*4.1. LFT Surface in Motion*

The layout of LFT in which the moving part is the rough surface is a setup adopted by numerous researchers Lorenz et al. [46,47] carried out a rubber friction study for a tread rubber sample sliding on an asphalt road specimen taking advantages from this kind of setup.

More in detail, the friction measurements have been carried out using an in-house developed test rig, schematically shown in Figure 9, composed by a lower steel sledge where the rough surface sample is clamped. The sledge is moved using a voice coil actuator, capable to generate a constant force. To control the actuator and its speed, the position of the sledge is measured using an analog magneto-strictive linear position encoder. On the upper side of the device there is an aluminium plate which on one side (the lower surface) allows to attach the rubber sample; on the other (upper surface), encapsulates a heating system; in this way it is possible to ensure a homogeneous distribution of the heat, ensured by inserting the device in a temperature-controlled chamber where the temperature can be controlled. The nominal normal load can be varied through the application of calibrated weights on the upper plate, allowing the contact force between tread sample and rough surface to be changed. The friction force is measured using a bi-axial load cell mounted in line with the sample holder. The main technical specifications are shown in Table 3.

**Figure 9.** Schematic representation of LFT surface in motion devices.

**Table 3.** Technical specification of the LFT adopted by Lorenz et al.


<sup>−</sup> <sup>−</sup> − With the LFT, the authors aimed to investigate contact forces between tread blocks and road surfaces. The tests were conducted under different sliding conditions, varying both the velocity in the range 10−<sup>6</sup> < v < 10−<sup>3</sup> m/s and temperature at three different levels, −8 ◦C, 20 ◦C, 48 ◦C, respectively. The rubber test sample is cut out from a tread compound used on summer tyres for passenger car. The measured friction forces are then shifted according to the Williams, Landel and Ferry (WLF) equation [48].

It is worth noting that this device allows measurements only in a small range of velocities and loads, therefore it is not suitable to simulate the real conditions of tyre/road contact. A similar LFT layout was adopted by Lang and Kluppel [49], designed for the experimental investigation of the load and temperature dependences upon the dry friction behaviour of racing tyre tread compound in contact with rough granite. The device employed for the study was constructed and developed at IMKT, University of Hannover. The scheme of the device is shown in Figure 10.

**Figure 10.** Schematic representation of the friction tester used for measurements [49].

The test rig is composed by the following main elements: An electric motor; an arm on which a rubber specimen is housed; a force transducer, interposed between the rubber sample and the arm; a tank, moved by the motor through an actuator, to host a road specimen; a granite road specimen and a temperature chamber. The nominal normal force acting on the tread sample can be varied. Through the application of different values of the load, on the upper side of the arm, it is possible to get a different value of the contact pressure between tread sample and rough surface. The main technical specifications of the test rig are shown in Table 4.

**Table 4.** IMKT, University of Hannover LFT technical specification.


The tests, aimed to investigate the friction coefficient, were conducted under different sliding conditions, varying both the velocity and the temperature aspects. Measurements in different velocity conditions from 0.1 mm/s to 300 mm/s have been carried out changing the load between 1 bar and 7 bar at six different temperatures (at 2 ◦C, 10 ◦C, 20 ◦C, 40 ◦C, 70 ◦C and 100 ◦C).

−

The rubber test sample was cut from a tread compound of a racing tyre and moved in sliding contact on two different granite surfaces: coarse and fine. In [49] is reported the friction coefficient carried out form the experimental activity, both for the coarse and for the fine granite, at different load levels in the range 1–7 bar and for six different reference temperature.

Compared to the other FTs presented in the review, the device adopted by Lang and Kluppel seems to be one of the most complete since it allows one to operate in both dry and wet conditions; to investigate the effects of the utilization of interface lubricants and the use of real asphalt and tread samples. Moreover, a wide range of applicable loads and the temperature control environment allows measurements in almost all tyre-road contact operating conditions of passenger or racing automotive applications.

The last device belonging to this category is reported in O'Neil et al.'s [50] studies, where the authors performed an experimental investigation to predict tyre behaviour on different road surfaces. To perform friction measurements, the authors have used a LFT. The device, constructed at the University of Surrey (Guildford, UK), has a layout very similar to the previous one, reported in its schematic representation and in the real arrangement in Figures 11–13.

