Black magic: How carbon fibre bicycle frames are made

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Pick up any marketing material from any number of bike brands offering a carbon fibre frame and you’re sure to be inundated with vague jargon about the materials and construction methods used. Take a deeper look and you’ll find that so many brands are actually talking about similar things, and yet, the end result is often so varied.

Just as a chef at a Michelin Star restaurant will tell you, the raw ingredients are only a single aspect of the end product. Give those identical ingredients to another chef and the outcome will certainly be a different one. It may not be worse, but the flavours, textures, and presentation will all vary noticeably. Using carbon fibre to make a frame is no different, and in this analogy, detailed engineering, correct material selection, layup design, and manufacturing consistency all combine to separate the impersonators from the experts, and even the experts from each other.

So how exactly is carbon fibre used to make a frame? What are the different construction methods? Why is the term itself so misleading in the world of cycling? And if the raw materials are the same, why does one frame perform better than another? To explore all of this and more, we got help from the US-based carbon manufacturing wizards at Allied Cycle Works (a brand of HIA Velo) and Australian-based carbon repair specialist Raoul Luescher of Luescher Teknik, to get their insights on the black magic of frame manufacturing.

What is carbon fibre?

Before getting in-depth about how a frame comes to be, we should start with an explanation of the raw material. Carbon fibre starts its journey as a polymer, which is processed through various heating steps into long strings of carbon atoms. These long strings, or filaments, sit at about 5-10 microns in diameter each, 10-20 times smaller than the average human hair.

These individual filaments are then bunched together to form a thin ribbon, or tow. And much like how a thread becomes a string, which then becomes a rope, carbon filaments work together to form something extremely lightweight and strong.

The number of filaments used per tow is a common metric in the cycling world, and is typically measured in the thousands. For example, a carbon tow with 3,000 filaments is typically given the designation of 3K; 6,000 filaments is 6K, and so on.

Actual strength and stiffness of the individual fibres can vary, too, with the stiffness being described as modulus. Higher modulus is achieved by increasingly refining the filament production process, stripping each filament further and further down, and progressively making it smoother and thinner. While more resource-intensive, these thinner filaments also sit more tightly together in a tow, and increase the stiffness of the tow overall. However, higher modulus is associated with increased brittleness, since each filament is thinner.

Modulus is a term often thrown around in marketing materials, and the key thing to know is that there is no standardization in how modulus is described, at least within the bicycle industry: one brand’s claimed “ultra high modulus” material may actually be more flexible than another brand’s “low modulus” carbon. More importantly, it’s how these varying stiffnesses of carbon are applied that matter most, and the best frames will always use a mix of moduli.

From carbon to composite

Tows of carbon fibre are hardly useful by themselves, as at this stage they’re merely dry, pliable bits of material. It’s here that one of the more misleading elements is revealed. All carbon fibre material used in cycling must be bonded in some regard, usually with a two-part epoxy resin. Adding resin to carbon fibre turns the material into a composite, or to use the more specific engineering term, carbon fibre reinforced polymer (CFRP). As the material is also usually layered, the composite is often referred to as a laminate, too.

Where carbon fibre is extremely strong and light, resin is comparatively heavy and weak. The goal with such a composite is to use as little resin as possible in order to hold the carbon fibre in place. It’s here that higher modulus carbon really shines, since the smaller gaps between filaments require less resin to fill.

Some manufacturers will vary the performance characteristics of the finished structure through the use of other fibre types and modified resins, such as composite epoxies infused with glass or carbon nanotubes (microscopic filaments). Allied Cycle Works uses a reinforcing material known as Innegra in its frames, while others have been known to include materials such as aramid to increase impact resistance of the laminate.

Most frame manufacturers build frames with sheets of carbon fibre that is pre-impregnated with uncured resin – better known as pre-preg – applied to a non-stick paper backing, and shipped on large rolls. The resin activates with heat, and so these pre-preg sheets are stored in a freezer until needed. This process helps to ensure even resin coverage throughout the frame, greater finite control over the lay-up, and reduced labour time.

In most cases, the fibres in those rolls are all unidirectional, with all of the fibres running in one parallel direction. This orientation provides maximum strength and stiffness in one direction, but at the expense of minimum strength and stiffness in the orthogonal direction. Alternatively, the tows can be woven together at various angles, often in a criss-crossed pattern, so that the material can be equally strong in multiple directions.

