2. Occupant packaging fundamentals

Side impact occupant-to-door clearance development begins with a precise, shared definition of where the occupant sits and where the door interior surface actually is. Two engineers can model the same vehicle and disagree by 30 mm simply because one used the design H-point and the other used an actual H-point measured from a physical buck. This chapter establishes the reference geometry that every downstream clearance calculation depends on: the H-point family of reference points, the seating reference point (SRS), and the door trim interface surface. Getting these definitions right — and keeping them consistent across CAD, CAE, and physical test — is the single most important prerequisite for credible side clearance work.

The H-point family: design intent versus physical reality

The H-point (hip point) is the pivot center of the human torso and thigh in the SAE J826 H-point machine, representing the occupant's hip joint location in side view. It is the origin from which almost all occupant packaging dimensions are referenced. In practice, three distinct H-points coexist in a vehicle program, and confusing them is a common source of error:

• Design H-point (H_D) — the target hip location defined in the CAD package at a nominal seat position, typically the mid-position of the seat track travel. It is a design intent value, not a measurement.
• Actual H-point (H_A) — the hip point measured on a physical vehicle using the SAE J826 H-point machine (or its digital surrogate) at the same seat position. H_A differs from H_D because of foam compression, trim build-up, seat frame compliance, and manufacturing variation.
• Seating reference point (SRS) — the design reference point used for regulatory and dimensional compliance, derived from H_D and the seat track geometry. The SRS is the point about which seat travel, backset, and cushion angle are formally defined.

The relationship between H_D and H_A is not a nuisance to be ignored — it is a design variable. A typical passenger car seat shows H_A displaced from H_D by roughly 5–15 mm vertically (usually downward, from foam compression) and a few millimeters longitudinally. For side clearance work, a downward H_A shift moves the occupant's hip and torso lower, which can either increase or decrease the gap to the door bolster depending on the door's tumblehome and the bolster's vertical profile. The correct practice is to run clearance checks at both H_D and the expected H_A envelope, not at H_D alone.

Seating reference points and the seat track coordinate frame

The SRS anchors the seat's coordinate system. Seat track travel is defined along the track axis (typically inclined a few degrees from horizontal), and the SRS is placed at the design H-point projected onto the track. From the SRS, the packaging engineer defines:

• Cushion angle — the angle of the seat cushion reference line, which sets thigh support and influences how far the occupant's hip sits from the door.
• Backset / back angle — the torso reference angle (commonly 23–25° from vertical for front seats), which determines how far the upper torso leans toward or away from the door.
• Track travel envelope — fore-aft and vertical adjustment range, which defines the worst-case occupant position for clearance (usually full rearward and full down, or full forward depending on the door geometry).

For side impact, the critical seat position is not always the nominal one. A seat moved fully rearward places the occupant's hip near the B-pillar and door armrest region, while a seat moved fully forward places the torso against the door's forward bolster. The packaging engineer must identify the worst-case seat position for each clearance pair (hip-to-armrest, torso-to-bolster, shoulder-to-window sill) rather than checking only the design position.

Door trim interface geometry

The door trim interface is the inboard surface of the door assembly that the occupant could contact: the armrest, the bolster, the map pocket, the window sill, and the upper trim roll. In CAD, this is usually represented as a set of offset surfaces or a simplified "trim envelope" solid. Three geometric characteristics dominate side clearance:

• Tumblehome — the inward lean of the door's upper section. Positive tumblehome brings the window sill inboard, reducing shoulder and head clearance at the top of the door.
• Bolster protrusion — the armrest and lumbar bolster project inboard into the occupant's hip and torso zone; this is typically the closest contact surface in side impact.
• Trim build variation — the difference between the nominal CAD surface and the as-built trim, driven by foam, substrate, and clip tolerances. A realistic clearance study uses a trim envelope that includes a build allowance (commonly 5–10 mm) rather than the nominal surface alone.

The interface geometry must be evaluated in the same coordinate frame as the occupant. Mixing a body-frame door surface with a seat-frame occupant model produces meaningless clearances. The standard practice is to transform the door trim envelope into the seat/SRS frame, or to evaluate both in the vehicle body frame with the occupant positioned via the SRS.

Building a consistent clearance measurement setup

With the reference geometry defined, the practical workflow for a side clearance study is as follows. The example parameters below are illustrative and should be replaced with program-specific values.

1. Fix the coordinate frame. Work in the vehicle body frame (X forward, Y lateral, Z up) and position the occupant via the SRS. Record the SRS coordinates and the seat track position used.
2. Select the occupant representation. Use an SAE J826 H-point machine surrogate for hip location, and an occupant model (e.g., a 50th percentile male or the program's target percentile) for torso and shoulder geometry. State the percentile explicitly.
3. Define the trim envelope. Offset the nominal door trim surface by the build allowance (e.g., 8 mm) to create the worst-case inboard surface. Include armrest and bolster protrusions.
4. Identify worst-case seat positions. Sweep the seat through its track travel and recline range; record the minimum clearance for each occupant-to-trim pair.
5. Measure and report. For each pair (hip-to-armrest, torso-to-bolster, shoulder-to-sill), report the minimum gap, the seat position at which it occurs, and the H-point definition used.

As a worked example, suppose the design H-point is at (X = 1200 mm, Z = 620 mm) in the body frame, the actual H-point is expected 10 mm lower at Z = 610 mm, and the door bolster nominal surface is at Y = 380 mm with an 8 mm build allowance bringing the effective surface to Y = 372 mm. If the occupant hip half-width at the H-point is 180 mm and the hip centerline is at Y = 180 mm, the nominal hip-to-bolster clearance is 372 − (180 + 180) = 12 mm. At the actual H-point, the bolster's vertical profile may be wider or narrower, so the same calculation must be repeated at Z = 610 mm. A 12 mm nominal clearance that drops to 4 mm at the actual H-point is a packaging concern that would be invisible if only the design H-point were checked.

Common failure modes and how to catch them

Three errors recur in side clearance studies. First, frame mixing: the door surface is taken from body-frame CAD while the occupant is positioned in a seat-local frame, producing a gap that is offset by the seat's installation angle. The symptom is a clearance that changes when the seat is re-installed in a different position without any geometry change. Second, nominal-surface measurement: using the CAD trim surface without a build allowance understates intrusion and hides marginal clearances. The symptom is a design that passes CAD review but shows contact in physical bucks. Third, single-position checking: evaluating only the design seat position misses the worst case at full travel. The symptom is a clearance that looks adequate at nominal but fails when the seat is moved to its extreme.

The corrective practice is procedural: lock the coordinate frame and H-point definition at the start of the study, apply the trim build allowance as a documented offset, and sweep the seat travel envelope rather than checking a single position. When these three controls are in place, the clearance numbers become comparable across CAD, CAE, and physical test — which is the foundation for the occupant-to-door clearance development that follows in later chapters.