Composites in bicycles
Definition : A composite consists of at least two different materials (often more) ; a matrix and a reinforcement (often a fibre).
Usually for composites, the reinforcement is a stiff fibre, inside a plastic matrix material. The mechanical properties of plastic matrix materials are poor, compared to the reinfor-cement. Composites of fibres and plastics have been given a very particular place in the construction of high-tech bicycle frames
A comparable analogy for the directionality of composite structures, is the variability in the performance of a wooden structure, depending on the directions of the wooden fibre grains (and even any weak areas caused by "knots" in the wood, equivalent to "voids" in a composite material).
The approach to the design for assemblies with composites requires a specific approach. It is not enough to simply replace an existing metal part, with one of the same design, but made from Carbon fibre material. Different designs, and different constructions must be used, if Carbon fibre is to be used to its fullest potential (such as saving weight in the wings of a Boeing Dreamliner jet airliner). There is an 'art' to designing really good Carbon fibre structures, that only comes with years of experience.
Some composites can be made using a light metal matrix material. Composite materials can also be made using a reinforcement of Si-C particles (Silicon Carbide), or of Al2-O3 (Sapphire). At Specialized in the nineties, they built bike frames with such "Metal Matrix (= M2)" material - using tubes from the American manufacturer Duralcan. Such a tube is made by heating a powder mix under high pressure, in a tubular mould, up to the melting point of the lowest melting component ; the bicycle industry no longer uses such types of alloy materials, (as Carbon fibre has superseded them).
Basically we can divide plastic materials into two different basic types ; "Thermo-plastics", and " Thermo-sets".
Thermo-plastics consists of long molecules, which upon heating, becomes easier to slide along one another. Composites using thermo-plastics are often reinforced with chopped fibres (such as glass). In Taiwan the firm HYC makes a polypropylene grain with carbonfibre in it.
To make a product from a thermo-plastic composite, the mixture of plastics and fibres is heated to a temperature of 150 to 250 °C, then the mixture is injected under high pressure into a mould to form the final product, (for example, plastic BMX wheels), the moulded mixture is then left to cool and harden. This application is limited to industrial mass production, on a large scale, as it requires expensive moulding equipment, that can only be paid for with products that sell in large quantities.
Reinforcing a plastic (such as Nylon) in this way, can greatly improve its mechanical performance properties, while maintaining its ease of manufacture, and relatively low cost.The fibre content of such products is usually low, a maximum of only 20 to 30 % (1/4) ; the reinforcing fibres are usually made of glass, (hence the name "Glass reinforced plastic" "GRP").
While the performance of such materials is very good compared to other weaker, and cheaper, types of plastics, it is generally greatly inferior to high quality engineering metals. Some thermo-plastics can be heated up, softened, and re-moulded, over and over, again and again, so they can be said to be somewhat " green" and environmentally friendly, but they still usually require petro-chemical raw material feed-stocks, and can be difficult to finally dispose of. There are some newer types of plastics, that use biological feed stocks, (such as plant material) and some types of plastics that are made to decompose and bio-degrade in landfill sites, but these usually have poor mechanical properties, and are often used for food packaging, not high performance composite structures.
Thermo-sets are plastics that three dimensionally "grow" or "cure" together. At high temperatures, they do not soften, (as thermo-plastics do) but instead char, or partly burn.They chemically fuse together, rather than become solid at lower temperatures, as thermo-plastics do. To break down the matrix structure of thermo-set plastics, the constituent chemicals need to be separated. Most composites use thermo-setting plastics (such as epoxy resin) as a matrix material.
The reinforcement material in composites can consist of : Glass fibre, Carbon fibre, Aramids, Polyethylene fibres, Vectran, or Magellan M5.
Glass fibres, are pulled as strands from liquid molten Glass. Chemically Glass consists mainly of Silicon Oxide (sand). E-Glass is the most common type of Glass fibre material. The strength of Glass fibre is reasonable, but the modulus of elasticity (E-modulus = i.e. stiffness) is only moderate, and it has a high density, but it is relatively cheap, and it is easy to use. By volume (if not value) Glass fibres are the most used composite material.
