1. |
- Hindér, Gustav, et al.
(författare)
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On Ski–Snow Contact Mechanics During the Double Poling Cycle in Cross-Country Skiing
- 2024
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Ingår i: Tribology letters. - : Springer. - 1023-8883 .- 1573-2711. ; 72:2
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Tidskriftsartikel (refereegranskat)abstract
- Of the medals awarded during the Winter Olympics Games, most are awarded for sports involving cross-country (XC) skiing. The Double Poling (DP) technique, which is one of the sub-techniques used most frequently in XC skiing, has not yet been studied using simulations of the ski–snow contact mechanics. This work introduces a novel method for analysing how changes in the distribution of pressure on the sole of the foot (Plantar Pressure Distribution or PPD) during the DP motion affect the contact between the ski and the snow. The PPD recorded as the athlete performed DP, along with an Artificial Neural Network trained to predict the geometry of the ski (ski-camber profile), were used as input data for a solver based on the boundary element method, which models the interaction between the ski and the snow. This solver provides insights into how the area of contact and the distribution of pressure on the ski-snow interface change over time. The results reveal that variations in PPD, the type of ski, and the stiffness of the snow all have a significant impact on the contact between the ski and the snow. This information can be used to improve the Double Poling technique and make better choices of skis for specific snow conditions, ultimately leading to improved performance.
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2. |
- Kalliorinne, Kalle, 1995-, et al.
(författare)
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A Novel Method for Quantifying Ski-Snow Friction Using an Rtk-Gnss Equipped Sled
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In most winter sports, the athletes interact with snow with their equipment. In some of these sport, e.g. in cross-country skiing, a large amount of energy is spent by the athlete to overcome the restive force of friction. Consequently, a reduction in friction can be the difference between winning and coming second in a race. Over the years, researchers have come up with many ways of measuring the friction between snow and sports equipment, such as different types of skis. However, only a few of these experimental setups can be used to test the glide of real-sized skis under natural conditions during both accelerating and deceleration motion. In the present work, a novel experimental setup consisting of a sled and a base station that uses GNSS receivers communicating internally by radio, thus making up an RTK-GNSS system which can measure the position of the sled with centimetre accuracy, was established. The sled is equipped with authentic cross-country skis and accelerated and decelerated on a track with natural height variation prepared with a conventional snow-track setter mounted on a snowcat. The recorded altitude and velocity data are used to quantify the coefficient of friction (COF), both for accelerating and decelerating motion, with a model based on the preservation of energy. The results show that the COF during acceleration was more than 28% higher than during deceleration, while the difference in the COF during deceleration on flat ground at loading conditions resembling the acceleration and deceleration phases was less than 5%. This is an important discovery, as when all types of skiing techniques are executed, the athlete is either accelerating or decelerating while moving forward along the track. The ability of the current experimental set-up to distinguish between them may thus have positive implications for further development.
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3. |
- Kalliorinne, Kalle, et al.
(författare)
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Characterisation of the Contact between Cross-Country Skis and Snow: A Macro-Scale Investigation of the Apparent Contact
- 2022
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Ingår i: Lubricants. - : MDPI. - 2075-4442. ; 10:11
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Tidskriftsartikel (refereegranskat)abstract
- In a cross-country skiing competition, the time difference between the winner and the skier coming in at second place is typically very small. Since the skier spends much of the energy on overcoming resistive forces, a relatively small reduction in these forces can have a significant impact on the results. The resistive forces come partly from the friction, at the tribological interface between the ski and the snow, and as with many tribological processes, the characterisation of its origin plays an important role in determining the frictional properties. Furthermore, in cross-country ski friction, there are several scales impacting the frictional performance, with the major contributors being the ski-camber profile and ski-base structure. Macro-scale measurements of the ski-camber profile under loading are often used to determine how adequate the ski is for use under specific conditions. The characteristic properties usually assessed are the force required to collapse the ski in order to obtain a certain camber height, the topography of the kick-wax zone, and the length (determined by simple means) of the frictional interfaces associated with the rear- and front glide zones, i.e., the apparent contact length. These measurements are, however, commonly performed by loading the ski against a much stiffer counter surface than snow and this affects the quantification of the characteristic properties. To date, some mathematical models have been proposed, but there is no reliable approach for determining the macro-scale properties of the contact between a cross-country ski and a counter surface using simulations. In the present paper, an Artificial Neural Network (ANN) has been trained to predict the ski-camber profile for various loads applied at different positions. A well-established deterministic approach has been employed to simulate the contact between the ANN-predicted ski-camber profile and a linearly elastic body with a flat upper surface, representing the snow. Our findings indicate that this method is feasible for the determination of relevant macro-scale contact characteristics of different skis with snow. Moreover, we show that the apparent contact area does not linearly depend on the load and that the material properties of the counter surface also exert a large impact when quantifying the apparent contact area and the average apparent contact pressure.
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4. |
- Kalliorinne, Kalle, et al.
