3. Occupant packaging and ergonomics

Occupant packaging is the discipline that fixes where the driver sits, where the steering wheel and pedals land, and how much room the knee bolster leaves for the legs — before any restraint performance is tuned. In a frontal protection program, packaging decisions set the boundary conditions for the airbag module, the steering column stroke, and the knee bolster load path. If the workspace is laid out poorly, no amount of restraint calibration will recover a good result: the driver may sit too close to the airbag, the knees may contact the bolster before the column collapses, or the instrument cluster may block the sightline to the road. This chapter explains how the workspace is defined, which reference points and dimensions govern it, and how to check clearance and reach in practice.

Reference points and the coordinate convention

All packaging work is done in the vehicle coordinate system: X positive forward, Y positive to the driver's left (ISO convention), Z positive upward, with the origin at the front axle or a declared datum. Two reference points anchor everything else. The H-point (hip point) is the pivot of the SAE J826 two-dimensional manikin seated in the actual seat; the R-point is the manufacturer's design hip point. The difference between them, measured in X and Z, tells you how far the real occupant deviates from the design intent. The steering wheel center and the heel point (ball of foot on the pedal) complete the primary triangle that defines the driving posture.

A practical rule: never quote a clearance without stating the reference point and the manikin percentile it was measured with. A 50th-percentile male and a 5th-percentile female can differ by more than 100 mm in knee position at the same seat setting.

Driver workspace layout

The workspace is built from three coupled dimensions: the seat height above the heel point, the seat cushion angle, and the pedal package. Raising the seat improves visibility over the hood but reduces headroom and moves the knee closer to the bolster. A typical passenger-car layout uses a seat height (H30) of 250–320 mm above the heel point, a cushion angle of 12–15°, and a pedal plane inclined 20–30° from vertical. These are starting values, not targets; the final layout must satisfy the reach and clearance checks below.

The steering wheel position is then set relative to the H-point. The steering wheel angle (from vertical) is usually 20–30° for a passenger car, and the wheel center sits roughly 550–650 mm forward of the H-point and 550–650 mm above the heel point. The column axis should pass close to the H-point so that the airbag deploys along the occupant's chest line rather than toward the chin or abdomen.

Knee bolster clearance

The knee bolster is the lower instrument-panel surface that the occupant's knees contact in a frontal impact. Its job is to absorb energy and distribute load across the femur, but only after the column and belt have begun to manage the torso. Clearance is therefore a two-part requirement: static clearance in the normal driving posture, and dynamic clearance during the crash pulse.

Static clearance is measured from the knee surface (the manikin's knee pivot, not the skin) to the bolster face, with the seat at the design position. A common target is ≥ 50 mm for the 50th-percentile occupant and ≥ 25 mm for the 5th-percentile female at full-forward seat travel. Below these values, the knee can contact the bolster before the belt loads the torso, which raises femur load and can push the occupant out of position.

Dynamic clearance accounts for column collapse and occupant ride-down. If the column strokes 80 mm and the occupant moves forward 120 mm relative to the body, the effective gap closes by roughly the difference in their motions. The check is to plot knee X-position versus time from the restraint simulation and confirm the knee does not reach the bolster face before the column has absorbed its stroke. A useful design margin is to keep at least 30 mm of dynamic gap at the time of peak femur load.

Reach and vision requirements

Reach is checked with the SAE J287 hand-reach envelopes and the J826 manikin. The driver must be able to grasp the wheel at the 9-and-3 or 10-and-2 positions with the elbow at 120–150° and the shoulder not lifted off the seatback. A simple check: with the seat at the design position, the wrist should reach the wheel rim with the arm nearly straight but not locked. If the wheel is too far, the driver leans forward and loses the benefit of the belt; if too close, the airbag is within the danger zone.

Vision is governed by the eyellipse — the statistical envelope of eye positions for the occupant population — and by the binocular field requirements. The instrument cluster must be visible through the upper portion of the steering wheel opening, and the road ahead must be visible over the wheel rim. A practical check is to confirm that the 95th-percentile eyellipse tangent to the wheel rim still leaves a clear sightline to a target 10 m ahead at ground level. The A-pillar and header must not intrude into the forward field more than the regulatory limit.

Working through a packaging check

Suppose a program has fixed the seat at H30 = 290 mm, cushion angle 13°, and the wheel center at X = 590 mm forward of the H-point, Z = 600 mm above the heel point. The knee bolster face is at X = 540 mm from the H-point. The 50th-percentile knee pivot sits at X = 380 mm, giving a static clearance of 160 mm — comfortable. Now move the seat to full-forward travel for the 5th-percentile female: the knee pivot moves to X = 470 mm, and the clearance drops to 70 mm. Still acceptable, but the dynamic check is what matters. If the column strokes 70 mm and the occupant rides down 110 mm, the knee closes 40 mm relative to the bolster, leaving 30 mm at peak load — right at the margin. The design action is either to increase the bolster setback by 15 mm or to add a collapsible bolster insert that yields at 2–3 kN.

This is the typical loop: measure static clearance at both seat extremes, run the restraint simulation to get knee X versus time, compare the closing rate against the column stroke, and adjust the bolster or column before the design is frozen. The numbers above are illustrative and must be replaced with the program's actual manikin data and simulation output.

Common failure modes and how to catch them

• Knee contacts bolster before belt loads torso. Symptom: high femur load in simulation, knee X reaches bolster face before peak belt force. Fix: increase static clearance or soften the bolster's initial crush.
• Wheel too close to the occupant. Symptom: airbag module within 250 mm of the chest at design position; driver leans back to compensate. Fix: move the column forward or reduce the wheel angle.
• Cluster blocked by wheel rim. Symptom: 95th-percentile eyellipse tangent cuts through the cluster. Fix: raise the wheel or lower the cluster, then recheck the sightline.
• Pedal reach forces the seat forward. Symptom: 5th-percentile driver cannot reach the pedal at full-forward seat travel without lifting the heel. Fix: adjust the pedal plane or add an adjustable pedal box.

The packaging work is complete when the static clearance, dynamic clearance, reach envelope, and sightline all pass at both seat extremes, and when the resulting geometry has been handed to the restraint team as a fixed boundary condition. From that point, the column stroke and bolster crush characteristics are tuned against a workspace that no longer moves — which is exactly the condition the next chapter's restraint integration work depends on.