Analysis and Optimization of a CFRP Mountain Bicycle Frame
Topic(s) : Material and Structural Behavior - Simulation & Testing
Co-authors :
Digby SYMONS (NEW ZEALAND), James KRIPPNERAbstract :
Off-road cycling is a sport that is demanding of both athlete and equipment. Bicycles must resist a range of high magnitude load scenarios without failure or excessive deflection but also be as light as possible. Constructing a bicycle frame from carbon fibre composite allows much more design freedom than metal tube construction and an opportunity to take advantage of carbon fibres' high specific strength and stiffness. However, determining the best distribution of composite material is a non-trivial task. This paper presents a case study investigation into the use of computational optimization of the composite laminate lay-up for a full-suspension mountain bicycle frame. The aim is to minimise weight whilst achieving adequate strength and stiffness.
A candidate bicycle frame geometry was defined using Solidworks (Dassault Systèmes) software. An initial laminate design was adopted with a quasi-isotropic carbon fibre lay-up of thickness 2.8mm in the main tubes and 5.5mm in reinforced areas around bearings, resulting in a mass of 1.1kg.
Analysis of laminate stresses and optimization of lay-up was performed using the Hyperworks (Altair Engineering) finite element analysis programme. Shell elements were used and a mesh convergence study conducted to determine the required element size for accurate results. Ply failure was predicted using the Hoffman composite failure criterion. The finite element modelling approach was validated by comparison of predicted strength with a published test to failure of a similar composite frame.
For a selected rider weight limit of 120kg a range of overload and fatigue load scenarios were defined: frontal impact, jump landing, pedalling and cornering. These multiple load cases present a multi-objective optimization problem. This was tackled by assigning selected weights to each load case and using a weighted compliance value as the objective to minimize. Stiffness under pedalling loads was selected as the most important factor and the largest weight was therefore assigned to this load case. Key constraints for optimization were that the frame mass did not exceed that of the initial design (1.1kg) and that a factor of safety (FOS) of at least two was achieved for each load case.
Analysis of the initial design revealed more than adequate strength under all load cases (with FOS of up to 7) but deflections of up to 10mm. The design was refined in two steps: an ideal free-size optimization and a subsequent manufacturable size optimization resulting in a practical laminate with discrete ply thicknesses and boundaries. After optimization deflections for all load cases were reduced to less than 6mm, with the average stiffness increased by 30%, and FOS for all load cases now in the range of 2 to 3.5.
Although load-case definition and weighting remain significant subjective manual tasks the case study demonstrates the benefit of computational optimization for a designer of a composite bicycle.
A candidate bicycle frame geometry was defined using Solidworks (Dassault Systèmes) software. An initial laminate design was adopted with a quasi-isotropic carbon fibre lay-up of thickness 2.8mm in the main tubes and 5.5mm in reinforced areas around bearings, resulting in a mass of 1.1kg.
Analysis of laminate stresses and optimization of lay-up was performed using the Hyperworks (Altair Engineering) finite element analysis programme. Shell elements were used and a mesh convergence study conducted to determine the required element size for accurate results. Ply failure was predicted using the Hoffman composite failure criterion. The finite element modelling approach was validated by comparison of predicted strength with a published test to failure of a similar composite frame.
For a selected rider weight limit of 120kg a range of overload and fatigue load scenarios were defined: frontal impact, jump landing, pedalling and cornering. These multiple load cases present a multi-objective optimization problem. This was tackled by assigning selected weights to each load case and using a weighted compliance value as the objective to minimize. Stiffness under pedalling loads was selected as the most important factor and the largest weight was therefore assigned to this load case. Key constraints for optimization were that the frame mass did not exceed that of the initial design (1.1kg) and that a factor of safety (FOS) of at least two was achieved for each load case.
Analysis of the initial design revealed more than adequate strength under all load cases (with FOS of up to 7) but deflections of up to 10mm. The design was refined in two steps: an ideal free-size optimization and a subsequent manufacturable size optimization resulting in a practical laminate with discrete ply thicknesses and boundaries. After optimization deflections for all load cases were reduced to less than 6mm, with the average stiffness increased by 30%, and FOS for all load cases now in the range of 2 to 3.5.
Although load-case definition and weighting remain significant subjective manual tasks the case study demonstrates the benefit of computational optimization for a designer of a composite bicycle.