1. Introduction
Olympic disciplines are subject to high performance pressure, as athletes strive to achieve maximum success in competition. There are two basic ways to improve: optimizing physical performance and optimizing sports equipment. Both possibilities are subject to physical limits, which are physiological on the one hand and technological on the other. While athletes are close to their performance limits from a physiological perspective, the performance potential in the technical disciplines can be increased by applying innovative development methods to the design process of sports equipment within the framework of the regulations.
Olympic Street Skateboarding involves a lot of tricks, such as rotations and flips of the board. Complex tricks require long airtime as well as quick rotation of the skateboard. To increase the angular velocity ω around the rotational axis, the skater can either increase the net angular momentum L or use skateboards with a low moment of inertia J (Equation 1):
$$\omega = {L \over J}$$
The net angular momentum is physiologically, and the moment of Inertia is technologically limited. For this reason, skateboards are lightweight designs (LWD) that have evolved over decades, as illustrated in Figure 1. In the evolution of skateboards, it can be seen, that there is a direct correlation between the increasing development and the number of invented tricks. As skateboards became easier to handle, skaters were able to perform more and more creative tricks. In this way, the development of tricks and skateboards have naturally driven each other. This article is about a lightweight design of the truck component, the metal part that connects the wheels to the deck and enables turning, to reduce the moment of inertia of the skateboard by reducing the mass of the truck.
Figure 1. Technological evolution of the skateboard (Mysoberromance, 2022)
The 1945 Skeeter Skate was an early form of transportation with an aluminum base and moving axles controlled by a removable bar. This design is considered the forerunner of the modern skateboard. In the 1950s, Californians attached steel roller skate wheels to wooden planks to transfer wave riding to the street (Marcus & Griggi, Reference Marcus and Griggi2011, p. 34), leading to the establishment of skate culture (Reference BurkeBurke, 1999, p. 10). These skateboards had no roller bearings, a narrow wheelbase and were heavy, making them difficult to ride. In 1963, the first commercially available skateboard called the Roller Derby was offered (Iain Borden, 2019). Technology and materials developed rapidly: in the 1970s, polyurethane wheels with built-in rolling bearings and grip tape significantly improved the riding experience and revolutionized skateboarding (Reference Marcus and GriggiMarcus & Griggi, 2011). Later, skateboards designed specifically for tricks and high speeds, such as the kicktail board and the popsicle stick board, became popular. There have been no major innovations since 1989, only incremental improvements in lightweight design.
In general, a skateboard as known today is a sports equipment which consists of three main parts: deck, wheels (including the bearings) and trucks (DIN Deutsches Institut für Normung e.V., 2009). In addition to the norm, there are numerous patents and studies on skateboards. In an evolutionary study, the technical evolution of a skateboard deck was presented (Reference Prentiss, Skelton, Eldredge and QuinnPrentiss et al., 2011). Based on the data of the investigation, wood laminate is the most widely used material. 7-ply Canadian maple decks are the state of the art because of their low density and high flexibility (Reference KennedyKennedy, 1965) which is needed to perform tricks. This may be the cause to displace fiberglass and fiberglass laminate decks in the 1970s (Reference Prentiss, Skelton, Eldredge and QuinnPrentiss et al., 2011) which both exhibit a high stiffness. However, the latest trend is to replace two layers of maple with carbon fiber not to make it lightweight but increase its lifetime. Other studies show that materials such as bamboo and banana stem are also possible, but without consideration of lightweight design (Rafiquzzaman et al., Reference Rafiquzzaman, Zannat, Islam and Hossain2022; Reference Zulfikar, Umroh and SiahaanZulfikar et al., 2019). Overall, it can be assumed that the standard deck will continue to be built using maple wood. It offers a good compromise between sustainability, energy absorption, bending stiffness and weight (Reference Rubén, Pérez-Peña, Wentzel and NegroRubén A. et al., 2022). Representing the lightest part of the skateboard, both wheels and bearings offer hardly any weight-saving potential underlying given geometries. Most wheels are manufactured out of polyurethane being the best mix between use life, abrasion resistance and stiffness while offering a high load capacity (Reference Thomas, Martinez and HadfieldThomas et al., 2012). To maintain the high radial and axial loads, eight industrial deep groove ball bearings (608) are built into all types of skateboards. Considering the truck, it remains the same geometry for over four decades despite new concepts and designs being developed, especially for longboards (Downs, Reference Downs2011; Fleischmann et al., Reference Fleischmann, Ehemann, Kaufmann and Cebulla2018; Turney, Reference Turney2018; Winter Fox, Reference Winter2018). With focus on lightweight design a bionic inspired truck was built (Reference TurneyTurney, 2018), and Iturrate et al. (Reference Iturrate, Amezua, Garikano, Oriozabala, Martin, Solaberrieta, Roucoules, Paredes, Eynard, Morer and Rizzi2021) use Generative Design to design a truck based on the static load of 67 kg.
