Upon examination of their properties, Carbon Fibre was dismissed as a choice, due to its high cost and lower availability as compared to the other two choices.
There was a close comparison between Chromoly Steel and Aluminium. The foldable bicycle is intended for city commuting, which would result in the rider carrying the bicycle often, and the high density, or weight, of Chromoly steel would make it impractical for use in a foldable bicycle's frame.
Therefore, Aluminium was chosen as the appropriate choice of material for this foldable bicycle.
Introduction
In this report, we will be analyzing the different materials that can be used for the frame of the foldable bicycle.
The different materials used in designing the frame of the bicycle needs to be taken into consideration carefully, as they each have different properties, both mechanically and physically. These properties include stiffness, strength, and weight. Other factors will also need to be taken into account for the material, such as, the cost, availability etc. Taking all these factors into consideration would ensure that the best material is chosen and proper functionality of the bike.
The main materials used in bicycle frames are Chromoly Steel, Aluminum, and Carbon Fiber. The properties of these three materials will be evaluated in this report, and one will be proven better suited for the bicycle's frame, based on this evaluation.
Discussion
Although there are many more materials that can be considered and used, the three materials being considered for use in our bicycle's frame are Chromoly Steel, Aluminum and Carbon Fiber.
The physical and mechanical properties of each of these three materials will now be analyzed in more detail.
Chromoly Steel
Chromoly Steel is type of Iron alloy containing Chromium, Molybdenum, Manganese, Silicon as well as Carbon. It is used extensively in bi-cycle frames, however it is heavier than most other materials used for bike frames. Chromoly Steel is also known as CRMO or 41xx Steel, where in the latter the last two digits 'xx' represent the Carbon content, in 100ths of a percent, for any particular Chromoly Steel alloy. 4130 and 4140 Chromoly are the most typical grades for bicycle frames. 4130 Chromoly is more easily welded than 4140, although specific welding constraints still apply, so it will be one of our proposed materials.
The following properties refer to 4130 Chromoly Steel.
Property(Physical)
Value
Metric Unit
Density
7.872 *10³
kg/m³
Modulus of elasticity
205
GPa
Thermal expansion (20 °C)
11.2*10-6
°Cˉ¹
Specific heat capacity
477
J/(kg*K)
Thermal conductivity
42.7
W/(m*K)
Electric resistivity
2.23*10-7
Ohm*m
Property (Mechanical)
Value
Metric Unit
Tensile strength (annealed)
561
MPa
Yield strength (annealed)
361
MPa
Elongation (annealed)
28
%
Hardness (annealed)
82
RB
Tensile strength (normalized)
669
MPa
Yield strength (normalized)
436
MPa
Elongation (normalized)
25
%
Hardness (normalized)
93
RB
Aluminium
Alloys comprising of aluminium and magnesium are or have been very commonly used in the production of bikes for their characteristics are suitable for those desired for a bike. They are low cost as they are an abundant resource and are easy to manufacture. They are lightweight which is highly useful for a human operated bicycle and although the strength values are lower than steel alloys, the strength to weight ratio is superior. Although it commonly found that a very high level of skill in workmanship is required in the manufacture of using these materials in order for the end result to be a high quality product. The overall end product will vary in thickness and strength depending upon the quality of the alloy used.
Magnesium is not as dense as aluminium and so, it has been found to be not suitable a material for the construction of bike frames within the last few decades. Aluminium alloys 6061 and 7005 are the most commonly used aluminium alloys which result in a highly stiff frame, which some cyclists regard as undesirable as it gives a less desirable feel to the ride.
