Source:
http://www.designnews.com/article/CA491475.html
Cover Story
Breaking Away
Striving to improve cycling performance, engineers push the envelope on
lightweight materials
Design News
January 10, 2005
A man, a bike, and six yellow jerseys: one athlete claims that honor.
Lance Armstrong, of course, who pedaled to six consecutive wins in the
Tour de France, beginning in 1999. And Trek, a company out of Waterloo,
Wisconsin that's been building bikes for over 25 years, was along for
the ride-thanks to its innovative use of carbon fiber composites and
manufacturing prowess.
Scroll down for the audio clip of Lance Armstrong
"Just about perfected a century ago, bikes are the most
energy-efficient transportation known," says Trek's Doug Cusack, senior
R&D engineer. "To put things into perspective, a car realizing the
efficiency of a man on a bike at 15 mph would get around 900 mpg."
But despite the inherent efficiency of the bicycle, over the years
engineers continually have sought to improve cycling performance of
road racers and weekend warriors, through both material and geometry
changes. Although the vast majority of frames today are built of alloy
steel, engineers are also exploiting aluminum, titanium, and carbon
fiber composites to produce lighter and stiffer machines capable of
transferring more of a peddler's energy to the back wheel.
Although the road machines that Armstrong and his Postal Service
teammates ride must conform to a set of rules promulgated by the UCI, a
Swiss federation that governs bicycle racing worldwide, those rules
still leave plenty of room for innovation. And for Trek, that means
exploiting a lightweight material like carbon fiber.
Trek manufactures its carbon fiber frames (including the ones it made
for Armstrong for the past six Tour de France races) by a patented
process called Optimum Compaction Low Void, or OCLV. According to
Manufacturing Engineer Scott Nielson, the process makes frames with
void content of less than half a percent. The resulting quality meets
aerospace standards.
Managing void content in the manufacturing of carbon fiber frames is
critical, since voids can lead to cracks. If they form, catastrophic
failure waits in the wings.
The head tube on Kestrel's triathlon bike shows the high degree to
which carbon can be shaped.
"Composites do not fail in a ductile manner," Nielson explains. That's
why OCLV-which uses an inflatable bladder to force layers of epoxy
and carbon fiber cloth against the walls of an interior mold-must
push so forcibly to squeeze out any entrained air. The result is a
frame member, sawn in half and polished, that shows no voids, he says.
The bikes go together in classic tube and lug style. Round tubes are
produced by roll wrapping. The hand-laid lugs allow engineers to
thicken areas where stresses will be highest. UCI standards let the
bike weigh no less than 6.8 kg.
Unlike isotropic alloys used in frame making, carbon fiber is
anisotropic-its strength varies by direction. Deployed strategically,
that directional strength turns into an advantage, Nielson says. "Each
ply goes into the mold in proper order according to a recipe," he says
of the lay-up procedure. Unlike aerospace manufacturing, where Neilson
spent part of his career, frame building in carbon fiber doesn't lend
itself to automation. Nevertheless, Trek engineers "work hard to
eliminate variability in the process," he says.
The latest efforts have put a material on the road that once was
exclusive to satellites-carbon fiber cloth with an areal density of
55 gsm, half that of ordinary carbon fiber fabric. That's significant,
because the lower the areal density, the higher the tensile strength
and tensile modulus. But the 55 gsm cloth hadn't made any progress in
the world of sports until Trek-which builds half a million carbon
fiber parts, enough for 50,000 bikes, annually-stepped away from the
crowd by pushing it in its latest SSL (super super light) frame. Not
that it was easy. The challenge in working with a lower density
material is you have to lay up more fabric-making manufacturing that
much fussier.
Cut in half to show different thicknesses, seat tube (left) and bottom
brackets (right) show no voids, thanks to Trek's use of the OCLV
process--which uses an inflatable bladder to force layers of epoxy and
carbon fiber cloth against the walls of an interior mold and push out
any entrapped air.
More Carbon Fiber Innovations
Trek isn't the only company to gain performance-enhancing benefits from
carbon fiber. Santa Cruz, CA-based Kestrel, which began selling carbon
fiber frames in 1986 also uses the material for road bikes. And
engineers have come up with novel deviations from triangular frames for
its triathlon and mountain bikes, which aren't as closely regulated.
Kestrel's latest triathlon bicycle, for example, attempts not only to
slice through the air but also to harness the energy in that air.
Preston Sandusky, product and marketing director, says the bike frame's
aerodynamic shape produces lift in cross winds. This reduces drag but
can make an unstable ride. Changes in the front fork and steering
geometry can accommodate instability somewhat, but not without
compromising performance. Instead, engineers locate the center of
pressure of the frame as far back as possible to take advantage of the
sail effect without diminishing handling. They've accomplished this by
eliminating the seat tube, something that the UCI regulations won't
allow but is permissable by many triathlon rules.
Kestrel's monocoque carbon frame eliminates the seat tube. The
projection above the bottom bracket holds the front derailleur that
mounts to the vertical tube on a traditional diamond frame.