One common application for Glass fibre composites, is in the construction of the hulls of small boats and yachts, because it resists the effects of water damage well, and it can easily be formed into the complex curves required in a boat hull. It is considerably cheaper than Carbon fibre, and it can be repaired if necessary.
Carbon fibres, the production of which is carried out by carbonising a yarn of PAN or Dralon material in an oven (cheaper fibres are sometimes even made from Pitch, which are only about 85 % Carbon). The charred yarn is then further heated to approximately 1600 °C. The Carbon is then recrystallised to form Graphite (Carbon). The production process consumes energy, which is why Carbon fibre is expensive, but it is worth the cost, because it is strong and rigid. A video about the process can be found here : Carbon Fiber: Conventional Carbon Fiber Conversion - YouTube
A new use of Carbon is working with Graphene ; this is an ultra thin Carbon Graphite layer, having a thickness of a single atomic layer, only one Carbon atom thick. Many wild claims have been made for Graphene, as a new wonder material, but years after its first creation, it still has not fulfilled most of these wild claims, and become established as a common industrial material. The best uses for Graphene maybe for its unusual electrical properties, rather than for structural applications.
Another carbon product that might be used in the future are carbon nano-tubes, see: http://mknano.com/
Aramids, such as 'Kevlar 49' (DuPont) and 'Twaron HM' (AKZO), are both chemically similar. Aramids have high tensile strength, and a reasonable modulus of elasticity, but composites made with Aramids are poor at compressive loading.
Polyethylene fibres, are made from long chains of PE (> 10^6, more than a million units long) and are dissolved in Paraffin ('Spectra' fibre) or Decalin ('Dyneema' fibre). They have a high tensile strength, a reasonable E-modulus, and a low weight. Polyethylene is a thermo-plastic ; the properties of the composite rapidly deteriorate at temperatures above 90 °C (so it is not suitable for high temperature environments, like engines).
Vectran, (LCP = Liquid Crystal Polymer) is owned by the American firm Kuraray. It is a strong fibre, but not so stiff ; it is a competitor of Aramid fibres in many ways. Vectran is used among others things in the plastic spokes of 'Spox' bike wheels, and in the more expensive vehicle tyres from Continental. In tyres its lack of rigidity is an advantage ; all tyres are indeed composites, even cheap ones!.
The product Magellan M5 (PIPD), is still in the development stage (the huge American company DuPont bought this Dutch invention in 2005). It is stronger, and much more resistant to breakage than Carbon fibre, and is thought to be stiffer too, with excellent compressive properties, see this link. However, very little has been heard about its development, it is not clear why this is ; the product may have had serious short-comings, or there may be production problems, or the Americans might want to retain the exclusive rights of this material for military applications (such as ballistic protection uses), or they may want to restrict its sale for commercial reasons (such as not to lose sales of other products from their range).
Working With Composites
Carbon and Aramid fibres are supplied in grades HS / HT, (High Strength / High Tenacity = high tensile strength) and quality HM (High Modulus = high E-modulus).
As manufacturers of bicycles, we are not so interested in high tensile strengths. "Light and stiff" is our motto ! So we only choose HM fibres ; the winning fibre is Carbon HM. Alternatively, we can also make fibre combinations of Carbon and Aramid, or Carbon and Polyethylene. This latter Carbon/PE fabric has good resistance to compression ; a trait where many composites score badly, and thus often fail in constructions. While many designers are obsessed by high tensile strengths, in reality many constructions are just as frequently subjected to compression loads.
In fact the fibres are only really rigid in the longitudinal direction, along the length of the fibres. In the transverse direction to the fibres (at 90 ° to the longitudinal plane), the stiffness is very much less. With a composite material containing 60 % fibre, and 40 % matrix (specific gravity 1.4). The E-modulus of the composite, as it is incorporated in the structure, will be much lower than the value mentioned in TABLE 2. (usually less than a quarter of this value ; only 25 % !).
