Bicycle Frame Construction and Repair
As a new employee in a bicycle shop, customers frequently seek your advice on a number of matters. What type of bicycle is best for them? What material is the bicycle made from? How has it been manufactured? And can it be repaired when it has been damaged? One of the key components of a bicycle is the frame—the backbone of the bike—which provides the necessary strength and stiffness. Early frames were made of wood or cast iron, which transitioned to steel tubing; then to thinner-wall and higher-strength steel tubing; and now to aluminum, titanium, carbon-fiber, and other exotic materials. In addition to strength and stiffness, customers also want their bike to offer light weight and a smooth ride—properties that often must be compromised with the original strength and stiffness. Steel is the most common bike frame material. Carbon steel is the cheapest but also results in one of the heavier bike frames. Chrome-molybdenum steel (alloy steel) can provide the necessary strength with thinner tubing and has become a popular frame material. Aluminum is widely used to provide light weight and adequate strength, coupled with good corrosion resistance. Its fatigue resistance is poor, however. Titanium offers the strength of steel with a weight between steel and aluminum and good corrosion resistance but at a significantly higher cost. Carbon fiber can produce one of the lightest frames, but the composite is brittle and the design must accommodate this feature. Bent or broken frames appear to be a common problem, and you learn that inexpensive bicycles generally have frames made from welded low- or medium-carbon steel tubing. This material can be heated to straighten and is easily repair-welded using a wide variety of welding techniques. As you move to the more costly, lightweight, or high-performance models, you learn that repair is generally not quite as simple.
1. In one case, the frame of a high-quality, lightweight bicycle had been fabricated from aluminum alloy tubing, with joints made by adhesive bonding of the tubes to connectors that incorporate either internal lugs or external sleeves. When the frame fractured near one of the joints, the owner contacted a local auto body shop and requested that a repair be made using conventional gas tungsten arc welding. The welder was familiar with the welding of aluminum, and the repair seemed to be of good quality. Shortly thereafter, however, the frame broke again. This time the fracture was adjacent to the repair weld and the characteristics of the break were different. While the first fracture was somewhat brittle in nature, the second appeared to be more ductile, with evidence of metal flow prior to fracture. Because the second fracture occurred in the tube material and not the weld itself, the welder felt that the failure was not related to the attempted repair and the material in the tubing must be defective.
a. If the material had been cold-drawn aluminum tubing (i.e., strain hardened), explain what may have occurred during the repair. What is the probable cause of the second fracture? Was the weld in any way defective? Was the second failure related to the welding repair?
b. If the tubing had been strengthened by an age hardening heat treatment, could the same results have occurred? Explain.
c. Is there a better means of repairing the original fracture? What would have been your recommendation?
2. Magnesium, while not as strong as steel or aluminum, is the lightest weight engineering metal. Would magnesium be an appropriate material for bicycle frame construction? If so, how would the frames be assembled?
3. If the bicycle frame deflects, motion of the cyclist and related energy can be wasted. Therefore, a rigid frame may be quite desirable. Beryllium is an extremely rigid, lightweight metal. Could it be used as a material for bicycle frames? How would you fabricate the tubing and assemble the frame? How would cost compare to steel or aluminum?
4. Titanium offers the strength of heat-treated steel at approximately half of the weight. How would you propose to join the tubular segments of a titanium bicycle frame?
5. Composite materials can be used to produce tailored sets of properties. Fiber-reinforced composites can have extremely high rigidity in the direction of fiber orientation, coupled with extremely light weight.
a. If you were to assemble a composite frame using fiber-reinforced tubing, such as graphite fiber reinforced epoxy, how would you join the assembly?
b. The strength and stiffness of carbon fiber can be extremely directional. Maximum strength can be provided in directions of maximum stress. Bicycle frames, however, often experience a variety of stresses in different directions. Consider the properties in directions transverse to the fiber orientation. Are they equivalent to alternate materials or would they be compromised?
6. Premium quality racing bikes (such as Tour De France models) have used one-piece carbon fiber composite frames (i.e., no joints at all!). What is the benefit of such a design?
7. Numerous joints appear in bicycle frames, some of which may involve dissimilar materials. Consider each of the following possibilities and discuss possible joining methods and the properties of each.
a. Aluminum to aluminum.
b. Carbon fiber-to-aluminum lugs.
c. Carbon fiber-to-carbon fiber.
8. Some adhesive-bonded joints may require a thermal treatment to complete the cure. Welded or brazed frames may benefit from a stress-relief heat treatment, or a complete reheat treatment to establish a uniform set of properties throughout the joined assembly. What materials would benefit from a subsequent heat treatment? What concerns might you have regarding these processes?
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