“Unidirectional (UD) pre-preg is common because it has higher specific properties and is easier to lay-up a specific fibre angle,” says Luescher. “[Woven fabric] is easier to lay-up at complex geometry locations and where the loads are less defined. It also provides better damage tolerance as it is less likely to delaminate due to the mechanical interlocking of the fibres. Woven fabrics are often used at locations throughout the frame such as inserts, bottom bracket shells, head tubes, and wherever holes are drilled for bottle mounts, cable guides, etc.”

While prepreg is by far the most common material in the cycling industry, other construction methods start with dry fibres.

Filament winding, for example, wraps sheets or ribbons of dry carbon fibre around a solid mandrel that is nominally cylindrical in shape. Resin is applied during the wrapping process, and then the entire assembly is cured under heat and pressure.

In yet another method, Time weaves its own carbon tubes in-house from dry carbon tow – sort of like how socks are made. That dry tube is then secured in a mould, and resin is injected under high pressure using a process Time calls Resin Transfer Moulding.

Regardless of the method used to form a structure’s final shape, it’s up to the engineer to ensure the right types of carbon fibre (and resins) are used in the right places and in the right orientations for the best end result. Frame designers need to weigh a wide range of parameters, such as stiffness vs. brittleness, and weight vs. durability. Impact resistance, and of course cost, must factor into the equation, too. In general, though, the design possibilities of a carbon frame are wide open, and when done right, the life expectancy of a carbon frame can be nearly infinite.

The design process in a nutshell

Designing a frame is no quick feat and so it’s impossible to do the subject justice here. Regardless of frame make or model, the process is an extensive one and varies greatly between the different brands.

Most carbon fibre frames arguably have a similar genesis – the brand defines the purpose of the frame and that there’s demand for it. After all, if you’re going to invest extensive resources, you’d better be sure you can commercialise it.

The next step would see brands define what the new frame needs to achieve. Given the maturity of carbon fibre bicycle frames at this point, it’s usually continual improvement that drives change, and rarely is genuine innovation achieved. This is why every few years you see a brand update an existing model with iterative and incremental improvements, rather than wholesale redesigns of products that are already quite refined. This is as much the result of learning from past mistakes or previous design limitations, as it is a sign of the continual development in the use of carbon fibre.

Luescher explains that the advancement in carbon fibre frames is mostly down to more consistent process control.

“Although there have been advancements in fibre grades, which is often the focus of the marketing departments, reliable compaction and moulding outweigh the theoretical gains from a raw material change alone,” he said. “The increased uniformity of the compaction has led to reduced flaws, more consistent laminate properties, and hence increased structural performance. By being able to produce more consistent laminates, structural models are better able to optimise the frame layup to produce lighter, stronger, and more fatigue resistant frames that do not require as large a factor of safety as previously required.”

According to Sam Pickman, director of product and engineering at Allied Cycle Works, digital development plays an enormous role after the initial concept is finalised.

“Here we dive into design in a major way including the 3D FEA analysis, CFD [computational fluid dynamics, used for aerodynamic design and testing] if necessary, and most importantly how we are going to make it. We decide if and where the frame will be split up [in its construction], what materials we want to use, how we will pre-form it, what we want the tooling to look like, et cetera.”

Rideable prototypes are expensive, and typically come much later in the process. According to Pickman, Allied first uses a 3D-printed sample of the bike to test component fitment, general aesthetics, and a manufacturing plan.

“Once we clear this, tooling design and manufacturing starts and the ply manuals are created. Once the tools are completed, we begin part development. This is when we are physically making and breaking parts. After all the digital development, we are pretty confident, but a few revisions are usually necessary to get the performance we need. Once we pass testing, we start riding the bikes and gathering feedback. At the same time, we begin training staff on the new processes. When we have cleared everything, we launch a pilot run to work out the kinks.”

Processes of manufacturing

There are a number of ways to turn those raw ingredients of carbon fibre and resin into a bike frame. While there are a few niche players with unconventional techniques, the vast majority of the industry have adopted the monocoque method.

Monocoque manufacturing

A term commonly used to describe modern carbon fibre bicycle frames, monocoque design effectively means the item handles its loads and forces through its single skin. In reality, true monocoque road bike frames are extremely rare, and the majority of what is seen in cycling only feature a monocoque front triangle, with the seatstays and chainstays produced separately and later bonded together. These, once built into a complete frame, are more correctly termed a semi-monocoque, or modular monocoque, structure. This the technique used by Allied Cycle Works, and is far and away the most common in the bicycle industry.

