I’ve raced on some amazing bikes over the years. I’ve also had a few dogs (and I’m talking strictly of bikes here, just for the record). But riding poor equipment is what sometimes happens when you’re not the one paying the bill; you get what you’re given. Here at Sunday we are often asked to explain why titanium is better than carbon. Most customers are surprised when I say, “it isn’t”.
To be more accurate, I should say “carbon could be better”. The thing is, before we can have a constructive conversation as to which material is best, we would need to find out a little more about your needs, and more importantly just what you mean by carbon.
All other things being equal, if I started racing again and could have any bike I could, if it was paid for, and replaced when I broke it, then I may well go for a carbon. Why ? Well it can be super stiff, comfortable, light and the bloke at my local café will think it looks cool. That’s right isn’t it?
Well, it could be, yeah……………..
The problem is we all use the term “carbon” like it’s a single material. We do a similar thing with titanium. But bike manufacturers usually only use 3Al, 2.5V, and occasionally 6Al, 4V Ti. But that’s really about it. So assuming all the manufacturers use ASTM grade material (so the impurities are tightly controlled), chemically you’re getting the same stuff. But carbon is a little bit more complicated. To say a frame is made from carbon is about as accurate as saying my frame is made from metal. It could be anything.
Let me explain………….
Carbon bike frames are made from what as an engineer, I would refer to as a fibre reinforced composite. In essence these material comprise of a matrix (the gluey bit) and a reinforcing fibre (the carbon fibre bit). The properties of the resulting material is dictated by five factors.
The individual properties of the matrix and the fibre.
The proportions of the matrix and fibre in the material.
The length of the individual fibres.
The geometric arrangement of the fibres in the matrix.
The interface between the fibres and the matrix.
The basic approach is to attempt to improve the strength and stiffness of a material (the matrix) by incorporating within it fibres that are actually stronger and stiffer than the matrix.
The carbon is often made by carbonising a polyacrylonitrile (PAN) filament to produce good strength and amazingly high modulus fibre but be aware of cheaper pre-cursers. Typically there are three types of carbon fibres produced, high modulus (HM), high strength (HT) and type A, which has properties somewhere between the two.
Orientation of these fibres is often commented upon in marketing literature. So, to clarify, when tiny fibres are used, they result in optimised isotropy of properties in theory. ie the properties of the material should be the same in all directions and this type of material can be found in frames and forks. However, processes such as injection moulding of such material usually result in great disparity of properties due to flow variation cased by the geometry of the moulded part. In small fibre systems, the percentage of fibres (to matrix) rarely exceeds 40% (by weight). So, most of the frame is glue. Woven reinforcement results, typically, to a maximum of 65% fibre to matrix. Unidirectional alignment of fibres results in the most dense fibre to matrix compositions (up to 90%) and offers the greatest enhancement of properties but at the cost of maximising the anisotropy of the material (it’s very directional!) This is often used to describe a great engineering advantage. Although this could be the case, it is often bad practice to incorporate materials of different stiffness’s as it can produce incompatible levels of deformation resulting in localised concentrations of stress which can lead to local failure.
The matrix is often an epoxy type resin although there are many different materials that could be used.
It is worth noting that controlling the manufacturing processes of carbon fibre materials appears much more difficult that metal forming techniques. I recently saw some data from a world famous composite tennis racket manufacturer where the same batch of production frames were cut up and identical pieces from many rackets were measured for a number of their physical properties (stiffness, etc). The properties varied greatly (and I mean GREATLY). The same factory manufactures bike frames.
Ok, so this brief intro has, I hope, demonstrated that a “carbon” material could have a hugely diverse range of physical properties due to the complex potential arrangements of the reinforcing fibres and composition of the fibres and matrix (properties can vary several 100%). And this is before we have even begun to select material cross sections, and tube geometries. Of course, these parameters can be changed for any other material, but should be done with the materials properties in mind as they can be material specific.
The carbon frames on the market range greatly in terms of their weight, stiffness, ride quality and cost. Although there are some that are fantastic they are not without drawbacks and the market is full of poorer quality products. But all things being equal, carbon could well offer the potential to deliver your dream ride. But it’s a minefield and even the best designs have some limitations.
So is carbon still the material for me? The thing is I’m not racing any more, I have to pay for my equipment, and I don’t (or can’t) replace it several times during the season. I also seem to spend a lot of time flying to Europe to ride the roads I used to race and train on and want something durable enough to cope with the occasional frustrated baggage handler. I ride a 3al, 2,5V titanium frame because I know what it is, what it’s capable of and have confidence that it will remain that way for year to come. I can’t at the moment say the same for most carbon fibre frames.
Yesterday, the guy at my local café picked my “Mondays Child” up. He was amazed how light it was and added that “it looks cool”. That’ll do me.
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