Carbon Fiber Composite
Carbon fiber isn’t a metal. It is a composite material made up of strands of carbon fiber (or other materials that can include Kevlar and boron) encased in a “matrix” of resin (also called epoxy). Matrix is a fancy engineering term that means that the carbon fibers essentially float within the resin. Strands of fiber are gathered to form what is called a tow. These tows are either woven together or placed side-by-side to form individual plies or layers of carbon fiber. This weave is what you see when you look at a clear-coated carbon fiber frame. Plies are added together to make a laminate (think plywood).
Most of the rules that apply to steel, aluminum, and titanium do not apply to carbon fiber. The material is so different in application and the variables so numerous that carbon fiber bicycles are, for the most part, the domain of large bicycle manufacturers who can afford to keep composites engineers on staff.
Some of the basic forms of analysis do apply. Carbon fiber has a little more than half the density of aluminum: carbon is around .064 pounds per cubic inch (lbs./cu. in.), while aluminum is .098 lbs./cu. in. And its tensile strength is higher than steel’s. While metals are isotropic, meaning they have uniform tensile strength (you can pull the material in any direction and get the same result), carbon fiber is anisotropic—it is strong in only one direction (along its length) and only when under tension. Under compression, carbon fiber fractures like dry spaghetti noodles. Carbon fiber is little different than rope; it is only useful when pulled upon, so rope can be said to be strong in one direction—anisotropic.
Modulus of elasticity (stiffness) is based on the proportion of fibers to resin (epoxy) and is measured lengthwise, along the axis the fibers run. In this orientation, the modulus numbers are great, better than steel. But as the direction of the measurement changes (say anywhere from 30 to 90 degrees off axis) the modulus number drops to a fraction of its former heartiness, often as low as 5 percent of its original value. Practically speaking it means that you can experience the stiffness of a frame when pedaling out of the saddle and yet flex the tube wall with your hand by squeezing it—stiff in one direction, but not another.
What carbon fiber absolutely doesn’t do is stretch. Elongation is a quality belonging to other materials. While carbon fiber structures can be built to flex, they won’t bend… ever. What they do instead of bending is break. It’s called catastrophic failure and has been the bike industry’s great reservation where carbon fiber is concerned. Steel and titanium will generally bend before failing, but carbon fiber snaps like a suddenly angry dog.
Frames can be made from carbon fiber from several different methods. Plies can be assembled around a round piece of metal called a mandrel. Some manufacturers wind the fiber around the mandrel prior to infusing the fibers with resin. In the case of monocoque (or one-piece) structures, individual plies are grouped together in a specific configuration in a mold to form the laminate. Once “laid-up” (the process of assembling the plies in the mold) around inflatable bladders that press the layers against the mold when inflated, the mold is closed, the bladders inflated, and heat is applied to cure, or harden, the resin.
As mentioned before, carbon fiber’s off-axis figures are dismal. To overcome this, engineers will vary the orientation of plies, so that some will be oriented 45 degrees left and right of the original axis. This variation allows the designer to choose in which directions tubes, and ultimately, the bike, will be stiff or flexible.
Ply orientation has also helped engineers solve the catastrophic failure problem. Traditionally, impacts led to cracks that spread through each layer of fiber, ending with a nasty shearing of the tube. By inserting plies oriented in different directions, cracks are much less likely to occur. Many frames and components are designed in such a way as to direct stress away from certain areas and toward areas that are reinforced, so that if there is a crack, it is much less likely to result in total failure. While a crack means the death of the frame, such engineering can allow the rider to stop safely rather than winding up on the ground.
There are two primary styles of building carbon fiber frames. The first method to emerge (in the 1970s) was to create individual tubes and bond those to lugs. This method is still in use today, though it is losing favor. In 1987, Kestrel debuted the first one-piece frame. With its flowing aerodynamic profile, the bike caused a sensation and became quite arguably the most coveted bike around (at least to the technologically hip—Luddites cried blasphemy).
Each method has its drawbacks. The presence of lugs increases weight. More recently, some companies have assembled frames by wrapping the tubes with carbon fiber, literally lashing them together. Full monocoque structures have sometimes suffered from poor compaction of the layers—unless the layers are fully compressed into a single layer, they lack strength. Manufacturers such as Giant, Kestrel, and Felt mold the front triangle of the frame in a single piece and then bond the seatstays and chainstays later. Manufacturers attempt to differentiate themselves in how they join these sections. Some simply epoxy the pieces together. Others use a process called co-molding where the joints are wrapped with added layers of epoxy-impregnated carbon fiber in addition to the application of epoxy.
The ability to mold carbon fiber frame components in any shape, configuration, or angle gives the manufacturer much greater control over frame design than is available in working with metals. From being able to place extra material in high-stress areas (such as on the bottom of the downtube where it joins the headtube) to unrestricted geometric choices and even aerodynamic fairings, carbon fiber offers the most limitless palette—and the greatest risk of frame failure if the design has been executed improperly.
One of the new frontiers in carbon fiber technology is a process called “net molding.” In net molding, carbon fiber is forced under great pressure into precise shapes; shapes that contain hard edges. Due to its anisotropic property, carbon fiber isn’t usually suited to making parts with hard edges. The vast majority of carbon fiber bicycles use metal (usually aluminum) inserts at each component interface. The inserts give the manufacturer the opportunity to ensure a secure and properly aligned fit. Net molding was developed by aerospace giant Northrop Grumman as a way to reduce the need to machine composite parts after curing. The process speeds manufacturing and, in the case of bicycles, reduces weight. As of this writing, Trek Bicycles is the only bicycle manufacturer using the technology in bicycles.