Although some publications deal with the development of a lightweight truck, realistic loads and geometries as well as methodical and product-orientated design are often neglected.
The following research questions are therefore addressed:
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How can the handling of a skateboard be improved by reducing weight while maintaining the same functionality in comparison to the chosen benchmark?
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How can the boundary conditions like loads and geometries for the development of the truck be determined?
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How can a holistic development process for optimizing a piece of sports equipment, such as a skateboard, be considered?
Among the skateboard disciplines of slalom, longboard, freestyle, vert, street and park, where the equipment is designed differently, the lightweight optimization for the discipline street offers the greatest benefits. This is because street skateboarding is a very agile discipline, which means that a low moment of inertia is sought when developing the skateboard. For this reason, the truck of a skateboard for the Olympic discipline street skateboarding is designed under real boundary conditions in this article. The use of ultra-lightweight design enables the use of innovative manufacturing technologies, such as additive manufacturing using selective laser sintering (SLS). Therefore, a topology optimization (TO) using the SIMP method is considered as an efficient method to redesign the functional truck. To maintain the established handling of a prevalent and established piece of sports equipment, the following methodology is employed.
2. Materials and methods
2.1. Methodology
The challenge of accurately determining the conditions and identifying the parts where mass reduction is required, particularly for sports equipment, requires a methodological approach that is both robust and flexible. To meet this challenge, this study integrates reverse engineering, load determination, ABC analysis of masses, TO and its geometry reconstruction process into the product design process.
This framework is designed to be adaptable to all types of sport disciplines where technical equipment is used and where low moment of inertia is essential for competition.
The starting point is the definition of boundary conditions, which are recorded in a requirements list. A benchmark analysis of all existing trucks on the market is then carried out, taking into account the relevant geometry and weight parameters. Materials and production technologies as well as costs are analyzed. For the development, it is also necessary to clarify which parts of the functional structure should be newly designed as in-house production parts, and which should be procured as purchased parts. For this purpose, the mass proportions of the components of the benchmark truck are heuristically arranged according to mass in an ABC analysis. The ABC analysis enables the estimation of lightweight design potential of individual components in relation to the overall system. The components are assigned to the classes of structural lightweight design, lightweight material design and purchased parts. The parts for structural lightweight design are subjected to TO. The geometry will not be changed in the case of lightweight material design, but material with lower density is used if stresses do not exceed. Two principles can be used to determine the necessary loads: Reverse engineering of the benchmark and physics-based literature research. With reverse engineering it is possible to determine the maximum bearable load based on the cross-sections and materials used in the benchmark, while this load may also be determined from publications or measurements. In order to optimize the individual components using TO, a design space of the truck is created as a CAD model based on measurement data. In this case, the A-parts and some B-parts of the ABC analysis are subjected to TO, considering the most important load cases that occur. Non design spaces are considered from the user’s point of view so as not to threaten handling. In this step, the material selection take place via the most heavily loaded areas, which, however, only works after the TO result has been fed back into a volume model. The method of Mayer et al. (Reference Mayer and Wartzack2020, Reference Mayer and Wartzack2023) can be used for this geometry reconstruction process. The parametrically variable volume model is then optimized using FE analysis and wall thickness is iteratively adjusted. Finally, stresses and deformations are considered for validation and the lightweight development is compared with the benchmark truck before the lightweight design is additively manufactured.
2.2. Material: the benchmark street skateboard truck
This chapter provides an overview of the skateboard and its components (Figure 2), focusing on the truck, which is optimized in this article using the methodology presented.
Figure 2. Skateboard components with a detailed view of the benchmark truck
The base of the benchmark truck is the base plate, which is usually provided with four through-holes to connect it to the deck using bolts. The material of the base plates is usually aluminum, which can be produced by machining, forming or primary forming. The base plate holds the kingpin and the pivot cup, which enables rotation. Together with the hanger, the cup forms the pivot joint of the truck. The kingpin is a high-strength steel bolt that attaches the suspension to the base plate. It is pressed into the hole provided for it in the base plate with an oversize fit and has a thread to exert a preload on the steering mechanism sitting on the kingpin. The steering mechanism is formed by two steering rubbers made of polyurethane (PU), each with a concave steel washer, with one bushing placed under and one above the suspension on the kingpin. In order to save weight and to transfer the preload force from the nut evenly to the suspension, the upper bushing is often tapered. A self-locking nut is screwed onto the end of the kingpin to pre-tension the steering mechanism.
