Introduction
One of the major requirements related to the design of any product is choosing the proper material to withstand all loads and stresses, either permanent or accidental, which may occur during its use. In order to reduce the time needed for the manufacture and trial of the products, a series of simulations are conducted by the designers and thus significantly reducing the number of possible materials the future products may be made of.
In general terms a bike is defined as a human powered road vehicle with two wheels arranged one after the other, set into motion by the application of work on two foot pedals. Bicycles are estimated to be the most energy-efficient mode of transport: one US study found that to cycle one mile burns 35 calories, to walk uses 100 calories, while a car’s engine burns 1,860 calories.
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Problem Definition
At present the following materials are mostly used for making bike frames:
- Al 6061 alloy - for the bikes used in Trial, Street, XC, Tr, Am and Fr;
- Carbon fiber - for the bikes used in marathons;
- Titanium – it has been increasingly used for making bike frames.
The mechanical properties of common bike frame materials are listed in Table 1. Among all the aluminium alloys used for making bike frames, series 6000 is most preferred - represented by aluminium alloyed with magnesium, chromium, manganese, copper and silicon. 6061 aluminium is the most used alloy in this series due to its excellent machinability and cost. After its machining, the material is subject to a thermal treatment – hardening – to increase the strength properties.
Carbon fiber was initially employed in the automotive industry, on Formula 1 single-seaters, as it has a reduced weight, has an increased strength and especially an increased safety level due to its capacity to disintegrate into small particles, thus absorbing the energy during a possible impact. The sports models took over elements from motorsport to reduce the weight and obtain improved dynamic performances.
For the material selection process, we made certain assumptions to ensure that the problem didn’t get too overtly complicated. They were namely;
- Frame was considered to be constructed from a uniform cross-section. This assumption was made for the simplicity of material selection. The selection depended on the shape of the of the part to be fabricated, and if we were to consider the varying shapes, the selection of a specific material would be difficult.
- The downtube is the most severely loaded component of the bicycle and hence selecting a material such that it sustained these loads were of most importance. Thus, the downtube was analyzed for its failure.
- Entirety of the frame is manufactured from the same material.
- The downtube is a simply supported beam.
- The bike analyzed would be used for performance focused/racing applications.
Requirements
Functional requirements are directly related to the required characteristics of the component, subassembly, or product. An important requirement for materials used in bicycle frame is its strength. Compressive strength is the basic measurement of strength of a material. It is specifically a measurement of the force required to push apart a material. In frame design, the higher the strength betters the performance of the frame. More strength allows less material to be used resulting less weight of the component. (Maleque, M.A.; Dyuti 2010)
Toughness is the property that defines exactly how much a material can stretch before failing. Titanium is an incredibly tough material whereas aluminium has good toughness as a raw material. However, some extra care needs to be taken during manufacturing of the aluminium frame to make sure not let the tube wall get too thin.
CFRP is found to be the best material based upon the ranking from Table 3 but since the material is already in use in the industry, the next best choice is found to be Silicon Nitride (Si3N4) and can be processed by Laser/pressure forming (Hot pressing) and sintering then subsequently machined as required.
Cost Trade-off
As part of the report, minimising the weight was considered to be of utmost priority without considering the cost that would be incurred for the same. Because, after referring to the Ashby Material selection textbook and by analysing this graph, it was found that coupling both cost and weight as objectives didn’t have a relevant impact on the procedure. As seen in the graph, for bikes below 10kgs the slope of cost per kg is significantly steeper, i.e. it gets expensive to reduce the bike weight below 10kgs. In the range below 10kgs, reducing the cost significantly hardly reduces the weight of the bicycle, but in that range the buyer won’t be concerned about the cost of the bike but rather on its performance. Considering purely performance-oriented bikes in which case minimising the weight trumps the cost of the bicycle and thus if both performance bikes as well as bikes for casual use were to be considered, then using this trade-off curve would be helpful in finding a middle ground. Fig 9: Price vs Bicycle Mass chart
Conclusion
The process followed is automatically checked for correctness because it is found that Carbon fiber reinforced plastics are the best choice for performance bicycles with the given selection criteria and the next best choice is found to be Silicon Nitride and then Titanium Alloy Ti6Al4V
References
- Rontescu, C., Cicic, T., Amza, C., Chivu, O., and Dobrotă, D. (2015). “Choosing the optimum material for making a bicycle frame.” Metalurgija, 54(4), 679–682.
- Maleque, M.A.; Dyuti, S. (2010). “Materials for Bicycle Frame System – A Case Study on the Development of Selection Method.” The third international conference on structure, processing and properties of materials SPPM2010 , (March 2014).
- Brower, M. (2005). Advancements in Materials Used in Bicycle Frames.
- Andrew Cantrell. (2003). “Bicycle Materials Case Study.” (Apr. 9, 2019).
- 'Ashby, M. F., and Jones, D. R. H. (2012). “Processing Metals 2.” Engineering Materials 2, 279–296.'
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