Road bike carbon frame construction: What you need to know

Road bike carbon frame construction: What you need to know

When we look at custom carbon frame builders we more often consider the geometry and how this affects the appearance and performance of a bike. Yet there are other processes involved in bike building, alongside materials, which can affect a riders handling and comfort. Here we look at carbon, and how the material is used in frame building.


Carbon fibre is saluted in the cycling industry because it provides components and frames the advantage of being high in strength, lightweight, no corrosion, the ability to adapt shapes and, when built correctly, provides excellent fatigue resistance. With most of the emphasis in ride performance marketed towards the measurements or geometry of a frame, little is mentioned about the choosing and handling of materials pre-construction, and how this also impacts on frame characteristics. Let's change that a little by diving in to carbon and understanding how both custom frame builders and manufacturers use it.

Cipollini Frames proceed after testing period. Our visits to carbon manufacturers and frame builders shares insights into methods of construction and resulting frame characteristics Image: Alison McGregor

There are two notable methods to skin a cat or, in this case, build a carbon frame. The first and more common method is bladder and foam core moulding. This method creates tubes using a hand layering application of carbon sheets, pre-cut and strategically placed inside the mould. This application requires intensive heating as well as pressure applied to the tubes (by way of bladders or foam cores).

The second rarer method used by select brands involves filament strands of carbon, or roll wrapping. In the former, filaments originate from dozens of spools that move in order to weave into the shape of the tube. In the latter, the tube is wrapped around a mandrel. All methods require pressure and heat. We won't dwell on this method because although very interesting, it isn't common practice.

Monocoque frames also use the process of layering carbon in moulds, but instead of the common tube-to-tube construction using multiple moulds to assemble the frame, the monocoque mould is singular, meaning the frame comes from the oven preformed. Of course, each size in a monocoque frame, such as Cipollini Ad.One, will require a mould.

Tube to tube construction will require bonding of the tubes or in few cases lugs. Tube wrapping is a method used by custom carbon frame builders, Titici (in the making of my Vento frame), as you will see in the images. The application requires knowledge about the use of the frame, and in some cases can be adapted to accomodate a specific rider by tailoring to weight, strength and stiffness requirements.

Speaking with Roberto at Titici, the layering can determine both feel and handling of a bike. "During the assembling of the tubes, we can manage the wrapping layers according to customer's request. We can realize the BB more rigid or less wrapping layer to reduce weight. Obviously, all these requests lead to having frames with different characteristics in terms of weight, stiffness, handling based on the amount of wrapping layers used during the construction process." Hence the tube to tube construction can allow for personalisations by custom frame builders not only in sizes and shapes, but in stiffness and weight. There are also other deciding factors made by manufacturers that we will see later, alter a bike's strength.

Carbon layup of carbon tubes. Image: Titici


Carbon sheets: Pre preg carbon

When employing the widely used moulding method, carbon sheets are often ordered pre-preg. This means carbon sheets are already pregnated with chosen resin. This is also called carbon fibre reinforced polymer (CFRP). More on resin in a moment...

The sheets may come as woven or unidirectional. Depending on the required need, the sheets are cut to a tube shape much like the design of a jersey on fabric. The shapes are then precisely layered by hand within the mould.

Carbon sheets cut to tube shape before layering in a mould. Image: Titici

What does K mean: Filaments and Tow

There are many "types" of woven carbon and in frame building are commonly identified by a number followed by the letter k. The difference between 3k and 1k for instance is the counted strands that make up the weave.

Carbon 3k finish: Titici. Read more on our Titici Review Image: Alison Mcgregor

To make sheets, carbon begins as a bundle of fine filaments - each is just visible to the human eye. Imagine holding a bundle of hair but far finer. The bundles of fibres are interwoven and called tows. Tow, as a definition, is instructed by its number of filaments. Keep in mind that the abbreviation for 1000 is k. You may have 3000 filaments in a tow (3k) that is later woven to create a wide pattern, compared to a 1000 tow (1K) which will exhibit a far finer weave. Have a look at the handcrafted cages by Alpitude Components to see the different finish for both UD, 3K and 6K.


More than appearance: Strength and Modality

Carbon sheets look visibly different by way of varied patterns or thickness in weave, or they may be unidirectional. These are just some factors that affect the strength or flexibilities as needed in defined areas of the frame. The choice and placement of these carbon strips can affect how a frame looks, but it can also alter how a frame performs or feels beneath the rider by adding directional stiffness or allowance for flexibility.

Other factors to consider are FAW - Fibre area weight, as well as the mix of resins, and the ratio of fibre carbon to resin, called resin content (RC). All three factors can again affect a frames character for example flexibility, brittleness and strength. There are quite a few reports of frames employing low ratio carbon to resin that are cheaper price point, but result in breakage.

