Material comforts

Put new materials in the frame

by Michael Marshall

Technical advances are making the already popular sports of cycling and mountain biking even more fun. Major improvements in cycling technology, notably in suspension and braking, already make for a safer and more comfortable ride, but it is in the field of materials science where the real impact is felt. Sciencebase guest writer Michael Marshall takes us on a smoother than smooth ride through the latest material gains in cycling technology.

Cycling has enjoyed a resurge in popularity over the last few decades, mostly due to the increasing interest in mountain biking. This has been matched by considerable improvements in cycling technology, notably in the fields of suspension and braking. However, some of the most crucial advances have been in the field of materials science; bikes nowadays are made from a wide range of different substances, with strikingly different properties and capabilities.

Traditionally, bicycles were made from steel, which is simply an alloy of iron and carbon. Steel is extremely tough and fatigue-resistant, and also has the key advantage of excellent damping properties. This means that much of the bumpiness of the ride is absorbed within the frame, rather than being transmitted through to the rider, and it consequently feels smooth to ride. There are many different types of steel, often with extra substances added to the alloy in small quantities. I have to confess an allegiance; my own bicycle is made of CroMo steel, which contains chromium and molybdenum and is consequently very strong and light.

However, steel has fallen out of favour with manufacturers. This is partly because it has a tendency to rust, but principally because of the high costs of steel frame manufacture. In its place, aluminium has swept the board to become the most widely-used material, principally because aluminium frames are easy to manufacture. The vast majority are assembled by the Tungsten Inert Gas (TIG) welding process, which was developed in the late 1940s.

TIG is a form of electric arc welding, in which two electrodes are brought into contact and a large current passed through them. The current produces intense heating; when the electrodes are separated, the current travels between them through the gap, forming a high-energy arc. This arc can reach temperatures of 2-3,000°C, ideal for welding work.

In TIG, the arc is formed between a single electrode, made of tungsten, and the metal to be welded. An inert gas, typically argon, is used to prevent unwanted chemical reactions from occurring. This distinguishes TIG from other forms of electric arc welding. Ordinarily, it is necessary to coat the electrode with a substance called a 'flux', which acts as a shield to prevent chemical reactions. TIG dispenses with this and is consequently more efficient. In fact, in most cases it is now an automated process, and has proved extremely reliable.

Aluminium is much lighter than steel and doesn't rust. However, it is much more prone to fatigue. Furthermore, when it breaks it does so suddenly and rapidly; steel has the good grace to crack slowly, giving the rider a chance to spot the damage and obtain a repair.

Being very stiff, aluminium also lacks the damping properties of steel; as a result, every little bump is transmitted to the rider and the bicycle is much less comfortable to ride. This becomes less of a problem on full-suspension mountain bikes, where the front and rear suspension, coupled with the large tyres, serve to cushion the rider. However, one tends to prefer that such bikes, the cycling equivalent of Land Rovers, will be fatigue-resistant! On hardtails (front suspension only) or rigid bikes, and particularly on road bikes with their thin tyres, aluminium proves a rather uncomfortable ride.

If steel is expensive, and aluminium uncomfortable, are there other options? Several other materials have come onto the market in recent years. All are still rather expensive due to the costs of continuing development, but prices are falling rapidly.

Carbon fibre has already achieved a certain degree of popularity. It is a matting of carbon fibres, encased in epoxy resin. It is possible to make it stiff in one direction but springy in another, which sounds ideal, as the substance should be very strong but also give a comfortable ride. Indeed, it is quite common for road bikes to have their forks made of carbon fibre; this ensures that the bike rides reasonably comfortably even if the rest of the frame is made of aluminium.

However, carbon fibre has acquired some notoriety for its brittleness; it has a tendency to shatter with no warning. It's for this reason that Gerard Vroomen of Cervélo, specialists in carbon fibre frames, said in a recent interview "...I'd rather have a two thousand dollar aluminium bike rather than a two thousand dollar carbon bike, because you know that the really bad ones have been eliminated. The same can't be said for carbon fibre."

Another material with a great deal of promise is magnesium. It is much lighter than steel and aluminium but is strong and has excellent damping properties. However, there are many practical difficulties with the manufacture of magnesium frames. At present, it is difficult and expensive to extrude magnesium tubes, leading to vastly elevated costs.

Perhaps the ideal material, however, is titanium. It is light but extremely strong; it is fatigue-resistant; it is not brittle, it never rusts and it even rides comfortably. It has a reputation as being a rare and expensive metal, but is in fact the 9th most common element in the earth's crust, and the 4th most common metal. Its high cost comes about as a result of the complicated extraction procedures necessary to obtain the pure metal, and the difficulties of working with the metal. It seems unlikely that these costs will decrease significantly in the near future, and titanium bicycles will therefore remain a high-cost, high-quality option.

It seems there is no single ideal substance from which to make bicycle frames. Each of the materials discussed is useful for certain types of bicycle, or for individual components (though carbon fibre still needs a great deal more development work before it can be considered reliable). However, thanks to the materials advances discussed, we are now in a position where the majority of cyclists can identify a frame that will suit their needs.