Designing in Selective Strength
Deviating from traditional tube-and-lug construction, Kestrel builds a
bladder-molded monocoque frame. This design allows engineers to put
strength and function where it's needed without running into the limits
of plain round tubes. For instance, the right hand chainstay-the rear
horizontal member that extends from the bottom bracket to the rear
drop-outs-is ordinarily dimpled to permit a pedal crank to rotate
past it. Instead of having to undergo mechanical deformation in this
area, it can be molded into shapes that accommodate the drive train
without giving up strength.
A carbon structure is thus tuned, Sandusky says, and stiffness,
strength, and rigidity placed where needed (stiffness through downtube
and toptube and strength through the headtube). "A bicycle frame is a
stiffness-critical structure, so we capitalize on the higher specific
stiffness of carbon and still have plenty of strength," he explains,
adding that Kestrel normally designs its frames to have twice the yield
strength of comparable metal frames.
Even mountain bikes that are exposed to hostile conditions beyond what
any road dishes out make full use of carbon fiber's ability to
strengthen locally. Kestrel adds fiber plies to areas that might be hit
with stones and flying debris.
Trek, too, builds carbon fiber mountain bikes. One design uses carbon
fiber for most of the frame but relies on the durability of aluminum
for the bottom bracket- an area of the frame exposed to stumps and
other obstacles littering the trail.
Quoted message said:From road bikes, whose shapes must conform to rigid rules, to mountain
bikes built rugged to match tough terrain, carbon fiber clearly has
opened up new possibilities in frame design-and that's clearly where
the fun begins for engineers.
Six Carbon-Fiber Frames for Armstrong: A look at six frames Lance
Armstrong rode in his six Tour de France wins.
1999
Notable Features:
-Shimano components package includes crank, chainrings, brakes,
derailleur
-Full OCVL (optimum compaction low void, a patented process for
reducing void content in carbon fiber composites) 150 carbon fiber
frame
-Trek carbon fiber fork
2000
Notable Features:
-Now that Armstrong's a Tour winner, Trek designs this special time
trial machine to replace the titanium frame he used in 1999
-OCLV 150 honeycomb frame
-Hear an audio clip of Lance's feedback to the Trek engineers after a
test ride
2001
Notable Features:
-Oversize steering tube, from 1 to 1.125 inch in diameter, lightens the
bike while improving road feel and steering
-Switch to 120 gsm carbon fiber cloth lops 0.5 lb off the frame weight
2002
Notable Features:
-Trek uses OCLV 110 gsm fabric for the first time
2003
Notable Features:
-Pronounced web at the juncture of the top, head, and down tubes
replaced by a more organic shape
-Subtle aero shapes develop behind the rear water bottle and ahead of
the front bottle, but stay within rules that restrict tube width and
diameter
2004
Notable Features:
-The 950 gm frame weighs so little that weight must be added in
components to meet UIC 6.8 kg minimum
-Frame features same stiffness at half the weight
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Web Resources
Lance's 1999 TdF victory was accomplished on stock Trek bikes. All
except his titanium-frame TT (Time Trial). In the months following the
TdF win, Trek engineers developed the company's first custom bike for
Lance -in ½ the normal development time. Today you can find the very
same Time Trial frame that Lance rides at your local Trek dealer.
The attached audio file is a recording of the phone call that LA made
to Trek HQ after receiving his new machine.
Metal to the Pedal
While carbon fiber is popping up on bike frames everywhere these days,
metals innovations continue apace in the industry.
While denser than aluminum or titanium, 4130 chrome-moly steel exhibits
a comparatively high Young's modulus, on the order of 30 Mpsi. The
modulus for titanium is about half that and for aluminum about a third.
A similar relationship holds when comparing the densities of the three
materials.
Done according to the book, titanium welds well. Even in an age of
carbon fiber, titanium remains a favorable frame material for
discerning cyclists.
Tubes can be stiffened by going to larger diameters. For the same wall
thickness, dou-bling the diameter of a 1-inch tube only increases its
weight by two but increases its stiffness by a factor of eight.
Buckling of tube walls-the beer can effect-becomes a concern as the
ratio of diameter-to-wall thickness approaches 60:1. Aluminum frame
builders circumvent this simply by adding material to the wall; steel
frame makers can't do that without suffering a more severe weight
penalty.
Equally important to frame builders is material elongation, which
basically measures how far a material will stretch before it breaks,
and is measured by tensile tests. Steel usually runs around 9-15
percent; aluminum typically less. Titanium, in contrast, can run 20-30
percent. The higher the elongation, the less chance for brittle
fracture, something frame builders-and riders-judiciously strive to
avoid.
Aluminum, though finicky to work, is another favorite of some frame
designers because of its mechanical properties. But unlike steel and
titanium, it lacks a well-defined endurance limit. That means under
repeated stress loadings, no matter how slight, aluminum will fatigue.
Aluminum frame builders combat this by stiffening the frames to reduce
flexing and by spreading out stresses over butts and gussets. They also
generally design around a higher factor of safety.