The bicycle fascinates me so much because with it we can illustrate just about every fundamental concept of Materials Science & Engineering. The purpose of this page is to provide an overview of some of the metallurgical and processing aspects of the bicycle spoke which I investigated as part of a metals processing and properties course at the Department of Materials Science & Engineering, Lehigh University.

For this study I chose two of the most "advanced" high performance spokes on the market. The first spoke is a DT Revolution* swaged stainless steel spoke (2mm/1.5mm). I also examined a Union Ti-dye^**Titanium spoke. Although I could not cover all the topics I wanted to in this web page, I would be happy to answer any questions or hear any suggestions/comments readers of this page may have. Also let me know if you have any other ideas for other materials-related topics I could discuss on this page.

-Rafael Raban (raban@andrew.cmu.edu).

Drawing and Annealing

Drawing of spoke wire involves a series of draws through decreasing diameter dies until the final net diameter of 2mm is achieved. Work hardening in Stainless Steel spokes (Figure 1) is evidenced by deformation twins (red pointers) which represent a common mode of deformation in FCC metals such as austenitic stainless steels (e.g. 304 Stainless). Likewise, annealing twins (blue pointer) show that the spokes were annealed to "soften" the material at a stage of the drawing process thereby preventing internal tearing commonly known as "burst."

Drawing of Titanium spokes also requires annealing. Ti/Al/V alloys are annealed above the recrystallization temperature in the alpha-beta region of the ternary phase diagram (~925 C isotherm). This assumption is evidenced in Figure 2 by beta phase precipitates that appear as outlined particles within the alpha matrix (Metals Handbook, vol. 9, 9th ed.).

Swaging

Swaging is a process used to thin the midsection of a spoke thereby decreasing the weight and enhancing spoke elasticity. A two piece ovalized die (shown below) compresses and releases the spoke as the spoke rotates. A sizable reduction of 25% is achieved using the swaging method. Associated with the formation of a "spiral" texture in Figure 4 is a loss of ductility due to strain hardening. Hence, severe cracking is shown in Figure 3. Such cracking, however, is not critical since this region of the spoke rarely fails in normal use. Given the crack's orientation with the tensile axis, it is also unlikely to propagate.

Cold Heading

Cold heading is a forging operation by which an end of the wire is pressed into a die. Benefits of low temperature deformation include strain hardening and increased fracture toughness through mechanical fibering as shown in Figure 5. By contrast, the Titanium spoke (Figure 6) lacks texturing which suggests that it was annealed at some point after the head forging. Given Titanium's high strength in comparison with Stainless Steel (~900 MPa vs. ~600 MPa), strengthening via strain hardening is not necessary. If fact, too much strain hardening would result in a brittle material susceptible to fatigue failure.

Thread Rolling

Thread rolling is a cold work shaping process used to form the spoke threads. Figures 7 and 8 show mechanical fibering oriented perpendicular to the loading axis (horizontal). Such texture impedes fatigue crack propagation -- the most common cause of failure in bicycles spokes.

While the microstructure of the Stainless Steel spoke in Figure 7 is defect-free, Figure 8 reveals several cracks in the thread valley and crest of the Titanium spoke. Cracking may be attributed to poor die design, inadequate lubrication during the rolling process, and/or material properties such as formability. Typical Ti/Al/V alloys, for example, exhibit roughly 1/4 the elongation of 304 Stainless Steel.

Acknowledgments

I would like to thank Arlan Benscoter, director of the Light Optical Microscopy and Metallography Laboratories at Lehigh for sharing his expertise in sample preparation, etc. I would also like to thank Professor David Thomas for his advice and consultation.

copyright 1997 Rafael Raban.

(please do not copy or plagerize the information on this page, not only is it highly unethical, its also illegal)

* "Revolution" is a registered trademark of DT.

** "Ti-dye" is a registered trademark of Marwi USA.