The composition of alloy 6061 is:
Silicon minimum 0.4%, maximum 0.8% by weight
Iron no minimum, maximum 0.7%
Copper minimum 0.15%, maximum 0.40%
Manganese no minimum, maximum 0.15%
Magnesium minimum 0.8%, maximum 1.2%
Chromium minimum 0.04%, maximum 0.35%
Zinc no minimum, maximum 0.25%
Titanium no minimum, maximum 0.15%
Other elements no more than 0.05% each, 0.15% total
Remainder Aluminium (95.85%-98.56%)
Property(Physical)
Value
Metric Unit
Density
2.7 x 10³
kg/m³
Modulus of elasticity
70-80
GPa
Thermal expansion (20 °C)
23.5*10-6
°Cˉ¹
Specific heat capacity
896
J/(kg*K)
Thermal conductivity
152-173
W/(m*K)
Electric resistivity
3.7-4.0*10-6
Ohm*m
Property (Mechanical)
Value
Metric Unit
Tensile strength
125-300
MPa
Yield strength
55-310
MPa
Elongation
8-30
%
Hardness
30-97
RB
Alloy 7005
Element
Weight %
Al
93.3
Mn
0.45
Mg
1.4
Cr
0.13
Zn
4.5
Ti
0.04
Zr
0.14
Property(Physical)
Value
Metric Unit
Density
2.7 x 10³
kg/m³
Modulus of elasticity
70-80
GPa
Thermal expansion (20 °C)
23.5*10-6
°Cˉ¹
Specific heat capacity
896
J/(kg*K)
Thermal conductivity
166
W/(m*K)
Electric resistivity
40*10-9
Ohm*m
Property (Mechanical)
Value
Metric Unit
Tensile strength
193
MPa
Yield strength
83
MPa
Elongation
20
%
Hardness
30-97
RB
Carbon Fibre
Carbon fibre is a composite material and is made up of three different precursor materials. These three materials are Rayon, Polyacrylonitrile (PAN), and isotropic and liquid crystalline pitches. Low-modulus carbon fibres are produced by Rayon and isotropic precursors where as high-modulus carbon fibres are produced by PAN or liquid crystalline pitches. In both cases, precursor fibres are spun and heated to 800 degrees Celsius to produce a carbon fibre. The fibres modulus increases with higher temperatures from 1000 to 3000 degrees Celsius. More specifically the Young's modulus of Carbon fibre can range from anywhere between 30-50 GPa and 180-250 GPa and an ultimate tensile strength between 300-420MPa and 2200-2800MPa. The variance in values is due to direction of fibres, temperature of manufacture and purity of materials.
Property (Physical)
Value Metric Unit
Density
1.78 x 103
Modulus of elasticity
150
Thermal expansion (20 °C)
0.224*10-6
Specific heat capacity
0.71
Thermal conductivity
24
Electric resistivity
1*10-14
Property (Mechanical)
Value
Metric Unit
Tensile strength (normalized)
1600
MPa
Yield strength (normalized)
350
MPa
Elongation (normalized)
0.5
%
Hardness (normalized)
unmeasurable
RB
Advantages of Carbon Fibre:
Physical strength, toughness, light weight. A carbon fibre part with the same strength and toughness as a steel part is more than five times lighter.
Low coefficient of thermal expansion. Carbon fibre has a coefficient of thermal expansion more than 50 times less than that of aluminium.
Electrical conductivity.
Biological inertness and x-ray ability.
Fatigue resistance, self-lubrication, high damping.
Electromagnetic properties.
Chemical inertness, high corrosion resistance.
Disadvantages of Carbon Fibre
The main disadvantage of carbon fibre is the manufacturing of the material. This is due to the difficulty and skilled manual work required to produce it. Skilled craftsmanship is required to carefully place each layer of fibres in a specific sequence, therefore causing the cost of this material to be quite expensive.
Selection Criteria:
The mechanical, physical and other properties of the three materials, and their alloys, need to be considered and evaluated in a selection criterion before a choice can be made.
The selection criteria involve properties such as:
Density: The material needs to have a moderate density, or weight, so that it is not too heavy, but at the same time, not too light. The density is not usually a limiting factor for normal conditions, as the rider will not feel the heaviness when riding. However, for a foldable bicycle, it is important that the bicycle is light and strong, and the rider will often have to carry the bicycle.
Modulus of Elasticity: The Young's Modulus needs to be high, as it will determine how much force the material can withstand, before it undergoes plastic deformation. The Elastic Modulus is also called the stiffness. An elastic bicycle would be useful when riding in terrains, or rough roads. However, for the purpose of commuting in the city, a stiffer frame would be the better choice. This doesn't change with the different alloys, so just the general material can be considered.
Strength: The amount of force the material can sustain before breaking, or deforming plastically determines how strong a material is, therefore we will use the Yield Strength for the evaluation. This property will be particularly significant if the bicycle is involved in an accident, so a stronger bicycle frame is needed to resist maximum deformation during impact.
