The beneficial properties of composites as a frame building material are exceptional. That is, if they are designed with carbon's specific characteristics in mind. With proper design, a carbon-fiber-tube bike can be stronger, lighter, stiffer, more fatigue resistant and more comfortable than a steel-, aluminum-, or titanium-tubing bicycle. Certainly these advantages are significant. These attributes can be realized through good design, because they are intrinsic to the carbon-fiber material.
However, the development of carbon bicycles has come with some challenges. Besides the simple failure to optimize ride quality, many designs have been plagued by spotty reliability including: tube and lug bonding failures, parting line cracks, loose component attachments, delaminating bottom bracket and head tube sleeves.
The following section discusses some primary design considerations - most of which are inter-related - that should be addressed in building a carbon bicycle.
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Experience and Real-World Testing
Sorting out the variables to satisfy the opposing goals of a bicycle frame has fallen onto the shoulders of composite engineers. Using extensive knowledge of composites and computers, some have engaged in a sophisticated analysis of a bicycle frame using a finite element analysis program. Ultimately, though, experience gained through trial and error and extensive ride testing is the best route to tuning a vehicle's ride qualities. Sophisticated computer analysis techniques can provide a jump on the design process, but the trial-and-error can not be ignored. The most important test is rider judgment.
Maximizing the benefits of carbon fiber in a bicycle requires extensive knowledge of composites combined with knowledge of how this applies to real life cycling. Manufacturers have a tendency to place too much emphasis on only a one of these areas while neglecting the other. Extensive product testing under extreme and demanding conditions needs to be followed by design refinements based on that experience - and the process needs to be ongoing. Such design elements as bike handling, fit, comfort, stiffness, durability, location of attachments, ease of maintenance, etc., are all issues that simply can't be learned on a computer alone.
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Ride Quality
Engineers have sought new ways to use carbon fiber to reduce weight without compromising rigidity. Some early models gave carbon fiber a reputation for excessive flex, a rap that's largely been remedied by today's frames. However, others have taken it to the other extreme and made frames too stiff laterally and vertically, giving them a "dead" feel.
Some of these problems can be attributed to manufacturers inappropriately taking previous tubing dimensions from aluminum and steel (standard and oversized) bikes and replacing them with similar dimension carbon fiber tubes. Although good shock damping characteristics are inherent in many advanced composites, these characteristics must be optimized by designing them into the specific structure of the frame. The challenge lies in creating tubes that offer an appropriately stiff ride horizontally, retaining vertical compliance (a forgiving ride), while at the same time withstanding the rigors and abuse of every day riding, racing, and transporting the bicycle.
Other ride quality problems stem from bonded joints. Where two materials of non-similar properties (i.e., carbon and aluminum) are combined, an area of material discontinuity is created. Stresses are therefore unable to flow smoothly throughout the frame, resulting in stress risers at those areas. Besides higher possibility for frame failure in this area, it also leads to a bike that does not absorb road vibration and shock well.
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Sizing
Similar to buying an expensive suit or dress, proper fit is one of the most important factors in choosing the correct bicycle frame. However, with a bicycle frame, correct fit is tantamount since the cyclist's biomechanics, aerodynamics, comfort, and handling abilities are all greatly affected by frame geometry. Forcing multiple body sizes, shapes, and riding styles onto an incorrectly fitted frame greatly sacrifices the cyclist's overall performance. Given the correct size, many cyclists can do well with the fit of a properly proportioned production bike. Others, who don't fall into the range of "normal" physical proportions, are better suited to a custom tailored frame.
Most carbon fiber frame designs-particularly those employing full size molds-are limited in their ability to offer a full size range. Many molded aerodynamic frame manufacturers are cost-constrained by complex, expensive molds. As a result, they can only offer limited sizes and some have been forced to subscribe to the one-size-fits-all theory. Some have done so while touting such things as aerodynamic frame benefits. In reality, the difference between good and fair rider position accounts for a much greater gap in athletic efficiency than the difference between an "aero" bike and a round tube bike. Professional athletes spend far more time and effort refining their position on the bike to be biomechanically efficient as well as aerodynamically efficient. Proper fit should never be compromised for flashy tube shapes. Luckily, most companies now offer a wide range of sizes with a select few able to make a truly custom frame.
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Interfacing Carbon with Metal Parts
One of the greatest drawbacks of a metal-tubed frame is the joints. Almost all frame failures occur near the joints, usually for one of two reasons: fabrication of the joint may weaken the tubing (through overheating), or the design of the joint may cause a concentration of stress that leads to failure. As the old saying goes, a chain is only as strong as its weakest link.
