Tuesday, January 22, 2008

Racers, rough riders, and recumbents.

By Kim, Irene
Publication: Mechanical Engineering-CIME
Date: Tuesday, May 1 1990


That most basic form of transportation, the bicycle, is becoming a test-bed for high technology as engineers and designers attempt to combine speed and durability in new models that are lighter, stronger, and more streamlined.

The quest has led manufacturers to a host of new materials originally developed for the aerospace and automotive industries, including titanium, carbon fiber composites, aluminum, metal matrix composites, and plastic. The materials are being used to build rugged, lightweight frames as well as components such as wheels, lugs, chainrings, and cogs.

Often, the new frames and components are taking shape as full-fledged "project bikes" that are used to test out and improve new designs. The list of models includes streamlined racing bicycles featuring clipless pedals and grip-mounted shifters, mountain bikes with front forks and suspensions engineered to handle the mud and shocks of harsh off-road environments, and recumbent bicycles that provide a low center of gravity for faster cornering and maneuvering.

Building a new bicycle to accommodate the often conflicting design goals of good aerodynamics, high strength, low weight, wear resistance, and reduced maintenance is an engineering challenge. As a result, even an everyday set of wheels can cost about $400. And when no-holds-barred performance is the goal, the most advanced of the new high-end bicycles can cost as much as $10,000.

Space-Age Materials

One of the aerospace materials that bicycle makers are turning to is titanium. Its high strength-to-weight ratio makes it an attractive alternative to the steel tubing used in a conventional bicycle frame. Titanium tubes can be made very thin while retaining sufficient strength and stiffness to support a frame and components. Three-year-old Merlin Metalworks (Somerville, Mass.), manufactures frames using 3/2.5 titanium alloy, which consists of 3 percent aluminum, 2.5 percent vanadium, 94.5 percent titanium, and 0.25 percent trace elements.

The titanium alloy frames are highly resistant to corrosion and therefore can be sold unpainted. Leaving the frames bare also simplifies the repair process considerably. "For example, we could cut out a damaged down tube [the main vertical frame member] and just weld a new one in place without having to repaint or file anything," said product development engineer Rob Vandermark.

Titanium, though extremely durable, is difficult to work with because it tends to machine-harden. "You have to do it right the first time," said Vandermark. "Once it goes somewhere, it likes to stay there." Because titanium quickly wears down tools, machine shops are generally reluctant to work with it. As a result, Merlin must machine many of its own parts. U.S. bicycle manufacturers such as Merlin have a significant advantage over overseas competitors because they are permitted to buy some of the titanium tubing produced for the U.S. Department of Defense. Japanese manufacturers, who cannot obtain the same titanium alloys, must use commercially pure titanium, which is even more difficult to work with.

Another new material, the carbon fiber composite, which is produced by embedding carbon fibers in an epoxy matrix, is also popular with high-end frame makers. Carbon fiber requires more engineering effort than traditional materials but offers valuable weight savings as well as high strength. When composites are manufactured, the fibers can be placed to tailor tubes to endure specific stresses. A seat tube, for example, primarily undergoes compression, while a down tube mostly receives torsion and bending.

Most manufacturers build their tubes with standard carbon fiber. A high-modulus carbon fiber, which is stronger than the standard material, as well as more expensive, also is available. Using the high-modulus fiber, Specialized Bicycle Components (Morgan Hill, Calif.) was able to build a 2.7-lb frame (not including fork) for their Stumpjumper mountain bike.

Huffy Corp. (Dayton, Ohio) has produced the Triton, a high-end carbon fiber bike. The complete bike, which has tubes of carbon fiber and sports rear dropouts, headset, and seat mast of titanium, retails for $8000 to $10,000. It can be built to specifications as a road bike, a time-trial bike with different-sized wheels, or a mountain bike. The Triton uses a monocoque design, in which a single mold is used for the whole frame. Its teardrop-shaped tubing is based on an airfoil section.

Unlike most other carbon fiber models lugged with aluminum or steel, the Triton uses carbon fiber lugs. Bicycle lugs, which are used to connect tubes at the frame's joints, usually have the tubes brazed or bonded into them, and are typically made of metal. Lugged designs are considered aesthetically superior to welded frames by many cyclists.