The largest component in terms of volume and the most important for the flow of force is the hanger, which includes the axle pin. The force is transferred from the axle pin, which runs through the hanger, into the suspension and via its geometry into the steering rubbers, the kingpin and the pivot cup. The axle pin is usually molded positively around the suspension. For cost reasons, the hanger is made from an aluminum sand casting (A356 T6) optimized for mass production on standard trucks.
Analyzing the market with the 12 best-selling trucks in street skateboarding for 7.875–8.250-inch decks, an average mass of 344 g per truck was determined by market research. Some manufacturers offer a lightweight truck in addition to the standard version. The truck with the lowest mass is from Tensor (288 g), although it is not as popular as the truck from Independent (346 g) due to its lower stiffness resulting from the use of magnesium instead of aluminum as a casting material. For this reason, and because it is one of the most widely used trucks, Independent’s lightweight truck 139 Stage 11 Hollow is used as a reference for the reverse engineering process as well as the validation. The truck is designed for mass production so that it is available at an affordable price of around $35 per unit, which includes all the highlighted truck components shown in the illustration above, plus the washer. The other components must be purchased separately to assemble a complete skateboard. For this article, the highlighted truck components are considered as the material.
The economic design of the benchmark truck exists because of the necessary consideration of economic efficiency in a company. An extreme solution is rarely achieved, so that an cost-optimized rather than an ultra-lightweight design is usually realized (Klein & Gänsicke, Reference Klein, Gänsicke, Klein and Gänsicke2019; Kurek, Reference Kurek2011).
According to Sauer (Reference Sauer2018) further optimization potential is accepted in the hobby area even with costs of ∞ $/kg weight reduction. This is called ultra-lightweight-design and is used in some Olympic disciplines like archery (Reference Edelmann-nusser, Heller, Clement, Vajna and JordanEdelmann-Nusser et al., 2004) and track cycling (3D-Laserdruck, 2024) due to the high performance pressure and the need to design the best possible equipment. Both approaches from Sauer and Klein & Gänsicke are combined in Figure 3.
Figure 3. Relation between costs and mass in lightweight design (Sauer, Reference Sauer2018, p. 18) under consideration of lightweight design economics (Klein & Gänsicke, Reference Klein, Gänsicke, Klein and Gänsicke2019, p. 29)
Accordingly, the framework may be applied to extreme sports such as the Olympic Games without consideration of the economic-LWD conditions. In order to facilitate a more comprehensive grasp of the novel approach and to initiate the preliminary stages of verification, the application of the new framework will be demonstrated using the skateboard, with a particular focus on the truck.
3. Application of the framework on the design process of an ultra-LWD skateboard truck
The framework introduced in the previous chapter, tailored for competitive disciplines where a low moment of inertia provides a competitive advantage, offers promising avenues for further development in these sports. The method entails the redesign of existing, functioning mechanisms rather than their complete reinvention, which often does not fit the requirements of the userbase. Skaters for example do not like to adapt to new mechanisms as mentioned above, which is one reason why skateboards have remained the same since 1989. Only incremental lightweight developments have been established over the past years. As time progresses, the number of tricks is increasing annually. However, due to the physical limitations of the human body, it is necessary to reduce the moment of inertia of the skateboard for further trick improvements. This is achieved by developing a lightweight truck.
The goal is to design the worlds lightest street skateboard truck while maintaining the same properties as the benchmark that ideally the handling gets better rather than worse. Therefore, a requirements list is developed in cooperation with skaters by conducting interviews and hands on product discussions. The list consists of 180 lines separated to the components of the skateboard. To focus on the main requirements it is shown that the skateboard should fit to a person with 1.81 m body length and shoe size of EUR 42±1, and 81 kg which is usual for men between 20 and 29 years old. In addition, the list of requirements is iteratively concretized and supplemented, for example, by the dimensions of the selected benchmark truck Independent 139 Stage 11 Hollow. This truck is selected in collaboration with skaters and through marked research, because it is handy, durable and the most popular. It is already a lightweight design with mass of 346 g which is 32 g less than the standard model Independent 139 Stage 11. The differences that contribute to the weight saving are a hollow kingpin and axle pin.