**Figure 11.** Schematic representation of the friction tester at University of Surrey.

**Figure 12.** View of the LFT at University of Surrey [51].

**Figure 13.** Internal test chamber view [51].

A direct-drive linear motor, located outside the climate chamber, moves a sledge where the test surface is clamped. The sledge is embedded in the climate chamber and is supported by a linear rail system. The rubber sample is placed above the test surface by a rigid frame, between the sample holder and the frame is interposed a three-axis force sensor. The tread sample is pressed against the test surface through the application of calibrated weights on the upper side of the sample holder; in this way it is possible to vary the contact pressure between tread sample and rough surface. The main technical specifications are summarized in the Table 5.

**Table 5.** Technical specification of the LFT at University of Surrey.


In [50], tests were conducted on a rubber sample cut from a passenger car tyre and driven on sandpaper. Measurements have been carried out for different velocities from 0.03 mm/s to 10 mm/s at sixteen different temperatures ranging from −40 ◦C to +50 ◦C. The main technical difference between the above test rig and the one at IMKT (University of Hannover), regards only the maximum sliding speed reachable (about 0.05 m/s).

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## *4.2. LFT Rubber Sample in Motion*

In the current section a second family of LFT is analysed, characterized by the fact that the moving part is the tread block sample. A first example of this kind of layout is reported in the studies conducted by Lahayne et al. [52,53], in which the results of the friction coefficient measurements are carried out using a LFT developed at the Institute for Mechanics of Materials and Structures at Vienna University of Technology [54]. The schematic representation and the core of the test bench are shown in Figure 14.

**Figure 14.** Schematic representation of LFT at Vienna University (adapted from [55]).

− The LFT consists of a rigid frame that contains both interchangeable test surfaces and a linear motion unit. The linear unit holds and pulls a sledge that incorporates both the tread block specimen and the test equipment. This sledge contains: a pneumatic system used to generate the normal load, a sample holder and a piezoelectric force sensor which measures the contact force between tread sample and rough surface. The frame was specially designed to hold a high-speed camera and a pyrometer to acquire the deformation and the temperature of the sample during the tests. The device is located in a climate chamber.

− The main technical specifications are summarized in Table 6.


**Table 6.** Technical specification of the LFT at Vienna University.

The tests were performed by the authors under different sliding conditions, varying velocity, temperature, rubber materials and rough surfaces. In order to know how the temperature of the tread samples changes during the tests, an optical pyrometer pointing at the tread sample, measures the temperature on the contact surface in the initial position; while a thermocouple introduced into the tread block to record the temperature data during the test. In [52] are discussed the friction coefficient measurements for six tyre materials done on concrete and asphalt at 18 ◦C and 27 ◦C. Also this test bench offers the possibility to operate in both dry and wet conditions.

At Aalto University of Technology (Helsinki Finland), another example of this kind of LFT, called mini-µ-road (MMR) FT, was developed [56,57]. The MMR, presented in Figure 15, was specifically designed for low-friction testing [58]. μ

**Figure 15.** Representation of Mini-mu-road linear friction tester (adapted from [58]).

The experimental device consists of an aluminium frame that contains both the interchangeable test surfaces and a linear motion unit. It is equipped with a servomotor, whit a power of 4 kW, to drive a sledge supported by a linear rail system. The sledge holds and pulls a rigid frame that incorporates both the tread block specimen and the test equipment. This frame allows to host: a pneumatic cylinder used to generate the normal load, a sample holder, two piezoelectric load cells which measure the contact force between tread sample and rough surface and an amplifier module. The entire system is located in a climate chamber and is controlled by LabView. The main technical specifications are summarized in Table 7.


**Table 7.** Technical specification of mini-mu-road linear friction tester.

− In their paper [59] Kärkimaa, and Tuononen presented the results of a study conducted on tread rubber sample sliding on an asphalt road specimen. The tyre tread samples were provided by Nokian Tyres. The tests were conducted under different sliding conditions, varying the velocity and temperature. Measurements for different velocities from 1 mm/s to 1000 mm/s have been carried out at different temperatures with uncertainty about +/− 1 ◦C.

