Feb 7, 2023
This project focused on solving a real-world safety issue: preventing lower-limb injuries caused by sudden braking on a company’s bike model. Our mission was to design a bike crank engineered to break safely and predictably under excessive force (around 8.7 lbs), protecting the rider while maintaining durability in normal use. The crank needed to break near its midpoint, at least 1 cm away from the attachment pins, with minimal mass.
Ours excelled in all metrics:
broke in smallest margin of target failure load (8.7±1lb)
3.2g mass achieved was 4x lighter than target mass
failure located within millimeters of center
Design Process
We began by calculating the force at which the crank should fail. After determining the target failure force of ~39.13N (8.80 lbs), we studied stress distribution from bending and torsion forces to inform design direction.
Key Insights:
Maximum stress occurs near edges and away from the neutral axis.
To encourage a break near the center, we focused on adding material where stress needed to be reduced and removing material where stress needed to concentrate.
Our conceptual designs included simple geometric cutouts and truss-like structures. Ultimately, we prioritized simplicity, ease of manufacturing, and predictable stress distribution.
 |  |
Iterations & Prototyping
1. Acrylic Two-Triangle Cutout Design
Initial laser-cut acrylic prototypes broke too early (under 5 lbs), revealing misalignment between our FEA analysis and real-world behavior.
2. Multi-Layered Acrylic Designs
Layering increased strength but led to parts that were too robust and heavier than ideal.
3. Wide and Tapered Designs
Adjustments didn’t result in consistent center-breaking performance.
4. "Bones" and Hybrid Designs
These 3D-printed attempts either failed prematurely or were impractical to manufacture.
5. Tube Design
Promising in theory, but printing issues and time constraints prevented testing.
6. PLA Two-Triangle Cutout
Switching to PLA provided better material behavior, allowing for elastic deformation and a predictable break near 9 lbs in the center.
Final Design
Our final design featured:
Two-triangle cutout for predictable stress concentration.
PLA construction for ductility and reliable performance.
Weight of only 3.2 grams.
Breaking force of 9 lbs, within ±1.0 lb of target.
The design met all criteria: minimal mass, predictable failure location, and safe breakaway force.
In final testing, our crank broke exactly as intended — in the center, at 9 lbs. The part exhibited elastic deformation prior to failure, demonstrating the material’s ideal performance. The final prototype validated our design strategy and iterative process.
Key Takeaways
Balance between geometry and material selection is crucial.
Real-world testing is essential to verify and adjust theoretical and FEA results.
Iteration and adaptability lead to success — sometimes returning to early ideas can yield the best results.
