The price of carbon fibre bikes has dropped dramatically over the last few years. What was the preserve of a lucky few on the club run has become the norm thanks to companies like PlanetX and Boardman making good quality, but affordable models. With the majority of big name bike manufacturers choosing Chinese factories to produce their frames and components, these factories have quickly become experts in their construction and pulled the prices down as the rate of manufacture has been scaled up.As carbon bikes have flooded the market, so it seems has rumour, misinformation, and a fair amount of fear mongering. If you look on any cycle forum or Facebook group there will be discussions about the ‘structural integrity’ of carbon bikes.
There seems to be a perception that carbon fibre is like an egg shell. That the slightest knock or bash and that’s it. The structural integrity is gone. Unseen cracks have formed, hidden below the surface, which are going to silently grow, and when you least expect it the frame will break. It might not look or feel broken, but somehow it is. Can this be true?
Carbon fibre is used extensively in high-tech industries from aerospace to formula one to SpaceX. It is chosen for its incredible strength, durability and low weight. All fantastic properties for building quick bikes.
Is your carbon fibre frame really a ticking time bomb, waiting to fail spectacularly and unexpectedly while you are riding it? Is Carbon fibre really as fragile as some people seem to make out, and if not, where is this perception coming from?
Perhaps we are stuck in an old mindset. Bike frames made from steel and aluminium all have their own characteristics, but they also have similarities in how their material properties change over time. All three are metals after all.
As metals are flexed and compressed, the areas exposed to these forces become slowly work hardened. Areas of a bike that undergo more stress -around where the down tube meets the bottom bracket for example- become harder and less flexible than areas which experience less stress. Over time, hardened areas can develop small cracks through the material which spread and join to form larger cracks, and eventually the part will fail.
Many cyclists will have experienced this at one stage or another, the day their trusty alloy steed needed to be retired due to a crack across the base of the down tube.
Carbon, however, is not like steel or aluminium in the way it reacts to stresses because it is not a metal. It is a composite material. Carbon frames can break certainly, and we’ve seen more than a few torn, crushed or punctured tubes come through our office, but the method of failure is different.
I spent a good afternoon surfing the internet trying to find examples of carbon frames or forks which had snapped suddenly while being ridden due to an unnoticed fault or previous unseen damage. While there were plenty of people talking about it, and swearing blind it’s a possibility, I didn’t come across anyone claiming that it had happened to them. Not even a ‘my cousin’s friend’ anecdote. There were plenty of examples of carbon bikes that have smashed, splintered and crushed on impact, but that isn’t the scenario I’m looking at. What I did find, was examples of where carbon and metal components which were bonded together had separated.
This is what we saw a few years ago which lead to the mass recall of Kinesis race light forks. Their design involved moulded carbon parts bonded with resin to aluminium parts, forming an inevitable weak link. A design flaw and problems with manufacturing had lead to sudden catastrophic failures while riding. The carbon pieces themselves however, remained undamaged. Perhaps it is scenarios like this that the perception of ‘weak carbon’ is coming from?
When carbon breaks it does so with a tear, crush or puncture. Carbon does not develop small cracks which could fail later like a steel or alloy frame might, by nature of it being a composite material.
The Romans worked this out when they were experimenting with concrete. The cement on its own is a very hard material once it sets. It is resistant to scratching, but has no flexibility. On its own, the cement will quickly develop microscopic cracks through its body which will join into larger lines of weakness along which the material will fail.
What made the Roman’s concrete so strong was the introduction of impurities. Shards of broken pottery, bits of gravel, and other detritus.
When tiny cracks form inside the very hard cement, they expand until they meet an impurity, then stop. The cracks don’t have the opportunity to join and grow, and so the integrity of the material remains intact. This is how they were able to build huge, complex concrete structures like the dome on the Pantheon, and it still be intact two thousand years later.
Like the concrete, carbon fibre is made up of a very hard but brittle material, the resin, and an incredibly strong but flexible material, the carbon fibres. Together, the properties of the different materials support one another. The resin locks the fibres in place, giving the composite rigidity, and the fibres prevent the propagation of cracks in the resin, giving the material strength.
We have taken in plenty of torn, frayed and splintered carbon frames, forks and wheels, but never have we had a sudden failure or a customer’s bike. From our experience and looking at the properties of the material, sudden failure isn’t a problem.
Where a carbon tube has been crushed on an aeroplane or torn on the road in a crash, this damage causes a weakness. If you continue to ride, the stresses are no longer distributed evenly along the tube which causes it to twist. The split or puncture grows turning into a tear, and if, for some reason, you are still riding, eventually the tube will break in two. The upside, this process isn’t sudden or catastrophic. It is slow, noticeable, and the sort of damage which could lead to this circumstance is obvious.
I came across a picture someone had posted in a Facebook group for my local club. It showed scratches in the gloss coat over the paint which covers the carbon on their bike, and they were asking if it was safe to ride. It seems a shame that a cyclists can be so unsure of a material, that once the showroom finish begins to wear off, they no longer trust it.