Adjustable Stiffening Device for Alpine Snow Ski
Design Team
Ryan Conroy, Michael Dores, Michael Grant, Scott Kelley, Lara Taylor Design Advisor
Prof. Mohammad Taslim
Abstract
The purpose of this project was to explore methods of varying the stiffness of an alpine ski in order to improve performance based on snow condition. Ski stiffness influences the performance of the ski in different snow conditions such as loose powder or hard packed snow. A stiff ski is more effective on hard packed snow while a flexible ski is more effective in soft powder. A variable stiffness mechanism provides the skier with a means of handling a variety of snow conditions with one pair of skis, reducing overall cost of the sport. Since mechanical stiffness data of skis is either unknown or unavailable to consumers, a mix of skis, designed for different snow conditions, were collected and tested to quantify their stiffness characteristics. In conjunction with this data collection, three designs for varying ski stiffness were built and tested to determine which had the greatest stiffening capabilities. The stiffness testing in conjunction with additional design criteria determined the final prototype mechanism. This design incorporates a pair of bent rods, which run from the tip to binding region with a variable spring rate element between them that when translated affects the overall stiffness of the system.
The Need for Project
A ski is needed whose stiffness can be adapted to varying snow and weather conditions.
Skis are designed with specific snow and terrain conditions in mind for which they will be used. Many avid skiers own several pairs of skis to accommodate varying weather conditions. The primary difference between skis is stiffness, which directly influences the ski’s reaction to an applied load. A device or system is required that can be coupled with a ski to adjust the ski’s stiffness such that a single pair of skis can accommodate a range of snow conditions.
The Design Project Objectives and Requirements
The intention of this project is to design a device that can vary ski stiffness without significantly increasing ski cost, and while preserving the physical characteristics of the ski.
Design Objectives
This device was designed to be couple with a soft, powder ski with a low original. Preserving the general profile and appearance of the ski was important so as to not deter potential customers. The final ski should use standard bindings, which means that the device cannot interfere with the binding region of the ski. For usability, the device must be operated in a tool free manner such that skiers can adjust settings on the go. The system must be able to survive standard skiing conditions including cold temperatures, ice, snow, dirt, and debris.
Since ski stiffness data is not made available by manufacturers, another objective of this project was to develop a device and procedure for quantifying the stiffness of individual skis on the market and of the final prototype.
Design Requirements
In order to be set apart from competitors, this device must have a minimum of four different stiffness settings. Physically, the prototype was constrained by the footprint of a powder ski which is characterized by 105-115 mm underfoot width and a maximum width of 130 mm towards the tip. The device must be attached without penetrating completely through the ski's core. Since typical skis are no more than 1.5 cm - 3 cm thick, the device must mount in a fashion that makes use of this thickness without compromising the structural integrity of the ski.
Three design concepts were chosen to be prototyped and tested. Two designs were successful in varying the ski’s stiffness.
An initial list of twenty-five brainstormed design ideas were considered and ranked. The three top ranking ideas were the mass moment of inertia (MMOI) concept, inserted plate concept, and the buckling member concept, all of which progressed to an initial round of prototyping and testing.
The MMOI concept utilizes two rotatable members running the length of the ski. When rotated to different positions, the mass moment of inertia of these beams would be altered and would thereby change the bending stiffness of the ski. This concept was prototyped using beams with rectangular cross sections inserted into cylindrical rods, which in theory provides variable stiffness of the system as a function of the members’ rotation between 0° - 90°. The settings of this design proved to be negligibly different and the concept was discredited.
The inserted plate concept consists of a series of rectangular plates made of various materials and geometries, which were clamped to the ski’s surface. In order to change the ski’s stiffness, the user would replace the plate that was mounted to the ski with a plate made of an alternate material or geometry. This design was able to stiffen the ski by 18.18%; however, this design was modular rather than self-enclosed. This would force the user to carry various plates while on the mountain in order to achieve variable ski stiffness.
The buckling member concept consists of two pre-bent rods mounted via pin connections to the top surface of the ski. A restrictor plate in the middle of the rods ensures that they never bend out of the desired plane and never are able to invert the direction in which they are bent. In order to vary the system's stiffness, springs of varying spring constants are placed at the midpoint between the two members which resists their ability to bend. This concept was able to stiffen the ski by 17.77%.
