Rocky road – surface roughness impacts on rubber friction

Mona Mahboob Kanafi

Research output: ThesisDoctoral ThesisCollection of Articles


Rubber friction is a topic of great importance for tyre/road friction. Tyre manufacturers and road authorities are striving to improve the friction between tyre and road pavement, without sacrificing other design parameters. Despite advancements in the rubber friction theories, yet, they cannot adequately predict friction even in laboratory tests. This arises from the complexity of filled rubbers, the multi-scale road surface roughness and the partial contact of the tyre with the road.This dissertation aims to improve the current understanding about surface roughness impacts on rubber friction. A field experiment reported actual macro- and micro-roughness variations of road surfaces. Applicability of surface roughness power spectrum (PSD) as a roughness characterisation technique was evaluated for tyre/road friction studies. Top topography PSD was employed to characterise roughness only on the positive texture, where tyre meets road. Unique potential of three-dimensional (3D-) printing technology for tyre/road friction studies was introduced. Randomly rough fractal surfaces with different macro-roughness were 3D-printed for rubber friction tests. Measurements were performed at various sliding velocities, temperatures and with different rubber compounds. Results were compared to the predictions of viscoelastic friction to address sources of uncertainties in the friction prediction. Based on the field experiment, the full surface topography of pavements undergoes changes in macro- and micro-scale. Hence, the PSD calculated from the full surface topography cannot show the real connection between surface roughness and friction. However, when top PSD was used for surface characterisation, a strong correlation was found between the friction and the roughness, but only at short-scale surface roughness. It was concluded that rubber penetration depth onto each pavement is a necessary pre-knowledge for finding meaningful relation between friction and longer wavelengths. Using 3D-printed substrates, for the first time, effect of macro-roughness on dry rubber friction was confirmed experimentally. Macro-roughness tended to reduce rubber friction. Yet, the magnitudes of the impacts were dependent on the test conditions. Based on the experiments, the friction variations as a function of the velocity exhibited a "bell curve" shape over 6 velocity decades. Measurements showed that the friction-velocity curve shifted roughly by one decade in velocity for 15°C reduction in temperature. Two main sources of uncertainties in the friction prediction were identified as I. inadequate knowledge about the large strain viscoelastic modulus of rubbers, and II. difficulty in determining the short-wavelength cut-off q1 of the surface roughness. Macro-roughness effects seem to be more visible when the hysteresis friction along with the frictional heating, are the dominant phenomena. The research results have direct relevance for tyre/road friction, particularly for road surface roughness characterisation and fundamental studies about roughness impacts on rubber friction.
Translated title of the contributionRocky road – surface roughness impacts on rubber friction
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Tammi, Kari, Supervising Professor
  • Tuononen, Ari J., Thesis Advisor
Print ISBNs978-952-60-7333-0
Electronic ISBNs978-952-60-7332-3
Publication statusPublished - 2017
MoE publication typeG5 Doctoral dissertation (article)


  • tyre/road friction
  • rubber friction
  • road surface roughness
  • 3D printing

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