Snow Friction Research Articles

Wearable snow friction measurement device for cross-country skiing

Friction on snow surface, especially in relation to winter sports, is difficult to measure directly because of its complexity and variability. In this work, a wearable snow friction measurement device for cross-country skiing was designed and constructed to calculate the friction force and monitor movement signals through the logical placement of three-component sensors and connection parts on skis. Both theoretical simulation and experiments have been conducted, which proved that the deformation of cross-country skis outputs few abnormal force signals during applying large load. Through the static validation in the laboratory, the output accuracy and stability of the device were confirmed. To verify the availability of the device, measurements on a skiing treadmill were conducted, and the results showed that the device can work well and distinguish the different friction behaviors of various waxes. More important, through the ski field test, the device was able to monitor the technical actions and evaluate the frictional coefficient of different ski waxes on snow surface comprehensively, which provides a scientific reference for athletes and coaches in routine training.

New database for the estimation of dynamic coefficient of friction of snow

New database for the estimation of dynamic coefficient of friction of snow

Characterisation of the Contact between Cross-Country Skis and Snow: On the Multi-Scale Interaction between Ski Geometry and Ski-Base Texture

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.

A Novel Free-Gliding Ski Tribometer for Quantification of Ski–Snow Friction with High Precision

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.Graphical abstract

Precise prediction of launch speed for athletes in the aerials event of freestyle skiing based on deep transfer learning

Automatically obtaining the launch speed are powerful guarantees for athletes in the aerials event of freestyle skiing to achieve good results. In most of the published studies describing athletes getting high scores, the assisting sliding distance depends entirely on the coach and even the athlete’s own experience, which may not be optimal. The main goal of the present paper is to use an acquisition system and develop an artificial neural network (ANN) model to automatically obtain the corresponding relationship between assisting sliding distance and speed. The influence of snow friction coefficient, wind speed, wind direction, slope, height and weight can be simulated in the Unity3D engine. The influence of temperature, humidity and tilt angle needs to be measured in real world by professional testers which is strenuous. The neural network is first trained by sufficient simulation data to obtain the encoded feature. Then, the information learned in simulation environment is transferred to another network. The second network uses the data taken from twenty professional testers. Compared with the model without transfer learning, the performance of proposed method has significant improvement. The mean squared error for the testing set is 0.692. It is observed that the speed predicted by the designed deep transfer learning (DTL) model is in good agreement with the experimental measurement results. The results indicate that the proposed transfer learning method is an efficient model to be used as a tool for predicting the assisting sliding distance and launch speed for athletes in the aerials event of freestyle skiing.

Effects of ski warming on ski–snow friction: some preliminary results

Some experiments were performed to determine the effect on the friction coefficient of a little temperature increase caused by a small electrical resistance inserted in different ways in a ski or a ski-like device. It appears that at a low speed, this warming is able to reduce significantly the friction coefficient. The conditions of the experiments cover only reduced speed and temperature intervals. A possible interpretation of this phenomenon is based on the modification of the quasi-liquid layer existing on the surface of snow at temperatures higher than −30°C. A wider set of parameter conditions should be analyzed to confirm this interpretation.

Investigation of the Influence of Snow Track Density on Tire Tread Block Traction by Experiments and Discrete Element Method Simulation

ABSTRACT While in nature, snow properties change from day to day or even minute by minute, one of the great advantages of lab tests is the stability and reproducibility of testing conditions. In our labs at the Institute of Dynamics and Vibration Research, Leibniz Universität Hannover, we currently run three test rigs that are able to conduct tests with tire tread blocks on snow and ice tracks: High-Speed Linear Tester (HiLiTe), Portable Friction Tester (PFT), and Reproducible Tread Block Mechanics in Lab (RepTiL). In the past years, we have run a project on the influence of snow track properties on friction and traction test results with those test rigs. In this article, we will present a first excerpt of the results concentrating on the RepTiL test rig. Because this rig reproduces the movement of rolling tire tread blocks, we executed a test campaign with special samples for the analysis of snow friction mechanics. We evaluated penetration into the snow, maximum longitudinal force level, and longitudinal force gradient. On the other hand, we varied the snow density while preparing our tracks to assess the influence of the snow track density on the friction mechanics. In parallel, we have accompanied our experiments with discrete element method simulations to better visualize and understand the physics behind the interaction between snow and samples. The simulation shows the distribution of induced stress within the snow tracks and resulting movement of snow particles. Hypotheses for the explanation of the friction behavior in the experiments were confirmed. Both tests and simulations showed, with good agreement, a strong influence of snow density and sample geometry.