In the bunch, there isn't much discussion when it comes to resins, RC or FAW. Instead, the amount of filaments in tow is widely agreed upon as the differentiator between carbons - hence the misunderstanding of how carbon is capable of reacting to its own construction, let alone its use in a frame construction. Breakage or weaker points in a frame are often heard as solely blamed on process of manufacture (voids, ripples, or fatigue points where tubes are bound or connected), rather than the decisions surrounding materials that make up the carbon, which come before frame construction. An example of this is dry resin, where the pre preg sheets may be lacking in resin content which is necessary to hold the carbons together but also to share the load.

Unfortunate decisions to use less than quality products by imposter or cheap advertised brands seems to be based on cost savings and rider's attraction for low weight.  Buying bikes for substantially competitive prices through online only purchase can be a warning for buyer's wary of substandard materials and potential future risk of accidents on the road.

Interesting to note, high-end performance bikes from reputable brands or builders do not cheapen products that weigh less. In fact there is plenty of evidence to show that manufacturers with high quality standards increase the price of lighter frames because they also invest in understanding how to paradoxically balance this against strength, stiffness or comfort. Bikes are proof in the old adage that the less you get the more you pay.

"Buying bikes for substantially competitive prices through online only purchase can be a warning for buyer's wary of substandard materials and potential future risk of accidents on the road."

An example of this adage is found amongst top tier brands. The Filante SLR is one of the highest models produced by Wilier Triestina and uses HUS-MOD carbon fibre and Liquid Crystal Polymer (LCP) to provide an excellent stiffness to weight ratio. Another is TREK lightest weight frame the Emonda SLR which boasts a new 800 Series OCLV Carbon; their secret "sauce" for laying up various pre preg carbons. Lastly the Specialised Atheos, which succeeds in eliminating ‘lazy fibres’ and delivers "The lightest disc road frame ever produced with builds as light as 5.9kg, and all with the telepathic handling, response, and stiffness targets you’ve come to expect".

The Wilier Filante has had success they contribute partially to the RC and specific carbon and resin types combined.

Tube shape

When it comes to the infinite possible carbon tube shapes, there is never ending changes that can be made by bicycle producers. In the development of carbon materials, construction technique and tube shape, considerable headway has occurred since the beginning of carbon bicycle manufacture. Most changes we promote from our carbon builders positively affect what our rider's value; a balance of achieving performance goals while retaining a sustainable ride position.

To achieve a rider's requirements, an expert builder will choose their materials, construction methods, tube shape and of course geometry. For our clients who are more often looking to accomplish endurance challenges, the recipe for the perfect frame is a balance between both reactivity and diffusion. 

Take for instance the Titici PAT top tube, which never fails to bring comments regarding the fineness of the top tube. The comments perfectly illustrate the misconception regarding carbons strength. It highlights the general inability to recognise why carbon is more likely to fail at junctions or through cheap construction than qualified and longstanding custom frame builders such as Titici.

However Titici engineered the tube shape and tested the strength and intended dampening effects. From the University of Parma, not only did they prove that the PAT technology successfully absorb 18% more vibrations than a standard frame, it is also strong. As Roberto highlights in our conversation, "In the test we had seen that the lateral stiffness of the titici frame with pat is equal to a frame with traditional tubes too."

With an understanding for carbon available, the methods of construction, the material structure, expert custom carbon frame builders have the capability to cater to a clients specific requirements in terms of necessary torsional and vertical stiffness in a frame.

Of course, carbon is widely used in the cycling industry for products other than frames. Components as small as stem spacers, all the way to carbon wheels or saddles are available in carbon fibre. Take for example Andrea Sega who constructs saddles and rails using a variety of wrapping and moulding methods.

Overseas Outsourcing

Applying heat and pressure is a science that can not be left to approximations. Like the layering technique, each stage of a frame build should result in thorough testing and, from there, follow the exact method. Testing can be costly, and with individual frames is even more expensive. Without testing assurances there is no way to understand if a frame may present voids or withstand rider use. It does go to say that buying goods from unknown manufacturers is not widely recommended as the quality controls are unknown.

While large household names have warranties and stringent quality controls, there are also many comparatively smaller and unknown brands without the budget to manage such controls or procedures of accountability. There exist brands and "frame builders" who do not manage construction in-house but outsource their production overseas. The issue with unknown outsourcing or even counter-fits, is that a client entrusts a brand with a huge number of variable that cannot be assured.

Relying on attractive marketing or pretty graphics isn't a guarantee that a frame will come together to form a bike deemed both safe and reliable. So its best to understand if your frame or components are being constructed by an experienced specialist or trusted manufacturer, before committing to purchase carbon products. When you can buy quality trusted carbon frames that can be painted in any graphic or colour and built with any spec it begs the questions, would you risk your life to save yourself money?


Frame construction plays a large role in a bikes overall appearance, reliability, feel and handling. Other factors that play part are obviously geometry and set-up. But an underrated influence is determined by layup processes, carbon resin ratio, as well as the type of carbon employed. Resin dry is one such potential issue. As a general rule, we always recommend buying from brands that are transparent in their processes, or expert custom bike builders who are transparent with who builds their frames and the methods employed.


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