تجزیه و تحلیل رفتار سازه از بوت اسکی تقویت شده نوآورانه

The effect on the boot structural behavior of a stiffening aluminum bootboard has been investigated by laboratory and field tests. Stiffness tests on the boot with the bootboard screwed to the shell (state ON) showed a 20% increase with respect to the unscrewed state (OFF). Lateral stiffness tests conducted on a servohydraulic test bench together with motion capture techniques did not show significant increases due to the bootboard. Strain gauges applied to the bootboard for measuring torsion and bending moments in the field confirmed the intervention of the bootboard torsional stiffness at the edge changes during slalom turns.

The ski boot is a very special piece of footwear that has been evolving since the early development of skiing. The different functions of the ski-boot can be stated as (i) transmitting control loads to the ski, (ii) enabling a quick connection and a safety release of boots from bindings, (iii) protecting the foot-ankle-shank complex from injuries due to overloads during falls and (iv) maintaining the foot pressure, thermal and humidity optimal conditions. The effort of manufacturers is towards the maximization of all performance and comfort parameters of ski-boots with mass and cost reduction. A crucial role in the field of performance is played by the boot stiffness parameters as they influence the ability of quick transmission of control loads from the foot to the binding-plate-ski assembly. The present work was carried out for evaluating the structural behaviour of an innovative ski-boot presenting an extruded aluminium bootboard (known also as “zeppa”) having not only a simple support function to the foot but also a reinforcing function due to the presence of four screws that can be connecting the bootboard to the shell sole. When connected (state “ON”), the overall stiffness of the boot was much higher that when not connected (“OFF”). The positive effect on the athlete of the new ski-boot was already studied in a previous work by means of a comparative EMG analysis of the same subject in special slaloms [1], leading to a reduction of main muscle activations when performing similar time trials.

The aim of the work was the evaluation of engineering parameters correlated to the enhanced performances that a boot reinforced with an innovative bootboard had shown in the snow tests. The increase of torsional stiffness of the boot shell was supposed to be the main reason of better skiing performances. The stiffness tests results shown in Fig. 4.a confirmed the significant increase (20%) of Tip-Heel Torsional stiffness of the closed boot. On the other side, lateral stiffness tests gave no evidence of any global lateral stiffening, therefore inducing to consider the experienced advantage in skiing as correlated more with the contribution of the reinforced ski-boot to the overall ski-shovel torsional stiffness during transition from edge to edge rather than to its contribution to lateral stiffness during the steady curve conduction. The application of strain gauges to the bootboard and its calibration allowed to measure the torques transmitted by the bootboard in the field and gave further confirmation to this interpretation. In fact, as it can be seen in Fig. 5.b, the peak values of the torque transmitted by the bootboard are found in correspondence of the edge transition from external to internal turn. In correspondence with the maximum roll values, both during internal or external turns, the torsion moment show minimum values, therefore suggesting that the bootboard stiffening effect is not requested when the ski is in full carving conditions. In those instants, bending contribution of the bootboard to the boot stiffness is maximized as expected, due to the peaks reached by the forces normal to the boot sole. The application of motion capture techniques to the structural analysis of such deformable structures can be seen as a powerful advance in the functional analysis of boots: results as those of Fig. 4.b can be achieved relatively simply and give an insight into the boots deflection patterns: the trend of local twist angles along the shell can highlight areas where stiffening (or softening) is appropriate. In particular, about this boot, the highest torsional stiffness of the shell under lateral bending were shown in Outward bending, corresponding to an internal ski turn: this may be seen as unexpected given the fact that highest values of binding loads are reached during external ski turns [2]. In addition to that, the analysis of deformation of cross sections along the boot during loading cycles can give quantitative information about possible discomfort reasons at the medio/lateral malleoli. In any case, these type of measurements can act as validating tools for the numerical model of such complex structures as the ski boots are. In terms of boot technical development, the study confirmed the correlation between high values of torsion stiffness and top slalom performances.