All three concepts were assembled, attached to skis and Flexure Arm Tracking System Bent rods
Recommended Design Concept
The final design involves two rods that bend in relation to a flexure between them. The location where the rods and flexure contact determines the stiffness of the ski.
Design Description
The chosen design, the buckling member concept, is comprised of two pre-bent rods which run lengthwise down the ski from binding region to the tip. The rods are assembled such that they are bent inwards towards each other and a flexure seated between the two rods provides resistance to the system. As the flexure is translated between the rods, the rod’s contact location on the side of the flexure is changed, which effectively changes the stiffness of the ski region in which the device is mounted.
Analytical Investigations
Finite element analysis was leveraged to verify the results of the experimental investigation. The base ski material properties were modeled such that the displacement reaction under a given load and position matched the experimental data. The prototype was then mounted to the ski and the simulation was re-run under various stiffness settings. The analytical and experimental results agreed within 2.36% for the least resistant condition and within 4.95% for the most resistant position.
Calculations were also done by hand to determine the stresses in the buckling members and the flexure to determine the fatigue life of the parts. The maximum stress within the
tested to determine their individual stiffening capabilities. The resultant test data showed that the MMOI concept’s stiffness settings proved to be very similar and did not have much effect on ski stiffness. Both the inserted plate and buckling member concepts were able to stiffen the ski by approximately 17-18%. These two designs were then ranked in a decision matrix based on a wide range of criteria, resulting in the buckling member concept being pursued as the final design for further optimization.
FEA overlays showing ski displacement at various
settings
Testing Setup Key Components
1. Ski 2. Winch 3. Spring Scale
buckling members was 58.6 ksi and within the flexure was 39.1 ksi. The buckling members will have infinite fatigue life while the flexure will see failure after 20,000 cycles. However, this part was designed to be serviceable seasonally.
Experimental Investigations
To determine the prototype’s effectiveness in stiffening, a testing procedure and apparatus was developed. A hand crank was used to apply a load at a known position along the length of a ski. The amount of force applied was measured with a spring scale while the displacement of the ski under the load was measured with an analog caliper. The load was varied from 0 – 125 lbs in 5 lb increments and the loading position was varied from 20% to 80% of the distance along the length of the ski in 10% increments. From this testing, load vs. displacement plots were constructed that showed the change in stiffness, quantified as the change in displacement under identical loading conditions. The results showed that as the variable stiffness flexure was translated from its disengaged position to its fully engaged position, the stiffness of the ski increased by 12%.
Key Advantages of Recommended Concept
The buckling member concept provides several key advantages over other designs considered in the project and over products currently on the market. This design is completely contained on top of the ski and does not require the user to carry additional components for optimal use. The design is actuated simply by hand and therefore does not require the skier to carry tools while on the mountain.
Compared to similar products on the market, this design provides more stiffness settings and variation, allowing the user to customize their experience even further.
4. Caliper
Financial Issues
Materials for a single buckling member prototype were acquired for $63.41.
The total material cost of the buckling member prototype, excluding ski costs, is $63.41 per ski. Additionally, materials to construct a test apparatus and two other design concepts were acquired. The cost of these materials was $549.52.
While appropriately sourcing materials (primarily the titanium rods), could reduce the prototyping costs, the low expected production volumes of this ski, due to its niche market, hinders further production price reduction.
Recommended Improvements
Improvements could be made in terms of protecting the device from the elements to avoid premature failure.
The intended user environment for this product is wet, cold, and dirty. The wearing effects of particulate buildup within the system and the buildup of ice on the external structure are topics of trouble for a production concept. The materials of the prototype have been chosen to reduce the effects of wear and corrosion, but ultimately the premature failure of the product before a realistic lifetime is a concern. An improved version of the model could work to encase the external stiffening mechanism in order to seal it off from the environment and reduce the risks associated with debris and corrosion.
A second area for further improvement is the method of actuating the variable stiffness element. The current design utilizes a friction lock controlled with a thumbscrew, which was chosen to minimize the buildup of dirt and ice in crevices and holes in the system. A more elegant design could be conceived, especially if the system was sealed and the risk of external contaminants was minimized.
