How surface roughness affects rubber friction beyond the spectrum

Friction between rubber and rough surfaces is a critical topic for vehicle safety, yet it remains unclear which roughness parameters govern it. While the power spectral density (PSD) describes the wavelength content of surface asperities, it is not known whether PSD alone predicts friction or if the height distribution (PDF) also plays a significant role. To address this, we performed dry-contact and boundary-lubricated friction tests on 3D-printed surfaces designed to share identical PSDs but different PDFs. This approach allowed us to isolate the effect of height distribution while keeping the spectral content constant. The tests revealed substantial differences in total friction despite identical PSDs, demonstrating that the PDF and related roughness parameters significantly influence rubber–road interaction. To support the interpretation of the experimental findings, the results were further compared with spectrum-based friction theories and qualitative Boundary Element Method (BEM) contact simulations. While PSD-based approaches correctly reproduced the hysteretic trends, the BEM analysis revealed systematic differences in contact-area evolution among surfaces sharing identical PSDs but different PDFs, showing qualitative consistency with the experimentally observed friction ranking. Direct measurements of the contact area using pressure-sensitive films further confirmed these differences, suggesting that contact-area evolution is the main mechanism underlying the observed dry-friction variability. These findings provide new experimental evidence beyond the PSD and have implications for tyre design and predictive friction models.