Lugs made of material other than carbon will pull apart from the carbon fiber tubes because of galvanic corrosion, said Michael Melton, coordinator of Huffy's technical development center. Relatively large changes in temperature, such as those that occur when a bike is brought from subzero weather into a hot room, could also cause the dissimilar materials to pull apart because of their different coefficients of expansion.

Although a single-piece carbon fiber frame would be stronger than one made of individual tubes, its cost would be prohibitive. Because only one size can be produced from each mold, a manufacturer would have to invest several hundred thousand dollars in molds in order to begin production. High-end Italian bike companies such as Cinelli (Milan), Ernesto Colnago SRL (Milan), and T. Carnielli & C. SPA (Vittorio Veneto) have been experimenting with monocoque designs, creating futuristic road, time-trial, and mountain models. While these designs are innovative, they are not meant for large-scale production, said Salvatore Corso, owner of Corso Bicycle Distributors Inc. (New York, N.Y.), a large distributor of Italian bicycles.

Fat Tubes

Aluminum, a more traditional bicycle-frame material than carbon or titanium, has also been taking on new shapes. Aluminum is lighter than steel - the standard frame material - but it also is weaker. In order to get the strength required for a frame, aluminum tubes must either have thicker walls or a larger diameter, both of which tend to add weight. Gary Klein, designer and founder of the Klein Bicycle Corp. (Chehalis, Wash.), is generally acknowledged as the inventor of the large-diameter aluminum mountain bike. Klein said he opted for the bigger diameter tube because it yields better overall performance in mountain bikes. Klein has employed the design in his Attitude bicycle, which uses wide aluminum tubes on frame and fork to keep weight down and strength up. A fully equipped Attitude retails for about $1825 and weighs 24.5 lb. Although the tubes, which can be as large as 1 3/4 in. in diameter, might seem to be less aerodynamic than smaller ones, their low weight reduces rolling resistance and results in a faster bicycle. Frontal wind resistance, said Klein, is probably due more to the rider than to the size of the bike's tubing.

Aluminum also acts as a good shock absorber, according to the Cannondale Corp. (Georgetown, Conn.), which manufactures complete aluminum bikes that use down tubes as large as 2 in. in diameter for as little as $429. The main reason for their low price is the relatively low cost of aluminum. Cost is reduced further by the use of lower-priced components.

The aluminum frame is welded, eliminating the need for lugs. Since lugs are expensive to manufacture, lug makers often will offer a limited range of sizes, which then limits the frame builder. In addition, the welding process makes a frame that has the strength advantages of unibody construction, said Cannondale. Welding is also a relatively inexpensive and quick alternative to brazing by hand.

Cannondale decided to use aluminum after applying finite-element analysis and other testing methods to carbon fiber designs. They then began to use their FEA software on possible aluminum designs, and found that they had not yet exhausted all of the metal's possibilities. Engineer David Graham, Cannondale's director of research and development, said he had developed his own Basic-language program to design frames, although "most of the design happens inside your head."

Frames go through an exhaustive testing procedure, including the fatigue and impact tests that are standard at most companies. In fatigue testing, a frame with a dummy crankset is positioned as if someone were actually riding it. An air cylinder is placed over each pedal and simulates a rider standing with the pedal at the four o'clock position, which is where the frame receives the greatest stress. The stress load alternates back and forth until the frame breaks. In impact tests, a pendulum is swung to hit the frame, and its deformation is measured.

Similar tests are run on forks. The steering tube is immobilized, a weight is hung from the dropouts (the ends of the fork's blades), and the extent of deflection is measured. A machine then moves the dropouts up and down until the fork breaks. In impact tests, the fork is hit with a pendulum, and the elastic and permanent displacements are measured. At Merlin Metalworks, a tube is tested for fatigue in a mill machine. The tube, rotating on an off-center chuck, is allowed to wobble until it snaps.

Stronger and Lighter

Metal matrix composites promise even greater strength and lightness for frames. While current frames can weigh as little as 2.7 lb, the new metal matrix composites could cut that amount in half, according to Fred Zahradnik, technical editor of Bicycling magazine. Originally developed by the aerospace industry, the material is made by reinforcing a matrix material with fibers or particles of lighter, stronger material. Filaments lie adjacent to, but not touching, each other within the matrix. Since the metal matrix is significantly stronger than resin, tubes can be made even thinner than carbon-fiber composite tubes.

