4. CAE-driven concept phase

The concept phase of an occupant safety program is where architecture decisions are cheapest to make and most expensive to reverse. Long before a single prototype seat or airbag module exists, CAE must answer a deceptively simple question: can the restraint system we intend to build actually fit, deploy, and protect an occupant inside the body we intend to build? This chapter covers how CAE drives that answer through restraint system layout, package studies, and early feasibility assessments — the analytical work that converts a styling surface and a set of targets into a defensible safety architecture.

What the Concept Phase Must Resolve

At concept stage, the vehicle body-in-white (BIW) is typically a coarse CAD surface with nominal sections, no closures, and no trim. The restraint system is even less defined: an airbag volume target, a seat reference point (S-R point), a belt anchorage intent, and a set of regulatory and consumer targets. CAE's job is not to produce a final validated model but to establish feasibility envelopes — the ranges of geometry, position, and timing within which the restraint system can meet its targets.

Three questions dominate:

• Layout: Where do the belt anchorages, retractor, buckle, airbag modules, and seat structure physically sit relative to the occupant and the body?
• Package: Does the intended hardware fit inside the available volume without violating styling, visibility, or adjacent-system constraints?
• Feasibility: Given the layout and package, can the occupant targets (HIC, chest deflection, femur load, etc.) plausibly be met, and what are the dominant sensitivities?

Restraint System Layout: From S-R Point to Anchorage Geometry

Layout work begins with the seating reference point and the occupant positioning envelope. The S-R point is established from the seat design intent, and the occupant is placed using a standardized H-point machine or a CAE equivalent. From there, the belt geometry is defined by the anchorage locations: the D-ring (upper shoulder anchorage), the retractor, the buckle, and the outboard and inboard lower anchorages.

The critical layout parameters are the belt angles. For a 50th percentile male occupant, the shoulder belt should pass over the clavicle and engage the sternum without riding up the neck or slipping off the shoulder. The lap belt should sit low across the pelvis, below the anterior superior iliac spine (ASIS), to load the pelvis rather than the abdomen. In CAE, these are checked by measuring the belt path against the occupant's skeletal landmarks in the seated posture.

A practical layout study uses a parametric belt model: the D-ring position is varied in X (fore-aft), Y (lateral), and Z (vertical), and the resulting belt path is evaluated for shoulder engagement and lap belt angle. The lap belt angle relative to horizontal is typically targeted between 45° and 65° for a 50th percentile occupant, with the buckle position adjusted to keep the lap belt in contact with the pelvis. These are not arbitrary numbers — they come from regulatory test experience and biomechanical loading paths.

Package Studies: Fitting Hardware into the Body

Package studies answer whether the restraint hardware fits. The most constrained items are usually the side airbag module in the seat, the curtain airbag along the roof rail, and the frontal airbag in the steering wheel or instrument panel.

For a side airbag, the package study defines the module envelope, the deployment path, and the clearance to the occupant and the door trim. CAE represents the airbag as a folded envelope and simulates deployment into free space and against a representative occupant. The key outputs are the deployment trajectory, the time to full inflation, and any interference with the seat frame or trim. A common failure mode is the airbag "shooting low" and loading the occupant's abdomen instead of the thorax — detectable in CAE as an early contact point below the rib cage.

For a curtain airbag, the package study is dominated by the roof rail geometry and the trim cover. The airbag must deploy downward and forward to cover the side window and the A- and B-pillars. CAE checks the deployment path against the headliner and the pillar trim, and evaluates whether the airbag reaches its target coverage area within the required time. The typical target is full coverage of the side window opening within 30–40 ms of deployment initiation.

Frontal airbag package studies focus on the module volume, the tear seam location, and the distance from the module to the occupant. The "stand-off" distance — the gap between the airbag module and the occupant's chest — is a primary parameter. Too small a stand-off risks early, high-force loading; too large a stand-off risks the occupant moving forward before the airbag is fully inflated. CAE sweeps this distance and reports the occupant response.

Early Feasibility Assessments: Sensitivity Before Optimization

Once the layout and package are roughly defined, CAE runs early feasibility assessments. These are not full validation simulations; they are coarse, fast-running models — often rigid-body occupant models coupled to a simplified restraint representation — used to identify which parameters dominate the occupant response.

A typical feasibility study uses a sled-equivalent pulse and a MADYMO or LS-DYNA occupant model. The restraint system is represented by a belt model with defined stiffness and a simplified airbag with a pressure-volume curve. The study sweeps key parameters: belt anchorage position, airbag vent size, airbag inflation timing, and seat cushion stiffness. The output is a set of occupant response curves — head acceleration, chest deflection, femur load — and a sensitivity ranking.

For example, a feasibility study might show that a 20 mm change in D-ring Z-position reduces chest deflection by 8%, while a 10% change in airbag vent area changes head acceleration by 15%. This tells the program where to invest design effort and where the architecture has margin. It also identifies feasibility risks: if the required chest deflection cannot be met even with the best-case parameter set, the architecture must change — a larger airbag, a different belt geometry, or a different seat position.

Deliverables and Decision Gates

The concept phase concludes with a restraint system layout package and a feasibility assessment report. The layout package includes the belt anchorage coordinates, the airbag module envelopes, the deployment paths, and the occupant positioning envelope. The feasibility report includes the sensitivity ranking, the identified risks, and the recommended architecture changes.

A practical deliverable is a restraint layout parameter table that captures the nominal values, the feasible ranges, and the sensitivity of each parameter. This table becomes the input to the next phase, where the architecture is frozen and detailed design begins.

These values are illustrative and would be replaced by program-specific data. The point is that the concept phase must produce a quantified, traceable basis for the architecture — not a qualitative "it looks feasible" statement. When the program later faces a package conflict or a target shortfall, this table is the reference that shows what can move, by how much, and at what cost to occupant performance.

In practice, the most valuable outcome of CAE-driven concept work is not a single answer but a feasibility map: a clear picture of where the design has margin, where it is tight, and where it is already infeasible. That map lets the program make informed trade-offs early, when changes are still cheap, rather than discovering package or performance conflicts after the first prototype build.