The next step is to analyze the benchmark in order to reproduce it. For this purpose, the components and the entire truck are measured and weighed to subsequently create both a parametric 3D model and an ABC analysis (Reference Swamidass and SwamidassSwamidass, 2000) of the components (Figure 4). This analysis highlights the shares of the components in the total weight of the truck from the largest contributor to the lowest. By specifying that the functional structure of the benchmark truck is retained for this development, parts without significant weight-saving potential can be specified as purchased parts, while others are newly designed.
Figure 4. ABC analysis of truck components according to their percentage of total mass
On this basis, the hanger and the base plate are selected as structural lightweight design components, and the two washers are selected as components to be optimized with material lightweight design. The remaining components have no significant lightweight potential because they are either already fully optimized like the hollow kingpin or have a too low mass, given the low density of the material used. For this reason, the parts are bought in, which is possible due to the consistent functional structure. The purchased parts also result in cost benefits.
In the next step, the loads are determined to carry out the TO and validation using FEM. Skateboarding is a highly dynamic sport with different levels of stress during performance. On the one hand, rolling alone does not result in a high load, whereas landing after a jump is the decisive criterion for the design of the truck. Good quality trucks can withstand landings from well over 1 m, even when repeated several times. To find out how much the benchmark truck can withstand, the maximum load can be determined using technical mechanics via geometry and material characteristics. With this method, the maximum bending moment of an independent truck is calculated at the most heavily loaded point just before the hanger begins. In practice, the truck breaks at this point in some cases.
First, conditions are defined which are valid for all calculations. The worst case is assumed, in which the landing takes place on one truck only. As this can hardly occur in reality due to the even distribution of force on both trucks during the impulse, a safety factor of around 1.5...2 is applied to all calculated loads. In addition, it is taken into account that an inclined landing can occur which does not take place centrally on the center of the fixed base plate. Thus, the acting maximum force (Figure 5) can shift along the y-axis, which causes an increase in the support force on one side of the bearing. For this reason, an application factor for inclined landings KIL is introduced that takes this superelevation into account.
Figure 5. Force applied to the skateboard truck when landing a jump
The acting moment at this point is borne solely by the steel axle pin. The maximum force that the material can bear at the point y=12 mm in relation to the resulting bending moment is calculated (Equation 2-4). The result is 5,531 N corresponding to a ratio of maximum bearable force to mass of 7.1 g. For the truck with a full axle pin, the result is 5,900 N, which is 6.7% higher than for the hollow lightweight version.
$$I_{{\rm{xx}}_{{\rm{SCM}}435} } = {{\pi \cdot (d^4 - d_i ^4 )} \over {64}}$$
$$M_{1_{{\rm{SCM}}435} } = R_{e_{{\rm{SCM}}435} } \cdot {{I_{{\rm{xx}}_{{\rm{SCM}}435} } } \over {z_{\max } }}$$
$$F_{\max } = {{{{M_{1_{{\rm{SCM}}435} } } \over {12mm}} \cdot 2} \over {K_{{\rm{IL}}} }}$$
The maximum load is the result of reverse engineering. Furthermore, the load can be determined from the literature, too. In a study by Determan et al., both landings and bail outs were measured using a force plate by study participants jumping from a height of 90 cm after grinding a rail. The average maximum force of a successful landing on the skateboard is 5,344 N, whereas the landing on the feet is 8,212 N. Determann states that the system consisting of the skateboard components has a considerable shock-absorbing effect. This results from the interaction of the flexible deck, the bending of the truck and the bushings. (Reference Determan, Frederick, Cox and NevittDeterman et al., 2010)
This knowledge is transferred to the mean maximum bearable load of 5,531 N per truck to determine that for Determan’s load case there is a safety factor of approximately 2, as the forces are distributed over time on both trucks during landing procedure. On the other hand, higher jumps can be performed with the Independent 139 truck, as reality shows. The safety factor for the respective higher load case decreases until failure occurs. The comparison with real measured data thus validates the determined load of 5,531 N per truck, so that these are used for the FEM validation of the ultra-LWD truck.
In addition, these loads are highly dynamic and do not consider Wöhler curves. It should be noted that the load is not based on a defined percentile of the user group but is methodically derived through reverse engineering. In the next step, a design space for the truck is modeled for the TO, from which the design and non-design spaces for the hanger and base plate are derived as shown in Figure 6.
Figure 6. Cross-section of the design spaces for the base plate and hanger with their integration of the purchased parts including the workflow resulting in an ultra-LWD
It can be seen that the functional structure of the benchmark truck is retained and selected components are declared as purchase parts based on the ABC analysis of masses. The masses of the two washers of the steering mechanism can be reduced with material-LWD without much additional effort. A- and B-parts are subjected to structural optimization if this appears sensible according to the designer. Extreme load cases that occur in reality are regarded in the TO of the two components: Landing a jump, steering, manual, grind, and slide. This enables user-oriented product development and durability during the execution of street skateboarding.
