Tandem Bicycle

Research and Ideation
I began by researching types and designs of tandem bicycles, and creating a master sketch to model the original bike on Solidworks.
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Boundary conditions were applied to simulate real bike use as if the model has seats, wheels and handles, and prevented singular matrix errors.

Loads and Boundary Conditions
To simulate rider weight on raised seats, two
forces were applied above each seat member face.  A gravity load was applied to ensure the mass of the frame itself was considered.

To model pedalling, remote oscillating forces were applied, as the 'worst case' scenario: when the forces of both cyclists are applied synchronously on one side of the bicycle. Rider weights were added to account for the riders getting on and off.

Results
Mesh refinement studies accurately discretised the model without excessive computing time. The element size and growth ratios were reduced and explored until stress converged and aspect ratios and  Jacobean ratios at a minimum. The most common areas of issue were the seat and gear stays, so mesh control was applied.

The aluminium bike weighs 3.99 kg, a frequency of 40.672 Hz and has infinite life. The titanium bike weighs 6.29 kg, a frequency of 38.674 Hz and also has infinite life.

Improved Design
Rationale for improvement was derived from the analysis of the fundamental frequency of a cantilever beam. My aim was to increase the fundamental frequency of the bicycle. The general members were changed to 36 mm external radius, 1 mm thickness; seat and gear stays 26 mm external radius, thickness 1 mm. The fillet radius was also increased to 10 mm to reduce stress concentrators.

The improved aluminium bike's frequency is increased to 46,142 Hz, with a weight decrease to 3.29 kg, but fails at 935,000 cycles due to the thinned walls. The improved titanium frame's frequency increases to 42.9 Hz with a weight decrease to 5.18 kg. It has infinite life, and a near zero damage percentage of 0.00384%