If you’re anything like us, you’re taking the opportunity (with today’s rest day in the Tour de France) to catch up on pretty pictures of pro bikes and the emerging technology that sponsors roll out to their teams for the Tour de France. To complement this, we’ll be publishing a rest day review of some carbon fiber componentry later today, so we figured we’d take a quick opportunity to give some background information on the construction methods commonly employed in construction.
In the last 20 years, carbon frames in the pro peloton has gone from an option for well funded teams in the mountains, to the de facto year-round choice. While the occasional aluminum frame may show up under certain riders during the spring classics, most pro teams outfit their riders exclusively with carbon.
That said, while carbon frames have entirely supplanted steel and aluminum in the pro ranks, teams have been slower to adopt composite componentry, particularly in the cockpit, opting to use aluminum bars and stem. Reasons cited include carbon components being too flexible, too stiff, subject to extreme failure modes, and just not made to last – a myriad of criticisms that apply perhaps to some, but not all, carbon components. The last few years, however, have seen a number of teams switch over to carbon components without the predicted failures.
A number of the upsides to carbon in frames also applies to componentry. Carbon bars and stems can be crafted in any shape imagined, allowing them to be more aerodynamic than a comparable aluminum component, while maintaining their feather weight. The carbon layup can be optimized to provide stiffness where appropriate, or a little bit of give to improve comfort. With the diminishing costs of carbon components, some of the concerns about replacing components after a crash fade away – a crash that would destroy or put a carbon bar in to question would likely do the same to an aluminum one. Much like carbon frames earned an initially poor reputation based on the earliest attempts at building them, carbon components have suffered due to a lack of carbon-specific engineering put forth in the earliest days. If you design a carbon frame, stem, bar, wheel or seat post as if they’re made from a traditional material, they’ll perform poorly. Take advantage of the specific characteristics of carbon, however, and you can build items that are light, strong, and not so bank-breaking so as to require a 2nd mortgage on your house.
Carbon components are usually constructed via one of two techniques, though modern component design can blur the two together.
The first method is similar to the tube-to-tube construction method employed by some frame manufacturers, particularly in the early days of carbon frames. In this method, carbon fiber tubes are joined to aluminum castings. For a stem, this would allow the extension to be created from a carbon tube, with the steerer and bar clamping sections made out of aluminum. It’s a simple construction method in that it allows the use of aluminum in high stress, threaded portions – eliminating slippage of bars or fork, and reduces the likelihood of stripping a bolt hole. The weight savings, however, are somewhat limited, and the options in shape for the extension portion can cause some of the flexibility used as a criticism.
The other is a method employed by most modern frame manufacturers in the building of monocoque frames, and involves using carbon fiber sheets that are pre-impregnated with temperature sensitive resin (often called pre-preg carbon fiber). These sheets are placed in a specific layup pattern in a 2-piece mold, with an inflatable bladder, foam, or other materials where the hollow voids will ultimately be. The mold is clamped together, the bladder inflated to a high enough pressure to push the carbon sheets in to the crevices of the mold, and the whole thing is baked at the temperature required to cause the resin to set. For stems, this method requires unique molds for every length and angle available, but does allow for a huge amount of flexibility in the design. By adding pieces of carbon fiber in specific locations, areas that are subject to higher stresses can be reinforced. Some manufacturers utilize aluminum inserts, either to provide a little extra strength, introduce aluminum in to areas likely to have slipping issues, or to introduce threaded inserts, thus reducing the likelihood of stripage. While these stems are often on the lighter side, some riders have issues with bars slipping, due to lower friction and limits on how highly bolts should be torqued. The use of carbon prep pastes, which contain small beads or other corse material, usually addresses these issues.
Both tube-to-tube and mold-and-bladder construction methods can turn out a stem that is both strong and light, if proper engineering is performed, and they are assembled in a controlled environment. The resins used in tube-to-tube construction are temperature and contaminant sensitive. When combined with aluminum inserts, care must be taken to ensure suitable fit that allows the correct amount of resin to exist between the pieces, while not leaving voids. In addition, the potential for galvanic corrosion between bare aluminum and carbon dictates that some sort of barrier be placed between them – this can be a single layer of fiberglass to insulate carbon from aluminum, or in some cases anodization or a surface prep specifically designed to reduce the likelihood of a reaction. Much like liquid resin, pre-preg sheets are temperature sensitive, as exposure to temperature variation can cause partial hardening of the impregnated resin. A failure to allow sufficient baking time can result in not all resin properly curing. Insufficient bladder pressure, or bladders that aren’t properly tailored to the stem shape can result in insufficient compaction, resulting in voids that compromise the strength of the stem. Finally, the lack of a well designed layup schedule (the “recipe” that determines the shape, alignment and order of carbon fiber fabric used in construction) can result in a compomemt that is lacking in rigidity and strength when subject to the stresses the component is likely to see.
In the proper hands, however, carbon allows for components that are strong, stiff, designed to cheat the wind or conform to the body in ways that cannot be economically achieved in other materials, and do so in a svelte, lightweight manner.
Coming soon: a review of carbon components.