There are many composites made with thermo-set materials ; most thermo-sets are reinforced with fillers and fibres. The fabrication often occurs at high temperatures and at high pressures, but there are two main groups of thermo-sets that cure at atmospheric pressure (1 Bar) and room temperature (20° C) : the Epoxy resins (EP), and the unsaturated Polyester resins (UP = unsaturated Polyester).
Polyester resins are used primarily in combination with Glass or Polyester fibres (which is a cheaper combination than Carbon/Epoxy, but also a lower performance).
High performance composites (like Carbon fibre, Aramid fibre, or Polyethylene), are usually used with Epoxy resin, which also adheres well to metals. The combination of Carbon fibre and metal can result in damaging galvanic corrosion (as they both conduct electricity). It is recommended that metal parts be protected with a thin layer of Fibreglass, or Aramid fibre, as electrical insulation.
The raw materials of Epoxy resin are long chains of molecules, that are cross-linked to each other, by the addition of a "hardener" to the mixture. When Epoxy is used with Carbon fibre, often a curing agent is used, which only becomes active at higher temperatures, this is easier to work with, as there is more working time before the resin is cured, such as when using "pre-preg" fabric (which is already impregnated with Epoxy resin - just as some wallpapers have glue already applied to them - and cured in an oven heated above 100 °C).
Pre-pregs have an optimum content of resin/ fibre of about 60 - 65 %, (1/3 resin and 2/3 fibre). Such a high fibre content is supplied already impregnated with resin, but the finished external texture is not smooth, or aesthetically pleasing ; so many smooth finished and painted constructions are only around 50 % percent fibre by volume, the use of extra Epoxy resin produces a nice exterior finish, (which is important for some applications, such as boat hulls, where surface friction must be reduced).
Using the simple "hand lay-up" construction method, usually the content of fibre does not exceed 40 % fibre content (i.e. 60 % resin content). The elongation-to-breaking of the matrix must be greater than, or equal to, that of the fibre ; which here is almost always the case.Certain UHM Carbon (ultra-high modulus) fibres, decrease the elongation-to-breaking to only about, 1 % - 1.5 % ; this is referred to as "friable", because the the material is so rigid and inelastic.
The inter-play between the matrix and the fibres is critical. The matrix has to soak the fibres completely, and to adhere well to the fibres, (even for many years of use). The matrix must distribute the load onto the fibres well (both in tension and in compression). In addition, the resin has to protect the fibres against chemical and mechanical damage, (a bicycle frame gets covered in all kinds of chemicals from road spray, including car oils, brake fluid, de-icer, petrol, car cleaners, detergents, farm fertilisers, weed killers, road-salt, etc).
The resin re-distributes all the forces it is subjected to, to the surrounding fibres, which then further re-distribute those forces elsewhere in the structure. It is the ability of a structure to re-distribute high loads, that helps determine its success or its failure. Appropriate material selection is critically important, especially for highly loaded safety-critical components.
Polyethylene fibres get a corona treatment (high voltage electric arc), in order to promote adhesion between the fibres, and the matrix material, by roughening the surface of the fibres to increase their contact surface area.
A major problem for all composite mate-rials, are the differences in the coefficients of expansion of the matrix, and the fibre. If the fibres become loosened within the matrix, they become unable to bear any more loading (similar to a wire spoke coming loose, in a bicycle wheel). This creates a weak spot in the composite, which may not be visible to see from the outside, but which may result in a sudden break in the structure (no re-distribution of the loading is possible, to the rest of the structure !).
The diameter of the individual wires (filaments) is only about 0.007 mm. (Only about 1/100 of a millimetre, and about 10 times thinner than a single human hair strand.)Bundles of filaments are called "rovings", these rovings can then be further processed into fabric, by weaving them together (into roving-mats).
Fine fabric materials contain approximately 1000 (1 K) filaments per strand ; in the construction of bicycle frames, this 1 K fabric can be used, up to coarser fabrics of 3 K. Some boat building applications even use fabrics as coarse as 24 K, and more ........
