Aluminum bicycle frames are not as strong as steel, titanium or carbon fiber, making them vulnerable to certain types of failure. In addition to supporting the rider, aluminum frames have platforms that support the bike's components. Various kinds of frame failure can result from impacts or improper maintenance.
Frame Geometry
If you stripped a bicycle down to the bare aluminum frame, you'd notice its geometric configuration. A closer look shows that the steering headset and the bottom bracket are the primary connection points of the upper and lower frame tubes. Aside from this basic configuration, road bikes, BMX and mountain bikes often have dissimilar geometry with regard to the rear frame design. With the exception of molded carbon fiber frames that have no welds, the connections on all bicycle frames are welded, and in some instances, the welds on an aluminum frame can be compromised.
Cracks and Bends
Bicycle frames are designed to flex, which reduces stress at the connection points. Hitting roadside curbs or hard off-road obstacles challenges the welded connections. and in some instances, cracks can develop. Fine hairline cracks in a weld can be hard to spot even with the closest of inspections. It's possible to continue riding a road bike without incident, but cracked welds on a mountain bike frame could result in a complete failure of the weld upon impact. In addition to the connections, aluminum bike frames are vulnerable to bends form spills and certain types of impact. Even though a bend might not be significant, it might affect the frame alignments that are critical the bike's performance. You could opt for a bike with a thicker aluminum frame that's less likely to bend. But a thicker frame cannot compensate for platforms that have not been properly maintained.
Platforms
The steering headset is a critical platform at the front of a bicycle frame. The front forks attach at the underside of the headset, and the handlebars attach at the top. Steering headsets have threads for nuts that secure each attachment, and internal bearings that require periodic lubrication. If the nuts are cross-threaded or the bearings are dry, a front wheel impact could cause the fork stem to dislodge and damage the headset, resulting in frame failure. The bottom bracket shell at the base of the frame tubes and the rear frame arms are the primary platforms for drive system on your bike. Bottom bracket shells are thicker than frame tubes, and failure at this part of the frame is rare. The rear frame arms have dropouts for attaching the rear wheel axle. The frame arms and dropouts can fail after certain types of spills and impact.
Observations
Compared with other types of metal bike frames, aluminum has relatively low fatigue endurance in terms of resisting bends and maintaining the integrity of welded connections. Many aluminum bicycle frames are designed without suspension because the manufacturer believes this adds fatigue endurance. Other manufacturers use thicker tubing, and the result is an aluminum frame that weighs more than a steel frame. The tensile strength of aluminum tubing is comparable to other metals, but most frame failures are the result of fatigue from impacts or improperly maintained attachments.
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