Tensile Strength: The tensile strength may seem useless to consider, as bicycles don't usually just break in the centre while riding. However, the tensile strength test encompasses all the values tested for strength, stiffness, ductility and other parameters obtained during heat treatment, so it is important to include in our evaluation.
Availability: Availability could be affected by other factors, like the limited skilled workers to produce the material for use, cost etc. For the selection criteria, the material should be available worldwide, and in bulk quantities.
Cost: The material should be not too expensive for the average rider, and should also be cost-effective for its price.
The three materials are marked against the selection criteria using a scale of 1 - 5 (Table 4). Number 5 representing the most suitable material for that criterion, while 1 the least suitable.
Material
Density
Modulus of Elasticity
Tensile Strength
Performance
Availability
Cost
Chromoly Steel 4130
2
4
4
5
5
4
Aluminium Alloys
5
3
3
4
5
5
Carbon Fibre
3
4
5
5
3
2
Selection Criteria Evaluation:
Chromoly Steel: 2 + 4 + 4 + 5 + 5 + 4 = 24
Aluminium Alloys: 5 + 3 + 3 + 4 + 5 + 5 = 25
Carbon Fibre: 3 + 4 + 5 + 5 + 3 + 2 = 22
From the selection criteria evaluation results, although Aluminium scored the highest, it can be seen that all three materials had close results. From further discussion of these results and the factors influencing their values in the selection criteria, we can determine why Aluminium is the best material to be chosen for the bicycle frame.
Chromoly Steel and Carbon Fibre both have higher values of Young's Modulus and Tensile Strength. This would mean that these two materials would perform better than Aluminium in case of a crash, and they both have higher strength and stiffness, giving them. A stiffer frame would give the rider better control and handling over the bicycle when turning corners, as it would resist torsional twist, preventing damage to the bicycle's frame. A higher strength and stiffness would also help in preventing any breakage or fracture in the bicycle's frame due to continuous folding and unfolding of the frame.
This correlates to their higher values of performance of these two materials than Aluminium, although the rating of Aluminium is just one value below the other two. A stiff alloy of aluminium could be used instead, but if the frame is too stiff, it would make the bicycle feel rigid, inflexible and uncomfortable for the rider. This would be due to the bicycle's inability to absorb shock during the ride, giving a rough or "rocky" ride.
The rating for Carbon Fibre decreases due to its lower availability as compared to Aluminium and steel. This lack of availability, along with the previous discussed good qualities result in it having a higher price than the other two materials. Due to its high price and lower availability, it is consequently eliminated as a material choice for the foldable bicycle's frame.
This leaves Chromoly Steel and Aluminium as the two remaining material choices.
The availability of both materials is very similar to each other, so we cannot use it as a defining factor. Research shows that the cost however for Steel is higher than the cost of Aluminium. This difference is not high though, so some may argue that the benefits of the Modulus of Elasticity, Ultimate Tensile Strength and Performance outweigh the small difference in cost. This leaves the final factor in the selection criteria to be discussed, the density.
For a case of general application, either Chromoly Steel or Aluminium may be used, but it is vital that the materials are considered specific to the situation of foldable bicycles. This makes Aluminium the clear choice between the three choices, due to its density value. The density value directly relates to the weight of the bicycle, and the density of Chromoly Steel, 7.87 x 103 kg/m3, is obviously too high. It would be impractical to have a frame of that that density for a foldable bicycle which requires frequent carrying. The density of aluminium is apt for this application, as it is between the density values of Chromoly Steel and Carbon Fibre.
The high density eliminates Chromoly Steel as a choice as well.
The properties of Chromoly Steel and Carbon Fibre would make them appropriate for racing bicycles and standard riding bicycles, as they have excellent strength to weight ratio.
Nonetheless, the selection criteria evaluation decidedly proves that Aluminium as the best material choice for the foldable bicycle out of the three choices.
Conclusion
After much research about typical materials that are used in bicycles frames, attention was focused on the main three materials, Chromoly Steel, Aluminium and Carbon Fibre.
In accordance to the results from a specifically created selection criteria, Aluminium received the highest score. This was due to its beneficial density, availability and cost, which outweigh its slightly lower performance, Modulus of Elasticity and Ultimate Tensile Strength compared to the other two materials. Therefore, Aluminium was found to be the most suitable material for use in building the frame of the foldable bicycle.
