The bicycle industry has embraced carbon fibre in the same way that steel once served traditional manufacturers. Renowned for its high strength to weight ratio, carbon has revolutionised the bicycle industry, but how long can a buyer expect a frame to last? CTech editor Matt Wikstrom investigates the answer by talking with three engineers working in the bicycle industry.
Almost three decades have passed since the emergence of carbon fibre and while the material dominates all but the low end of the road bike market, the reputation of carbon fibre still seems marred by the industry’s early efforts. I regularly encounter owners concerned about the robustness and longevity of their carbon bikes while others expect to retire their bikes after just a few years of use.
The first carbon bikes (such as Look’s KG86) were far from robust or reliable. Carbon tubing was bonded to aluminium lugs and while the frames were significantly lighter than their steel counterparts, it was a devastating mismatch of materials. Galvanic corrosion (and to a lesser extent, UV exposure) would defeat the epoxies in use at the time and the frames would fall apart.
Faith in carbon fibre
Carbon composites and manufacturing methods have evolved considerably since the mid-‘80s and independent testing has consistently demonstrated superior fatigue resistance for the material. “The fatigue tests that we run here are almost a matter of going through the motions,” said Chuck Texiera, a senior engineer at Specialized. “We just about never see a failure or fatigue. Once you hit expected ultimate strength, the fatigue is like a gimme. If you subjected any type of metal frame—including titanium—to the same total cycles, typically they would not withstand it. It’s quite amazing.”
When I put the question of the lifespan of a carbon bike to Benoit Grelier, the person in charge of Scott’s bicycle engineering, his answer was clear, “I think it can last your life, actually.”
Scott Nielson has worked with carbon fibre for over a decade, starting with Trek, and is now the vice president of research and development and engineering at Enve. “If you look at carbon materials in general,” he said, “they’re very good in fatigue, much better than any aluminium or steel would be. If done properly, a frame could last you forever.”
The reason for such confidence is the extraordinary durability of carbon fibre. “Composites do not behave like metals,” explained Chuck Texiera. “In fact, they don’t actually fatigue like metals in the same classic sense of the word. The fatigue life of the fibre itself is just about infinite.”
Interplay between carbon fibre and the resin
Strictly speaking, bicycle frames are not made from carbon fibre but a composite comprising the fibre and resin. The result is akin to reinforced concrete, where the carbon fibre acts as reinforcement for the resin. “It is truly a matrix,” said Grelier. “The resin is the material that joins all the fibres together. They have to match each other really well, then you will have a better material.”
It is an understanding of the interplay between the two components of the composite that has developed in recent years, improving the quality of the reliability of carbon composites. A strong fibre is of course critical, but there must be thorough penetration of the fibres by the resin, minimisation of any voids (literally, air bubbles), and complete curing of the resin.
“If the resin is not fully cross-linked,” explained Texiera, “then it compromises its ability to withstand the elements and to also hold up with time. There’s a way we can tell in the laboratory but for the end user it’s really impossible for them. We can do a failure analysis that can actually tell whether the resin is fully cured or not.”
Then there is variation in the starting materials to contend with. For Scott Nielson and Enve, they have found there can be considerable variation from one batch of carbon fibre to the next. “That’s always a challenge with composites,” he said. “We do a lot of sampling, take a certain number of products every month and test them to make sure there isn’t any drift in the materials or the process.”
The weaknesses of carbon composites
While carbon fibre composites have a high strength to weight ratio, they are highly susceptible to high loads over a small area, such as an impact. Once the integrity of the composite is compromised, the matrix essentially starts to crumble and must be repaired or replaced.
In the absence of any impact, the matrix can deteriorate with use, but it takes an extremely long time. “The epoxy matrix will at some point start to form little cracks,” explained Chuck Texiera, “and then over time it will just have the connectivity of the fibre. So really what’s happening, over really extended periods of time, you can expect the stiffness of the frame to change ever so slightly but it’s such a small number. We can measure it but I really wouldn’t think it would be perceivable. But it takes hundreds of thousands of cycles to even get to that. Two years would be far too short for that to occur with any kind of typical age group racer.”
As robust as carbon composites can be, Chuck Texiera pointed out that there are some threats that are easy to overlook. “The greater hazards are just maintenance, people taking things apart and overcooking clamping,” he said. “Also, travel is not very good on bikes.”
The key message here is to avoid any kind of impact or excessive local forces to preserve the integrity of the composite. But what about the environment, especially constant exposure to sunlight? According to Texiera, there’s nothing to worry about. “Most epoxies and resins today are extremely good. I’m sure you could find some place like the moon where really high radiation levels could burn off all the resin. Bikes are typically painted with UVA-resistant paints, even if they’re clear-coated. Then the resins have a certain UV-tolerance as well.”
