“Wow, it’s a lot lighter than it looks!” says my non biking friend, looking impressed, as he hefts my full bouncer with one hand.
“That’s because the frame, cranks and wheels are made of carbon fibre.” I nod sagely, looking slightly smug.
“Amazing! I didn’t think you could make mountain bikes out of carbon fibre. I thought they’d break.”
“Er, no, its, um, stronger carbon fibre than in road bikes, so they can, you know, use it for mountain bikes and stuff.”

I’m getting a bit out of my depth here.
“Oh, right. So how do they do that then?” He knows he’s got me.
I suddenly find that I’m terribly busy and have to take a phone call.
Then I do some research.
Amazing, if you think about it, how little we know about the frames that we ride around on. What they are made of, how they are constructed, why they don’t just break, why they aren’t just like coke bottles (because it’s all plastic, right?). It is time, my friends, to right that wrong. Welcome to the dark arts of carbon fibre frame production. Welcome to a new level of understanding.

When we talk about carbon fibre, we should really be talking about carbon fibres. That’s right. Each carbon frame contains thousands upon thousands of individual fibres made up of tiny carbon filaments, woven together into strands that we call carbon fibres. Not only that, but manufacturers don’t just use ‘pure’ carbon fibre, but introduce filaments of other fibres (glass fibre, aramid, Kevlar) to strengthen, change characteristics or increase impact resistance and then stick the whole thing together with resin, so it doesn’t unravel. Almost all carbon fibres are, in fact, composites of materials, blended together to create the ideal fibres for the job.

Once you have these, the next job is to decide how the fibres are turned into actual products. There are a number of options facing bike manufacturers (and indeed anyone working with carbon fibre, as there is no such thing as ‘cycle specific carbon fibre’ – no matter what certain manufacturers might tell you). The main techniques are:
Bladder moulding (including foam core moulding) – The carbon substrate, as yet unformed and semi-liquid, is put into a pre-formed mould (usually steel or aluminium) which is then closed around an inflatable bladder. This bladder is then inflated and the carbon forms into shapes under the pressure of the moulding. The bladder is then removed through a pre-set point (often at the head tube) to leave a monocoque in place. The foam core moulding operates in a very similar manner, but the foam is heated to cause expansion. With foam core moulding, the resulting monocoque still contains the foam, which can help with vibration damping, strengthening of key areas, impact resistance and even frame rebuilding in the event of damage. In both cases the frame must be ‘cured’ by applying the right combination of heat (around 220◦)Celsius and pressure(usually about 150psi or so) until the carbon has set to the mould.

This process can be used to create whole monocoque frames, monocoque halves or even frame sections. These can then be bonded together to form a finished product in the shape of the manufacturer’s choosing and metal parts (linkages, BBs threads, brake boss threads) can be bonded in afterwards. Again, this requires more heat and pressure.
Roll Wrapping & Filament Winding – As a kind of inverse to the bladder/foam process, the approach involves wrapping the sheets or individual carbon fibres/ filaments around the metal mould or ‘mandrel’ and then applying the pressure heat combination externally using heat shrinking tape. The tape will often apply slightly unevenly to the exterior, but these inequalities can be sanded away once complete to give a totally even surface inside and out. Again, this process can produce complete frames, or individual parts that can be bonded together afterwards. This is quite similar to the way in which steel, alloy or ti frames are made, in that the tubes and stays are preformed and then welded together. Instead of welding, however, the segments are bonded using heat and pressure, so much heat and pressure. 

So what works best? Well, as with anything in this world, it’s a combination of costs and benefits. Bladder and foam forming are cheaper and faster than wrapping. There are excellent mass production methods and produce very similar outcomes time and time again. There are disadvantages, however. While the external finish is consistent and needs very little attention, hidden away inside the frames/ sections there may be inconsistencies that are externally invisible. In most cases this will have no effect upon the frame performance, but might shorten the ‘life’ of sections or components (it is a myth that carbon fibre has a shorted warranty life than metals, it is all a quality issue – better carbon/ metal lasts longer than less well made stuff). The worst case scenario, however, is that there are weak points in the frame which cause higher levels of vibration, flex more easily or even fail unexpectedly. That said, most bladder/ foam processes use established moulds, have excellent quality control and suffer very low failure/ inconsistency rates. If you want a ‘quality check’ mark, road bike builder Parlee, whose bikes frequently retail for upwards of $20,000, use bladder moulds to form their tubes.

