**6. Summary and Conclusions**

In this review, the studies and the developments carried out with the aim to test and analyse frictional and wear behaviour of viscoelastic materials, and of tyres tread in particular, were examined by comparing the different working principles of different so-called friction testers, with an in-depth focus on a selection of them, representative of each category.

In the authors' research activity on tyre analysis and friction modelling, the need of a wide overview of the state of the art in tribological testing on viscoelastic materials was experienced. Due to the lack of such kind of information, the aim of the review is to resume devices and methodologies developed in the far and recent past, in order to give suggestions and ideas to who could be interested to build a friction testing bench or to evaluate the optimal methodology to get data concerning interaction phenomena between a rigid and a viscoelastic material.

It has been found that the main variables affecting tyre friction in local contact are sliding speed, contact pressure, tread temperature, road (or in general counter-surface) roughness and compound viscoelastic characteristics. Consequently, most of the developed friction test benches allow to the user to vary such variables in a range due to space and to technological constraints.

Friction coefficient is expressed as the ratio between the tangential and the normal load acting on the tested specimen, and for this reason the rigs are usually equipped with load cells and with control systems that guarantee stability and precision in the test setup and in the conduction of the experiments.

A key obstacle encountered in the tyre friction testing regards the possibility to vary single working conditions, keeping the others constant. The difficulty is due to the mutual and deep interconnections among the phenomena arising at tyre/road contact, and in many of the described applications the solution has been fund involving very small variations in the working conditions. On the other hand, real tyre working conditions are characterized by significant values of speed and pressure, not easily

reachable and controllable by laboratory benches. For this reason, "on field" friction testers have been developed, able to observe real contact conditions, despite the lower level of repeatability and precision of the measurements linked to the noise and the random nature of the asphalt profile.

The variation of contact pressure in the indoor benches is usually controlled by the actuation of controlled pneumatic systems, while the speed management is highly linked to the motion principle of the bench itself (linear, rotational, etc . . . ).

Some of the illustrated devices, moreover, are installed in a climate chamber that allows a reliable control of temperature, highly affecting the tread polymers viscoelastic behaviour. Such solution improves the repeatability of the measurements and gives the opportunity to test the specimens also in uncommon or hardly reachable environmental conditions, but increases significantly the cost and the volume of the experimental setup.

Finally, some friction testers have been developed re-adapting systems conceived for different scopes (like pin on disk benches, often adopted for hardness and wear measurements in metal-metal sliding contact), substituting some elements with specimens of rubber to be tested.

As a final summary, a table of FT features (Table 15) has been developed, with the aim to provide a global overview of the described benches, each with its peculiar characteristics and advantages/disadvantages.


**Table 15.** Friction testers features resume.

In conclusion, friction, and in particular friction between a viscoelastic material and a randomly rough and rigid surface, still represents a field in which significant efforts in terms of research and testing are daily spent. Each of the benches reported and analysed is useful to study some specific aspect of very complex phenomena, but it seems that an ultimate technological solution, able to reproduce the global effects involved in tyre/road sliding contact in a fully satisfying way, has not been developed yet. The improvements in control systems and in the accuracy of the measurement devices are leading to increasingly evolved benches, able to lead researchers and industries, interested in a fundamental topic for mobility, road safety and polymers science, towards a deeper understanding of contact mechanics.

**Author Contributions:** Conceptualization, A.G. and F.F.; methodology, A.G. and G.A.D.; validation, A.S. and F.F.; formal analysis, A.S. and F.F.; investigation, A.G. and G.A.D.; data curation, G.A.D.; writing—original draft preparation, G.A.D., A.G., F.F. and A.S.; writing—review and editing, A.G., F.F. and A.S.; visualization, G.A.D. and F.F.; supervision, A.G. and A.S. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