Assessing the Mechanisms Thought to Govern Ice and Snow Friction and Their Interplay With Substrate Brittle Behavior

Sliding friction on ice and snow is characteristically low at temperatures common on Earth’s surface. This slipperiness underlies efficient sleds, winter sports, and the need for specialized tires. Friction can also play a micro-mechanical role affecting ice compressive and crushing strengths. Researchers have proposed several mechanisms thought to govern ice and snow friction, but directly validating the underlying mechanics has been difficult. This may be changing, as instruments capable of micro-scale measurements and imaging are now being brought to bear on friction studies. Nevertheless, given the broad regimes of practical interest (interaction length, temperature, speed, pressure, slider properties, etc.), it may be unrealistic to expect that a single mechanism accounts for why ice and snow are slippery. Because bulk ice, and the ice grains that constitute snow, are solids near their melting point at terrestrial temperatures, most research has focused on whether a lubricating water film forms at the interface with a slider. However, ice is extremely brittle, and dry-contact abrasion and wear at the front of sliders could prevent or delay a transition to lubricated contact. Also, water is a poor lubricant, and lubricating films thick enough to separate surface asperities may not form for many systems of interest. This article aims to assess our knowledge of the mechanics underlying ice and snow friction. We begin with a brief summary of the mechanical behavior of ice and snow substrates, behavior which perhaps has not received sufficient attention in friction studies. We then assess the strengths and weaknesses of five ice- and snow-friction hypotheses: pressure-melting, self-lubrication, quasi-liquid layers, abrasion, and ice-rich slurries. We discuss their assumptions and review evidence to determine whether they are consistent with the postulated mechanics. Lastly, we identify key issues that warrant additional research to resolve the specific mechanics and the transitions between them that control ice and snow friction across regimes of practical interest.

Balanced carving turns in alpine skiing

ABSTRACT In this paper, we analyse the model of pure carving turns in alpine skiing and snowboarding based on the usual assumption of approximate balance between forces and torques acting on the skier during the turn. The approximation of torque balance yields both lower and upper limits on the skier speed, which depend only on the sidecut radius of skis and the slope gradient. We use the model to simulate carving runs on slopes of constant gradient and find that pure carving is possible only on slopes of relatively small gradient, with the critical slope angle in the range of . The exact value depends mostly on the coefficient of snow friction and to a lesser degree on the sidecut radius of skis. Comparison with the practice of ski racing shows that the upper speed limit and the related upper limit on the slope gradient set by the model are too restrictive and so must be the assumption of torque balance used in the model. A more advanced theory is needed.

Snow Load Modeling on Different Surfaces

The snow load distribution model on the buildings and structures coatings is considered. The main factors affecting the snow load formation on the coatings of random shape are revealed. The dependence of the snow thickness on the coating on the angle of the roof inclination, the coating curvature radius, and the snow friction coefficient on the surface of the roof is determined. The level of individual factors influence on the snow load distribution is revealed. Recommendations on the coating structures calculation are given. These allow taking into account possible unfavorable loading options during the highest snowfall period.

Influence of Roof Shape on Snow Accumulation

The paper describes the snow accumulation issues on the buildings and structures covers. The main factors influencing the nature of the snow load formation on random shape coverings are revealed. The holding capacity dependence of incoherent snow on the coating configuration and the snow friction coefficient on the roof surface is established. The individual factors influence degree on the snow accumulation nature is revealed. An analytical dependence of the snow capacity to be retained on the coating surface from the roof outline and the snow adhesion forces on the coating material surface is proposed. Recommendations for taking into account the coating structures outline in the buildings and structures design allowing to clarify the snow accumulation features that are characteristic of this roof shape are given. The dependence in the parametric shape of the cover inclination critical angle from the magnitude of the snow friction coefficient on the roof surface is established. The obtained results allow clarifying the snow load for incoherent snow depending on the friction coefficient magnitude.

Application of dGNSS in Alpine Ski Racing: Basis for Evaluating Physical Demands and Safety.

