Skateboard Project

Design Requirements:

1. The board shall support the weight of a user who has shoe size US12 and weight 180 lbs

2. The board shall not deflect more than 0.375in across its span

3. The board shall have a minimum factor of safety of 3 (that is, the stress is never greater than 1/3 the yield stress)

These requirements were the initial guiding principles used for the design of this board.

3 FEA iterations were run through to optimize the thickness such that the deck was as light as possible. First, a thickness of 0.45in was used, as was predicted by the Bending Equations. This results in a predicted maximum deflection of 0.395in, which is close to the limit of 0.375 but just over it. The thickness was thus increased to 0.47 in, and the FEA was rerun. This time, the max deflection was only 0.36, which met the requirements. Additionally, the factor of safety was 4.7 which also met the requirements. Finally, the thickness was decreased to 0.465in, yielding a maximum deflection of 0.37in, just below the threshold of 0.375 in. The final factor of safety was 4.6, easily meeting the requirement, and thus this was chosen for the final design thickness.

Conclusions

For this project, FEA was used extensively to evaluate different iterations of a design to optimize its characteristics within given constraints. The initial predicted deflection using the Euler-Bernoulli beam bending equation was quite close to that computed using Finite Element Analysis. However, this is likely just a coincidence; while I would expect the bending equation to yield a result that is roughly in the right ballpark, maybe by a factor of 2, it is unlikely that the simplification to a 1D uniform cross-section beam is equivalent to my deck design. My deck has both variable cross-sectional profile, thickness, and width, and thus should only roughly conform to the beam equations. It did, however, allow us to pick a good starting point for FEA analysis, even if this starting point was already near the optimum solution.

Professional Longboard deck manufacturers do not simply make their decks out of one uniform material. Rather, many materials are layered, laminated, and compressed together, to create a non-isotropic composite material that is much stronger than any single material, while still being flexible enough to ride. In the future, one could take this longboard deck design and attempt to incorporate this layered structure in either a more intricate FEA analysis, or by building one and measuring its bending characteristics and performance.