Continuous silicon-carbide filaments result in composites that are three times stronger than steel in tension and 10 times stronger in compression, but comparable in weight to aluminum, according to Melvin Mittnick, director of marketing for Textron Specialty Materials (Lowell, Mass.). Silicon-carbide-reinforced aluminum is three to four times as strong as plain aluminum. It can be welded or bonded like other materials, and may even be used with commercial lug sets.

The filaments can be used in a titanium composite as well. Textron has provided reinforced titanium composites for the U.S. Advanced Tactical Fighter and for the National Aerospace Plane. Reinforced titanium composites, which can withstand temperatures up to 1500(degrees)F, are more heat-resistant than reinforced aluminum composites, which can be used for applications up to 500(degrees)F. That property makes them more attractive for aerospace uses.

Aluminum composite bicycle tubing has been manufactured by Textron. Because titanium composites are so much more expensive than aluminum composites, however, they have not yet been used in bicycle frames. The titanium filament itself costs thousands of dollars per pound, said Mittnick. Production costs are expected to drop eventually.

Manufacturers seeking corrosion resistance with high strength and low weight may find some answers in plastics, according to Chester Kyle, an engineer and editor of Cycling Science (Mount Shasta, Calif.). Merlin's Vandermark agreed, predicting that a completely plastic frame would appear some time in the coming decade. Parts such as crankarms, chainrings, and cogs could be made from injection-molded thermoplastic composites, said Leigh Sargent, president of Compositech Inc. (Indianapolis, Ind.). Continuous fibers could be used to reinforce plastic as well. Sargent hopes to have thermoplastic composites developed for the market in about a year.

Because of their resistance to corrosion, plastics could play an important part in the hostile environment of mud and water that mountain bikes face, said John Olsen, a mechanical engineer and contributing editor to Mountain & City Biking (Canoga Park, Calif.). The plastics could be used for components in the drive train, which must withstand especially high stresses caused by rides over rough terrain.

Shifting Gears

In addition to new materials, many bicycle makers are experimenting with design improvements. True Temper Sports (Memphis, Tenn.) has developed an aluminum and steel project bike that incorporates new components and engineering concepts.

The company designed the bicycle in collaboration with triathlete/biathlete champion Scott Tinley, who currently rides the bike in races. It uses triathlon handlebars, which place a pad under each elbow and direct the arms into a more aerodynamic position. In concert with a more vertical seat-tube angle, the handlebars put the rider in a streamlined position similar to a downhill-skiing tuck, according to Dave Bahniuk, a mechanical engineer at True Temper.

A novel gear shift system installed on the handlebars lets the rider shift by twisting the grips. The shifters eliminate the need to reach for the down tube to shift, which always disturbs the rider's aerodynamic position. Produced by Sram Corp. (Chicago, III.) the Grip Shift is very light, with only one moving part, a helical cam that converts a rotation of the grip into pull on the cable. In addition to being easier to reach, the shifters allow racers to change gears while cornering, climbing, or sprinting, according to the manufacturer. Sram manufactures the Grip Shift for touring, road, and mountain bikes, as well as triathlon models. Grip shifters could eliminate the thumb fatigue caused by the thumb-operated, handlebar-mounted shifters used on most mountain bikes, said Sram.

The True Temper project bicycle also uses clipless pedals, derived from designs developed for ski bindings, which lock automatically onto cleats on the rider's shoes. To release the foot, riders twist their heels away and up from the bicycle. Several companies manufacture clipless versions of pedals, which are safer than strapped or clipped versions. Oldstyle pedals could prevent the rider from jumping free of the bike if it falls. Shimano American Corp. (Irvine, Calif.) manufactures the SPD clipless pedal and a compatible shoe with recessed cleat. The pedal and shoe, which can be adjusted to provide the correct amount of tension, allow about six degrees of freedom on either side of the foot, freeing the rider to move in three dimensions. A former misconception in bike design, according to Kyle, was that a rider moves only in the plane of the bike. Strapping the foot, therefore, often resulted in many knee problems.

To reduce wind resistance, True Temper's bike uses 24-in, compositespoke wheels, which require larger chainrings than conventional 27-in wheels. The frame eliminates the weight of lugs, while retaining strength in the joints through butted tubing, said Bahniuk. Riding the True Temper bike, Tinley won the 1989 Ironman triathlon in Hawaii and the 1990 world biathlon in Cathedral City, Calif. He said he gained 15 to 20 seconds over his 40-km time on a more traditional model. An aerodynamic riding position and the lightness of the frame were the two main factors that increased his speed.