External forces, such as ground reaction force or air drag acting on athletes' bodies in sports, determine the sport-specific demands on athletes' physical fitness. In order to establish appropriate physical conditioning regimes, which adequately prepare athletes for the loads and physical demands occurring in their sports and help reduce the risk of injury, sport-and/or discipline-specific knowledge of the external forces is needed. However, due to methodological shortcomings in biomechanical research, data comprehensively describing the external forces that occur in alpine super-G (SG) and downhill (DH) are so far lacking. Therefore, this study applied new and accurate wearable sensor-based technology to determine the external forces acting on skiers during World Cup (WC) alpine skiing competitions in the disciplines of SG and DH and to compare these with those occurring in giant slalom (GS), for which previous research knowledge exists. External forces were determined using WC forerunners carrying a differential global navigation satellite system (dGNSS). Combining the dGNSS data with a digital terrain model of the snow surface and an air drag model, the magnitudes of ground reaction forces were computed. It was found that the applied methodology may not only be used to track physical demands and loads on athletes, but also to simultaneously investigate safety aspects, such as the effectiveness of speed control through increased air drag and ski–snow friction forces in the respective disciplines. Therefore, the component of the ground reaction force in the direction of travel (ski–snow friction) and air drag force were computed. This study showed that (1) the validity of high-end dGNSS systems allows meaningful investigations such as characterization of physical demands and effectiveness of safety measures in highly dynamic sports; (2) physical demands were substantially different between GS, SG, and DH; and (3) safety-related reduction of skiing speed might be most effectively achieved by increasing the ski–snow friction force in GS and SG. For DH an increase in the ski–snow friction force might be equally as effective as an increase in air drag force.

Accurate Measurement of Temperature Change of Gliding Surface of Ski Associated with Snow Friction

Accurate Measurement of Temperature Change of Gliding Surface of Ski Associated with Snow Friction

The mechanics of snow friction as revealed by micro-scale interface observations

ABSTRACTThe mechanics of snow friction are central to competitive skiing, safe winter driving and efficient polar sleds. For nearly 80 years, prevailing theory has postulated that self-lubrication accounts for low kinetic friction on snow: dry-contact sliding warms snow grains to the melting point, and further sliding produces meltwater layers that lubricate the interface. We sought to verify that self-lubrication occurs at the grain scale and to quantify the evolution of real contact area to aid modeling. We used high-resolution (15 µm) infrared thermography to observe the warming of stationary snow under a rotating polyethylene slider. Surprisingly, we did not observe melting at contacting snow grains despite low friction values. In some cases, slider shear failed inter-granular bonds and produced widespread snow movement with no persistent contacts to melt (μ < 0.03). When the snow grains did not move and persistent contacts evolved, the slider abraded rather than melted the grains at low resistance (μ < 0.05). Optical microscopy revealed that the abraded particles deposited in air pockets between grains and thereby carried heat away from the interface, a process not included in current models. Overall, our results challenge whether self-lubrication is indeed the dominant mechanism underlying low snow kinetic friction.

Kinetic Friction of Sport Fabrics on Snow

After falls, skiers or snowboarders often slide on the slope and may collide with obstacles. Thus, the skier’s friction on snow is an important factor to reduce incidence and severity of impact injuries. The purpose of this study was to measure snow friction of different fabrics of ski garments with respect to roughness, speed, and contact pressure. Three types of fabrics were investigated: a commercially available ski overall, a smooth downhill racing suit, and a dimpled downhill racing suit. Friction was measured for fabrics taped on a short ski using a linear tribometer. The fabrics’ roughness was determined by focus variation microscopy. Friction coefficients were between 0.19 and 0.48. Roughness, friction coefficient, and friction force were highest for the dimpled race suit. The friction force of the fabrics was higher for the higher contact pressure than for the lower one at all speeds. It was concluded that the main friction mechanism for the fabrics was dry friction. Only the fabric with the roughest surface showed friction coefficients, which were high enough to sufficiently decelerate a sliding skier on beginner and intermediate slopes.

Effect of ski geometry and standing height on kinetic energy: equipment designed to reduce risk of severe traumatic injuries in alpine downhill ski racing

BackgroundInjuries in downhill (DH) are often related to high speed and, therefore, to high energy and forces which are involved in injury situations. Yet to date, no study has investigated...

