The old saying “a chain is only as strong as its weakest link” has an analog in bike frames, and on metal bikes often times it is the area around the right rear dropout. Millions of bikes don’t fail at the dropout, but I’m never surprised when I see one break. Fatigue failures basically require two things to occur: a cyclical load on the structure and a focal point where the stress can overwhelm the strength of the material. A bicycle frame is a deceptively complex problem to analyze stresses, with a huge number of variables that are difficult to quantify. I suspect that the vast majority of dropouts in the history of bicycle design were designed with aesthetics and intuition in mind rather than detailed stress analysis. Accumulated experiences would be incorporated into new designs. By now, you’d think that the dropout would be perfected, but manufacturing methods, frame styles, and drivetrain components have continued to evolve. Setting aside the carbon fibre frame construction and the rapid onset of thru-axles, dropouts on metal bikes still fail too.
This first photo shows a fixed-gear/single speed bike with a fractured dropout. It is a steel bike from a somewhat niche yet inexpensive brand, a brand that has the wherewithal to have its own dropouts made rather than buy generic frame fittings. The brand’s marketing taps into the everyman appeal of traditional steel framesets, which is made accessible through the economy of Taiwanese manufacturing. The dropout design incorporates a practical integral axle tensioner bolt and stylized cutout windows beside the chain stay and seat stay attachments that harken to the delicate embellishments the framebuilders of yore added to forged or stamped fittings with much hand-filing labour. But these windows are not added by hand, nor is the dropout is likely stamped or forged. Most likely is investment cast, since modern manufacturing methods allow near-net shape items to be quickly developed and economically produced. However, cast steel parts frequently have less strength than forgings. In all likelihood, the strength of the material was just not sufficient with the windows removing so much metal in the area. Interestingly, the left dropout partially fractured in the exact same place. Basing statements on anecdotal evidence can be shady business, but I did see two more bikes with broken dropouts like this not long after. And current production of this model eschews the window cutouts adjacent to the chainstay, suggesting that the manufacturer felt compelled to change the design.
The picture below shows a dropout that cracked right beside the chain stay. The dropout design seems rather conservative, but this particular failure was actually one of six to eight that I personally saw. Most likely these dropout failures occurred because either the metal used in the dropout had some sort of metallurgical defect affecting the strength and/or microstructure, or the area was brazed too long or too hot such that the dropout material weakened or became embrittled. I’m inclined to suspect the material issue rather than a manufacturing error, since the factory has a very long history of making traditional lugged steel bicycles sold under their name and as well as other brands.
The next two photographs show a titanium frame from a well-known manufacturer. This particular bike is a discontinued flagship model that showcased all their weight-saving techniques. But at a certain point weight-saving becomes a tricky game. First costs go up as you use more expensive materials and methods, and then you’ll have to knowingly sacrifice strength and/or durability to remove material mass. That’s probably what happened here. The seat stays on this frame are crazy light; I’ve seen bubble tea straws that were stouter. Also, the seat stay seems to be swaged down to a point rather than using a bullet. You pretty much can’t do that unless the titanium tubing has been annealed to lower yield strength to avoid cracking during the shaping process. And since resistance to fatigue is proportional to material strength and with the tube walls being so whisper thin, this is probably a case of too weak or too little material.
In this particular case, a person brought the bike to show a local titanium framebuilder to see if it could be repaired, but there was no way to economically repair. I don’t remember why the person couldn’t address the problem to the original manufacturer, whether it was because it was out of warrantee or if he wasn’t the original owner, but he was understandably disappointed. One often hears that “titanium lasts forever” but the truth is more complicated than that. Good design and manufacturing are more important than material alone.
This last photo depicts a titanium dropout from a recently defunct framebuilder. With nowhere to address a warrantee claim, three different owners brought bikes in with the same problem within three months to see if DKCBikes could execute a solution, so I am confident that this has been an issue elsewhere for the bikes using this dropout. I am not sure if the shape of the dropout is the issue or the material. Though I kinda like the style, which is something of a cross between a hooded and plate dropout, I wonder if the perhaps the section wasn’t thinned out too much. Alternately, perhaps the edges of the dropout should have been rounded more to reduce the likelihood that they act as a stress riser. Lastly, perhaps the particular alloy of titanium was not of sufficient strength, instead being chosen for ease of machining.
Dropouts seem so simple, but if that were true you would think that after 150 years of bicycle evolution failures would be unheard of. Up till now, I think most of that happened when frame manufacturers either get too ambitious in weight-reduction or too clever in making the dropout look cool or unique. From here on out, the requirements of disc brake mounting and adoption of thru-axles are going to burden designers in ways that were never before considered.