Fabric with nearly all the filaments in the same direction is called "Uni-Directional" (UD) cloth ; this is used in order to improve the stiffness in a particular directional plane, where the loading is highest. In a bicycle frame where there are a lot of different loads, in many different directions, it is therefore appropriate to use "square" fabric, where the warp and the weft are equal (such fabric has a checker-board appearance, and is sometimes used on the outside of frames, for its visual appeal).
FIG.1a From 1989 a monocoque carbon frame the Cinetica Giotto.
FIG.1b Another carbon monocoque Lotus (UCI banned).
The bikes of FIG.1a/b show how different the bicycle frames can be, if you use the freedom the material offers to the designers. In FIG.1 the outer layer of the tube (A) is made from a square-weave fabric, this has a stiffness which is at most only a quarter (25 %) that of UD fabric (C) in the longitudinal direction (along the frame tube), but in every direction it is about the same strength. It is also possible to incorporate a few layers of UD fabric at different angles (B), to create a more robust overall structure, that can resist loads in many different directions. To optimise their structures, many modern Carbon fibre frame makers don't use only a few thick layers of Carbon fabric, but instead use many thin layers of CF fabric (this also has the effect of more accurately determining the resin composition, and distribution of resin within the complete structure).
The Epoxy matrix material should not be subjected to shear loading. This means that, in a tube-lug frame design, the lugs should be at an angle of 45 ° to the fibres of the tubes. Transverse to the fibres (at 90 °), the matrix bears the load, not the fibres, so the matrix has to try to transmit these forces to other fibres, rather than carry them itself. In many modern frames, the outer-most layer is only really a coating, and not really necessary for strength. This layer makes the tube less sensitive to scratches, (and it can be used for graphic designs and patterns).
A simple construction method is the "hand lay-up" method. Using a simple (and low cost) mould, made of Wood, or Plaster. The inside surface of the mould is covered with a release agent, typically Poly-Vinyl-Alcohol (PVA), because it is soluble in water.
To this release-layer, is then applied a thin layer of liquid resin, onto which the roving-mats or fabric is laid. The fabric mat is then firmly pressed down onto the resin (such as by using a small hand-held roller), in order to properly penetrate the fabric cloth with resin, and to remove any air bubbles trapped inside the resin, before the resin hardens ("zero voids").
Now a new layer of liquid resin is applied, onto the fabric previously laid down, and then another new layer of fabric, onto this resin. So layer-by-layer, the structure in created inside the mould.
By sucking all of the air out of the fabric, and the resin, using a vacuum pump, (in conjunction with the use of a large sealed plastic bag, around the entire mould), one can avoid having bubbles in the resin, and prevent excess resin use (extra weight). With this technique, absorbent cloths can be placed between the fabric and the plastic bag, which can help to draw out any excess resin, away from the fabric.
This "hand lay-up" method is widely used to make streamliner HPVs, it is also used for other small volume, high performance structures, such as racing cars or glider aircraft. Usually there are two templates : a 'left' and a 'right hand' ; or a 'lower' and an 'upper half'. This splitting of the mould into two halves, makes gaining access to the inside of the moulds much easier, for laying the fabric material into the moulds, and then later releasing the cured structure from the moulds.
The curing of the Epoxy resin can be at room temperature (20 °C), but often it is done in a large special temperature-controlled oven (called an "Autoclave") ; where the curing reaction proceeds faster, and the strength of the set resin is improved.
It is also possible to build a structure in the opposite direction ; "from-the-inside-outwards", rather than "from-the-outside-inwards". This involves using an inner core (or " buck"), around which the composite structure is built up. The core can be made from many different materials, some of the most common ones, are from relatively soft and easy to work, Wood or Foam plastic materials.