Another recent design effort, at Hooker Industries (Ontario, Calif.), sought to develop a bicycle with reduced wind drag. The Elite, which is called a "funny bike" because it has small front wheel in combination with a regular-sized rear disk wheel, is a time-trial project bike designed by Hooker in conjunction with Kyle. Its airfoil-shaped handlebars, aluminum tubes, and fork all contribute to the bike's streamlining. All of its cables are hidden within the tubing. Company president Gary Hooker said he was able to shave as much as 75 seconds from his 40-km time using the 16-lb Elite.

To make their bicycles faster and more durable, some manufacturers are looking to one-piece wheels made of a composite material. Composite wheels molded as a single piece transfer more of the rider's power from the pedals directly to the ground than do traditional, many-spoked wheels.

Compositech currently offers composite-spoke and disk wheels made of carbon fiber and Kevlar in an epoxy matrix. The wheels, said Sargent, are more aerodynamic, stronger, and longer-lasting than traditional spoked wheels.

In developing the bicycle wheels, Sargent drew upon his experience over the past decade supplying Indianapolis 500 race cars with composite components. The bicycle wheels come in two versions with weights of 1150 or 950 g. The lighter model is more expensive. "The wheel is designed to fail safely - progressively and predictably," said Sargent. "You could take a pickax, punch four big holes in the wheel, and go off and ride it without a problem."

Composite-spoke wheels are also made by Specialized Bicycle Components. Specialized carbon fiber and Kevlar wheels with aluminum rims were developed in collaboration with the DuPont Corp. The Specialized wheel weighs about 1100 grams, which is about 300 grams heavier than a conventional spoked wheel. However, the company claims that the aerodynamic advantage of the composite model more than compensates for the added weight.

Rocky Terrain

Over the past decade, off-road mountain biking has presented a significant design challenge. These bikes, which have wide tires, straight handlebars, and as many as 21 gears, began to appear in the late 1970s in the San Francisco Bay area. Mountain bikes put the rider in a more upright position than do road bikes. They have wide, durable tires that have lower rolling resistance. Wider tires require lower pressure than thinner tires, so are less vulnerable to punctures. "At first, people took the old paperboy bikes, and used modern equipment derived from BMX (motocross) bikes," said Olsen. These eventually evolved into today's mountain bikes, which now account for about 25 to 30 percent of bike sales, according to Kyle.

Mountain bikes have a different geometry and different environmental requirements than other bikes The frame generally has a smaller head angle, which is the angle formed at the joint of the handlebag stem and the head tube. Head angle of 69 to 72 deg are used for mountain bikes; angles of 73 to 74 deg are common for road bikes. In addition mountain bikes have a lower placement for the top tube, which allows more standover clearance, and their bottom brackets are slightly higher.

Components used in the early mountain bikes were not intended for riding down mountains and through streams. Endurance problems cropped up, and companies started to manufacture shift systems, chain rings, cogs, and brakes specifically for rough terrain. "You're in a horrible, hostile environment in which your derailleurs are constantly breaking off, and your brake pads and rims wear away," said Olsen. "Everything is basically operating in a constant slurry of grinding compound." These problems have not been completely solved even in today's most modern designs.

To address the problem of component wear, Bicicletas Corp. (Taos, N.M.) manufactures mountain-bike chain rings that contain titanium and are also coated with an external layer of it to increase wear resistance and lubricity. Designed to outlast the bicycles that use them, the chain rings retail for $130 for a 24-teeth ring to $220 for a 50-teeth ring. The numerically machined chainrings have an ultimate tensile strength about four times as great as comparable chainrings made of 6061-T6 aluminum, according to Bicicletas. In collaboration with Los Alamos National Laboratory (Los Alamos, N.M.), Bicicletas is currently investigating metal injection molding of these and other parts with titanium.

The Alien ACX, a mountain bike manufactured by Nishiki (a division of Derby Cycle, Carson, Calif.), uses a novel frame geometry and cross section. The aluminum front triangle has rectangular instead of round tubing, which the company claims makes the frame stronger. On the steel rear triangle, the chainstays are elevated above the chain and rings, allowing for easier chain maintenance, and decreasing their chances of becoming clogged with mud.

