Developed by ViaTech in collaboration with the Norwegian Public Road Administration, ViaFriction is a friction measurement device used as reference for continuously measuring the longitudinal friction on road and airports according to standards (ASTM E1551-08, ASTM F408-99, CEN TS 15901-2, CEN TS 15901-14). This system is also used for the approval of newly built roads and for accident reconstruction. Based on the seasonal period, there are two ViaFriction systems which can be used: the summer and winter system. In this article the winter continuous measurement device is described.
The measurement system is portable and consists of a light trailer attached on the back side of a vehicle. The system unit is designed for measure friction in wheel tracks, however, the friction unit can be mounted anywhere within the width of the vehicle. If required, multiple systems can be mounted on a same vehicle. The measuring unit can also be attached to a vehicle with a lift up system. The friction system is controlled by a PC. The PC is connected to the friction system either using an ethernet cable or using WLAN. The PC enhances the control of the system directly from the driver position. The measurement system consists of a grooved standardised testing wheel with a diameter of eight inches, which is designed for measure friction with a speed range between 15 and 80 km/h. With a specifically designed braking system, the measurement wheel is forced to rotate at lower speeds compared to the vehicle speed. The difference in speed between the vehicle speed and the rotating speed of the testing wheel enhances the control of the slip ratio from 0.01 to 0.75. Typically, the system is using a slip ratio of 0.2 in order to measure the maximum available friction over the measuring underlying surface.
When the operator is ready to measure friction, the measuring wheel is lowered to the underlying pavement (if using lifting mechanism) and a normal load of 75kg is applied to the wheel. This guarantees the continuous contact between the tyre and the pavement for all the measurement session. When the measurement vehicle is driving, the friction unit is activated and starts to record the longitudinal friction.
The monitoring of long road networks is enhanced with the use of the surveillance mode. When activated, road friction is measured only if is below a threshold level specified by the system operator. When the vehicle is driving over a high friction surface, the measuring wheel is freely rolling. When the vehicle is driving over a slippery surface, the system controller forces the measuring wheel to measure friction at a certain slip ratio. The strength of the surveillance mode is the ratio between the long driven distance and the significantly lowered testing tyre wear. In such conditions the road operator is able to monitor the road friction over more than 400km per day.
In order to check the reliability and reproducibility between all the different units, every year, in Norway, the units gather together in a test area for the harmonisation process. The harmonisation process is defined as the calibration of each single system against the reference unit. At the end of the harmonisation process, all the units are able to provide the same results while measuring the road friction. Therefore, for the same weather, time and road surface condition, the measurements obtained from a single ViaFriction unit are highly repeatable.
Today, the system is used by authorities, road maintenance, research institutes and industrial partners for several purposes such as road monitoring road inspections, accidents reconstruction and testing for car manufacturing. The tests are performed in any condition such as bare, wet, snowy, icy salted and sanded roads.