If the "Wood-core" method is used, (or PUR Foam, or Balsa Wood, are used), then the core can be left in place after the composite structure has been built. But this adds extra weight (and some stiffness too). Because of the extra weight, sometimes low density Foam plastic materials are chosen for the core, and designed to form an integral part of the finished structure, (for example, as in the expensive deep section aero wheels made by 'Lightweight').
Some cores are made from other materials (such as Wax) that can later be removed after the outer Carbon structure had been formed, and has cured hard. For example the French 'Time' bicycle company, uses this method for some of its expensive Carbon fibre frames.
Optimisation of a bicycle frame is only possible if thin roving-mats are used ! The price of thin mats is almost as high as that of thicker mats. It is more expensive, and much more work, to stick all of these different thin layers together onto each other.
Most modern high-quality road bike frames are made in a mould, and as a whole (monocoque) structure, that is cured in an oven, in one single heating operation.
To get the optimum weight / stiffness balance, a compressed gas balloon (air-bag) is fitted on the inside of the frame, it is inflated once all the fabric, and the resin, has been placed into the moulds, it is used to press out all the excess epoxy resin, before the resin has cured hard. The inflatable bag (or "Bladder") can be made from a variety of different materials, such as thick Rubber (later removed), or from thin Nylon (which is sometimes left in place, after curing). One small company even uses rubber bicycle tyre inner tubes, for the inflatable bladders.
This process facilitates the use of the maximum achievable Carbon fibre percentage of 60 % (i.e. to reduce the resin content down to 40 %). For reasons of safety, it is necessary to ensure that enough resin has been used, and that no resin is missing from the frame structure. So for example, when 'Trek' joined different sections of Carbon fibre frame together, it used more resin than was needed, even though this added a small amount of extra weight, and some extra cost of resin too.
Cheaper Carbon frames are often made by selecting thicker Carbon layers (which require less work to lay-up). Considering the fact that the ultimate strength at the sensitive points is the same (strong enough), the weight is increased, because of the extra unneeded Carbon fibre, at the low-loaded frame parts. The difference between expensive and cheaper Carbon frames is therefore, not only the quality difference in the Carbon fibres, and the Epoxy resin materials, but also the quality of the production processes too ! For example, while in cheap frames the inner air-bag, (or the PUR-Foam core), will remain in place ; in expensive frames these manufacturing parts will be removed, so making the frame lighter, (and this also applies to Carbon fibre wheels too, which can be sold with a thin plastic liner still inside them).
There are bicycle manufacturers that do all the Carbon fibre work in-house, (rather than just outsource it), from processing the Carbon fibre mats, to completing the entire frame ; including all the cutting and gluing, filling moulds, and curing frames (for example some 'Giant' frames are made this way).
But also many frame builders don't build everything themselves, and buy many of the pieces ready built for them by other companies. For example, the famous Italian tubing manufacturer Columbus makes frame kits, for others to assemble into finished frames. The front-forks and rear-stays are supplied ready built from the factory, and the main frame tubes too. The whole frame need only be glued together, this means that the variations possible are only very limited, (such as the lengths of the tubes, to suit the rider). For example, some of the smaller Italian bicycle companies use these Carbon tube kits, and assemble their Carbon frames in a similar way to a traditional Steel frame, using the same sort of jig as for a Steel frame, but using Epoxy glue instead of brazing rod.
Of course, if a separate mould is used for every different model of frame, and every different size of every model, then this is prohibitively expensive for smaller companies, especially if new moulds need to be made regularly, whenever there is a change of model (such as every year). Which is another reason why many smaller companies use tube kits, rather than moulds. In the Far East some manufacturers also have so called "Open Moulds", in which any customer can have their Carbon fibre parts built in, (such as frames or rims). This greatly reduces the costs for customers, because they don't have to finance the cost of having many different moulds, but it also means that the products aren't unique, and can't be differentiated by design, or by lower price, from other identical products. Only the brand name on the parts is different, inside all the parts are the same !
Once a mould is made, it is very difficult to alter any dimensions, to suit different riders, but it far easier to alter details when building using a tubing kit. Even the top professional riders sponsored by the major bike brands, have to use the frames that come out of the mould, just like any ordinary retail bike buyer.