Improving the impact resistance of carbon composites
The impact resistance of carbon composites has improved in recent years to the point where MTBers are now truly embracing the material. The advances are largely due to innovations in resin technology. “We have the materials that are stiff enough,” said Benoit Grelier, “but the goal is now to work with some materials that have strength in case of an impact. We have had some good results by playing with the resin and nano-components.”
“Standard resin is like oil and nano-resin is like water,” explained Grelier. “If you throw oil onto a mesh, it won’t go inside because it is thicker, whereas the water will go directly inside the mesh. If I use a nano-resin, it will go deeper into the fibres and the final bonding will be better.”
Chuck Texiera has seen the same kind of results. “The fibre is quite good and quite tough and it actually hasn’t changed much in thirty years,” he said. “It’s the resin systems that have continued to evolve, become tougher, to fill in voids and create better bridging. But there’s still room for improvement.”
Scott Nielson agrees. “They’ve been working on nanomaterials for years and now we’re seeing new materials coming out that are taking advantage of some of those nano-enhancement or nano-tougheners. It’ll be interesting to see what happens. I think it’s a good start but there’s a long way to go for those materials to really truly yield a dramatic improvement.”
The influence of cost
The value of a high-cost carbon frame over a low-cost one is normally expressed in terms of weight savings and improvements in performance. But does cost have any bearing on the longevity of the bike? Can buyers expect a high-cost frame to last longer than a low-cost one?
“A big part of the cost is how precise the plies [of carbon fibre] are laid down and how many plies there are,” explained Chuck Texiera. “Usually for a low cost frame, there are a few thick plies, as opposed to a high cost frame where there are a greater number of thin plies. So there’s far more plies doing the work for you.”
Cost also influences the quality of construction as well. “Usually a low cost frame, they are really not that diligent at getting all the trapped air and resin out of the system,” said Chuck Texiera. Thus, a high-cost frame might last longer thanks to higher quality construction and materials, but the current market trend is to satisfy consumer demand for weight savings.
As a consequence, high-cost frames are generally constructed from less material than low cost-frames. “Both of them are really safe,” said Benoit Grelier. “But I would say the lightweight frame has a bigger risk of damage when you have a crash.”
“The best way to get the most out of carbon,” said Chuck Texiera, “is to carry a good balance between a light weight, crash resistance, and longevity. Typically we don’t chase the ultra-ultra-lightweight platforms because, sure you could have this very efficient eggshell design, but if it’s subjected to anything like unexpected forces then that thing goes to hell. That’s really not a responsible place to be as a manufacturer.”
Designing for failure
The susceptibility of composites to impact damage creates enormous potential for catastrophic failure. A quick survey of the Internet will yield a multitude of instances where carbon frames and forks have snapped into pieces during a crash. However, it is possible to prevent such catastrophic failures and protect the rider as well as preserve a brand’s reputation.
“You can have the same product but depending on how you work with the layers you can have different fail modes,” explained Benoit Grelier. “In one case it could collapse and crack into two different pieces and in the other case it could crack but still be in one piece.”
“Anybody can break any product if they tried hard enough,” said Steve Nielson. “Products aren’t indestructible, especially when you’re looking at high end products. Even though we have a minimum requirement, we’re expected to go above that requirement, so we continue to test until failure. And when it does fail, it has to fail in a safe manner. We want it to break in a manner so that the rider will stay upright and won’t get blown to pieces.”
The future for composites in the bicycle industry
By comparing today’s carbon road bikes with the industry’s earliest efforts, it is clear that enormous advances have been made in all aspects ranging from the design and engineering to improved materials and manufacturing. And while carbon bikes still extract a hefty premium, there has also been remarkable growth in low-cost carbon bikes.
So what can consumers expect for the future of carbon bikes? “A lot of research has been done at the university level,” said Steve Nielson, “but a lot of it is still very academic. It’ll be interesting to see how the research translates. I think there are new materials on the horizon and there’s a lot of things that people are looking at to make a composite more ductile. It’ll be interesting to see how the material changes as more and more of it is used in the automotive industry.”
“We are studying some materials which are not fully carbon fibre based,” said Benoit Grelier. “Kevlar could be an interesting player. We did some tests with self-healing composites but it hasn’t moved forward a lot.”
Carbon fibre composites have emerged as a near-ideal material for building bikes thanks to its high strength to weight ratio and the flexibility it affords construction. Where once carbon frames were assembled, now they are sculpted and moulded. Advances in materials have improved upon the impact resistance of carbon composites, and while that Achilles heel still remains, the nature of the materials ensures a frameset that will not deteriorate with use.
Acknowledgements: This article was prompted by a suggestion from Josh Annells. The author would like to thank Benoit Grelier, Scott Nielson, and Chuck Texiera for all of their comments and insight.