**4. Linear Friction Testers**

The first linear friction tester (LFT) was developed by Leonardo da Vinci at the end of the fifteenth century and the concept of his device is still valid and adopted in recent studies [43,44]. Indeed, in their recent study [45], Tolpekina and Persson analysed adhesion and friction in tyre tread compound, conducting an experimental investigation trough a LFT like the Leonardo da Vinci type. The schematically representation of the tester is available in Figure 7.

**Figure 7.** Schematic representation of the Leonardo da Vinci set-up [45].

The layout of the device is as simple as smart: it consists of a table on which a substrate is fixed and on which a rubber sample can slide. The latter can be loaded by means of a calibrated weight and moved by adding a weight in a container connected to its free end. Nowadays this type of test rigs is widely used, perhaps is the most used device in the study of friction and wear in tyre tread block.

In the modern era, Grosch's tribometer is considered one of main examples of this category of testers [25]. The device, shown in Figure 8, consists of a test sample pad of about 2.5 cm<sup>2</sup> and 0.5 mm thick, attached to a sample holder and pressed against the test surface by a calibrated load. To measure the frictional force a U-spring dynamometer is connected to the holder by means of two steel wires. The test surface is driven by a motor coupled to a multi-ratio gear box, that provides sliding speeds ranging from 10−<sup>6</sup> to 3 cm/s. The sample and the track surface are enclosed in a temperature and humidity-controlled chamber. −

**Figure 8.** Schematic representation of Grosch's tribometer. (**a**) Specimen holder, (**b**) narrow tunnel for preheating the surfaces, (**c**) dynamometer, (**d**) self-tightening drive, coupled to gear box and motor; normal load applied at (**e**) [25].

Over the years, researchers have adopted various layouts to improve the friction coefficient measurements, evolving from the principles developed by Grosch's works. Given the wide range of existing LFT devices for convenience, in the following, they are distinguished in two categories:

• LFT in which the moving part is the rough surface

−

• LFT in which the moving part is the rubber sample/specimen
