RSS Feeds
Breaking Away
Striving to improve cycling performance, engineers push the envelope on lightweight materials
By Design News Staff -- Design News, January 9, 2005
Hear an audio clip of Lance's feedback to the Trek engineers after a test ride
|
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.
"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.
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 |
| 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
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.
