3. Contact zone mapping from interior geometry

Contact zone mapping is the step that converts a finished interior surface model into a defensible list of head-impact test regions. It answers a deceptively simple question: for every point on the occupant head path, which trim surface can the head actually reach, and under what geometric conditions? The output is not a single drawing but a structured map — a set of surface patches, each tagged with its parent component, its local curvature, its distance from the head path, and the regulatory or internal target that governs it. Getting this map right determines whether later countermeasure work (padding, ribbing, material change, or geometry shift) is applied where it matters or wasted on surfaces the head never touches.

The mapping relies on two data sources that must be brought into a common coordinate frame: the interior geometry (CAD surfaces of pillars, roof rails, headers, and consoles) and the occupant packaging data (headform path envelopes, seating reference points, and the head impact zone boundaries defined by the applicable regulation or internal standard). The most common failure in this step is not a modeling error but a frame mismatch — a pillar surface modeled in body coordinates compared against a head path defined in a vehicle-local packaging frame, producing a map that looks plausible but is offset by tens of millimeters.

Establishing the shared reference frame

Before any surface is queried, fix the coordinate convention and state it explicitly. A practical convention for this work is: X forward, Y to the occupant's left, Z up, origin at the vehicle datum used by the packaging group. The head path envelope is then expressed in the same frame. If the packaging data arrives in a seat-relative frame, apply the seat-to-body transform once, at import, and record the transform matrix alongside the map so the result is reproducible. Every subsequent distance and angle in the map inherits this frame; switching frames mid-analysis is the single most reliable way to produce an inconsistent contact map.

Building the head path envelope

The head path is not a single point but a swept volume. For a given seating position, the headform center travels along an arc, and the headform radius (typically 165 mm for a 50th-percentile adult headform in many interior impact protocols, though the exact value must come from the governing standard) defines a spherical envelope around that arc. The contact zone is the set of interior surfaces that intersect this swept sphere. In CAD, this is most robustly computed as an offset-and-intersect operation: offset the head path curve by the headform radius to create the envelope solid, then intersect the envelope with the interior surface set. The intersection curves are the contact boundaries.

Two practical refinements matter. First, the head path should be sampled at discrete stations (for example, every 25 mm along the arc) rather than treated as a continuous sweep, because the resulting station-by-station intersection gives you a contact map with resolvable patches rather than one undifferentiated blob. Second, the envelope must be evaluated for each seating position and each applicable headform size in the protocol; a map built only for the 50th-percentile occupant will miss contact zones that only the small or large headform reaches.

Classifying surfaces by component and role

Once the intersection patches exist, each patch is attributed to its parent surface. The four surface families in this course behave differently and should be tagged separately:

• Pillars (A-, B-, C-pillar): typically high-curvature, often with a hard structural substrate close beneath the trim. Contact patches here tend to be small and localized, and the local radius of curvature strongly influences whether the surface acts as a point contact or a distributed one.
• Roof rails: long, relatively low-curvature surfaces running fore-aft. Contact patches are elongated, and the relevant question is usually whether the head grazes the rail or strikes it squarely.
• Headers: transverse surfaces at the top of the windshield and rear window openings. These are frequent contact zones because the head path arc passes close to them at the top of its travel.
• Consoles (overhead and floor): overhead consoles sit directly in the head path for taller occupants; floor consoles are relevant mainly for out-of-position or rear-seat scenarios and must be mapped when the protocol includes them.

Attribution should be done by surface ID, not by visual proximity. A patch that straddles a pillar-to-header joint must be split at the component boundary, because the two components will almost certainly receive different countermeasures and different validation targets.

Quantifying each contact patch

A contact map that only says "this surface is contacted" is not actionable. For each patch, extract a small set of geometric descriptors that later countermeasure decisions depend on: the patch area, the minimum distance from the headform center to the surface (which indicates strike depth), the local principal curvatures at the patch centroid, the surface normal at the centroid relative to the head path direction (the impact obliquity), and the substrate depth beneath the trim at the centroid. These five numbers, recorded per patch, are what let an engineer decide between adding padding, adding a rib, softening the substrate, or reshaping the trim.

Worked example: mapping a B-pillar and header

Consider a simplified example (parameters are illustrative, not measured): a 50th-percentile headform with 165 mm radius, head path sampled at 25 mm stations, seat in the design position, body-frame coordinates. The offset-and-intersect operation returns 14 stations in contact with the B-pillar upper trim and 9 stations in contact with the header. The B-pillar patches show a minimum headform-center-to-surface distance of 12 mm at station 7, with a local radius of curvature of 40 mm and an impact obliquity of 35°. The header patches show a minimum distance of 28 mm, a local radius of 300 mm, and an obliquity of 10°.

Reading these numbers: the B-pillar contact is a near-normal, tightly curved strike — the kind of geometry where a small pad or a local rib is likely to be effective because the contact is concentrated. The header contact is a shallow, glancing strike on a broad surface — padding there would spread load over a large area, and the more efficient countermeasure is usually a substrate or material change rather than added thickness. The map has already begun to steer the countermeasure decision before any padding is modeled.

Common failure modes and how to catch them

Three symptoms recur in contact mapping. Missing patches at component joints usually indicate that the surface set was imported with gaps at seams; check that adjacent components share coincident boundary edges before intersecting. Patches that appear on surfaces the head cannot physically reach almost always trace to a frame offset or a stale seat transform; verify by placing a reference sphere at a known station and confirming it intersects the expected surface. Patches that vanish when the headform size changes are not errors — they are real size-dependent contact zones, and they should be retained in the map with their headform size tag rather than discarded.

From map to countermeasure targets

The completed contact map is the handoff artifact for the rest of the countermeasure work. Its value lies in the per-patch descriptors: a tightly curved, near-normal pillar strike and a broad, glancing header strike demand different responses, and the map makes that distinction explicit before any padding or ribbing is modeled. Keep the map versioned against the interior CAD release and the packaging data release it was built from; when either changes, the affected patches must be re-derived rather than assumed unchanged. A contact map that is traceable, frame-consistent, and descriptor-rich turns an ambiguous "check the head impact areas" instruction into a specific, reviewable engineering task.