set of hard geometric constraints that flow directly into module placement and venting strategy.

Reference Geometry: H-Point, Seating Position, and the Occupant Coordinate Frame

All packaging work is anchored to the H-point — the hip point of the SAE J826 HPM (H-point machine) seated in the seat at its design position. The H-point establishes the vehicle coordinate origin for occupant-related dimensions. In a typical SAE vehicle coordinate system, X is positive rearward, Y is positive to the right, and Z is positive upward, with the origin at the front axle ground line. Every airbag dimension — module face angle, deployment distance, vent location — is referenced to this frame, so the sign convention must be fixed once and used consistently in CAD, CAE, and test reports.

Two H-points matter in practice:

• Design H-point (H30): the nominal hip point height above ground for the 50th-percentile occupant, used for layout.
• Actual H-point: the hip point measured in a specific build, which can deviate from design due to seat cushion compression, track position, and recline angle.

Seating position is defined by the seat track fore/aft travel, cushion height adjustment, and backrest angle. For driver airbag (DAB) work, the critical derived quantity is the steering wheel to chest distance and the wheel to H-point relationship. A common design target is a steering wheel center located roughly 450–550 mm forward of and 150–250 mm above the H-point, but these values vary strongly with vehicle class. The passenger side uses the instrument panel to H-point relationship instead, since there is no wheel.

Steering Wheel and IP Layout as Module Constraints

The driver airbag module is packaged inside the steering wheel hub. Its placement is constrained by:

• Wheel center axis: the module face is normally perpendicular to the steering column axis, tilted by the column angle (typically 20–30° from horizontal).
• Hub depth: the folded cushion and inflator must fit within the hub envelope, usually 60–90 mm deep behind the cover.
• Cover tear seam: the seam geometry determines how the cover opens; it must not be obstructed by the wheel rim or spoke structure.
• Column tilt and telescoping: the module moves with the column, so the deployment path changes across the adjustment range.

On the passenger side, the module sits in the IP, typically above the glove box. Constraints include the IP upper surface profile, the windshield rake, the defroster duct, and the cross-car beam. The module must deploy through a chute or door that opens without striking the windshield or the occupant. A common packaging target is a module face angled 10–25° downward from horizontal, aimed at the occupant's chest and head region.

From Geometry to Module Placement

Once the H-point and seating position are fixed, module placement becomes a constrained optimization. The driver module must be centered on the column axis, but the column axis itself is set by the steering wheel position, which is set by the occupant's reach and visibility requirements. The passenger module must be positioned so that the deploying cushion contacts the occupant's chest and head before the occupant moves forward significantly.

A practical placement workflow looks like this:

1. Establish the H-point and the 50th-percentile occupant template in CAD.
2. Define the steering wheel center and column angle from ergonomic targets.
3. Place the DAB module on the column axis, respecting hub depth and cover seam constraints.
4. Define the IP surface and locate the PAB module above the glove box, checking windshield and cross-car beam clearance.
5. Verify deployment distance from module face to occupant chest for both driver and passenger.
6. Check that the vent exhaust path does not intersect the occupant or sensitive trim.

Deployment distance is the single most important derived parameter. For the driver, the target is typically a minimum of 250 mm from the module face to the chest in the design position, with more distance preferred. For the passenger, the target is often 300–400 mm from the IP surface to the chest, depending on vehicle class. If these distances cannot be met, the module must be repositioned, the IP reshaped, or the seat moved — in that order of preference.

Venting Strategy and Its Geometric Dependencies

Venting controls the internal pressure of the cushion and therefore the force applied to the occupant. The vent location is not arbitrary; it is constrained by the same geometry that constrains the module.

Common venting approaches:

• Module-integrated vent: holes or slots in the module housing or diffuser. Exhaust direction is fixed by the module orientation.
• Cushion vent: fabric orifices in the cushion, often on the side or rear. Exhaust direction depends on cushion shape and fold pattern.
• Active vent: a controllable vent that opens or closes based on occupant size or crash severity. Requires additional packaging space and a control signal.

The geometric constraint is straightforward: the vent exhaust must not blow directly onto the occupant's face, neck, or chest, and must not impinge on trim that could be damaged or displaced. For the driver module, the vent is typically located on the module housing facing away from the occupant, often downward or to the side. For the passenger module, the vent is often on the underside of the module or on the cushion rear, exhausting into the IP cavity or footwell.

Worked Example: Packaging Check for a Passenger Airbag Module

Consider a mid-size sedan with the following assumed packaging data (these are illustrative values, not measured results):

Check the deployment distance: 350 mm from IP surface to chest is within the 300–400 mm target band, so the module placement is acceptable from a distance standpoint. Check the windshield clearance: 45 mm exceeds the 30 mm minimum, so the chute can open without striking the glass. Check the cross-car beam clearance: 25 mm exceeds the 15 mm minimum, so the module can be mounted without interference. Check the vent path: the downward exhaust must have a clear route to the footwell; if the IP cavity is sealed, a relief vent or a different vent location is required.

If the deployment distance were below 300 mm, the first corrective action would be to move the module rearward (toward the occupant) if space allows, or to increase the IP surface height to raise the module. If the windshield clearance were below 30 mm, the module would need to be moved rearward or the chute angle changed. These are the real trade-offs that packaging engineers manage daily.

Key Takeaways

• The H-point is the origin of all occupant packaging dimensions; fix the coordinate convention and use it consistently.
• Steering wheel and IP layout directly constrain DAB and PAB module placement, face angle, and deployment path.
• Deployment distance (module face to chest) is the primary geometric acceptance criterion: typically ≥250 mm for driver, 300–400 mm for passenger.
• Vent location is constrained by exhaust direction and must not impinge on the occupant or sealed cavities.
• Packaging checks should be performed in CAD before any simulation, because geometry errors cannot be corrected by tuning the inflator.