Author(s): Willemijn Idenburg, Valentijn Goudriaan and Zofia Hulshoff

Abstract: Driving comfort during long journeys depends heavily on how the seat cushion distributes the occupant's weight across the ischial tuberosities and surrounding soft tissue. Conventional automotive seat cushions use a single-density polyurethane foam, which produces a pressure concentration directly beneath the sit bones that worsens over time as the foam compresses. This research investigated whether a through-thickness density gradient soft on top, firm at the bottom could reduce peak contact pressure and improve subjective comfort during sustained sitting. Three foam configurations were tested: a uniform 45 kg/m³ foam (control), Gradient A (35 kg/m³ top surface transitioning to 55 kg/m³ at the base), and Gradient B (30 kg/m³ top transitioning to 60 kg/m³ at the base). Thirty participants (15 male, 15 female, age 24 to 58 years, body mass 54 to 97 kg) sat on each cushion in a driving-simulator buck for 90 minutes while a 1,024-sensor pressure-mapping mat recorded contact pressure at 1 Hz. Subjective comfort was captured on a 100 mm visual analogue scale (VAS) at 15-minute intervals. The uniform foam produced a mean peak ischial pressure of 4.83 kPa (SD 1.18). Gradient A lowered this to 3.87 kPa (SD 0.91), a 19.9% reduction (p<0.001). Gradient B achieved the lowest peak pressure at 3.41 kPa (SD 0.72), a 29.4% reduction (p<0.001). VAS comfort scores at the 90-minute mark were 41.2 (uniform), 56.8 (Gradient A), and 63.7 (Gradient B), all pairwise differences reaching p<0.01. The correlation between peak ischial pressure and 90-minute VAS score was r = -0.81 (p<0.001), confirming that pressure distribution is a strong predictor of perceived long-duration comfort. These findings suggest that a density gradient with a top-to-bottom ratio near 1:2 offers the best trade-off between pressure relief and cushion support, and that the approach can be implemented in production seats with minimal tooling changes by pouring a dual-density foam in a single moulding step.

DOI: https://www.doi.org/10.22271/27078205.2026.v7.i2a.80

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