Regardless of whether the industry’s terminology is correct, typically the first steps see large sheets of pre-preg carbon cut into individual pieces, each of which are placed in a specific orientation within a mould. In the case of Allied Cycle Works, the specific choice of carbon, the layup, and orientation all go together in a ply manual, otherwise known as layup schedule. This specifically outlines exactly what pieces of pre-preg carbon go where within the mould. Think of it as a jigsaw puzzle, where every piece is numbered.

Carbon fibre frames are often perceived as being cheap and easy to manufacture, but the reality is that this layering process is extremely time-consuming and expensive. According to Allied Cycle Works, the Alfa road frameset uses 326 pieces of individual pre-preg carbon pieces in the frame and 170 in the fork, all of which are carefully laid by hand, in a specific sequence and in multiple layers, following the engineer’s ply manual.

“The way the plies lay on to another aids in how they unfurl into the [mould] when the resin viscosity drops,” explained Pickman. “The easier they can slide and fill the tool, the better consolidation you get. Pre-form size is just ensuring that the plies don’t need to move a long way to get to their final shape. The more they need to move, the more issues you get, including consolidation issues.”

Made to be model- and size-specific, the mould dictates the outside surface and shape of the frame. These moulds are typically machined of either steel or aluminium, built for repeated use and without variance.

However, the outer surface is only a part of the story, and the carbon must also be compressed from the inside to ensure correct compaction and that no voids (weaknesses) are created. Here, various techniques are used. Inflatable bladders, which are sometimes just left in the frame, are perhaps the most common. Other examples include foam or wax mandrels that can be melted away; flexible silicon mandrels; and sometimes even more solid mandrels, whether they be plastic or metal.

Allied’s process is fairly common amongst premium and large-scale frame options. The frame is layered up around a network of inflatable bladders and semi-solid pre-forms on one side of a two-piece, clamshell-like mould, and the other side of the mould is secured on top once the lay-up is complete.

From here, the mould is completely sealed with a vacuum bag before being moved onto the de-bulk phase. “De-bulking is a process in between lay-up and cure where you apply vacuum and some heat to the part and draw out as much air as possible before curing,” Pickman explained.

In Allied’s case, the mould is then removed from the vacuum bag and placed into a heated press. Again, the frame inside is heated to allow the flow of resin, while the internal bladders are pressurized to give final reassurance that correct material compaction is achieved. This curing process increases the internal pressure incrementally with the goal of pushing the plies to the outermost parts of the mould. Both this and the de-bulking work together to help eliminate air voids, fibre creases, or other potential stress risers in the material – all while removing excess resin.

After curing, the frame is extracted from its mould, and the internal air bladders and pre-forms are removed. The dropouts, seatstays, and chainstays are then bonded with the front triangle. Those bonds are overwrapped with additional strips of carbon fibre to provide both extra structural support and seamless surface finish, and all of that assembly is performed in a jig to ensure perfect alignment.

Now looking like a frame, the next step is sanding and paint prep. An arduous process of fine detailing ensures no excess resin or marks from the mould are visible. In particular, manufacturers will pay very close attention to the bonding joints, which often require the most treatment from frame assembly.

At this same point, drillings for water bottle cages, the front derailleur mount, and cable management systems take place. With a mixture of rivnuts (threaded rivets), rivets, and epoxy typically used to permanently affix the items, these are carefully added to areas that have already been reinforced in preparation during the lay-up stage.

All in, the creation of a single Allied Alfa frame, which is wholly produced in-house in the USA, is said to take approximately 24 hours in labour.

“In actual time, it takes about 10 days for a bike to go through the building,” states Pickman.

When done right, monocoque design produces an incredibly strong and lightweight product, all with minimum overlap of materials. It’s for this reason, plus the way that carbon fibre mechanical properties can be so carefully controlled, that monocoque manufacturing is the top pick for building a frame with the highest stiffness to weight ratio. If you look at the bikes used in the WorldTour, for example, all but the Colnago C60 of UAE Team Emirates use a modular-monocoque manufacturing technique.

Monocoque manufacturing is not without a few disadvantages, though, mainly related to accessibility and cost.

Firstly, as detailed above, this method is extremely labour-intensive. Even a well-staffed and efficient factory such as Allied’s takes a relatively long time to produce a frame. This is one of the key reasons why the majority of the world’s carbon fibre bicycles are made in Asia – when labour comprises the majority of the manufacturing cost, it makes sense to minimise your labour costs as much as possible.

