2. Side airbag types and architectures
Side airbags are not a single product but a family of restraint devices, each defined by the body region it protects, the crash mode it addresses, and the physical location where it is packaged. Getting the type and architecture right is the first architectural decision in any side restraint program, because it determines the inflator class, the cushion geometry, the trim and seat structure interfaces, and the CAE load cases you must eventually validate. This chapter maps the side airbag landscape and explains how packaging location interacts with protection strategy.
Protection Targets Drive the Type
Side impacts load the occupant laterally, and the injury mechanisms differ sharply by body region. The thorax is vulnerable to rib fracture and lung contusion from door intrusion and arm-to-chest loading. The pelvis tolerates higher loads but can suffer acetabular and pelvic ring fractures. The head, in a side impact or rollover, is at risk from contact with the B-pillar, roof rail, or glazing. Each of these mechanisms motivates a distinct cushion.
- Thorax airbag: A relatively flat cushion that deploys between the occupant's chest and the door trim. It distributes pressure across the rib cage and limits chest deflection. Typical inflated thickness is 60–120 mm, tuned to the gap between the seat and the door.
- Pelvis airbag: A smaller, stiffer cushion positioned lower, near the occupant's hip. It reacts against the door or the seat frame to control pelvis lateral displacement and reduce load transfer to the pelvis.
- Combo (thorax + pelvis) airbag: A single cushion with two chambers or a shaped profile that covers both the chest and the hip. This is now the most common seat-integrated side airbag in front seats because one module addresses two injury regions.
- Curtain airbag: A long, tubular or sail-shaped cushion that deploys from the roof rail and covers the side glazing from the A-pillar to the C-pillar. It protects the head in side impacts and rollovers and helps prevent ejection.
The choice among thorax, pelvis, and combo is driven by the vehicle's side structure performance and the seat-to-door gap. A strong door with limited intrusion may need only a thorax bag; a vehicle with a larger gap or a higher pelvis load target typically moves to a combo. The curtain is almost always added on top of a seat-mounted bag, not instead of it, because it addresses a different body region and a different crash mode.
Seat-Integrated vs. Door-Mounted Architectures
The packaging location is a separate decision from the protection target, and it changes the entire module design. In a seat-integrated configuration, the module is mounted to the seat back frame, usually on the outboard side, and the cushion deploys through a tear seam in the seat trim. In a door-mounted configuration, the module sits inside the door trim panel and deploys through a seam in the door card.
| Attribute | Seat-Integrated | Door-Mounted |
|---|---|---|
| Module location | Seat back frame, outboard side | Door trim panel, inner surface |
| Deployment reference | Moves with the seat; gap to door varies with seat position | Fixed to the door; gap to occupant varies with seat position |
| Trim interface | Seat trim tear seam, foam channel | Door card tear seam, door module carrier |
| Typical use | Front seats, combo bags, adjustable seats | Rear seats, fixed seating positions, some front applications |
| CAE challenge | Seat position sweep, trim seam opening | Door intrusion coupling, occupant-to-door gap |
The seat-integrated approach keeps the cushion at a consistent distance from the occupant regardless of seat travel, which is a major advantage for occupant coverage. Its weakness is that the gap to the door changes as the seat moves fore and aft, so the cushion must be robust across the full seat track range. The door-mounted approach fixes the cushion to the intruding structure, which can help in a stiff door, but the occupant-to-cushion distance grows when the seat is moved rearward, and the door trim must carry the deployment loads.
How the Architecture Maps to the CAE Model
The architecture decision propagates directly into the simulation setup. A seat-integrated thorax or combo bag is modeled with the cushion parented to the seat frame, a fabric material model for the cushion, and a fold pattern that matches the production fold. The tear seam is represented by a tied or breakable contact along the seat trim. A door-mounted bag is parented to the door trim, and the door intrusion curve becomes a primary input.
For a curtain airbag, the model is different again: the cushion is long and thin, often modeled with a gas-flow network across multiple chambers, and it interacts with the roof rail, the headliner, and the glazing. The inflator is typically a hybrid or pyrotechnic unit mounted at the roof rail, and the fill time and chamber pressure distribution are the key calibration targets.
Practical Selection and Modeling Notes
When selecting a side airbag architecture, the engineering team typically works through a sequence of questions. First, which body regions must be protected in the target crash modes? Second, what is the available package space between the occupant and the door or roof rail? Third, does the seat move, and if so, how does that change the deployment gap? Fourth, what does the door intrusion curve look like, and can the door trim carry the reaction loads?
In CAE, the answers translate into specific model choices. A combo bag is often modeled with a single cushion and a mapped fold, with the inflator mass flow rate calibrated so that the thorax chamber fills slightly before the pelvis chamber. A curtain bag is modeled with a gas-flow network and a chamber pressure target; the fill time is typically 20–40 ms depending on the inflator. The seat-integrated module requires a seat trim tear seam model, often a tied contact with a failure criterion, while the door-mounted module requires a door trim seam and a door intrusion input.
A useful verification step is to run a static deployment check before the full crash simulation. Inflate the cushion in the design position and confirm that the tear seam opens cleanly, the cushion does not snag on the seat frame or door hardware, and the inflated shape covers the intended body region. This catches packaging errors early, before the more expensive crash load cases are run. Typical failure symptoms at this stage include asymmetric deployment, cushion contact with the seat recliner mechanism, and tear seam propagation into the seat foam rather than along the intended seam line.
Understanding the type and architecture landscape is therefore not just a taxonomy exercise. It determines the inflator, the cushion, the trim interface, the seat or door structure requirements, and the CAE load cases and model fidelity you will need. The next step is to translate these choices into concrete cushion geometry and inflator sizing, which is where the design and simulation work begins in earnest.