Conventional carbon frame-building methods using carbon tubes bonded to aluminum lugs is a controversial method. Will the adhesive bond have adequate strength? Will there be an electrolytic reaction between the carbon fiber and the aluminum (galvanic corrosion)? Will the materials expand at different rates when subjected to temperature variations (thermal expansion)? Also, by simply switching the tubing material from metal to carbon fiber, there is no reason for the problems of joints simply to go away. The glued joints are located at the areas of highest stress - particularly the bottom bracket cluster and the head and down tube juncture - on a bicycle frame. Material discontinuity (where two dissimilar materials are joined) creates stress risers (concentrated areas of stress). Coupled with galvanic corrosion, thermal expansion, or an inadequate bond, these areas can catastrophically fail. Failure can, at best, be a distressing experience for a cyclist at high speeds and more than an irritation if it means downtime from their bike.
Bonding failures can stem from an act as simple as storing a bike in the trunk of a car. Under even a moderate sun, the interior can easily achieve temperatures more than 180 degrees Fahrenheit. Similarly, transporting a bike in the cargo space of a plane (which can become extremely cold at high-altitudes) can have detrimental effects on the structural integrity of the frame. At these temperatures-particularly if repeated and if followed by rapid heating or cooling afterwards (like hosing off a bike to clean it)-thermally induced changes can take place in the various frame materials and its bonded on parts. For frames with materials of different expansion coefficients, this can lead to failure. Failure can occur in areas ranging from tube and lug joints, to bottom bracket shells, head tube sleeves, shifter- and water-bottle bosses. Two ways of addressing this issue are to use materials with similar coefficients of thermal expansion to join the tubes without bonding and to use mechanical retention systems along with bonding for parts attachments. If metal must be used it should be titanium. Titanium, unlike steel and aluminum, is not susceptible to corrosion. Although a metal, it also has thermal expansion characteristics fairly similar to carbon fiber while aluminum and steel are quite different.
The attachment of parts poses another challenge in the building of composite bicycles. Aluminum readily corrodes when joined with carbon fiber due to the substantial difference in the galvanic corrosion potentials of the two materials. Corrosion poses a problem not only for aluminum lugs bonded to carbon tubes, but also in other areas like bottom bracket shells, head and seat tube sleeves, water bottle bosses and shifter bosses. Besides galvanic corrosion, the different fatigue characteristics and thermal expansion rates of the two materials increase the potential for failure to occur at the connections.
Bonding to carbon fiber has always been a problem for carbon frame builders. There are too many variables involved with adhesive bonding (shelf life, mixing and metering accuracy, transportation problems, human error, etc.). And, it's simply too hard to tell if a chemical is performing the required function correctly or incorrectly. The potential problem of having these bonds fail and the parts fall off while in motion could lead to disastrous consequences for the rider (and accompanying riders) if they were to fall into rotating spokes or under a wheel. If aluminum must be used, it should be insulated from the carbon to prevent galvanic corrosion. Some designers rely on anodizing to insulate the aluminum. This has proven to last only a few years at best. Others rely on the adhesive itself to insulate the aluminum from the carbon. This can be done with a special glass filled adhesive used in parts that are bonded in fixtures that prevent the parts from touching each other during the curing of the adhesive.
One attempt at solving the bonding dilemma is to drill holes in the structure and blind rivet the part onto the carbon fiber laminate and attempt to reinforce the area around the drilled hole with some form of backing plate. Drilling holes in composites: degrades the laminate by interrupting and cutting the fibers, provides a spot where delaminations can start, creates dramatic increases in stress risers in the respective area, and decreases fatigue life. This can occur in areas as large as tube and lug joints, or as small as bottom bracket shells, head tube sleeves, and bonded on shifter- and water-bottle bosses.
The best solution lies in creating a system where, if the attachment of metal parts is required, they should be formed with a material that exhibits similar properties to carbon fiber. An important requirement is that the material used should have a coefficient of thermal expansion similar to carbon. Titanium is the most appropriate metal for use in applications requiring metal mated to carbon since it is highly resistant to corrosion and it has a similar thermal expansion coefficient to carbon's. Additionally, if bonding is required, it should be combined with a mechanical retention system. Mechanically retained parts reduce or eliminate the reliance on bonding alone.
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Consistency in Manufacturing
Consistency in manufacturing with carbon fiber is an issue that continues to confront many manufacturers. Each method has its own inherent challenges.