(författare)
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Characterisation of the Contact between Cross-Country Skis and Snow: A Micro-Scale Study Considering the Ski-Base Texture
- 2023
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Ingår i: Lubricants. - : MDPI. - 2075-4442. ; 11:5
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Tidskriftsartikel (refereegranskat)abstract
- In winter sports, the equipment often comes into contact with snow or ice, and this contact generates a force that resists motion. In some sports, such as cross-country skiing, this resistive force can significantly affect the outcome of a race, as a small reduction in this force can give an athlete an advantage. Researchers have examined the contact between skis and snow in detail, and to fully understand this friction, the entire ski must be studied at various scales. At the macro scale, the entire geometry of the ski is considered and the apparent contact between the ski and the snow is considered and at the micro-scale the contact between the snow and the ski-base textures. In the present work, a method for characterising the contact between the ski-base texture and virtual snow will be presented. Six different ski-base textures will be considered. Five of them are stone-ground ski bases, and three of them have longitudinal linear textures with a varying number of lines and peak-to-valley heights, and the other two are factory-ground “universal” ski bases. The sixth ski base has been fabricated by a steel-scraping procedure. In general, the results show that a ski base texture with a higher ??? value has less real contact area, and that the mutual differences can be large for surfaces with similar ?? values. The average interfacial separation is, in general, correlated with the ?? value, where a “rougher” surface exhibits a larger average interfacial separation. The results for the reciprocal average interfacial separation, which is related to the Couette type of viscous friction, were in line with the general consensus that a “rougher” texture performs better at high speed than a “smoother” one, and it was found that a texture with high ?? and ??? values resulted in a low reciprocal average interfacial separation and consequently low viscous friction. The reciprocal average interfacial separation was found to increase with increasing real contact area, indicating a correlation between the real area of contact and the Couette part of the viscous friction.
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5. |
- Kalliorinne, Kalle, et al.
(författare)
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Characterisation of the Contact between Cross-Country Skis and Snow: On the Multi-Scale Interaction between Ski Geometry and Ski-Base Texture
- 2023
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Ingår i: Lubricants. - : Multidisciplinary Digital Publishing Institute (MDPI). - 2075-4442. ; 11:10
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Tidskriftsartikel (refereegranskat)abstract
- In elite endurance sports, marginal differences in finishing times drive ongoing equipment improvement to enhance athlete performance. In cross-country skiing, researchers, since the 1930s, have faced the challenge of minimising the resistance caused by friction in the contact between skis and snow. This study was designed to evaluate the multi-scale interaction between the macro-scale ski-camber profile and the micro-scale ski-base texture. Considerations included real contact area, average interfacial separation, and total reciprocal interfacial separation between the ski and snow, which are properties that are intimately coupled to ski–snow friction. We found that both the profile of the ski camber and the texture of the ski base play decisive roles in determining viscous friction. At the same time, the texture of the ski base exerts a greater impact on the average real contact pressure, real contact area, and minimal average interfacial separation between the ski and snow than the ski-camber profile.
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6. |
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7. |
- Kalliorinne, Kalle, et al.
(författare)
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The impact of cross-country skiers' tucking position on ski-camber profile, apparent contact area and load partitioning
- 2023
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Ingår i: Proceedings of the Institution of Mechanical Engineers, Part P. - : SAGE Publications. - 1754-3371.
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Tidskriftsartikel (refereegranskat)abstract
- In cross-country skiing races, the difference between the fastest and the second fastest time can be minuscule. As in all endurance sports, cross-country skiing requires the use of energy to overcome resistive forces, in this case primarily aerodynamic drag and friction between the skis and snow. Even a slight reduction in either of these can determine the outcome of a race. The geometry of the ski exerts a profound influence on the friction between the skis and snow. As a result of the flexible modern cross-country skis, the camber profile and gliding properties to be influenced by the skiers' position. Here, based on the location of the normal force corresponding to the plantar pressure, we characterize the ski camber while performing three variations of the downhill tucking position. We found that when gliding on a classic ski, the risk of contact between the kick wax and snow can be reduced by tucking in a leaning backwards position (i.e. by moving the skier's center of mass backwards). With the tucking position, the percentage of the skier's body weight that is distributed onto the friction interface at the rear of the skis varies between 63.5% in Gear 7 (leaning forward) on a skating ski and 93.0% in Gear 7 (leaning backwards) on a classic ski.
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8. |
- Sandberg, Joakim, et al.
(författare)
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A Novel Free-Gliding Ski Tribometer for Quantification of Ski–Snow Friction with High Precision
- 2023
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Ingår i: Tribology letters. - : Springer Nature. - 1023-8883 .- 1573-2711. ; 71:4
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Tidskriftsartikel (refereegranskat)abstract
- The nature of snow and the ever-changing environment makes measuring friction on snow and ice challenging. Additionally, due to the low friction involved, the equipment used must exhibit high sensitivity. Previous investigations of ski–snow friction have ranged from small-scale model experiments performed in the laboratory to experiments with full-sized skis outdoors. However, few have been conducted under conditions similar to those encountered during actual skiing. Here, we present a novel sled tribometer which provides highly reproducible coefficient of friction (COF) values for full-sized skis gliding at relevant speeds (approximately 5.9 m/s) in a controlled indoor environment. The relative standard deviation (RSD) of the COF is as low as 0.5%. The continuous recording of velocity allows for innovative investigations into COF variations when skis are permitted to free-glide in a natural setting. Different methods of analysing the results are presented which shows that the precision is not a single number, but a function of the range of velocities over which the average COF is calculated.
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