Featherweight Skateboard Deck
Challenge
To explore material selection and design optimization, the class was tasked with designing a skateboard deck to be as light as possible. The deck must support 180lbs, fit at most a size 12 shoe, not exceed a vertical deformation of more than 0.375in, and maintain a factor of safety of 3. To simplify the design process, only the deck must be designed, and it can be modeled as being supported by two revolute joints above where the axles would be.
Background
I can’t claim to be much of a boarder myself, so first I sought to understand what different types of skateboards exist, what traits are desirable in different kinds of skateboard designs, and how those traits are achieved. Thankfully, the class was provided with a design project that explores the use of bamboo laminates in deck design. The abstract and historical information in the study was very helpful for understanding the different types of skateboard uses. Since its inception in California in the 1950-1960s, skateboarding has grown to achieve significant notoriety and relevance in modern culture.
Types of Board
Generally speaking, there are two main types of skating that use two similar but unique board designs. Longboard skateboards are (as the name implies) a longer type of board with bigger trucks and wheels. These boards are typically used for events like weaving through cones downhill or “carving” down hills. Commuter and trick skateboards are typically shorter than longboards. They have smaller trucks and wheels, and are meant for more level applications with less emphasis on “carving” and more on tricks or just getting around. These boards are typically stiffer and are less prone to vertical displacement
Typical Materials
In most cases, board decks are constructed from various types of wood, though carbon fiber and fiberglass are also options. The author of the study notes that carbon fiber and fiberglass are less sustainable than wood options.
Dorren, Brett. (2013). Flexural Properties of Self-Constructed Bamboo Laminates [Unpublished Senior Project Approval]. California Polytechnic State University.
Preliminary Design
Type of Board
Based on the background research, it seems prudent to use the commuter board style as the basis for the design. The typical length of these boards is about 20” and they’re typically meant less for carving, so truck position can be reduced to improve the bending properties.
Physical Approximation
To better understand the physical characteristics of the board and to be able to analyze deck designs, it is prudent to approximate the deck with a physical model. To conduct a design study, the skateboard deck is approximated as a simply supported beam with a single point force applied in the middle of the beam. This type of beam has a fixed joint on one end and a “roller” support on the other. Using this model, it is understood that maximum displacement also occurs at the mid length of the board. The two supports in the beam model represent the truck supports and the downward force represents the weight of the user. It is possible to use other physical models to approximate the characteristics of bending for this situation, but the simply supported beam is used because it gives a “worst case scenario” approximation that is simple to compute. The user weight could also be modeled as a constant force along the length of the beam, but the maximum displacement would turn out to be the same. In addition, a roller support is used on one end to simulates the tendency of the deck to deform horizontally as it bends under the weight of the user. A fixed support must be used on the other end in the mathematical model in order to restrict the movement of the board in space. However, in simulation a different technique is used that will be discussed later. The simply supported beam with a point force gives a nice combination of accuracy and simplicity. Show below are typical ways to model a simple beam. Using this model it is assumed that the load is static and concentrated at the center of the beam. In reality, this is not the case, but it is difficult to anticipate all types of loading, so the design will include a factor of safety.
Different types of simple beam models
Material Selection
Once the physical conditions to which the deck of the skateboard will be subjected are able to be approximated, it is possible to select a material best suited for this design by obtained a performance index. Performance index helps give an understanding of how physical, geometric, and material properties influence certain performance characteristics of the design. For this design, the goal is to minimize the weight of the board while staying within 0.375in” maximum vertical displacement and a FOS of 3. Using the physical model, it is possible to derive an expression for the maximum displacement of the deck in terms of geometric parameters and young’s modulus. It is also understood that the total weight of the board is a function of its geometric properties and density. It is also understood that the stiffness of the board is a function of maximum displacement and the applied force. Combining these factors and rearranging gives an expression for the mass in terms of geometric, physical, and functional properties of the model, and by isolating the desired material (physical) properties in the model, it is possible to obtain a performance index for mass in terms of young’s modulus and density of a given material. Next, this performance index is plotted as a line on a log scale graph with young’s modulus in terms density, and maximized. It is found that Ultra High Modulus Carbon fibers are a suitable material.
Preliminary Design and Verification of FEA Results
Once a material is selected, a basic CAD model of the deck is created in Solidworks so that FEA may be used to optimize the design of the board. To begin with, the board is modeled as a 22”x11.25”x0.1” plate with a 180lb force applied to a split line at mid length and with one of the bottom edges completed fixed and the other supporting loads in the vertical dimensions. Using the mathematical model, the expected displacement at the middle is calculated to be ~0.426” and the FEA results agree. T
Refining the CAD Model
To be able to better understand stress distribution in the board when a person is standing on it, an approximate sketch of a size 12 shoe is mocked up and used to create split geometry on the top face of the deck. Holes are also added to the model to simulate the truck attachment points. The hole diameter is 1” and the holes are 14” apart, symmetric about the middle length of the board.
The placement of the supports is meant to be conducive to minimizing the displacement of the board in the model. By examining the physical model and the expression for maximum displacement of a simply supported beam, it is understood that maximum displacement decreases proportionally to the distance between supports raised to the third power. With this in mind, placing the trucks too close to the center of the board is less conducive to turning and maneuverability. Therefore a separation of 14” is used based also on the sketch for the shoe placement of the user to try to find a middle ground between minimizing displacement and maintaining a good user experience.
To conduct the FEA analysis in Solidworks, a static loading simulation is used. Two fixed hinges are established at the hole cuts in the deck and one of the edges of board is fixed in place to fully constraint the part. Then a force of 180lbs total is applied to the top of the deck in split geometry created from the shoe mockup sketch.
After running a static simulation on the refined cad model, it is observed that stress is concentrated towards the middle of the board and much lower on peripheral areas. Based on this information, a center taper and end cuts are added to the deck in an attempt to reach a more uniform stress concentration throughout the board and reduce the use of excess material. Once these measures are incorporated into the CAD model, a design study is used to vary the thickness of the deck, the minimum width at the center of the deck, and the clearance between the end cuts and the truck holes. The study is constrained to a maximum absolute vertical displacement of 0.375” and a minimum factor of safety of 3. The goal of the study is to achieve the lowest weight.
Final Design and Future Work
After conducting a few design studies, a final design is reached. As shown below in the drawing, it was found that the ideal design has a minimum width at the center of 3.2”, a thickness of 0.08”, and a clearance between the end cut and truck holes of 0.1”. The weight of the deck is 0.867lbs with a minimum factor of safety of 3.16 and a maximum displacement of 0.056”. The final dimensions of the board as well as the FEA results are shown below