With molded frames a different mold is required for each frame size. Pressure is required inside the tubes to expand them such that they compress against the mold cavities. This pressure is usually applied by way of expandable rubber mandrels or bladders or by applying pressure by co-curing the composite with an internal foam core. These curing processes must be performed at elevated temperatures that cause the foam to expand and compress the composite material, while the bladder-molded frames are internally pressurized with air or gas. The foam remains inside the tube as a permanent part of the structure, as often do the bladders. It's difficult, however, for the bladders and foam to apply sufficient and consistent pressure. This is especially true at the tighter curves where the bladder will tend to bridge across the radius. The absence of adequate pressure results in irregular compaction and possible voids (air pockets) which could result in delamination of the plies of composite material.
Additionally, seams are usually created at the areas where the carbon is joined or overlapped. These raised seams require extensive and careful sanding or filing around most of the frame to assure a smooth finish. Too much sanding and filing can sever the fibers and weaken the respective area. This problem can be compounded if the carbon pieces become wrinkled or pinched - rather than properly overlapped - before being compacted.
Apart from the difficulty in ensuring consistent quality, bicycle frames manufactured by this process are generally more expensive and hard to produce in varied sizes and geometries. Manufacturers of molded frames have become much better at working with simple parts and shapes, such as the fork and seatstays. However, more complex shapes like a bicycle frame create more potential for inconsistency. The fewer the pieces the easier it is to control the process.
Consistency is generally very good in the manufacturing of structural tubes formed around a mandrel. Tubes can be created through one or more processes: hand lay-up, roll-wrapping or table rolling, braiding, or filament winding. Of these methods, filament winding and braiding offer several structural advantages over hand lay-up or table rolling methods. Filament winding maximizes load transfers in a composite structure since the fibers are typically placed under tension (the way in which carbon fiber best transfers loads). Winding the fibers under tension allows fiber to accept loading immediately after application before the matrix material is stressed beyond its ultimate limits. The matrix material's primary functions are to provide shape to the structure and position the fiber such that an applied load can be efficiently transferred to the fiber. When fiber in a load bearing structure is wrinkled or uneven, matrix failure can occur since the resin strength is significantly less than that of the fiber. This resin breakdown, although often gradual, will significantly reduce the functional life of the composite structure because fibers in the structure won't be loaded evenly, causing some fibers to fail before all the structure fibers begin to work together. This gradual breakdown of matrix and fiber can lead to lower fatigue life of the structural member.
Structural efficiency and maximum fatigue life in filament-wound tubular structures is further optimized since material seams or overlaps do not exist in material layers. Proper attention to the position of layer seams or overlaps in table rolled or layed-up tubular structures is necessary to maintain static load design properties. However, seams and overlaps add inefficiencies in weight over filament wound structures.
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Damage Resistance
Like damping, impact resistance is not found in every composite, but it can be designed in. The bicycle frame is historically subject to stress from minor crashes, falls, and simply abusive storage and transportation. Such things as the cumulative effects of numerous abrasions (leaning the bike against a parking meter or wall) or the single catastrophic event such as jamming a chain between a frame tube and the rotating chain rings are a concern for carbon manufacturers. Some manufactures have used an aluminum core in the tube wrapped with carbon fiber and Kevlar®. This metallic core provides insurance in case of severe abrasion, as well as eliminating problems associated with bonding non-similar materials, although this makes for appreciably heavier tubes. Fabricating the tubes with a margin of strength that will tolerate some minor abrasion despite the loss of a number of fibers should be easy. In order to prevent minor abrasion, the choices are: use urethane enamel coatings, use a sacrificial outer layer, or use a Kevlar or boron outer layer in the tubes. Like damping, impact resistance is not found in every composite, but it can be designed in.
Pre-fabricated tubes formed around a mandrel are generally the most damage-tolerant. Bladder molded frames, because of lower compression pressures and inconsistent wall thicknesses are generally more susceptible to impact damage. With any tubing, appropriate minimum wall thickness must be balanced with appropriate tube diameter (diameter/wall thickness ratio) to ensure a bike that combines optimal ride characteristics and damage resistance.
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Finish
How a frame is finished can tell a lot about the consistency of manufacturing. A frame made with lots of pinholes and surface voids must be treated with a body filler (Bondo) and painted with a primer/filler and finally a top coat. This tends to hide the problems of a frame's construction as well as add excessive weight. If a crack forms in the paint, it is difficult to tell whether it is a serious structural flaw or a minor cosmetic problem. One thing that paint does well is to protect the epoxy from ultra-violet rays. Clearcoats that filter UV rays are commonly available however. On a clearcoated frame, it is much harder to hide manufacturing defects. Non-coated frames can be protected from UV with wipe on, wipe off products such as 303 Protectant. Getting a good finish on composite frames is difficult and is one of the main reasons more companies do not attempt to build a full carbon frame.
