*4.3. On Field Friction Tester*

It was decided to group this type of FT into a separate category, although the type of movement of the sample is still linear since these FTs have been specially designed to allow measurement "on field".

The FT designed and developed at the Centre for Tire Research (CenTiRe), a consortium of tyre and tyre-related industry members with two word-class universities (Virginia Tech and the University of Akron), belongs to this type of tester. This innovative device is an unmanned ground vehicle [67]. The robot shown in Figure 20, aims to reproduce all relevant tyre testing conditions on any surface and in both indoor and outdoor experiments.

**Figure 20.** (**a**) The six-wheel small ground robot (**left**) [68]; (**b**) 3D representation of sample holder (adapted from [67]).

The device consists in a chassis with six wheels and an assembly of all the essential parts of the device, in which, four of these wheels are driven by a brushed permanent magnet DC motors, coupled directly to the wheels' shafts, and the other two are nondriven wheels. Two encoders are used to measure the rotational velocity of the wheels, one fixed to the driven wheel the other to the undriven wheel, so that the slip of the wheels can be calculated. Attached to the chassis a dragging arm holds and pulls the sample holder attached in series at load cell. The sample holder is built to fit a T-type sample with different size in the range of 25.4 × 25.4 mm<sup>2</sup> to 50.8 × 50.8 mm<sup>2</sup> .

The main technical specifications are summarized in Table 10.


**Table 10.** Technical specification of the six-wheel small ground robot.

The six-wheel small ground robot was developed to study friction and wear of a tread block sample on different surfaces and under different operating conditions. Emami et al. [67] showed the results of an experimental investigation conducted on a styrene-butadiene rubber (SBR) sample sliding on an asphalt track. The experimental results were conducted outdoor on a clean asphalt track at an outside temperature of 24 ◦C. The SBR sample used has dimension equal to 25.4 × 25.4 mm<sup>2</sup> and it is loaded with a weight of about 4.5 kg which corresponds to an equivalent pressure between the sample and the road of 81 kPa.

Although the robot was designed to perform measurements on field, it also allows indoor measurements, but these would require very large spaces specially if high speeds are to be achieved. The main limitation of this equipment consists of a particularly small normal load allowed.

Another type of on field FT was developed at the department of the "Forschungsgesellschaft Kraftfahrwesen mbH Aachen (fka) and the Institut für Kraftfahrzeuge (ika)" of RWTH Aachen University. The FT, called LiRep [69], is a portable device developed to study friction and wear of a tread block sample on real road surfaces under different operating conditions. The device, shown in Figure 21, consists of a four-wheeled frame to facilitate transport, therefore these wheels can be easily lifted or removed for performing the tests. The core of the device is a ball screw linear axis driven by a servo motor, that drags a "sensing head" (carriage/sledge) on which the tread-block sample, 3-axis force transducer and the loading weights are mounted. The "sensing-head" allows to install samples of a maximum size of 60 × 60 mm<sup>2</sup> , and to add a calibrated weight up to 60 kg. Depending on the contact surface of the sample, these weights correspond to an equivalent pressure between the sample and the road up to 3.5 bar. All data acquisition system, cables and the control unit are located on the chassis. Furthermore, the test area can be covered with a climate chamber to study the effect of temperature on the friction coefficient. The main technical specifications are summarized in Table 11.

(a) (b)

**Figure 21.** (**a**) LiRep general view (left); (**b**) detail of two different road surface configurations [70].


Compared to other FT shown in this category, the main difference is that this device is not a mobile device but rather is attached to the floor, so in this case, it is easier to carry out laboratory tests since the speed and length of the sliding are not linked to the movement of the device.