In the case of wrapping / winding, the process is much, much slower and significantly more expensive. The level of expertise needed to carry out the process effectively, is far greater and wastage is higher too. Wrapped or wound frames will necessarily cost far more than their moulded counterparts and it may be impossible to find any difference, as a mere rider, in terms of weight, vibration damping, appearance or performance. This is especially the case in full suspension frames, that make up the bulk of the carbon bikes that we sell at Biketart. The characteristics of the suspension design, fork & shock quality and chunky tyres would mask any difference in frame quality/feel, unless there was very serious flex indeed. The pay off, however, is in the quality of the finished product. There are no weak spots, no flats and the external appearance can be more precise and individual than a mould created frame…at a serious price.

Having said all of that, there is also the fundamental issue of raw material. The individual fibres of carbon are, as mentioned above, held together by resin and the vast majority of manufacturers take delivery of their carbon fibre pre-impregnated with resin (also known as pre-preg) in small sheets that can then be used in one of the methods above to create their frames & parts. The number of carbon frame builders who take raw filaments and impregnate it with resin themselves is tiny and, to be honest, only available on road frames which cost more than any of our bikes, so we’ll leave that and focus on bikes made using pre-preg. Either way, that carbon is going to come from one of just five carbon fibre manufacturers in the world. That’s right – five. So the bike companies have to choose one these from which to order their pre-preg carbon sheets (usually about 10cm squared).

The thing is, not all pre-preg is the same. Its not that there is one manufacturer of carbon that is better than all the rest, more that they all make multiple grades of carbon fibre and sell them to whoever wants to buy them, at prices to match the quality. The pre-preg will come in a variety of modulus options, with an equal variety of degrees of operation (0, +/-30, +/-45 and +/-90) for different purposes. The higher the modulus, the more expensive the pre-preg and, to be honest, the more of a pain it is to work with. The degrees don’t quite work in the same way, but essentially, higher degree are more impact resistant and flexible the pre-preg (but that is a massive oversimplification and we would need at least one more complete blog to explain it properly). Thus, 0 degree is usually used for the parts of the frame that will not get hit, but do need to be highly resiliant (seat stays, joins, linkages, the frame substructure) while the 90 degree stuff is layered on top to take the day-to-day knocks that most bikes suffer, er, day-to-day.

Are you still with me? Good. So, manufacturers need to select their modulus and degree for every section of the bike frame or part (any given carbon frame will be composed of thousands of pre-preg sheets, made up of thousands of carbon fibres, composed of many more thousands of filaments. The higher the modulus, the stiffer and lighter the frame but, guess what, the more expensive. More complex combinations of different moduli and degree will also result in harder wearing, longer lasting, better performing frames.

The bottom line, is that Biketart’s major manufacturers, Genesis, Santa Cruz, Ibis, Saracen & Ridley have all chosen to follow the monocoque route, using bladder or foam injection techniques. Santa Cruz have taken the interesting approach of using two different modulus/degree/layup techniques, giving them the more affordable ‘C’ frames and the seriously light ‘CC’ options. Ibis (who explain their own process in this great article) are happy to stick with one set of techniques, selecting the layup, modulus and degree combinations for their frames to suit usage and sticking with it. Ridley offer different bikes with different modulus compositions and the more costly bikes are usually made up of 3 different moduli matched to their intended ride characteristics. Genesis use exclusively monococque, bladder moulded frames with a variety of modulus options, including (like Ridley) some with multiple moduli sheets within the same frame. Saracen follow the same approach, but the degree differences are more nuanced in their mountain bikes because, to put it bluntly, they are more likely to end up wrapped around a tree than road or cross bikes.

So what about the ride? Will one carbon approach feel much different to another? Will you really be able to tell the different in stiffness as you grind up another steep climb or huck off a six foot drop? Of course not! Any rider who tells you that they can is either a liar or has a complex set of digital sensors built into the hands and feet. Sure, we can all flex a crank or bar on a static bike, but the reality is that out on the road or trail, we are too busy riding to carry out a running quality analysis of the way the frame material on our bike feels.

But try telling that to someone who has just blown £25,000 on a boutique hand built Somethingorother road bike from a workers collective in Portland…