Despite many attempts over the years to build bikes from all manner of materials, steel has held sway for most of the life of the bicycle. The quality of the material used has gradually improved so that now tubes just 0.5mm thick can be used (that's about 5 sheets of photocopy paper)!
Titanium and carbon fibre are generally reserved for the elite riders or enthusiasts with deeper pockets
Steel bicycle frames have been brazed together using lugs (shaped sockets that hold the tubes in their correct relative positions) for almost as long as bicycles have been made. The reasons for this are simple: (A) it works! And (B) It can be done with relatively unsophisticated equipment.
The down-side is that the residue left after the brazing procedure is difficult to remove and needs a complete sub-process to prepare the frame for painting. With advances in joining technology, welding (instead of brazing) became a much more viable option, as it gives a much cleaner joint.
Welding is a fusion process where the edges of the joint are fused or melted together with the addition of a filler rod of the same material. Brazing is a process where the joint is heated to the meting point of the filler material, brass in this case, which acts as a glue. The problem with welding was always the lack of fine control. It was great for joining pieces of ship or steel bridge girders together, but for the thin-walled steel tubes of a bicycle the problems outweighed the advantages until the process became refined enough through modern microelectronics.
There are a variety of welding processes, but the one commonly used in bicycle construction is TIG (Tungsten Inert Gas) where an electric current passes between a tungsten electrode and the job in the form of an arc. The temperature in this arc is sufficient to melt the joint edges and a separate filler rod is added. To prevent reaction with atmospheric oxygen a shield of argon gas is fed over the joint. The degree of control available with a good modern welding set is such that welding tubes just 0.5mm thick is no problem to an experienced welder. The net result is a clean, generally oxide free, joint that needs little or no subsequent processing. Practically all of the volume produced steel frames in the world are now made using this process.
The most abundant metallic element in the earth's crust, aluminium was a wonder metal when it was first extracted in the 1850s
Taking this a step further, if steel can be replaced with a material that is easier to cut and shape, is no more difficult to join, and is cheaper to obtain as a raw material, then further commercial gains are to be made.
Enter aluminium!
The most abundant metallic element in the earth's crust, aluminium was a wonder metal when it was first extracted in the 1850s. Bars of the metal were exhibited at the Paris Exhibition of 1855 next to the Crown Jewels - and Napoleon III had a dinner service made from it because it was then so rare! In its pure state, aluminium is virtually useless as a construction material, as it is far too soft. Mix it with other alloying elements however, such as copper, silicon, zinc and magnesium and the picture changes dramatically.
There are hundreds of aluminium alloys, each designed for a particular purpose. The qualities of the alloy can be tailored to suit the purpose. Most frame manufacturers favour 7000 series, which is a far cry from the material that was available 20 years ago.
The problem of joining aluminium has always been the oxide that forms on the surface of any aluminium exposed to the air. This oxide has a melting point far in excess of the melting point of the metal. As a result, when you melt aluminium it up in unsightly blobs of molten metal surrounded by a skin of oxide. This problem used to be handled in the gas welding process by using a chemical flux that dissolved the oxide. Gas welding aluminium is akin to balancing a sword on the point of another sword!
A TIG welder uses an alternating current to actually break down the oxide layer, so allowing the relatively easy welding of aluminium. So now we have a cheap, easily joined, easily manipulated material that is lighter than steel and needs no finishing after it is welded. No wonder it is catching up with steel in the popularity stakes for low to mid market sports and leisure bikes!
The downside (there had to be one!) of aluminium is in the ride quality when it is used for 'good' bikes. The frames are very 'stiff' and give quite a harsh ride. This may not be a problem for short distance racing bikes, but it can be for branches of the sport such as audax riding or touring, where the more resilient qualities of a top class steel tube set are more welcome.
The downside (there had to be one!) of aluminium
Aluminium also has a shorter fatigue life than steel, so the life expectancy of such a frame is less than steel, and its 'repairability' is much lower than steel. By and large, a steel frame is usually 'fixable' no matter how it is damaged. An aluminium frame is much more difficult to repair, if it can be done at all.
The other two materials used in bike construction, both for frames and components are titanium and carbon fibre. There are several titanium frame suppliers (mostly in the USA) and lots of 'high end' components such as seat pins, sprockets, use titanium bolts or parts. Titanium is extremely difficult to work with, both to machine and join. Hence it is expensive. Titanium is about 40% lighter than steel and it has a fatigue life of up to 200% more than steel. Like aluminium it has to be alloyed with other metals to get the best out of it. A popular alloy widely employed in bike frames is 3Al-2.5V, which contains 3% aluminium and 2.5% vanadium. This alloy was originally developed for aircraft hydraulic lines but as well as bike frames it is also used to make golf clubs, tennis rackets, pool cues and ski poles!
In addition to its light weight, a major plus for titanium is that it does not corrode; so most titanium frames are left unpainted, partly to show off the material and partly to save the weight of the paint. If the frame is carefully designed using custom tubing then the result can be extremely good, but this must balanced against its high cost. For most riders the motivation behind choosing titanium is its light weight but the material also has an inherent springiness quality which provides a very comfortable ride.
Carbon Fibre
Carbon Fibre is a composite material made up of a resin reinforced with fibres of almost pure carbon. It is produced by heating polyacrylonitrile fibres first to about 300°C in air to oxidise it, and then to about 3000°C in an inert atmosphere to carbonise it. It can be formed into tubes or sheets, and also moulded into specific complex shapes. This enables strength to be built into the structure in specific areas by increasing the amount (thickness) or the type of fibre, or by altering the orientation of the fibres.
Carbon fibre can have three times the tensile strength and only a quarter of the weight of steel! Sounds magic, so there must be a catch, right?
Carbon fibre composite can have three times the tensile strength and only a quarter of the weight of steel! Sounds magic, so there must be a catch, right? There is, carbon fibre needs to be treated very carefully as relatively minor surface damage can cause failure, and unlike steel it tends to fail 'catastrophically' i.e. no warning - bang - it's broken.
Carbon fibre is used quite extensively in higher end components. The Italian manufacturer Campagnolo uses carbon fibre in its top of the range shift levers, gear mechanisms, bottom brackets and seat pins and a number of specialist manufacturers make carbon fibre hubs and wheels. The material is now commonly used for front forks on racing bikes. Again, weight saving is the major motivating force for most buyers and some models can weigh as little as 300g.
Carbon fibre frames fall into two categories:
1. Conventional, where the structure is a diamond frame with carbon fibre tubes bonded into aluminium lugs, or moulded to look like a conventional frame.
The Lotus bike
2. Monocoque construction, a moulded construction which looks nothing like a conventional frame but still holds all the components in the same relative positions. The Lotus bike designed by Mike Burrows and ridden to gold in the 1992 Olympics by Chris Boardman is a good example of this type. However, such frames are not common due to the high costs of the moulding process and there being very few manufacturers and because of cost and other limitations so this sort of bike is really restricted to top level competition (or top level posing!)
No doubt new materials will come along in the future as technologies progress. But it's safe to say that for the moment the majority of frames will be made from steel or aluminium - and even these materials are continually being developed for improved strength to weight ratios - with titanium and carbon fibre generally reserved for the elite riders or enthusiasts with deeper pockets.