**3. Rolling Friction Testers**

This category of devices makes use of the principles of double disc tribometers, which consist of two discs rotating against each other. This kind of tribometers are also known with different names such as: ring on ring, roll on roll, rolling sliding apparatus, etc. In the field of tyres, these tests equipment reproduce, on a small scale, the design of the "tyre on drum" test machine.

Liu et al. [40,41] have developed a high-speed rolling test rig aiming to simulate the impact and release mechanisms of tread block. As shown in Figure 3, the device consists of a small wheel with a rubber belt coating, which drives a big wheel with a steel surface. The small wheel is mounted directly on a solid base and is driven by an electric motor, while the big wheel is mounted on a solid base by a moving rocking arm; lastly another couple of stiff arms are used to lock the wheel shaft position, to obtain the desired value of interference between the two wheels and to apply a compression to the tread block sample.

**Figure 3.** Rolling test rig (**a**), main component of driving wheel (**b**) [41].

The main elements of the driving wheel are:


The main technical specifications are summarized in the Table 1.


**Table 1.** High-speed rolling test rig technical specification.

With the aim of the investigation regarding the contact forces between the tread block and the road, the authors conducted several tests, varying both the speed (at three speed levels of 150, 300, 600 rpm), and the interferences in the 0.1–0.3 mm range.

With the high-speed rolling test rig, it is possible to simulate the impact and release mechanisms of a tread block. The main drawback of this device is related to the use of the asphalt surface, for two reasons: firstly, it is difficult to make a curved asphalt sample; secondly, when the big wheel is covered by a rough surface, the tread-block sample comes in contact with the wheel only partially.

A different approach for rolling FT is proposed by Lundberg et al. [42], involving a new experimental device that allows detailed studies of the rolling contact force between a tread block and a relative substrate. The device, called compact internal drum (CID), aims to simulate a realistic impact and release mechanism for the tread block-substrate contact and enables force measurements.

Figure 4 shows the core of the device, consisting of two wheels: a solid metal wheel (1) with a cut-out window to accommodate a force link (2) interposed between the tread block sample and the inner face of the wheel. The tread sample is obtained by cutting a tread strip from a truck tyre re-treading material, like the one shown in Figure 5. The solid wheel rolls on the inner surface of the second wheel (drum) (3), that can be covered by an interchangeable rough surface.

**Figure 4.** The core of the test rig [42].

**Figure 5.** Truck tyre retreading material (**a**), tread strip (**b**) tread strip main dimension (**c**) [42].

In Figure 6, the test rig is depicted in all its components, the highlighted (numbered) components complete the device description. The solid wheel is driven by an electric motor (4), controlled by a driver, which transmits the movement directly to the shaft of the solid wheel, through three belts, and subsequently from this to the drum through the friction forces established between the rubber mounted on the periphery of the solid wheel and on the inner surface of the drum. The drum shaft is equipped with an automotive-derived disc brake (5), used to deaccelerate the drum; furthermore, it is possible to vary the brake pressure by means of a manually adjustable hydraulic system (9). An optical sensor (7) is used to measure rotation speeds, while a resistive temperature sensor is introduced into a tread block adjacent to the sample tread block, allowing to estimate the real temperature without damaging the reference sample. The data from the sensors are transmitted from the rotating part to the non-rotating data acquisition system via an HBM SK12 slip ring (8). It is possible to prescribe the desired compression load between the tread block sample and the rough surface of the drum, by shifting vertically the shaft of the drum. The normal load between the sample of the tread block and the rough surface of the drum is quantified by measuring the bending deformation induced in the strain gauges mounted on the drum shaft. The test rig is mounted on a concrete block (6). The main technical specifications of the rig are listed in Table 2.

**Figure 6.** Layout of the test rig [42].

**Table 2.** Compact internal drum technical specification.


By adopting this kind of test rig the contact forces between tread block and roads under different conditions can be investigated. Indeed, the test rig layout allows to vary: the velocity, the normal load and the braking pressure, with or without driving torque. In. [42], Lundberg et al. showed the experimental results obtained varying the main parameters involved.

The main advantage in the use of the CID lies in the possibility to operate in a wide range of speeds; furthermore, if compared with the device analysed in the previous section, it offers the possibility to measure the forces generated in the tyre/road contact both in free-rolling conditions or in sliding conditions, simply varying the braking force. Again, a drawback of this layout is related to its geometry that imposes the use of curve asphalt samples which are particularly difficult to produce.