Once the TO has been successfully carried out, a tessellated surface model is available, which is fed back into a parametric solid model. Additional reinforcements can be modeled and finally validated using FEM. Many iteration steps are necessary to carry out ultra-lightweight design realistically. A parametrically traced model helps to reduce the time-consuming loop and saves a laborious restart of the TO (Figure 7). Typically, the TO has to be repeated, when there is no parametric solid model, which can be adjusted. With a method from Mayer et al. (Reference Mayer and Wartzack2023) this longer iteration is not necessary due to the flexibility of the reconstructed solid geometry once a good TO result has been achieved.
Figure 7. From TO to a validated design with/without parametric reconstruction shown at a base plate
Using this process also enables adding struts for higher stiffness like shown above. In the end, both the hanger and the base plate were optimized in several iterations and weak points as well as notch effects were improved manually so that a lighter version than the benchmark truck is developed, and stresses can be borne by the selected material TiAl6V4, which is validated via the final FE analysis. Lastly, the designer decides if the design is approved, considering the Design for Manufacturing guidelines.
4. Results and outlook
Validated designs are created for both the base plate and the hanger, considering the five defined load cases. The result is an ultra-LWD truck with a mass of 280 g, which corresponds to a weight reduction of 19% compared to the benchmark. This reduces the mass moments of inertia and simplifies the execution of rotations. In order to determine the profitable effect, the entire skateboard must be considered, which is why a demonstrator is built both in CAD and in reality (Table 1, Figure 8).
Table 1. Comparison of ultra-LWD and benchmark in assembled configuration
Figure 8. Assembled skateboard (left) in both the ultra-LWD configuration and the benchmark, and the final design of the topology optimized truck (right)
The newly designed skateboard is 134 g lighter than the skateboard with the reference truck as a result of the ultra-LWD truck. This represents a saving of 5.5% in relation to the entire skateboard mass. In addition, the skateboard’s centre of gravity has been optimized by placing it 2 mm closer to the deck, which is the heaviest component at 1,400 g. This results in a more favorable center of gravity and the moments of inertia with regard to the center of gravity around the three axles of the skateboard have been reduced by an additional 4.5% (x), 5.8% (y) and 5.5% (z). Overall, this allows the skater to achieve higher jumps and faster rotations with the same energy input. The extent to which the shift in the center of gravity and the reduced moments of inertia are noticeable for people must be tested in practice or quantified using parametric biomechanical simulations involving the entire human body (Reference Miehling, Schuhhardt, Paulus-Rohmer and WartzackMiehling et al., 2016).
The framework for developing a lightweight truck shows that retaining the basic geometry by means of reverse engineering and TO results in untapped potential for sports equipment in disciplines, which people in high competition sports like in the Olympics may be willing to pay for. Here, the consideration of real load cases as well as handling is essential to provide athletes with optimized equipment. New developments that involve changes in geometry entail the risk that athletes will have to adapt. For this reason, an existing design space was created based on the geometry of the most popular skateboard truck.
By involving users in all phases of product development and incorporating their comments into the requirements list, it was possible to design the lightest truck without sacrificing handling. With the lower moment of inertia, tricks such as kickflips are easier to perform, which is further supported by the center of gravity being closer to the deck respective to the foot that provides the impulse. Nevertheless, there is further potential for optimization, especially in the hanger.
From the perspective of method validation, it is noted that realistic product developments with ambitious goals, such as the outperformance of a benchmark, require a high level of detail in the design process. This is the only way to work out the strengths and weaknesses of the methods. In some scientific publications, unrealistic components are developed as demonstrators that do not do justice to the holistic approach of product development. In this development, the many iterations clearly show the advantages of parametric geometry reconstruction process of a TO and also validate Mayer’s (Reference Mayer and Wartzack2023) largely DfAM-compliant method by manufacturing a real product using SLS and assembling the components.
This framework should be largely transferable to other disciplines in the context of sports equipment, which still needs to be validated in further developments.
Acknowledgments
Bretterbude Skateshop, Erlangen, Germany, is thanked for the collaboration and the input for reengineering the truck. Dominic Bartels is thanked for the support during DfAM. Johannes Mayer is thanked for support during geometry reconstruction. Fabian Halmos is thanked for the support during the design concept phase.
Declaration of generative AI and AI-assisted technologies in the writing process
During the preparation of this work the authors used DeepL in order to improve readability, grammar and to support translation into well written English. After using these tools, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.
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