Recently, a fairly revolutionary bike was released that seems to have pushed the use of carbon-fiber in mountainbikes to a level unseen as of yet.
This design will surely raise a great number of eyebrows, and give some of us who tend to be cynical luddites ammunition to attack carbon-fiber bikes as being unsafe & overpriced. Already, there are a huge number of discussions among the various internet MTB forums on the growing usage of CF in the mountainbike industry. Many myths abound in both the supporting camp and the anti-CF crowd, and much of the information provided by self-proclaimed experts is gleaned from second & third-hand reports of carbon-fiber failing spectacularly; or carbon-fiber laying waste to aluminum’s claim on light-weight and durability.
Fortunately for the mountainbike industry, the use and manufacture of carbon-fiber has been ironed out by the aerospace, marine, and road-bike industries. In the early days of CF road-bikes, there are well documented instances of the afore-mentioned spectacular failures, as the application of said technology was fairly unexplored. Bikes are subject to many different forces dissimilar to those found on aircraft or marine craft. But within a decade’s time, the rate of failure had decreased by a large factor. The are still instances of carbon-composite bikes or components failing, and when they do, there is a general outcry of “See?! Told you so!”; even though aluminum bikes & components break just as frequently (if not more often). These events are kept low-key, and chalked up to just bad luck or abuse.
However, the foul taste of CF’s failures remained in a lot of rider’s memories, and they have carried on the prejudice until the present day.
The same prejudice still exists, to some extent, with another common material; aluminum. In the ’70s, bicycle manufacturers began using aluminum in earnest for their primary frame material; and there were a rash of failures that severely hurt aluminum’s reputation as an appropriate alloy for bikes. This was not the solely the fault of the metal; it was due to poorly designed frames and manufacturing, as well as non-standard alloying procedures. And as with carbon-fiber, there are a large number of bicyclists and frame-makers that refuse to utilize anything but steel. Now, aluminum alloys have gained a stable standard of manufacture, resulting in negligible variances between batches; and the methods of extruding, forming, and welding aluminum tubes has been perfected.
There are a huge variety of carbon-fiber composites now being used, just as there are a large number of different aluminum alloys; each with their own strengths and weaknesses. But whatever the end-use, carbon composites gain almost all of their strength from the actual carbon filaments. And just as pure aluminum is relatively worthless as a structural material without an alloying agent, CF is worthless without the epoxy that binds the carbon fibers themselves into a strong matrix. And until recently, there was a finite limit as to the end strength that could be achieved through available fibers and resins. However, in the last few years, advancements have been made in the manufacture of both fibers and resins.
A large part of a carbon-fiber composite’s durability is the result of the way raw filaments and epoxy were combined. When the fibers are impregnated with epoxy in a process called “wet-out”, and the epoxy must flow through the tightly packed filaments completely. Afterwards, the composite must be cooked to achieve it’s hardened state. Any pockets left dry will cause the composite to have weak-spots, which will lead to complete failure. Early in the development of carbon-fiber composites, the epoxies used were fairly high in viscosity, which allowed such dry voids to form. These dry spots may not cause immediate failure, but they will seriously increase crack-propagation. This was surely the cause of many frame & component failures. Recent advances in low-viscosity have caught up, and these new epoxies allow for even more densely packed filaments; which in turn increases a composite’s overall strength. Even so, the possibility of dry-spots exist, although the rate of incidence has been much decreased. When carbon-fiber composites fail, it is not the actual filaments that fail; in almost all cases it is the epoxy/resin matrix. New, low-resin content composites further reduce the likelyhood of failure.
Carbon-fiber filaments are rated by their strength and “modulus”, or stiffness. The tensile strength of CF filaments used in most high-end bicycles is rated at 500 thousand pounds per square inch (or ksi). Carbon-fiber comes in low, intermediate, and high modulus ratings. This is the measurement of the tensile modulus expressed in million pounds per square inch (msi) or ksi. During my research, I’ve found that CF’s strength and stiffness don’t always match each other. To balance this, the actual design of the composite must work to achieve equilibrium, so that the frame or component is both strong and durable. Also, according to a respected bike designer experienced in composites, most carbon-fiber is 33-MSI modulus. Refined further, with more filaments packed into a smaller space, 42-MSI fiber is the result, with a greater tensile stiffness. 42-MSI is known as Intermediate Modulus or IM fiber. With even more refining, the filaments get smaller and denser. This means that the material is stiffer yet again, but is also expensive, brittle, and not used often. These composites are in the 55-MSI range, or higher, and labelled “high modulus” fiber. According to my source; “Many companies refer to 33 and 42-MSI fiber as High Modulus because they cant get sued for false advertising by using this informal term.”
Something to think about……
Now I’ll move on to the different methods of actually putting a CF frame or component together.
Carbon-fiber composites usually come in one of two forms; sheet (prepreg) and tube. Composite tubes are pre-cooked in a generic form, and allow a manufacturer to simply cut the tubes to a desired length. The tubes are then fit and bonded together using lugs, which can be made of various materials like aluminum, titanium, carbon-fiber, steel, or even bamboo. This process provides a large amount of leeway when designing a bike’s geometry, but it limits the shape of the finished bike and concentrates stress at the bonded joints.