Secondly, specific moulds need to be created for each frame design, and within that, each frame size requires its own mould as well. Considering how some manufacturers offer 12 sizes, or even multiple geometries for each size, it’s easy to see the inherent expense in this process. According to Pickman, Allied’s moulding investment for a new frame and fork design across a full size range, including accompanying specific tooling, costs around US$160,000.

To overcome this, many manufacturers work on a two- or three-year lifecycle for a carbon frame design, in order to recoup costs over an extended period. It’s one of the key reasons why you don’t see the likes of Giant or Specialized coming out with a new frame model every year.

With such tooling costs, smaller brands and manufacturers have a tough time justifying the resources when there aren’t production quantities to back up the investment. In many cases, this is what leads to open-source or generic moulds being used by smaller or discount brands.

Tube to tube

Boutique manufacturers who specialise in custom geometries, fits, and lay-ups find it extremely difficult to produce monocoque designs at a marketable price, so they often turn to another method of frame manufacturing called tube-to-tube. In concept, it’s not all that different to how welded steel, titanium, and aluminium frames are made.

In this process, each carbon frame tube is produced separately, and sometimes sourced directly from a carbon tube manufacturer. This method gives a lower barrier to entry for builders to have control over a frame’s geometry, stiffness, and ride quality. Tube selection dictates the performance properties a frame builder seeks, and the customised tube length dictates the geometry.

With the tubes selected and cut to length, they’re mitered so as to fit seamlessly with each other. Then a jig is used as the tubes are joined together to create a frame. Builders often epoxy the tubes together, and then use pre-cut, pre-preg sheets to wrap the tubes together and reinforce the joints.

Some more advanced methods will see the frame then put into a vacuum bag or even a rigid or flexible mould to assist with compaction, while others will move straight to final preparation once the resin cures.

This method is popular for custom geometry frames as it allows a wide range of control on specific angles and tube lengths. However, it’s a process that requires a skilled approach to ensure long-term safety. Additionally, there will be more redundant material overlap in this method than what’s possible with the monocoque technique.

Lugged Carbon

Much like the tube-to-tube method, lugged carbon frames sees singular tubes joined piece by piece to create a frame. However, where tube-to-tube joints are individually wrapped, lugged carbon frames use more of a plug-and-play process where the mitered tubes are bonded into pre-formed lugs – again, just like their metallic analogues.

Often, the lugs of modern carbon frames are carbon, too, such as on the Colnago C60, but this is not always the case. Like tube-to-tube, lugged construction provides generous flexibility in terms of frame geometry, frame stiffness, and ride quality, with the possibilities only limited by what lugs are available.

One of the most high-tech recent examples is Bastion, out of Melbourne, Australia, who use 3D-printed titanium lugs for complete custom control with each order. The original BMC Teammachine, such as that ridden by Tyler Hamilton on Phonak, used aluminium lugs with carbon tubes, and much earlier than that, Trek pioneered the mass-production of the technology with its 2300 road frame.

Just like with tube-to-tube construction, though, lugged frames inherently feature more material overlap than monocoque ones, and therefore return a lower stiffness-to-weight ratio.

Quality control and Testing

What isn’t obvious are the steps some manufacturers take along the way to ensure that the finished frames actually meet the design intention – and, in other words, are safe to ride.

While some industry standards do exist in this area, such as CEN and ISO certifications, what Allied Cycle Works – and most other major brands – do could be considered the most common practices. In addition to frequent visual inspections, individual parts and sub-assemblies are individually weighed as a way to ensure the proper amount of resin has been infused into each component. Thanks in part to Allied’s smaller production volumes, raw materials are tracked as well.

German frame supplier Canyon even goes so far as to inspect forks and frames with an X-ray machine, which provides a more detailed, non-destructive way to examine finished composite parts.

A finished frame

All said and done, creating a carbon frame is a time consuming process, and one that remains surprisingly hands-on. For a material with so much versatility in its usage, there’s no doubt the devil is in the detail – especially when it comes to creating something that’s equally light, strong, compliant, and safe.

From afar, not much has changed in the making of carbon bikes over the years. However, look deeper, and you’ll see the finer understanding of the material application and improved quality control has led to a product that’s superior to what was available in years past. No matter what aesthetic shape a frame takes, it’s safe to say that carbon fibre’s true performance lies well below the surface.