Child-Safe Bicycle Crank
A lightweight crank designed to intentionally fail under high braking forces to prevent lower limb injuries
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Problem
Children are especially vulnerable to leg and foot injuries when a bike stops suddenly. Retrospective data from manufacturers shows that when braking forces exceed 12 N at the drive wheel, the resulting sudden deceleration often injures the lower limbs. Current crank designs are built to withstand far greater loads, unintentionally transferring that force directly to the child’s legs during an abrupt stop. The challenge was to design a crank that remains durable in daily use but intentionally fails at the critical threshold before causing injury.
Solution
We created a crank arm engineered to fail at around 40 N of pedal force (corresponding to 12 N of wheel deceleration) with a factor of safety just under 1.0. Material was strategically removed from the center of the arm and replaced with stress-guiding notches that focus the failure zone away from critical mounting interfaces. The final prototype weighed only 9.91 grams, featured smooth rounded edges for child safety, and broke cleanly at the targeted load without damaging surrounding components.
The project began with the goal of making bikes safer for young riders. We first analyzed the forces during sudden stops and calculated the expected pedal load, then ran hand analyses to understand stress behavior in a typical crank. Our first prototype used a circular cutout to thin the center, but testing showed it broke too early and unpredictably near the mounting holes.
Learning from this, we increased thickness around the holes, bonded double acrylic layers for strength, and added notches at the center to guide stress. This design performed closer to predictions, breaking around 8–9 pounds of force, but still occasionally failed early due to inconsistent epoxy curing.
For the final version, we reduced weight with mirrored cutouts, rounded all sharp edges, and carefully tuned the notch geometry to achieve a consistent factor of safety of about 0.99. It successfully broke at the center—over 1 cm from interfaces—protecting the bike from damage and the rider from harm.
Initial Ideation and Design Process
We explored ways to localize failure by removing material from the crank’s center to lower cross-sectional strength while keeping mounting ends strong. A Pugh matrix was used to evaluate concepts by safety, predictability of failure, manufacturability, and weight. This led to the decision to combine thickness reinforcement at the ends with notches at the middle as a controlled failure point.
Stress Analysis and FEA
Hand calculations identified the highest stresses at the outermost top and bottom fibers of the crank’s central cross-section. FEA confirmed this stress pattern and showed a factor of safety around 0.99, aligning with the target of failure at about 40 N pedal load. This location is over 1 cm from the interfaces, ensuring the crank fails safely without damaging other componentsProject 1 Report.docx.
CAD and Assembly
The final CAD design included mirrored side cutouts to reduce weight, rounded corners to minimize stress concentrations, and a defined notch geometry for controlled fracture. The crank fits standard child bike interfaces and was dimensioned to maintain stiffness during normal riding while ensuring predictable failure at the threshold force.
Manufacturing
The crank was laser cut from acrylic sheets, with two layers bonded using epoxy for sufficient thickness. The use of acrylic allowed precise control of dimensions while keeping the part lightweight. We discovered that epoxy curing time greatly affected strength, so consistent curing was enforced in later builds to improve repeatabilityProject 1 Report.docx.
Continuous Improvement
Future work could involve using injection-molded polymers with embedded stress concentrators for better consistency, automating epoxy application, and conducting larger batch tests to reduce variability. This would improve manufacturing control and reliability at scale.
Impact
This crank demonstrates how engineering design can actively prevent injuries rather than just withstand forces. By deliberately designing for controlled failure, the project rethinks safety for children’s products—turning a potential hazard into a protective feature.
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Our first prototype featured a central cutout to reduce strength, aiming to trigger failure at the midpoint under load. However, during testing it fractured prematurely near the mounting interface instead of the center. This unpredictable failure showed that stress was concentrating at the wrong location, leading us to reinforce the ends and redesign the geometry to guide breakage safely away from the critical attachment points.
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The image shows our crank failure testing setup, where the acrylic crank was clamped in a vise and loaded using a lever arm with incremental weights attached at the end. This setup allowed precise control of the applied torque, enabling us to measure the exact force at which the crank fractured and verify that it broke at the designed safety threshold.
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The crank successfully fractured at the designed weak point during testing, breaking cleanly at the targeted force threshold to confirm the intended safety behavior.