“Prepreg” carbon composites arrive at the manufacturer in a fairly raw state. The carbon filaments have been soaked in epoxy, and are laid in a uni-directional sheet form. Each sheet has a different angle at which the filaments are aligned, most commonly at a plus 45° angle, a minus 45° angle, 0°, 90°, and/or a plus or minus 22.5°. Each direction gives the composite a different attribute to the finished product. 0° sheets give strength lengthwise, 45°’s resist twisting, and 22.5° resist crushing forces. The 90° final layers are designed to withstand impacts, and is what is most often seen below the clear-coat.
When these sheets are finally cut, they are “laid-up” in a tightly controlled sequence. This is done in a way which balances strength, stiffness, while maintaining as light a weight as possible. The lay-up schedule is determined through many different, highly scientific methods, as well as simple trial-&-error. Once the ideal lay-up is figured, the next step is making sheets into a moncoque frame. Monocoque frames are simply shells that distribute stress over the whole frame structure. If properly designed, a monocoque frame eliminates dynamic stressors from concentrating at certain points; like the lugged/bonded joints found in traditional frames.
Monocoque designs are becoming popular in both aluminum and carbon-fiber frame building, as it not only imparts strength to the design but is also responsible for the flowing, organic lines featured in modern bikes. Hydroformed aluminum tubes still require welding at the tube joints, and the more radically shaped the tube, the harder it is to weld.
Finally, the various composite frame sections (example: top, seat, bottom, & head-tube) are laid by hand into silicone forms. This process is done in a fashion so that once the composite is cured, the sheets form a solid, uniform piece. The parts are set into a steel mold, and air-bladders are inserted into the formed component. These bladders are inflated, the mold closed, and then heated to over 200 degrees F. The heat causes the resin to cure and harden, and the seperate components combine into a single monocoque part.
After being removed from the mold, the part must be hand-finished, clear-coated, and possibly painted. It is then ready to be made into a complete bicycle.
One of the great features of carbon-fiber is that it can be repaired fairly easily, should a tube be fractured or crushed. Aluminum frames are much harder to repair, as they are heat-treated. If an aluminum tube is cracked/dented, the entire tube must be replaced (if possible) & realigned. The frame then needs to be heat-treated again, and repainted/anodized. Very few manufacturers will do this, and there are just as few welders who are willing to perform such a service. When an aluminum bike is damaged, most often the whole frame/triangle is replaced, hopefully under warranty. But many of us know that dealing with a warranty replacement often presents a knotty problem, and many of us are riding older bikes with expired warranties, or secondhand rides. Many manufacturers will not honor a warranty on a secondhand bike, nor replace it if the damage is found to be anything but a manufacturing defect.
This aspect of carbon-fiber is often overlooked or unknown to the majority of the bike-buying public. And carbon fiber has both phenomenal strength and superior fatigue resistance when compared to other commonly used frame materials. But as with these other frame materials, a crash can just as easily damage or destroy a bike. Another feature not well known about CF, is that some frame manufacturers add extra layers of impact-resistant composite in damage prone areas, as well as thicker & more durable clear-coats.
Carbon-fiber composites have come a long way since making the leap from aerospace to bicycles. Light weight, versatility, strength, stiffness, and aesthetics make it a very attractive alternative material. Titanium, for example, is expensive and difficult to weld. Aluminum requires careful heat-treating; certain alloys are susceptible to corrosion; is prone to undesirable flex; and as mentioned before, hard to repair.
The Arantix bike represents, in my opinion, the cutting-edge of carbon-fiber bikes. Though just looking at it makes me cringe at the thought of even bunny-hopping a curb, I still respect the ingenuity and effort put into the design. Each composite strand consists of carbon filaments which are tightly bound inside a helical wrapping of Kevlar string. The composite strands are then woven together to form the “Iso-Truss” lattice tubes, and then joined with carbon-composite lugs. Whether this bike will withstand the rigors of a pro-level XC race, or put up with the daily abuse imposed by an average rider is yet to be seen. With an entry fee of over $11,000, I highly doubt that I will be able to test the Arantix; or even see one up close. But as with all experimental/advanced technology, it could very well trickle-down to our level, resulting in yet more refining & perfection of the technology which reaches the end-user.
In any case; carbon, titanium, steel, aluminum….. even bamboo, are viable and effective materials with which to make bikes. There will always be people who are purists and will resist any thought of using anything but a particular material in their bikes. Then there will be the revolutionaries, who will constantly try to advance different ideas and materials; searching for the lightest & strongest way to make a bike. And in the middle will be people like you and me, who do our best to love (& sometimes destroy!) these rolling works of art. No matter which camp a rider belongs to, care must be taken, as a crash will hurt both bike and body…. even a bike made from the most advanced unobtanium.
Sources for this article in large part came from:
Calfee Design Carbon-Fiber Bike Repair
Calfee Design Carbon-Fiber Information
http://delta7sports.com/product.html
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Dec 26, 2007