4. Occupant packaging and belt fit

Belt geometry robustness begins with packaging. Before any restraint performance can be evaluated, the belt must physically land on the right landmarks of the occupant: the lap belt on the anterior superior iliac spines (ASIS) and the shoulder belt across the mid-clavicle and sternum. Whether it does so is determined almost entirely by the relationship between the seat, the occupant's H-point, the torso angle, and the anchorage locations. This section explains that relationship and shows how to reason about it quantitatively.

The packaging reference frame

All belt-fit reasoning depends on a consistent coordinate convention. In this course we use the vehicle coordinate system: X positive forward, Y positive to the occupant's right, Z positive upward, with the origin at the vehicle datum. The H-point is the hip pivot of the seated occupant (or of the SAE H-point machine); the seating reference point (S-R-P) is the manufacturer's design H-point. The torso angle (often called the seat-back angle) is measured from vertical, positive when the torso reclines rearward. Belt anchorages are specified as points in the same frame: the D-ring (shoulder anchor), the retractor, the buckle, and the two lap-belt outboard anchors.

Two derived quantities dominate belt fit. The first is the lap-belt angle in side view — the angle of the lap belt relative to horizontal as it crosses the pelvis. The second is the shoulder-belt angle in front view — the angle of the diagonal belt relative to horizontal as it crosses the chest. Both are functions of anchor position and occupant geometry, and both have well-established target ranges.

Why the H-point and torso angle matter

The H-point fixes the pelvis in space. The lap belt must pass over the ASIS, which sit roughly 30–50 mm forward and slightly above the H-point depending on the occupant. If the lap belt is too high, it crosses the soft abdomen; if too low, it slides off the iliac crest onto the thighs. The acceptable band is narrow — on the order of 40–60 mm of vertical tolerance for a mid-size male, less for smaller occupants.

The torso angle then determines where the shoulder belt crosses the chest. As the torso reclines, the clavicle moves rearward and downward relative to the D-ring, which changes the shoulder-belt angle and the lateral position of the belt on the chest. A reclined torso also rotates the pelvis, which can pull the lap belt upward unless the lap anchor geometry compensates.

How belt geometry maps to placement

Consider the lap belt in side view. The belt runs from the outboard lap anchor, across the pelvis, to the buckle. The lap-belt angle is the angle of that segment to horizontal. A common design target is roughly 45°–65° from horizontal for a mid-size occupant, with the belt passing over the ASIS. If the outboard anchor is too far forward or too high, the angle steepens and the belt rides up; if too far rearward or too low, the belt lies on the thighs and provides poor pelvic engagement.

In front view, the shoulder belt runs from the D-ring down across the chest to the buckle. The shoulder-belt angle to horizontal is typically targeted in the 50°–70° range for a mid-size occupant, with the belt crossing the mid-clavicle and the sternum. A D-ring that is too far inboard pulls the belt toward the neck; too far outboard and the belt slides off the shoulder. The vertical position of the D-ring sets how much of the belt crosses the clavicle versus the upper arm.

The figure below shows the packaging relationship as a side-view schematic: the seat, the H-point, the torso line at a given angle, the lap and shoulder belt paths, and the key anchorages. It is a geometry diagram, not a crash model — its purpose is to make the anchor-to-landmark relationship visible.

Working through a packaging example

Suppose a seat is designed with an S-R-P at X = 1200 mm, Z = 520 mm, and a nominal torso angle of 25°. The D-ring is at X = 980 mm, Z = 980 mm; the outboard lap anchor at X = 1080 mm, Z = 540 mm; the buckle at X = 1290 mm, Z = 560 mm. For a mid-size male, the H-point sits approximately at the S-R-P, and the ASIS is about 40 mm forward and 20 mm above it.

Compute the lap-belt angle in side view. The belt runs from the outboard anchor (1080, 540) to the buckle (1290, 560). The horizontal run is 210 mm, the vertical rise is 20 mm, so the angle to horizontal is arctan(20/210) ≈ 5.4°. That is far too shallow — the belt would lie nearly flat across the thighs rather than over the ASIS. The design needs the outboard anchor lower or the buckle higher, or both, to reach the 45°–65° target. Raising the buckle to Z = 620 mm and lowering the anchor to Z = 500 mm gives a rise of 120 mm over a run of 210 mm, or about 30° — still shallow, which tells the designer the anchor must move rearward as well.

Now the shoulder belt. From the D-ring (980, 980) to the buckle (1290, 620), the horizontal run is 310 mm and the vertical drop is 360 mm, giving an angle to horizontal of arctan(360/310) ≈ 49°. That is within the target band, but the belt's lateral position on the chest still depends on the Y-coordinate of the D-ring, which this side-view calculation does not capture. A front-view check is required.

Sensitivity and robustness

The reason this matters for anthropometry is that every quantity above shifts with occupant size. A smaller occupant has an H-point closer to the S-R-P but a shorter torso, so the clavicle sits lower relative to the D-ring and the shoulder-belt angle steepens. A larger occupant has a longer torso and a wider pelvis, so the lap belt must span a greater distance and the ASIS sits further forward. The same anchor set therefore produces different belt angles for different occupants.

A practical robustness check is to evaluate the lap-belt and shoulder-belt angles at the 5th-percentile female and 95th-percentile male H-point and torso-angle extremes, then confirm both remain inside the target bands. If one extreme fails, the fix is usually an anchor adjustment or a geometry feature — a lap-belt guide, a D-ring height adjustment, or a buckle stalk that positions the latch differently — rather than a change to the seat itself.

What to check before moving to restraint performance

Before any belt is evaluated for crash performance, confirm four things: the coordinate convention is stated and consistent; the H-point and torso angle used match the intended occupant percentile; the lap-belt angle and shoulder-belt angle have been computed in both side and front views; and the belt path has been checked against the ASIS and clavicle landmarks, not just against the anchor coordinates. If those four checks pass across the anthropometric range, the belt geometry is robust enough to carry forward into restraint tuning. If they fail at an extreme, no amount of airbag or pretensioner calibration will recover the pelvic or clavicular engagement that the packaging has already lost.