13. July 2021

Optimization of the ROTWILD stem S240

ROTWILD employee Martin Zimmermann worked on the weight optimization of the existing ROTWILD S240 stem as part of his master's thesis. For the first time, selective laser melting was used as a manufacturing technique. We explain what the process is all about and what advantages it offers in the development of lightweight components.

Selective laser melting (SLM) as an innovative manufacturing process

Martin Zimmermann, student at the TU Darmstadt in the Mechanical and Process Engineering course of studies and working student in the engineering department at ROTWILD since 2017, dedicated himself to the optimization of the ROTWILD S240 aluminum stem as part of his master's thesis. The topic of his master's thesis came about through a cooperation with the company Sauer Product and was developed in collaboration with the Department of Structural Lightweight Design and Construction (KLuB) of the TU Darmstadt, which is headed by Prof. Dr.-Ing. habil. Christian Mittelstedt is conducted. As supervisors of the work stood Martin Dr.-Ing. Alexander Großmann from the KLuB department at Darmstadt Technical University and, on the company side, our product manager Johannes Matschos. The aim of this work was to optimize the weight of the bicycle stem and to manufacture it using selective laser melting (SLM) at Sauer Product.

Selective laser melting (SLM) as an additive manufacturing process offers several advantages over the conventional production of aluminum components:

shorter product development times
resource-saving production
great creative freedom
possibility of function integration within a component

By using computer-aided topology optimization in the development of the component for additive manufacturing, it was possible to realize a weight-reduced and load-compatible lightweight design that cannot be manufactured in this way using conventional manufacturing processes. To explain: Computer-aided topology optimization is a development tool that accumulates material only at those points where it is necessary due to the existing load situation.

The demands on a stem are high. To reduce its weight, engineering employee Martin Zimmermann relies on an innovative manufacturing process, selective laser melting, in his master's thesis project.

The development process in seven steps

1. define requirements

First, the requirements of the optimization task were defined. The available installation space was largely predetermined by the geometry of the ROTWILD S240 stem and its interfaces. In addition, the loads to which a stem is exposed were determined. These are based on the loads defined in the DIN EN ISO 4210 standard for the counter-phase fatigue test (load during sway pedaling), the in-phase fatigue test (load during braking) and the lateral bending test (overload test). The requirements definition also specified the extent of the weight reduction (at least 25% compared with the initial component) and the material of the optimized component (aluminum alloy AlSi10Mg).

2. output analysis

At the beginning of the optimization task, an FEM simulation of the ROTWILD S240 bicycle stem was performed to gain an understanding of the stresses in the component, under the loads defined in the requirements.

Where there is potential for optimization and what loads need to be taken into account - calculations and simulations on the computer are an indispensable part of the engineering process.

3. performance of the computer-aided topology optimization.

To carry out the computer-aided topology optimization, the maximum available installation space was first constructed. Then the computer-aided topology optimization was performed, taking into account the boundary conditions of the calculation task and the defined optimization objective. The result was an optimal design of the stem for the given load cases. However, this calculation result could not be produced on a one-to-one basis. Rather, it was a design proposal that served as a template for the post-modeling.

Because a high degree of integration is desired for use on the E-MTB, there are special requirements for the stem.

4. post-modeling and adaptation of the design draft

The design proposal obtained by computer-aided topology optimization was reconstructed using CAD software.

5. strength verification by FEM simulation

After post-modeling, an FEM simulation was performed to identify critical component areas and mitigate or eliminate them by appropriate design measures.

Prototypes fresh from the printer. After a few passes of elaborate finishing work, it becomes apparent that the stem prototypes are significantly lighter than the ROTWILD S240.

6. manufacture of the optimized stem using the selective laser melting process

The optimized stem released for production was manufactured at the company Sauer Product in the selective laser melting process. The manufactured optimized stem prototypes are about 42% lighter than the conventional ROTWILD bicycle stem S240.

ROTWILD bicycle stem S240 (left), optimized stem (right).

7. test bench tests at EFBE Prüftechnik GmbH

To verify the calculation results of the FEM simulation, the prototypes were subjected to bench tests at EFBE Prüftechnik GmbH. The loads defined in the requirements were based on the DIN EN ISO 4210 standard. The optimized stem passed both the in-phase fatigue test and the lateral bending test. Only the loads of the antiphase fatigue test could not be withstood by the new optimized stem, which was manufactured by selective laser melting.

Test setups: lateral bending test (left), opposite-phase/equal-phase fatigue test (right).

Conclusion

The master thesis offers ROTWILD's engineering department the opportunity to deal with a comparatively young manufacturing technology, to gather new knowledge for further product developments and to apply this knowledge in the entire engineering team. Thus, the project consequently follows the ROTWILD product philosophy: Developing innovations, implementing them independently and using the latest technologies.

After successfully completing his master's thesis, Martin Zimmermann is a permanent member of the ROTWILD engineering team and will be able to contribute his know-how in the field of innovative manufacturing technologies to future product developments.