3. Side impact occupant restraint system architecture

Side impact crashes leave very little package space and very little time. The struck-side occupant is loaded by the intruding door within roughly 10–30 ms of bumper contact, so the restraint system must deploy earlier, faster, and more selectively than in frontal crashes. The architecture of a modern side restraint system is therefore built around three complementary hardware families: side airbags (thorax/pelvis, mounted either in the seat or in the door), side curtain airbags (roof-rail mounted, covering the window opening), and seatbelt pretensioners (retractor or buckle type) that remove belt slack and couple the occupant to the seat before the door intrudes. This chapter explains what each subsystem does, where it is mounted, how the mounting location changes its performance, and how the elements are coordinated into a single deployment strategy.

Side Airbags: Seat-Mounted vs. Door-Mounted

A side airbag's job is to place a cushion between the occupant's thorax and pelvis and the intruding door trim, spreading the load over a larger area and limiting rib deflection and pelvic force. The two dominant mounting architectures differ in a way that directly affects performance:

• Seat-mounted side airbag (SAB): the module is integrated into the outboard side of the seat back frame, usually in the bolster. Because the seat moves with the occupant during seat-track adjustment and recline, the cushion-to-occupant gap stays roughly constant regardless of seating position. This is the preferred architecture for thorax and thorax-pelvis combination bags, and it is essentially mandatory for vehicles with adjustable or "captain's chair" rear seating.
• Door-mounted side airbag: the module sits in the door trim panel. It is closer to the impact point and can begin cushioning earlier, but the gap to the occupant changes with seat position, and the door structure itself deforms during intrusion. Door mounting is common in older or cost-sensitive platforms and in some European B/C-segment vehicles, often as a thorax-only bag.

In practice, the choice is driven by package space, seat design, and the target occupant population. A seat-mounted bag must survive seat durability cycling and must not interfere with seat heating or lumbar mechanisms; a door-mounted bag must be protected from water management and door-slam loads. Both must pass the same out-of-position (OOP) requirements, which is where the mounting location becomes a real engineering constraint: a seat-mounted bag is closer to a child seat or a leaning occupant, so its deployment must be tuned more conservatively.

Side Curtain Airbags

The side curtain airbag (SCAB) is a long, roof-rail-mounted cushion that deploys downward to cover the window opening from the A-pillar to the C-pillar (or D-pillar in SUVs and MPVs). Its primary functions are head protection in side impacts and rollover, and ejection mitigation. Key architectural parameters include:

• Coverage length and height: the curtain must span the full glazing and overlap the door beltline and roof rail so that the head cannot slip between the curtain and the structure.
• Tethers and chambers: internal tethers shape the inflated curtain into a flat "sail" and keep it positioned during the impact; some designs use a separate lower chamber for the head.
• Inflator location and fill time: a hybrid or pyrotechnic inflator at the rear of the roof rail fills the curtain in roughly 20–30 ms; fill time and pressure must be balanced against OOP loading of a child's head.
• Rollover hold time: for ejection mitigation, the curtain must remain inflated for several seconds, which drives the use of a cold-gas or hybrid inflator with a sustained pressure curve.

The curtain is the single most important element for head injury and ejection in side impacts, and it is the element most sensitive to roof-rail packaging. Its deployment path must be clear of trim, grab handles, and sunroof mechanisms, and the roof rail must provide a reaction surface so the curtain does not simply push outward.

Seatbelt Pretensioners

Pretensioners remove belt slack and pull the occupant into the seat before the crash pulse fully develops. In side impact, they serve a different purpose than in frontal impact: rather than primarily controlling forward excursion, they couple the occupant to the seat so that the seat-mounted side airbag and the seat structure can do their job, and they reduce the lateral gap between the occupant and the door. Two common types are:

• Retractor pretensioner: a pyrotechnic or mechanical device that rotates the retractor spool to wind in webbing, typically pulling 60–100 mm of belt in 10–20 ms.
• Buckle pretensioner: pulls the buckle stalk downward, tightening the lap portion and reducing pelvis excursion; often used together with a retractor pretensioner.

In side impact, pretensioners are usually fired on the struck side only, and the firing decision is made by the side impact sensing system (pressure sensors in the door, acceleration sensors in the B-pillar or sill). Firing both sides would be unnecessary and could increase OOP risk on the non-struck side.

How the Elements Are Coordinated

The architecture is not a collection of independent devices; it is a timed sequence. A representative (illustrative, not measured) deployment timeline for a 50 km/h MDB side impact might look like this:

The exact timing is calibrated in CAE and confirmed on sled and full-vehicle tests. The key architectural insight is that the seat-mounted side airbag and the pretensioner must fire first, because they act on the occupant before the door reaches them; the curtain follows to protect the head as the upper body rotates and the head moves toward the glazing.

Illustrative Sizing Example

Consider a mid-size sedan with a seat-mounted thorax-pelvis bag and a roof-rail curtain. A simplified first-pass sizing exercise (illustrative values, to be replaced by CAE and test data) is:

Target: limit chest deflection to < 42 mm (ES-2re, 50 km/h MDB)
Seat-mounted SAB volume:        18–22 L
Curtain volume:                 25–35 L (single inflator, rear roof rail)
Pretensioner pull-in:           70 mm retractor + 40 mm buckle
Firing threshold (side pressure): 35–55 kPa, calibrated to avoid
                                  deployment on door-slam and rough road

These numbers are starting points for a CAE correlation loop, not validated results. The engineer adjusts bag volume, vent size, and inflator output until the chest deflection, pelvic force, and head injury criterion all fall inside target, then confirms the OOP cases (child seat, leaning occupant) remain acceptable.

Architecture Selection Summary

In summary, the side restraint architecture is a layered defense: pretensioners couple the occupant to the seat, the side airbag cushions the thorax and pelvis against the intruding door, and the curtain protects the head and prevents ejection. Seat-mounted side airbags are preferred where seat position varies and where a constant cushion-to-occupant gap matters; door-mounted bags remain a viable lower-cost option where package and seat design allow. The curtain is effectively mandatory for head protection and ejection mitigation in modern vehicles. The engineering task is not to choose one element but to time and size all three so that they act in the correct order within the very short side-impact window.