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MotorCycle swingarm Optimization

This project utilized the bottom-up modeling technique in Ansys Mechanical to optimize a motorcycle swingarm.

A motorcycle swingarm is a key component of the rear suspension of a motorcycle. Decreasing the volume and thus the mass of the swingarm will directly affect the speed, acceleration, and handling of the motorcycle. For this project, the swingarm was optimized to decrease the total volume of the structure while staying within acceptable ranges for maximum stress and deformation.

A preliminary model was generated in Solidworks to replicate the basic features of a motorcycle swingarm including the main pivot, the shock mounting holes, and a hole for the rear axle.

full arm.png

Modelling Process

The model was made using the bottom-up method in Ansys software. Instead of importing a CAD model into Ansys, the structure was created by manually defined key points, lines, areas, and volumes. Ansys APDL language was utilized for this process. This approach was critical to ensuring a high-quality hexahedral mesh that could render greater modelling accuracy in comparison to tetrahedral meshes.

 

A standard material for this application, Aluminum 7075 T6, was used in this model. All material properties were assumed to be isotropic for this analysis. For boundary conditions, all nodes of the inner ring of the main pivot hole were constrained in all translational degrees of freedom. Three main loads were considered, an accelerating force based on 5 m/s2, the normal force acting on the wheel, and the suspension force.

The normal force acting on the swingarm was calculated from a motorcycle dead weight of 300 kg and an average driver weight of 75 kg. The load due to the sprung portion of the motorcycle was assumed to be 85% of the total mass of the bike, and the weight distribution was 60% on the rear axle. The force due to acceleration was applied in the negative X-direction. Since all loads were applied in the X-Y plane, the model was simplified to reflect the right half of the swingarm.

Optimization

Initial Results

Prior to optimization, the model was evaluated to determine its response to the load case. The results can be seen in the table below.

max allowable_edited.jpg

The model demonstrated minimal deflection and magnitudes of stress significantly less than the yield strength (503 MPa) of the material. The results of this load case suggest that the design was overbuilt due to a low maximum stress value. However, these results reflect only one load case and they do not ensure an appropriate safety factor for all load scenarios. Additional load cases such as cornering, potholes, and other dynamic load situations should be considered as well. A plot showing the expected deformation of the initial design can be seen below.

pre-opt deflection.png
Optimization Results

Multiple design variables were used to dimension key features in the model. The optimization tool in Ansys varied the design variables using gradient-based methods to converge on an optimized design. Constraints were placed on the optimization to ensure that the maximum allowable deformation and von Mises stress were not exceeded. Based on multiple optimization trials, it was found that the maximum deformation value was a barrier to further volume reduction.

opt results_edited.jpg

The final design achieved a 66.21% decrease in volume, and a maximum von Mises stress significantly below the yield strength. This is important as at higher stress values the swingarm will failure at a lower number of loading cycles and thus have a shortened lifespan.

The optimized design has noticeable differences in the key features with a significant reduction in volume closest to the wheel attachment point, and an increase in the size of the cut-out feature. The von Mises stress is shown in the plot.

Note: Due to time constraints, the optimization shown above considers limited loading cases and thus does not represent a truly optimized motorcycle swingarm. Additionally, the long, thin sections of the arm may be subjected to buckling and therefore require further evaluation.

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