**6. Conclusions**

In this paper, a design optimization methodology is described that incorporated: (1) a non-destructive measuring technique using a laser vibrometer; (2) a reverse engineering technique for 3D model construction; and (3) a structural topology optimization. The design optimization methodology was applied to a sidecar suspension bell crank to minimize its structural weight and then subjected to constraints including volume, strain energy and von Mises stress.

A 3D bell crank model was created using a reverse engineering technique and the corresponding FE bell crank model was validated using laser vibrometer test results. The difference between the experimental and FE results of the natural frequency at the fourth mode was less than 5%, which was deemed to be valid for further analysis.

At the beginning of the optimization process, the design domain for material removal was selected. Using the topology optimization approach, the geometry designs produced by the optimization algorithm were transformed into CAD models for the smoothing of the geometry to eliminate holes, irregular shapes and complex edges before fabricating the prototype for further testing. The results obtained from the design optimization methodology demonstrated its applicability and capability to produce an optimized bell crank, which increased its overall sti ffness and natural frequency, while reducing its weight by 3%, maximum principal strain by 4.3% and maximum von Mises stress by 16.5%. This study demonstrated how reverse engineering and non-destructive testing methods can be used e ffectively for structural topology optimization to produce a suitable lightweight product. This is significant contribution to finding solution to complex designs without compromising structural performance and integrity.

Further work using the current methodology could be implemented in other sidecar components (e.g., brake bell, steering, wheel hub and spindle, brake rotor etc.), and additive manufacturing using 3D printing of lattice structures could achieve further net weight reduction.

**Author Contributions:** T.Y.P.: Conceptualization, methodology, software, writing—original draft preparation, M.F.: Methodology, writing, review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to acknowledge and give many thanks to Luka Grubic and Jamie Crass for their assistance on the project. The authors also acknowledged the contribution of the Laboratory Technician at RMIT Bundoora East Campus for their ongoing help and support in this project.

**Conflicts of Interest:** The authors declare no conflict of interest.
