Hydroforming is a process of creating a formed (metal) tube by way of a mould.
By using a ‘blank tube’, placing it in a mould and injecting a hydraulic fluid to expand the blank, it conforms to the shape of the mould creating a tube with form, allowing for various design details to be created. Primarily a hydorformed tube is the only way to create detailed formed and fluid shapes in aluminium tubes and also offer the designer that ability to engineer shapes that can bear specific loads, or overcome specific spatial problems in an elegant manner. But while there has been a massive adoption of hydroformed tubes, primarily in the goal to create more formed designs, it should be noted that a hydroformed tube may not necessarily be the strongest or the lightest solution. Quite often a hydroformed tube is required to have a thicker wall thickness in order for a desired form to be achieved.
While the ‘blank tube’ can utilise butting in order to lighten the finished tube, it does not mean that the final product is as efficient as a simpler round tube, though it may look much more interesting. Additionally, the cost of tooling for hydroforming is very high, hence the end product will cost more than a similar and potentially stronger traditional design; also as the cost and complexity of tooling is high minor changes are often difficult to achieve. Hydroforming requires careful and extensive engineering and design in order to create lightweight, efficient results. When done right though, the end result will often be near impossible to achieve in any other manner other than through the use of far more expensive carbon fibre forming.
A monocoque construction, also known as ‘stressed skin’, is made from a skin of material that is formed to create a shape and can be either formed as a single piece, when using materials like carbon fibre, or by the joining of two clamshells.
The original Mountain Cycle San Andreas is a classic example of a monocoque structure made from two halves and the biggest monocoques employed today are found in the aerospace sector, where most aircraft fuselage are monocoque structures. The key advantage to monocoque design is that a ‘single’ skin can be formed to carry all the loads required, hence can often create a very light and very stiff end product. If we look at the original San Andreas, the head tube, swingarm pivot, bottom bracket and seat pod, are all directly linked to the one single piece, the skin itself bearing all the loads. The big advantages of monocoque design are that the skin can be designed to handle specific loads in specific locations, with the potential to create an undulating cross section from a thin skin. Manufacture can be highly simplified and the accuracy of aligning critical points can be increased. The key downside of a monocoque design is that it is highly unadaptable and minor changes can result in extensive re-tooling. As a result the use of monocoque designs for bicycles has been limited, where there is the need for not only easy manufacturing adjustments but also the requirement for a range of sizes.
The darling of the hi-tech sector, carbon fibre is in many ways the ultimate engineering material.
Unlike metal forming, where the end product is ultimately dictated by the inherent properties of the material, carbon fibre can be specifically engineered to behave in a predicted and desired way. By using various fabrics, weaves and layup techniques, carbon fibre is perhaps the most flexible of all materials. Additionally, as it starts life as a fabric, it can be formed to adapt to any number of shapes, including simple tubes, monocoque as well as easily replicate hydroformed shapes and forms unachievable with any other material. The drawbacks of the material though are the the base tooling is expensive and complex. The manufacture is slow and requires exacting standards as any introduced impurity, air pocket or misaligned fabric can lead to catastrophic failure. Furthermore, one can not take carbon fibre and copy an existing metal design as it has its own demands and weaknesses, especially when it comes to mating aluminium harpoints such as pivots, head tubes and drop outs.
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