Bicycle Bottom Bracket Shaft Design for Weight Reduction
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Solution Overview
Problem
Existing bicycle bottom bracket assemblies face challenges in reducing weight while maintaining rigidity, and in minimizing bending moments at the shaft ends during pedaling.
Innovation Solution
The design features a shaft with an increased outer diameter, allowing for the use of lighter materials like aluminum or carbon fiber, and a central portion with a smaller diameter to reduce weight and stress, along with bearings positioned closer to the crank arms to decrease bending moments. The shaft can be made from metallic or composite materials with structural fibers, and the housing box can have a shorter length with adaptation elements for precise centering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the shaft outer diameter is increased to maintain rigidity, then the bending moment resistance is improved, but the weight of the assembly increases
Solution Approach 1:
The shaft features a variable diameter design where the central portion has a smaller diameter than the end portions. This local differentiation allows the shaft to have sufficient strength at the ends (where bearings are mounted) while reducing material usage and weight in the central region, resolving the contradiction between rigidity and weight.
Solution Approach 2:
The shaft is made from composite materials or materials with structural fibers, which provide high strength-to-weight ratio. This allows the shaft to maintain adequate rigidity and bending moment resistance while significantly reducing the overall weight compared to traditional solid metal shafts.
2Strength
If the distance between bearings is increased to reduce bending moments, then the shaft stress is reduced, but the housing box length must be increased
Solution Approach 1:
The bottom bracket assembly is divided into modular components including the housing box, adaptation elements, and bearings. This segmentation allows the bearings to be positioned optimally with sufficient distance between them to reduce shaft bending moments, while the housing box length is independently optimized through the use of adaptation elements that provide mounting flexibility.
3Weight of moving object
If lighter materials are used for the shaft, then the weight is reduced, but the structural integrity and rigidity may be compromised
Solution Approach 1:
The shaft utilizes composite materials or materials with structural fibers that provide exceptional strength-to-weight ratios. These materials maintain adequate structural integrity and rigidity while significantly reducing the shaft weight, directly resolving the contradiction between weight reduction and structural strength.
Solution Approach 2:
The variable diameter design concentrates material where it is most needed (at the end portions for bearing mounting) and reduces material in less critical areas (central portion), optimizing the strength-to-weight ratio and ensuring structural integrity is maintained with minimal material usage.
Data Source
AI summary
The invention concerns a bottom bracket assembly (1) of bicycle, comprising a shaft (2) extending along a predetermined longitudinal direction and a couple of bearings (31, 41) adapted to rotatably support said shaft (2) with respect to a housing box (10) provided in a bicycle frame, said bearings (31, 41) being mounted on said shaft (2) at opposite end portions (3, 4) of said shaft (2). Advantageously, said opposite end portions (3, 4) of said shaft (2) have an outer diameter (D2) comprised in the range between 28 mm and 37 mm, bounds being included, and the distance (D1) between the median planes (Y1, Y2) of said bearings (31, 41) along said longitudinal direction is comprised in the range between 80 mm and 100 mm, bounds being included.