I thought I would pull together some of the information on Carbon spokes which are becoming more common on wheels today.
Carbon fibre is the perfect material to make a spoke from. Unidirectional carbon fibres are three times the longitudinal strength of structural steel whilst being five times lighter - the carbon fibre part of the spoke could have a tensile strength of more than 2 tons. Each spoke comprises many fibres bonded together, but they all run in the same direction. The image below is an electron microscope cross-section of a Vortex spoke, which clearly shows the individual fibres. The other benefit is the spoke can be moulded into whatever shape the wheel designer requires round, flat or aerodynamic - steel spokes will always have to consider weight as you move to more extreme formats.
- Whilst carbon fibre is perfect for the spoke itself, the slower take-up is because connecting the spoke to the rim and hub is the more significant engineering challenge. Earlier wheels used wide blades like Spynergy Rev-x or HED tri-spoke wheels for the track and TT. The next to try was Mad-Fibre, but they ultimately failed due to manufacturing quality issues. This leads us to today and point number 3.
- There are four types of carbon spokes that you will see out in the wild today:
- Moulded in spokes - this is where the spokes are an integral part of the carbon fibre structure by having the fibres of the spokes being joined to the fibres in the rim and hub. Examples include Mavic, Corima, Lightweight. (1 in the diagram)
- Bonded at both ends - this is where the spoke is manufactured, and the metal ends are bonded on. With today's bonding capabilities, there is nothing wrong with this method, but the spoke will only ever be as strong as the bond. Therefore there is some loss of the benefit of UD carbon fibre. Examples include first-generation Winspace and Vortex. (2 in the diagram)
- Interference fit - the spoke ends are tapered to provide an interference fit with the metal end, which ultimately will provide a very secure fit. However, there is nothing to stop the spokes spinning or even seeing very small changes in tension as the wheel is ridden. (3 in the diagram)
- Wrap over at one end - the fibres wrap over the head of the spoke and are bonded at the end; this maximises the benefit of the UD carbon fibre. The carbon fibre still needs to be attached at one end, but this is now reduced. (4 in the diagram)
In conclusion, unidirectional monofilament carbon fibre is perfect for making a spoke, and it is the connections at each end that is a more important question. I think that there is still plenty of refinement of both types of carbon spoke, and this will improve the weight, stiffness and ride characteristics. There will be a move away from hubs, spokes and rims being three items, and you put them together. The optimisation of the total wheel system will always be better if the wheel producer is in charge of all three and develops them as one, or they may start with the spoke and develop the wheel around that (something which would not have been considered a few years ago). As we reach the point that wheels are all very good, differentiation will come in areas like ride quality, weight, durability, and stability, which carbon spokes can contribute to.
If you would like to find out more about wheels designed as a system with carbon spokes at its heart, then look at the Vortex Wheel range at Cycling100.com