Ever since the wheel was invented 5000 years ago Man has been looking for new ways to move himself and other objects around more easily. The first two-wheeled device was shown in Paris in 1808 and it was developed and introduced in England as the Draisenne or Dandy-horse in 1818. This device essentially consisted of two wheels joined together by a wooden beam and it was propelled by the ‘rider’ striding along the ground. The addition of cranks in the 1840’s allowed the cycle to be driven by the rider and the penny farthing was introduced in 1870. In the early days bicycles were made from wood and the ride was often very uncomfortable. The invention of pneumatic tyres in 1846 by Thomson (they were reinvented by Dunlop in 1888) and the introduction of iron in the 1860’s allowed designs to be improved and the bicycle became a viable mode of transport for all. In the 1890’s T I Reynolds started producing relatively lightweight frames from steel rather than iron, but after this few major advances in materials were made until after the Second World War when aluminium, titanium and composites were introduced. Modern day bicycles are made from a variety of materials but the design considerations (weight, stiffness, strength, aerodynamics, cost and safety) are essentially the same, it is only the relative importance of these which changes.
Most commercial cycles for everyday use are still made from steel and the grade varies from low carbon to low alloy to high strength steel depending on the cost and final use. The tubes for the conventional diamond shaped frame can either be made seamlessly by extrusion or by rolling followed by welding. In order to reduce the weight of the frame set the tubes are butted, which means they are thicker at the ends where more strength is required. The tubes are then joined by brazing or welding.
High performance cycling is a very competitive sport and new materials are continually being introduced to further improve performance. Different types of bicycles have different requirements, for example a road racing cycle will have different requirements to an off road bike which will have different requirements again to those for the speed racers used in the velodrome. A wide variety of more ‘exotic’ materials are now available for these applications.
- Aluminium is probably the closest competitor to steel as it is relatively cheap and easy to form. The density of aluminium is about one third of that of steel and even though the tubes have to be thicker to compensate for the lower strength, aluminium bikes are often quite light. Alloys from the 5000, 6000 and 7000 series are used.
- Titanium frames are stronger than aluminium and lighter than steel and this combined with the excellent corrosion and fatigue resistance makes this material an ideal choice for bicycles. Originally Ti+6%Al+4%V was used but this has been replaced by Ti+3%Al+2.5%V as it is easier to form
- Magnesium alloys opened up a whole new world in bike building as it has an excellent strength to weight ratio. Magnesium can be extracted from sea water and it only takes 1.5m3 of water to produce the 2kg of metal needed to build a bicycle frame. The alloy used is Mg+9%Al+1%Zn and this is heated to 650°C and die cast into a one piece frame. The main reason why magnesium bicycles are not more popular is the cost.
- Carbon fibre frames are made from carbon fibres laid down in the desired orientation fixed together using an epoxy resin. The frame is made in one piece using a mould made specifically to fit the size of the rider. The resulting frame is extremely light and stiff and the fibres can be oriented so that strengthening is achieved where it is required most. The main drawbacks of carbon fibre frames are the mode of failure (they snap without yielding), the material is very difficult to machine and join and the cost since the production process is both time and labour intensive. As a result carbon fibre bikes are only found at the top end of the market.