The combination of incredible strong new materials, new construction methods, and better design work has caused a carbon fiber revolution in the bicycle industry. Ten years ago, a 3-lb. frame was considered light. Today, some manufacturers offer frames that weigh less than a kilogram (2.2 lbs.)
If one true thing can be said about the ride of carbon fiber bicycles, it is that the ride quality is as varied as the kinds of music you hear on the radio. Carbon fiber bicycles have ranged in stiffness from sofa cushion to brick wall and in feel from cadaver to swing dancer.
Almost without fail, carbon fiber damps vibration in a way that no metal does. For some riders this smothering of vibration can be rather unsettling, by making it seem as if the bicycle’s tires are perpetually under inflated. The best carbon fiber frames balance vibration damping with torsional stiffness and vertical compliance.
Greg LeMond rode carbon fiber to victory at the 1989 and 1990 Tours de France. In 1999, Lance Armstrong initiated a nine-year unbroken streak for the material. Eight of those wins have come aboard Trek Bicycles, the world’s largest bicycle company in dollar revenue.
Ten years ago, most manufacturers’ top-shelf bicycle was likely titanium or aluminum. Today, almost without fail that bicycle is carbon fiber. Many manufacturers have abandoned steel and titanium entirely, leaving them to custom builders and using aluminum to offer a more affordable alternative.
Custom sizing of carbon fiber frames is offered by an increasing minority of builders, but in each instance the technology used begins with premanufactured tubes bonded together by some method.
Visual beauty: 4–5
Road feel: 4
Up until the mid-1980s bicycle frames had only been built of single materials. Frames were either steel or aluminum or titanium or carbon fiber. Even though on some bicycles carbon fiber tubes might be bonded to titanium lugs, all the tubes were carbon fiber. This changed when Trek introduced a frame that employed three carbon fiber tubes (top, down and seat tubes) mated to an aluminum head tube and rear triangle. The bikes were fairly light for the day but suffered from poor vibration damping in the rear triangle. Trek all but abandoned this design when it introduced its first full-carbon-fiber bicycle, the OCLV, in the early 1990s.
In 1997, an upstart custom manufacturer, Boston-based Seven Cycles, revolutionized ideas about material usage. By this time, bonding carbon fiber into other materials was largely considered passé. Then Seven introduced a model called the Odonata, which used titanium in every tube except the seat tube and seatstays. Their reasoning was simple: use carbon fiber in the three least-stressed tubes and use titanium in the tubes that most dictate a bicycle’s personality and are most vulnerable in a crash. The result was a design that was hailed for its exceptionally comfortable ride. In 1998, many manufacturers introduced models that substituted a carbon fiber wishbone seatstay for aluminum seatstays in their aluminum race models. By 1999, nearly every manufacturer offered a bike with an aluminum, steel, or titanium frame and a carbon fiber rear wishbone. The Odonata is arguably the single most influential road bike design of the last 20 years.
Most hybrids on the market work the theme of incorporating carbon fiber into a frame that would otherwise be made wholly of aluminum or titanium.
Lugs: The oldest method of joining tubes is lugged construction. Two tubes are inserted into a fitting known as a lug that will hold the tubes at specified angles to each other. The joints are heated with a blowtorch to either roughly 700°F or 1,600°F (depending on if silver or brass is being used to braze the joint) and then solder (think hot glue) of silver or brass is gently fed into the joint. This form of construction allows the builder considerable opportunity for artistry. The builder may thin, reshape, or cut windows into the points of each lug. Most importantly, this method of construction allows a builder considerable flexibility in designing the frame so that it may be custom sized to a particular rider. While it used to be rare for a steel frame to weigh less than 4 lbs. in a 56 cm-size, today’s steels can enable a builder to create a frame that weighs in the range of 3.5 lbs.
Fillet brazing: In fillet brazing, a builder carefully cuts steel tubes for the frames to exact angles and lengths and then uses layers of brass solder to glue the joint together. When performed by an expert, a fillet will look as if the steel tubes were molded together with smooth, flowing lines leading from one tube to another.
TIG-welding: TIG stands for Tungsten Inert Gas. In this form of construction, an electric arc literally fuses the two tubes together. This can be used with steel, aluminum, or titanium. Whereas brazing can be performed with minimal eye protection, welders must wear a darkened mask because the electric arc reaches temperatures upwards of 5,000°F and is as bright as the sun. A filler rod, much like solder, is used to complete the joint, and the welding torch bathes the joint in an inert gas (usually argon) to prevent fusion defects that can be caused by the presence of nitrogen or oxygen. This method is used with nearly all aluminum and titanium frames and a large number of steel frames. In steel frames, this method of construction results in a marginally (a few ounces) lighter frame.
Bonding: A bonded joint is one in which an adhesive is used to literally glue the tube into a lug. This method of construction has been used with aluminum and carbon fiber tubes and occasionally with titanium tubes, as well. Today, bonding of this sort is generally only performed with carbon fiber tubes. It’s accomplished either by bonding the tubes into carbon fiber lugs or by bonding the tubes into lugged joints in hybrid frames made predominantly of aluminum or titanium, or occasionally steel.
Monocoque molding: In monocoque molding, a whole bicycle can be molded as a single piece. This proved to be an inefficient way of working and the process has been revised some over the past 20 years since it was first applied. Today, sections of a frame are molded in a single piece, rather than as individual tubes, or as one big frame. The most common configurations are the main triangle (top, head, down and seat tubes plus the bottom bracket shell), a wishbone seatstay (the two seatstays meet at the point the brake is mounted) and sometimes a wishbone-style chainstay.