In Suspense

Another problem posed by rugged terrain is shock absorption. Unlike most road or time-trial models, mountain bikes must be equipped to handle obstacles and jumps. One answer is shock-absorbing components. Dia-Compe Inc. (Fletcher, N.C.), a subdivision of Yoshigai Kikai Kinzoku Co. Ltd. (Osaka, Japan), developed RockShox, a chrome-molybdenum steel fork with oil-dampened hydraulics. The fork is similar to those used for motorcycle shock systems. Although shock absorbers have only recently begun to play a significant role in the bicycle market, the idea is not new: Sir Alex Moulton of England is credited with developing front and rear suspensions for bikes in the 1960s. Along similar lines, British aerospace designer Michael Browning last year introduced a small-wheeled road bike with a hydro-pneumatic suspension on the front, and a compressible rubber plug to dampen movement on the rear triangle.

In the Allsop Soft-Ride suspension system for mountain bikes from Allsop Inc. (Bellingham, Wash.), the seat post is replaced with a horizontal, flexible seat strut that works to absorb shock. According to Olsen, this system allows additional damping through the rider's legs, and could avoid possible resonance problems caused by pedaling. The rider's legs work at about 50 to 100 rpm. Add to that frequency the rigid-body bounce of the rider, "and you could just go boinging down the road," said Olsen.

Future componentry may also include electric shift systems, according to Olsen. One such system, called the Browning Electronic Auto-Shift Transmission (BEAST), was developed by the sports-equipment company Browning (Morgan, Utah). It is currently marketed by Suntour USA Inc. (Novato, Calif.), a division of Maeda Industries Ltd. (Osaka, Japan). The design is very simple, enclosed, and rugged, said Olsen. One drawback of the BEAST, however, is that a power source is needed. "If your batteries run down, you simply can't shift," he said.

Reclining Riders

New components are one way to improve the bicycle, using a different geometry is another. In 1986, Gardner Martin of Easy Racers (Freedom, Calif.) showed what could be done with a recumbent bicycle. His Gold Rush, a recumbent covered with an aerodynamic Kevlar shell, was ridden to a world record for human-powered vehicles at 65,484 mph for 200 m.

The recumbent, which puts the rider in "easy-chair" posture, makes a great deal of ergonomic sense, said Kyle. Because the seat is located about one and a half feet off the pavement, the rider's center of gravity is very low, which enables faster cornering and maneuvering. Because of its reduced frontal area, a recumbent is more aerodynamic than a traditional bike. In the event of a crash, the rider will fall not head-first, but feet-first. In addition, drag will therefore likely be lessened. Kyle pointed out that an upright bike traveling at 20 mph encounters 80 percent of its drag through wind resistance.

Recumbents will become popular in the 1990s, predicted Kyle, though the bulk of the bicycle market will continue to be upright models. Because regulations of the governing bodies of bicycle racing - the U.S. Cycling Federation (Boulder, Colo.) and the UCI, or Union Cycliste Internationale (Geneva, Switzerland) - restrict qualified bicycles to the more familiar, diamond-shaped frame of the conventional bicycle, most of the bikes sold continue to be uprights.

Old Favorite

Even with the excitement over new materials and designs, many professional racers still favor traditional steel frame designs. Many large manufacturers maintain lines of steel bikes because of their popularity. "Steel is easy to machine, and easy to weld," said Merlin's Vandermark. And because frame builders have worked with the material for so many years, they are better acquainted with its strengths, weaknesses, and idiosyncracies.

According to Corso, many cyclists believe that steel frames produced by traditional Italian manufacturers are still the ultimate in bicycling. Family-based companies like DeRosa, Pinarello, and Bottecchia, who have been in the business for well over three decades, have not altered their main lines much in response to American trends. Most Italian companies are supplied by Columbus SPA (Milan, Italy), which has been manufacturing steel tubing for over 70 years, and uses lugged designs for its traditional frames. "Some of the old, lugged frames are like great sculpture," said Bahniuk. They are also of superior quality, according to Corso. He noted that the handcrafting of a brazed, lugged frame would take about 24 hours, including filing and finishing, as opposed to the three hours it would take to join the tubes together via tungsten inert gas welding. A DeRosa frame, for example, requires 14 separate brazing operations, and can cost as much as $3,000 - comparable to a high-end titanium or carbon-fiber bicycle.

In Italy, where bike racing is more popular than in the United States, manufacturers are more concerned with racing performance than cosmetics. On racing bikes, it is the design of the components, rather than the materials they are made of, that is the only thing liable to be considered outdated.

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