Occasionally a big star rider will have a frame specially modified for them, (such as by adding extra strengthening material) or altered for a special race (like Paris - Roubaix), but the top brands genuinely want to produce bikes and frames that can be raced at the highest level, from stock.
The down side of building each Carbon frame individually, using a tubing kit, is the cost of needing skilled labour to build the frame by hand, which is why mould made frames are usually cheaper, and often more profitable for the big brands that sell them.
In Taiwan the firm HYC produces a PolyPropylene pellet with carbonfiber in it; this can be molten in an extruder and formed into tubing, or used for injection moulding. If strenght and weight are comparable, a much promising new production technique. This frame will be recyclable, and cheap in production.
Information from books and the Internet about working with composites :
At the American recumbent site WISIL , there is a lot about DIY in composites.
Road bikes in Carbon : www.youtube.com/watch?v=mSpx-nPhE-Q&feature=related
An extensive story about carbon can also be found on : www.ibiscycles.com/support/technical_articles/
Naturally there are many composites builders on the Dutch site of the NV-HPV : www.ligfiets.net
A German mail order company for fibre and epoxies, and a computer program for weight : www.rg.de/de/laminatrechner.html
An English supplier: https://www.easycomposites.co.uk/
and special materials : www.specmaterials.com/index.htm
Also, a composite (used for bicycles for decades !) : www.instructables.com/id/How-to-Build-a-Bamboo-Bicycle/
The theory of composites
The E-modulus of Epoxy lies between 2 (adhesive) - 20 Gpa (sheet material) ; in composites it is about 4 GPa. FIG. 2a shows the different characteristics in stiffness of a composite material, as the ratio of fibre to matrix is varied. Using the example of a composite material with ; 60% fibre (E-modulus Carbon HM = 400 Gpa) and 40% resin matrix (marked on the graph with a line).
In modern engineering, some computer software programs, called "finite element analysis" (FEA)" can be used, where a computer simulates the loadings on a structure. The computer programs, see FIG.2b, can be of different levels of sophistication and power, from basic simple 2D models, to 3D static loads, to 4D dynamic loads and impacts, (sometimes even using very expensive super-computers). It is possible to simulate a complex car crash, using computer software alone.
A freeware version for designing in composites:
Fenite Elements Analyses software: https://academy.3ds.com/en/software/abaqus-student-edition
In length of the fibre the stiffness is: E composite= E fiber * Vol% fiber + E matrix * Vol% matrix , bij voorbeeld: E composite= 400 * 60/100 + 4 * 40/100 = 242Gpa
Transverse to the fibre the stiffness is: E composite= E fiber * E matrix / (E fiber * Vol% matrix + E matrix * Vol% fiber) = 400 * 4 / (400 * 40/100 + 4 * 60/100) = 10Gpa
This calculation shows an important pitfall, for novice designers of composite structures; transverse stiffness is only about 4.1 % that of in line stiffness.
There's another one : the compression loading. This is especially true for a UD fabric.
Consider for a moment ; you take a bunch of strings with a generous dollop of glue on all of them. Now, if we pull (<->) on this composite, we encounter a lot of resistance ; if we push (>-<) then this composite soon collapses. The strings cannot resist compression ; the adhesive glue (matrix) must bear all the compression, and the structure is loaded in shear of the matrix.
We now come to a material property that is called "Shear Modulus" (abbreviated as 'G'). "G" is measured in Gpa, just like the E-modulus. One important variable is the "Poisson-factor", or "Poisson Ratio", shown below with the Greek letter ν (nu) ; for Epoxy this is approximately 0.3.
This is the ratio of the change of the expansion, relative to the change of compression. For example the elongation of a Cork, as it is pressed into the neck of a wine bottle, or for example the necking and narrowing of a Rubber band, as it is stretched out tight.
