Until only a few years ago, pretty much all bikes were made of iron, steel, or titanium – relatively common metals and alloys that were already being used in the manufacturing industry. But all of a sudden that has changed, and rather rapidly. The bike you ride today is more than likely composed of carbon fiber. But how much do you know about this material? Stefan Christ and his team spend hours conducting research and experimenting with it, this most resistant of materials, losing sleep in a never-ending quest to find new ways to improve how it is used to build BMC’s bikes. We caught up with him to quiz his brain about the most talked about material in modern bike construction.
Stefan, we’ve heard the term ‘elastic modulus’ being used a lot recently when it comes to carbon fiber—what does it mean?
The elastic ‘modulus’ of carbon fiber indicates the level of elasticity of the material. It can be classified into two, well-known families: High resistance (low modulus, compliant, and strong) and High Modulus (very stiff but brittle). A good frame will always be made of an appropriate mix of the two types. No matter what marketing departments claim, a fully modulus frame is not rideable, way too stiff, and far too fragile to be of any real use.
What’s more important: the type of carbon used or the layout of the carbon?
Those two parameters can’t be separated and there is a crucial third factor which is shape and design. We only need to look to Mother Nature to see that material and structure cannot be dissociated. Think of all the different kinds of wood, minerals, plants etc. To build a bike frame, all three parameters need to be taken into account, neglecting just one of them leads to catastrophic results. BMC’s ACE project goes a long way to demonstrate our efforts to optimize all three factors simultaneously.
A lot of riders are still very concerned about the supposedly limited lifetime of carbon products. Should they be? What’s the lifespan of a carbon frame?
Carbon composite is clearly a delicate material because its reaction and performance is not predictable when impacted. But what people rarely understand is that the absolute strength properties of carbon composite are actually higher than those of aluminum alloy, and more importantly, it does not fatigue in the same way as metals. So, actually, the lifetime of a carbon frame is almost infinite, as long as it doesn’t sustain great impact or stress that forces it beyond what the product has been designed for (the first rule being, always assemble and apply the torque that has been recommended by the manufacturer). In contrast with carbon frames, the lifespan of a metal frame owned by an avid cyclist generally extends to one or two years before showing significant losses in stiffness and resistance due to fatigue of the alloy.
In any case, whatever the material and after any serious crash, we recommend you bring your bike to your dealer to inspect the frame for any structural damage, which is not often visible from the outside.
Nanotechnology and new materials? Are there any really interesting alternatives to carbon fibre out there at the moment?
Of course we are always keeping an eye out for new technologies that have the potential to increase the performance of our frames and components, and we are studying several at the moment. But not much of the new stuff can be called game changers; it’s more about refinement and optimization. We are lucky to own our own carbon prototype lab in our head office in Switzerland so we can test virtually anything we like. Just to give a small example, we recently did some tests incorporating organic fibers into carbon composites…more on that later!
Ok, but is there a ‘better’ carbon material out there on the market?
The production of carbon material nowadays is driven by bigger industries like aerospace and military and we feel that we have reached a standstill. It is hard to identify a ‘magic’ fiber that outperforms all others. Yes, you can use some reinforced resins or very fine pre-impregnated layers, but they are available to the entire industry. More than the material itself, what today makes the difference is the design of the product, and the application of the right ingredients to the right places, in the right quantity, etc. Manufacturing processes are also improving but at the same time, we are expecting some big breakthroughs that will improve performance or reduce costs.
What is the ‘K’ number and why is it important?
The letter ’K’ is an industrial unit used to describe the size of a carbon fiber thread. K stands for Kilo. A 3K carbon thread contains 3,000 single fibers. The bike industry commonly uses 1, 3, 6, and 12K threads, to stay within a realistic size scale.
Threads made of thousands of fibers are usually weaved (twill) or plainly laid out (unidirectional) into sheets or, directly braided into a shape as with the BMC impec tubes. There is no direct link between the K number and the performance of a frame, it is not important as such but it can help to improve the frame performance if used expertly to reinforce specific areas. More often, is it used to improve the bikes cosmetic appearance.
Carbon is often linked to aerodynamics. How does it stand up to compared to metal in terms of shaping?
Tube shapes, such as the ones used to build bike frames, are linked to performance, together with material and layout. To make a good frame, you need to master all three factors and find the best shape so that the frame will ‘behave’ and respond in the desired way. By layering carbon we have more flexibility to form particular shapes – even the latest metal forming processes cannot match that.
What are the differences between the various construction styles like ‘monocoque’ and ‘tube to tube’?
100% monocoque frames just don’t exist.
From a performance standpoint, a monocoque construction style should theoretically yield better results, because the fibers can be laid out in an organic and seamless manner. But from an industrial standpoint, it’s not possible to build one-piece fully-monocoque frames. So the current standard construction uses a monocoque front triangle connected to two or more rear triangles, each produced as monocoques too.
Tube-to-tube is another, more simplified process. It is less sophisticated because the fibers are interrupted from one tube to the other. It can be compared with welding because the tubes are first bonded together with glue, and then wrapped with fibers at the joints to reinforce the connections between each tube. It’s a very flexible process that requires fewer tooling, and is especially appropriate for tailor made and prototype fabrications.
The lugged frames (made of tubes connected by shells or lugs) are less used today. This construction was the first to appear 20 years ago when high quality carbon lugs were difficult to produce. At the time, it enabled the mixing of materials and the use of metal parts where appropriate. This technology has evolved today towards the current standard of bonding and joining of several monocoque parts.
It makes sense to certain point, to use highly-modulus material to create monocoque parts and tubes to provide stiffness, but all connections are best made with highly resistant material.