PISTE: A Snow-Physics Model Incorporating Human Factors for Groomed Ski Slopes*

Abstract Accurately calculating snow-surface temperature and liquid water content for a groomed ski run, known as a ski piste, is crucial to the preparation of fast skis for alpine racing. Ski technicians can use forecasts of these variables to reduce ski–snow friction by applying layers of wax ahead of time. A new one-dimensional numerical Lagrangian snowpack model, Prognostic Implementation for Snow Temperature Estimation (PISTE), is presented that solves the heat-, liquid water–, and ice-budget equations to calculate these snow variables. In addition, the human effects of skiing and grooming are modeled. Meteorological measurements from a 5-day, clear-sky case study at a ski piste on Whistler Mountain, British Columbia, Canada, are prescribed to PISTE as boundary conditions. Because of a lack of interior snowpack measurements, PISTE was spun up from a dry, isothermal snowpack using repeated boundary conditions from 1 day of measurements. Initial conditions for the main model run that used the subsequent 4 days were taken from this spinup. Simulated and measured snow-surface temperatures show very good agreement, with slight cold daytime and warm nighttime biases (averaging 0.5° and 1°C, respectively). The modeled behavior of snowpack temperature and liquid water content profiles is consistent with previous literature having similar radiative boundary conditions. The case study indicates that PISTE is useful under simple conditions. It shows the potential to be developed into a more sophisticated model that can incorporate complex boundary conditions such as cloudiness and precipitation and can be driven by numerical weather prediction output.

The influence of slope and speed on locomotive power in cross-country skiing

Purpose: A model was developed for cross-country skiing where locomotive power depends on speed and slope as variables, and further depends on snow friction, gravitation, mass, air drag, wind, and air density. Model parameters were estimated experimentally. Methods: Two differential equations were developed for how locomotive power depends on speed and slope. The equations are of the logistic form with a threshold determined by the skier’s technique, intensity, mass, metabolic rate, gross efficiency, and lactate threshold. Three parameters were estimated by minimizing the average summed square difference between the simulated speed, using the model, and experimental speed of an elite male skier in a 4218m track. Distance and height along the track were measured using a measuring wheel and an inclinometer generating 52 datapoints. Research assistants measured time from start at 14 different positions. Parameter values were determined from the literature. Results: We illustrated how speed and slope impact locomotive power. The model was shown to be superior to a model where locomotive power is a function of speed only. The joint dependence of locomotive power on speed and slope is crucial in the non-stationary conditions where the skier passes high and low points along the track where both speed and slope change rapidly. Conclusion: The model is useful to predict the impact of altering a subset of the 23 variables and parameters on the remaining variables, for example the impact of changed friction or terrain slope on the skier’s speed and thus time to complete a ski race.

A comparison between timed and IMU captured Nordic ski glide tests

An inertial measurement unit (IMU) with six degrees of freedom (three orthogonal accelerometers and three orthogonal rate gyroscopes) was used to calculate the acceleration loss as a function of position down the test slope. A traditional timed glide test was performed at the same time. The results from both test methods were compared. To increase the accuracy of the sensor system, two speed sensors were installed at known positions. The gradient of the track was measured and utilized by the sensor system to correct measurement errors. A mathematical model, developed by Apertus AS (Asker, Norway) was used to estimate the snow friction coefficient based on the acceleration loss. The results from the IMU sensor provided more information, including peak acceleration, maximum speed, time required to reach maximum speed, and speed loss, which can easily be seen. The use of IMU sensors in gliding tests may prove useful to differ between skis (Δμ ∼ 0.01). We conclude that in order to detect the minute differences distinguishing the best skis (Δμ ∼ 0.001), sensors with an even higher degree of sensitivity than those tested in this experiment should be used.

Possible contribution of triboelectricity to snow - air interactions

Environmental contextPolar near-surface snow can act as a chemical reactor that alters the composition and chemistry of snow and the overlying air. Although the mechanisms and driving forces of these reactions have long been debated, triboelectrification (production of electrostatic charges by friction) of snow by wind has not yet been considered as a factor. It is proposed that in polar regions, triboelectrification could significantly influence the composition and chemistry of snow. AbstractReactions that proceed in polar snow cover may significantly affect the chemistry of the overlying atmosphere, but mechanisms and driving forces of these reactions are still under discussion. The proposed hypothesis attempts to explain some experimental data that cannot be fully understood (e.g. the effect of wind on OH radicals, ozone and persistent organic pollutants levels) by taking into account the influence of electrical phenomena on the snow surface. We assumed that a combination of such factors as low humidity, high wind speed and low temperatures makes the influence of triboelectrification of snow significant for polar areas, where purity and the depth of snow cover prevent fast charge dissipation. The major points of the hypothesis are: (1) when the electric field reaches a value sufficient for the onset of corona discharge, various free radical processes are initiated resulting in changes in the concentrations of ozone, OH radical, nitrate, etc., and the decomposition of pollutant molecules; (2) the high electric field can stimulate transport of ions (such as bromide and nitrate) from the condensed phase to the gas phase; and (3) the ageing of charged snow can increase its electrical potential.