4. Seat Pan Geometry and Pelvis Loading Path

Why Seat Pan Geometry Governs the Pelvis Load Path

In a frontal impact the pelvis is the first major body region to load the restraint system. The lap belt routes force into the anterior superior iliac spines (ASIS), while the seat pan carries the ischial tuberosities and the posterior pelvis. The pelvis loading path is the sequence of contact and force transmission from belt-to-pelvis, pelvis-to-cushion, and cushion-to-seat frame. Seat pan contour (the transverse and longitudinal shape of the cushion surface) and cushion stiffness together determine how much of the occupant's kinetic energy is absorbed by controlled pelvis travel versus transmitted as a sharp acceleration spike into the spine and torso.

A pan that is too flat and too stiff produces a short, high-amplitude deceleration pulse: the pelvis stops almost immediately, and peak pelvic acceleration rises. A pan with a rearward "bucket" and a softer, progressive foam lets the pelvis sink and rotate slightly, extending the deceleration distance and lowering the peak. The design task is therefore a geometry-and-stiffness trade: enough support to prevent submarining (the concern of the previous case), but enough compliance and contour to stretch the pelvis deceleration over a longer stroke.

Analytical Model of the Pelvis–Cushion Interface

For a first-order design study, model the pelvis as a rigid mass m_p resting on a cushion that behaves as a nonlinear spring with a progressive force–deflection curve. The cushion reaction force is

F_c(δ) = k1·δ + k3·δ³        (progressive foam, δ = cushion compression)

and the pelvis equation of motion during the crash pulse a_vehicle(t) is

m_p · ẍ_p = −F_c(δ) ,   δ = x_p − x_seat ,   x_seat(t) = ∫∫ a_vehicle dt

The peak pelvic acceleration is governed by the maximum cushion force divided by mass, and the maximum force is set by how far the pelvis can compress the cushion before bottoming out on the pan. Two geometry levers change that stroke:

• Contour depth — a deeper rear bucket increases available compression stroke before the ischial tuberosities reach the pan.
• Local stiffness — a softer rear region (lower k1) reduces the early force slope, spreading load over more time.

Use the following teaching parameters (illustrative, not measured): m_p = 12 kg (pelvis plus lower torso share), k1 = 40 N/mm, k3 = 0.8 N/mm³, available stroke to pan δ_max = 45 mm for the baseline flat pan and 60 mm for a contoured pan. A 50 km/h frontal pulse with roughly 30 g peak and ~90 ms duration is applied.

Reading the Load Path Visually

The diagram below shows the two competing load paths through the pelvis and how contour and stiffness redirect energy. The left branch is the stiff flat pan: short stroke, early bottoming, high peak. The right branch is the contoured pan: longer stroke, progressive force, lower peak. The belt path (ASIS) is common to both and is shown for context.

Proposed Geometry Modification

Based on the load-path reasoning, the recommended change is a rear bucket contour with a softer rear foam zone:

1. Increase the rear cushion depression depth from ~15 mm to ~30 mm, giving the ischial tuberosities additional stroke before contacting the pan.
2. Reduce rear-zone foam stiffness from k1 = 40 N/mm to k1 = 28 N/mm, keeping the front and side bolsters firm to preserve lateral support and anti-submarining geometry.
3. Keep the pan frame unchanged; the contour is achieved in the foam and trim, so the modification is low-cost and does not alter the seat structure.

The expected effect is a longer pelvis deceleration stroke and a lower peak pelvic acceleration for the same crash pulse, without reintroducing submarining risk because the front bolster and cushion angle remain unchanged.

Verification and Failure Symptoms

To confirm the modification, run the same pulse through the occupant model and compare peak pelvic acceleration, pelvis forward excursion, and time-to-bottoming. Acceptance criteria for this study: peak pelvic acceleration reduced relative to baseline, no increase in submarining metric (lap-belt-to-ASIS engagement maintained), and no bottoming before pulse end.

Check	Baseline	Modified target	Failure symptom
Peak pelvic acceleration	High (early bottoming)	Lower, broadened	No reduction → contour too shallow or foam too stiff
Stroke used	45 mm (saturated)	Up to 60 mm	Still saturates → increase depth or soften rear zone
Submarining	Acceptable	Unchanged or better	Pelvis slides under belt → front bolster too soft
Bottoming time	Before pulse end	At or after pulse end	Early bottoming → insufficient stroke