INTRODUCTION: The rolling resistance of a roller ski wheel constitutes an important factor determining the outcome of roller ski competitions, yet there is a lack of equipment for effective characterization of the same. In the past this was less of a problem as most competitions were organized without restrictions on allowed wheel types. Each participant could freely select whichever wheel material they deemed most competitively advantageous, similar as is the case on snow. However, in recent years it has become increasingly common for roller ski competitions to restrict which wheel types are allowed, most commonly by restricting allowed wheels to so-called "standard-roll wheels" (also known as "grade-2 wheels"). Nevertheless, there exists no quantitative definition of standard-roll, and the variation among common available brands is large. The situation necessitates an instrument that can characterize the rolling resistance of roller skis with high precision and with efficiency in the testing procedure. A couple of attempts to develop such instruments have been done in the past, e.g., [1] presents an instrument based on a miniaturized treadmill and [2] presents an instrument based on a strain gauge. However, both these set-ups have drawbacks in terms of limited portability and/or that they require the wheels to be separated from the ski. The purpose of this work is to build a portable instrument for non-destructive characterization of roller ski resistance. METHODS: Fig. 1 illustrates the instrument designed in this work. It is based on an electrical motor driving a flywheel that drives a roller ski wheel. The power consumption of the electrical engine is a function of the frictional force between the roller ski wheel and the metallic flywheel. In its final implementation the motor is connected to a micro-computer programmed with a behavioral model for calculation of roller resistance as dependent on speed and load. RESULTS/DISCUSSION: The developed instrument has been validated by means of five different roller ski wheels characterized by the instrument as well as on treadmill, measuring rolling resistance using a dynamometer. The correlation is presented in Fig. 2. In addition to the strong correlation demonstrated in Fig. 2, the reliability of the instrument is also validated by means of repeated measurements, revealing a precision better than 1.1% assessed from 10 repeated measurements. CONCLUSION: A portable instrument for non-destructive rolling resistance characterization of roller skis has been developed. This tool can quantify the performance of different wheels and thus contribute to more fair roll-ski competitions in the future.
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