ht elsewhere.
For aluminum, the choice was between a limber Alan or Vitus or a super-stiff, ultra-expensive custom Klein. The
few exotics such as the carbon fiber Graftek and the Teledyne or Speedwell titanium were plush-riding but
costly curiosities with deserved reputations for frame failure and flexible handling.
Breakthroughs in materials and a growing market for high-tech cycling products accelerated the evolution of
bicycle frames through the 1980's. Cannondale and Trek led the industry in popularizing aluminum frames, while
better, less-costly grades of titanium and carbon fiber sparked interest in the potential of these space-age
materials. Steel manufacturers fought back with new higher strength alloys and heat treatments, sophisticated
shapes, and non-standard diameter tubes to reduce weight while increasing comfort and efficiency.
Now there are more choices and naturally more confusion. If one asks "What frame material is best?," a qualified
answer is required because how a given frame material is used can be as important as what material is used.
The ideal bicycle frame for a given rider would fit the rider's build and would be light. It would absorb road
shocks well, but it would handle crisply because of lateral stiffness and would deliver undiminished applied
pedal power to the drive train. It would be durable and not subject to fatigue failures and would be strong
enough to stand up to unexpected impacts and torsion forces. It would lend itself to attractive finishing and
would resist corrosion or attack by the elements. It would be formed in an attractive, functional way allowing it to
move through air resistance easily.
Material Facts
Steel, aluminum, titanium and carbon fiber all attempt to achieve the above criteria, but differ from each other in
strength, stiffness, weight, fatigue resistance, corrosion, etc. For example using aluminum or titanium in the same
tube dimensions as a traditional steel frame would reduce weight but would produce excessive flexibility. So
non-ferrous metal frames typically have larger tube diameters than steel ones to gain rigidity.
Metal frames usually do not fail due to a single catastrophic load but because of small, repeated stresses (called
"fatigue"). Steel and titanium have defined minimum fatigue limits if the stresses are smaller than these limits,
these smaller forces generally don't shorten the fatigue life of the frame. Aluminum has no such specific endurance
limit, so each stress cycle, however small, takes the material that much closer to fatigue failure. This sounds
worse than it is, however designers realize this limitation and attempt to "over build" their frames for a lifetime of
use.
Titanium's high strength, light weight, resilience, and resistance to corrosion make it a well-suited frame material.
However, being a metal, many of the same mechanical properties that limit steel and aluminum also limit titanium:
metals are equally strong and stiff in all directions (a property called "isotropy"). Once the cross section
geometry of a metal pipe is determined to meet strength or stiffness requirements in one plane, an engineer lacks
the freedom to meet varying demands for strength or stiffness in any other plane. In metal tubes, by setting
diameter and wall thicknesses to meet bending standards, this automatically determines torsional and lateral
bending stiffness.
Metal frames are just variations on a single theme compared to composites. Composites consist of reinforcing
fibers, particles, or whiskers that are embedded in a matrix material. Advanced composites are composed of
engineered fibers and polymer, metal, or ceramic matrices combined to form fabrics. Combining these woven
fabrics with a thermosetting resin (using the hair-like fibers of carbon, glass, and boron) create amazing strength
and stiffness. They make structures that are as strong and rigid as metal ones of equal size, but weigh much 2
less. Furthermore, until the binder is hardened by a chemical reaction or heat, the resin-soaked fibers can be
molded or formed into virtually any shape.
Unlike isotropic metals, composites are anisotropic their strength and stiffness is only realized along the axis of
the fibers which can be arranged in any desired pattern. Thus, to absorb the variable stresses of a given
bicycle frame, composite frames can use multiple layers with different fiber angles for each. This puts strength
only where it is needed while minimizing weight.
Along with traditional round tube and lug frame designs, composite frames can be molded with the use of internal
bladders and foam in either one-piece ("monocoque construction") or multi-section frames. Also, they can be
formed in a high pressure lamination process combining the frame tubes into one integral piece.
Industry Parallels
As with some other sporting industries, the future of cycling is moving away from metals. Continuing
advancements in the pace setting aerospace, automotive, and boating industries have nearly assured the role of
composites as the structural material of the future.
Other sporting good industries where new materials have supplanted the old, include: tennis, archery, skiing,
boating, golf, and fishing. Composites have replaced previous materials and eventually declined in price to
widely affordable levels. There is little or no reason why the bicycling industry should be an exception.
Composite bicycle frames have been a largely American phenomenon, because the technology emerged from
the aircraft and boating industries. Manufacturing of composites requires greater technical expertise and money
for product development. Consequently, these products usually must enter the market at the high end. As a
result, there have been few high-end American bike companies that have been willing to learn this technology to
develop innovative composite framesets.
New materials replace established ones for many reasons. In sporting goods substitutions occur because a new
material out performs an existing one. An example is the tennis racket where wood was once the only material
for racket frames. It offered excellent shock absorption, but it swelled and shrank with the weather, changing
string tension. Wood that was strong enough to meet the needed criteria was too heavy. Tubular steel and
aluminum rackets came into vogue in the early 1970's. They were lighter than wood, were unaffected by the
weather, and offered more potential power in the swing. However, the feel of metal didn't suit many players, and
many did not like the impact shock these rackets radiated into their hands and arms.
Composite rackets arrived in the late seventies and changed everything. They delivered resiliency and shock
damping of wood, and weather immunity of metals. Also, they were light! Within six years, composite rackets
became available at all but the lowest price points, and wood virtually disappeared. At present, 95 percent of all
tennis rackets are of composite construction.
Many bicycling engineers who have envisioned composite frames haven't enjoyed the proper circumstances to
create a widely marketable product. With more people becoming convinced that composites can wring even
more performance out of a bicycle, efforts are afoot to expand this technology in the cycling industry.
The Benefits of Carbon
A bike frame is a considerably complex structure with performance characteristics that include: lightness, rigidity,
durability, and shock absorption. Aluminum and titanium frames have become popular because they challenge
steel frames in at least two areas of performance lightness and shock absorption. But, at the high end of the
industry, composites will likely eclipse frames made from any metals in all performance areas.
The metallurgical composition of a metal tube can't be varied over the length of the tube. In contrast, composites
can be infinitely varied over the length of the tube. Some of the variations include: different fiber angles, different
plies, different ply thicknesses, different combinations of materials. So the properties of the end product made
from composites can be tailored to specifications.
Composite tubes are typically formed around a mandrel (a metal dowel, typically steel, that is later withdrawn)
by either "filament winding" (winding strands at various angles), "roll wrapping" or "braiding." Some tubes
combine methods, using a top woven layer for appearance and protection of the underlying wound ones. TECHNICAL WHITE PAPER
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Another method called "pultrusion" pulls fibers through a heated die that melts a thermoplastic matrix. Each
manufacturer has its own special number of layers and orientations of fibers to create its desired combination of
strength, weight, and stiffness. This is the beauty of carbon fiber: with metals the choices are much more limited,
but with carbon fiber they are almost limitless.
Tailoring of a bicycle frame is not new; it's been done with steel frames for years through the butting process,
where tubes are thickened at the joints to handle stress and thinned out in their long center spans to reduce
weight. What if the size and shape of each tube are matched precisely to the predicted loads of pedaling and
road shock? What if the material could be distributed precisely where it is needed. What if the rigidity of each
tube, through some complicated shaping or milling process, varied from one plane of bending to another or from
one end to another? The frame could be built to be rigid to late