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This is a hotly debated issue with all sides to the debate either emotionally or financially invested in the outcome, but what it really comes down to is safety.
Designers and engineers prefer to utilize carbon fiber whenever economically feasible because it is five times stronger than steel, twice as stiff, but weighs ⅔ less than steel. Carbon fiber is made up of thinner than human hair strands of carbon woven into a carbon cloth. Unlike steel and other metals, including aluminum, carbon yields itself to molding of highly aerodynamic shapes normally unachievable with other materials. The biggest use of carbon fiber is in the aerospace industry so much so that the fuselage of Boeing 787 Dreamliner airplane, designed to carry 335 passengers, is made out of carbon fiber.
This material allows for weight savings while still providing strength and rigidity. The use of carbon fiber to build bicycle frames means the frame is light, rigid, aerodynamic and strong. The only caveat is that unlike steel, aluminum or titanium, carbon fiber is prone to shattering when hit with a sharp object. Furthermore, the effect of a rider falling on the thinner parts of a carbon fiber bicycle can create devastating damage to the frame.
Once damage occurs, the question arises whether the damaged carbon fiber bicycle frame be returned to pre-accident structural integrity? The answer is possibly.
The repair of a carbon fiber bicycle frame depends on the area of the damage and the extent of the damage. At some point the economics of the repair might exceed the cost of replacing the frame. If carbon repair experts determine that a repair is economically feasible and can be guaranteed to be returned to pre damage strength, then why shouldn’t the frame be repaired?
Airlines don’t scrap a plane because a carbon fiber wing is damaged, America’s Cup challenge boats aren’t scraped after a collision, and amputees don’t discard their prosthetics when they get damaged. Of course each of these damage scenarios is evaluated by experts and the determination of whether to repair or replace is made based on the safety of the end user.
Above logic can be applied to carbon fiber bicycle repair. If a top tube is damaged and evaluated by the experts as repairable to its initial strength and integrity, then an undetectable repair should be acceptable to the bicycle owner. Riding a repaired frame is a personal choice so if the repair facilily can not provide quality assurance guarantees that you’re comfortable with, seek a second opinion or reconsider the repair.
A rarely asked question is what happens to all those carbon frames that can’t be repaired? The dirty little secret of carbon fiber is that it has the half-life of plutonium, is neither biodegradable or photodegradable and cannot be recycled to produce the same strength qualities of the original product. Some steps are being made into how to use carbon fiber as a recycled material but this is still in the research stage with some promising possibilities.