The compression (σ composite) which leads to collapse of the composite, is even lower : σ composite = G matrix * (1- Vol % fibers )
The relationship between the Shear Modulus (G), and the Poisson ratio ν , is expressed with the following formula : G = E : (2 . (1 + ν )) expressed in Gpa.
Information IN BOOKS OR ON THE INTERNET about the theory of composites :
The pdf download above comes from: https://compositesnl.nl/wp-content/uploads/2019/10/Composites-an-introduction-1st-edition-EN.pdf
A computerprogram as help for designing laminate constructions: https://tu-dresden.de/ing/maschinenwesen/ilr/lft/elamx2/elamx?set_language=en
An English book (Msc level) : "An Introduction to Composite Materials" D.Hull, and TW Clyne, Cambridge University Press (1996); a summary of this book is found at : www.matter.org.uk/matscicdrom/manual/co.html
A manual in German, and in English : www.ezentrumbilder3.de/rg/pdf/Handbuch_deutsch.pdf
Fibres are only very stiff in the longitudinal direction, transverse to the fibre (at 90°) the stiffness is much less. The fibres can be used in a variety of different directions (see the frame tube drawn in FIG. 1 ), in order to prevent the frame becoming too weak in certain directions (especially around highly loaded areas, like the bottom bracket, or the headset). In this case we get an average value of 60 Gpa, (much less than 244 Gpa), but still more than two times as stiff as the steel frame, with the same diameter of tubes, and the same mass.
In other words Carbon fibre composite materials can produce an excellent bicycle frame, if the customer can afford it, and the use isn't too rough or extreme. Scratching, hitting and crashing can easily damage a frame beyond repair: light carbon frames are fragile!
The bicycle manufacturers and their engineers, have to decide what parameters the design requires, (such as the maximum rider weight, the largest frame size, the optimal wind speed, the grades of Carbon fibre that can be afforded, the maximum weight that the frame can be, what parts the frame must work with - such as handlebars, disc brakes, or deep-section aero wheels, and of course restrictions caused by racing regulations, etc).
For bicycle frames, one use is to study the vibrations and shocks going through a frame, as it is ridden over rough ground or cobbles, and trying various different designs, to try to find the one that gives the smoothest ride, with the least loss of power, while also being lightweight, strong, tough, cheap, and easy to manufacture. The most advanced Carbon road racing frames have been designed in this way, such as Jaguar Cars helping to design the 'K-8' Pinarello frame, that Team Sky riders used in the bumpy Paris-Roubaix race of 2015.
Once all the FEA work is finished, the wind tunnel testing has been done, and the design choices made, the frame structure design can be optimised. The most heavily loaded points are strengthened, and any possible excess material is removed and reduced, on places with lower loadings. Sometimes external Carbon fibre specialists are consulted by bike makers, such as the Formula One racing car company McLaren, advising Specialized on the best lay-up patterns for Carbon fibre material, for their 'Venge' aero road frame.
Of course even after all this hard work, riders may not be happy with the finished product, for example while a frame may be very stiff, and good to sprint with, some riders may find the ride is too harsh and unforgiving, and isn't suited to long road races, that can last more than six hours.
In the competitive world market for the latest and best racing bikes, it is the companies that can develop new products the fastest, that have the best chance of being successful, and keeping up with changing trends and fashions. Of course every bike maker tests the products of rival firms, and tries to copy anything that they can, to improve their own products, and to save on R&D time and costs. Also the world of cycling is influenced by fashions and fads, just like other industries, and people will buy something because of the way it looks, or because of its prestige, rather than because of a genuine need by leisure riders, to be able to compete in a stage of the Tour De France, or to use special racing parts (like aero Carbon wheels, or electronic gears).
2.18 Colnago (and Trek) used to make their first carbon frames with a glued lug & tube construction, baked in an oven.
5.36 The modern way to built a bike is shown by Giant. Weaving, cutting, glueing and baking
1.34 Though Dyneema would deminish the vulnerability of carbon frames, it is little used. One other application is the use as protection for racing accidents.
20.33 Engineering a carbon composite frame.