This device has a layout very similar to the one at Vienna University, the only technical difference is the maximum sliding length that the device can reach, which in this case is about 1000 mm. Among the examined devices, this is the second for sliding length.

Le Gal [60] in his Ph.D. thesis worked on a characterization of the friction coefficient by means of two different testing methods. Previously, a stationary friction experiment was performed using a modified Zwick universal test rig and subsequently a modified version of the MTS biaxial servo-hydraulic testing facility was used to extend the range of measurements and to simulate the typical loads in tyre application. Figure 16 shows a complete view of the MTS modified test facility layout.

It consists of a vertical cylinder positioned at the base of machine's floor, connected in series with a biaxial load cell for measures both the normal and the tangential force. An aluminium tank mounted on the load cell holds an interchangeable test surface and allows the possibility to use a liquid in order to simulate wet condition. A second cylinder, perpendicular to the vertical axis, on which is mounted an aluminium plate allows both the horizontal movement, and the possibility to fix the rubber sample

with a maximum size of 80 × 80 mm<sup>2</sup> ; both axes (cylinder) are displacement controlled. The contact pressure between the rubber sample and rough surface can be varied assigning different displacement values to the vertical axis; on the other side, assigning different displacement time histories to the horizontal axis, it is possible to obtain different sliding speeds. The main technical specifications are summarized in Table 8.

**Figure 16.** Picture of a modified biaxial MTS testing machine for friction tests (**a**) and schematic representation (**b**) [60].


**Table 8.** Technical specification of the modified biaxial MTS testing machine for friction tests.

Figure 17 depicts a typical measure of the contact forces. Figure refers to tests conducted at the sliding speed of 4 mm/s on a styrene-butadiene rubber (SBR) sample filled with 60 phr N339 and at the experimental pressure of 0.25 MPa between the rubber sample and the fine asphalt sample.

**Figure 17.** Measured normal and friction forces at constant velocity of 4 mm/s [60].

The high-speed tribometer developed at the Institute of Dynamics and Vibration Research (IDS) at the Leibniz Universität Hannover [61], also belongs to the LFT rubber sample in motion category. Such tester, also called HiLiTe, along with HSLFT manufactured by Altracon [62,63] is representative

for a class of test rigs able to reach high velocities. The HiLiTe test machine, shown in Figure 18, was designed and developed in such a way to be able to simulate all relevant tyre testing conditions.

**Figure 18.** HiLiTe test rig [64].

− It consists of a 5 m long linear guide rail in which a carriage is driven by a servomotor with a toothed belt and performs a linear movement. The carriage mounts all the test equipment and the rubber sample. Embedded on the carriage is a pre-stressed helical spring by means of which it is possible to generate the normal load between the sample and the road surface. It is actuated by a pneumatic actuator through which it is possible to set the normal force in the range of 23–1000 N. The carriage also contains a bi-axial piezoelectric force transducer which measures the normal and tangential components of the contact force between the tread sample and the test surface. The track can be equipped with any surface. The entire test bench is located in a climatic chamber so that experimental investigations can be conducted in a temperature range from −25 ◦C to 60 ◦C. The climate chamber also allows humidity control. The main technical specifications are summarized in Table 9.


**Table 9.** Technical specification of HiLiTe LFT.

− The HiLiTe machine allows to use a number of different test tracks to reproduce a great variety of outdoor environmental conditions.

The Researchers of University of Hannover have performed several experimental investigations by means of HiLiTe [65,66] adopting concrete and asphalt tracks for dry and wet testing as well as test tracks made from ice and snow. In their paper Rosu et al. [64] performed an experimental investigation of the contact between an aircraft tyre rubber and rough surface. The experimental results, shown in Figure 19, were conducted by a rubber tread sample, measuring 20 × 20 × 10 mm<sup>3</sup> , on an asphalt test track. Measurements at fixed sliding speed of 4 m/s with varying the load, by step, at four different temperatures have been carried out.

This is the only test rig, among those discussed in this review, with a top speed reaching 10 m/s, and the only device with a five-meter-long test track. Its technical specifications allow to explore the widest range conditions of contact between tyre and road.

**Figure 19.** Friction coefficient measurements, as function of the temperature (**a**) as function of contact pressure (**b**) [64].
