The discrepancy between the measured friction coefficients and the theoretical predictions, see Fig. 2, may partially result from viscoelasticity of the rubber material used in the present study. Note that in the theoretical model it is assumed that the material is elastic so that the possible viscoelastic effects cannot be captured. At the same time, these effects may influence the actual deformation pattern and contact pressure, and, as a result, the lubrication conditions. It is also noted that the discrepancy is not caused by a possible inaccuracy of the estimated Young's modulus: it has been checked that the agreement of theoretical and experi- mental results cannot be significantly improved by varying the Young's modulus.