Why Carbon Fiber Bike Rims Age So Well

1. Why Carbon Fiber Bike Rims Age So Well Serious cyclists who put in hundreds of miles a week can expect a 3-5-year lifespan on alloy bike rims. If you get beyond five years with alloys, you are doing exceptionally well. But what about carbon fiber? It is an entirely different material. Carbon fiber bike rims age extremely well, to the extent that they might never need to be replaced. Although that seems far-fetched, it is theoretically possible for a set of carbon fiber rims to last a lifetime. It is all about fatigue limit. Carbon fiber's fatigue limit is nearly nonexistent while, for alloys, it is one of the leading factors that determines how quickly rims wear out. How the Materials Handle Energy An alloy is made by combining two or more metallic elements, sometimes along with one or more non-metallic elements. Aluminum 6061 is a good example. It is an alloy that combines aluminum, copper, chromium, iron, magnesium, and other metallic elements with manganese, silicon and other non-metallic elements. This combination creates an alloy that has good toughness and excellent corrosion resistance. While the combination of elements in 7075 Aluminum give it high strength and average corrosion resistance. Bike makers have long appreciated aluminum alloys because they absorb energy very well. A bike with an aluminum frame and rims is actually quite comfortable to ride. However, aluminum's energy absorbing properties are also its downfall. Aluminum alloys absorb energy mainly through compression. This is where fatigue limit comes into play. Even when alloys aren’t moved beyond their flexural limits fatigue sets in and over repeated cycles will shorten the life of the product. All aluminum alloys have flexural limits much smaller than carbon fiber composites. In fact, some aluminum alloys are considered brittle and will fatigue fracture rather easily. For instance, 7075 aluminum is very machinable, but also very brittle, while 5052 is easier to weld and form. Carbon fiber composites have much higher flexural limits than aluminum alloys. Carbon fiber products are designed with specific limits in mind. Fiber orientation allows the designer to optimize the design for strength, stiffness, damping, weight, or any combination of these attributes. Components made from carbon fiber composites are superior at vibration damping, reducing energy loss by allowing the material to return to its natural state quicker than aluminum alloys. However, it is not through compression. It dampens vibration through deflection. Thus, there is no fatigue limit involved.

2. This theoretically means that carbon fiber bike rims can last forever – just so long as they suffer no internal delamination or fractures due to events that are outside the design limits of the parts. Riding is Rough on Rims It goes without saying that regular riding is tough on bicycle rims. It is equally tough on forks and frames as well but let us stick with the rims for now. Every bump, every pothole, and every sharp turn around a corner puts stress on rims. How those rims respond to stress affects their overall wear. The major advantage alloys bring to the table is that, when they fail, the failure doesn't tend to be catastrophic. In other words, your aluminum rim will bend before breaking in two. It will warp rather than shatter. In all likelihood, you would be able to spot serious fatigue in an alloy rim long before it became a problem. Carbon fiber is just the opposite. It is superior to aluminum alloys at damping vibration. Carbon fiber also stands up to a tremendous amount of punishment. But when it fails, the failure is catastrophic. So why take your chances with carbon fiber? Because catastrophic failure is unlikely with normal riding and maintenance. Any event that would cause a carbon fiber rim to fail will also cause an aluminum alloy rim to fail. As with aluminum alloy rims, you should routinely inspect your rims and have them looked at by professionals when something looks out of sorts. As long as there is no internal damage, your carbon fiber rims will last forever. They age very well because they are not subject to fatigue limits. If you would like to know more about using carbon fiber to build bikes, give us a shout.