Bicycle Frame Construction and Repair

As a new employee in a bicycle shop, you learn that customers will frequently seek advice regarding various types of bicycle repair. Bent or broken frames appear to be a common problem, and you learn that inexpensive bicycles generally have frames made from welded low-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.

a. 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. Since the second fracture occurred in the tube material, not the weld itself, the welder felt that the failure was not related to the attempted repair and that the material in the tubing must be defective.

1. 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?

2. If the tubing had been strengthened by an age-hardening heat treatment, could the same results have occurred? Explain.

3. Is there a better means of repairing the original fracture? What would have been your recommendation?

b. Titanium offers the strength of heat-treated steel at approximately half of the weight. Magnesium, while not as strong as steel or aluminum, is the lightest-weight engineering metal. Would these materials be appropriate for bicycle frame construction? If so, how would they be assembled?

c. If the bicycle frame deflects, motion of the cyclist and related energy are wasted. Therefore, a rigid frame is 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?

d. 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. If you were to assemble a composite frame using fiberreinforced tubing, such as graphite fiber-reinforced epoxy, how would you join the assembly?

e. 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?