Rear hub has tangent aluminium spokes on drive side |
Distinctive Mavic styling |
Threadlock used on hub end spoke mount |
Threaded rim familiar from Ksyrium series |
Hub end non-threaded |
Splined threaded end attaches to rim |
Eight years on from shocking the cycling world with the flattened aluminium spokes of the Ksyrium wheelset, Mavic has done it again by building a wheel with spokes that do the apparently impossible by working in either tension or compression depending on the loading on the wheel.
It needs stating that this does not happen in a conventional wire-spoked wheel. All the spokes are in tension all of the time in a sound wheel. Any spoke that finds itself no longer in tension, as when the rim is deflected inwards enough to unload it, simply becomes entirely slack. If this happens momentarily to a wheel that hits a big enough bump, the spoke’s nipple may even start to unscrew, which is one reason wheels with low spoke tension, which slacken under a lesser load, quickly go out of true.
It is obvious on inspection that there is no mechanism by which a conventional wire spoke can work in compression, that is to say be compressed from the ends. Since the nipple is free to move outwards away from the rim, the rim cannot press inwards on the end of the spoke. Even if it could, the thin wire spoke would be unable to resist such compression without buckling, as is easily demonstrated with a single spoke.
The latest wheelset, labelled R-SYS, from the innovative French firm turns convention on its head with a concept that is not only able to compress the spokes, but uses spokes designed to be compressed at times. This novel idea, named TraComp from Traction Compression, relies, perhaps inevitably, on the incredible stiffness of carbon-fibre. Each spoke is made by moulding unidirectional fibres into a thin tube of about 3mm diameter. The hole down the middle is about 1mm diameter. The spoke would work equally well without the hole, but the hole, obviously, saves weight. Since the fibres all run along the length of the tube, it is very stiff in tension. Therefore, a sharp increase in tensile load results in a very small increase in the length of the spoke.
The spoke is also stiff in compression and, thanks to the diameter of the spoke and the high modulus of elasticity of carbon, resistant to buckling. A back of an envelope calculation of Euler’s theorem says that a steel spoke of slightly greater diameter able to resist buckling to the same extent would need to be solid and around 5 times the weight. Not surprisingly, the carbon spoke wins, since a steel spoke of 3mm diameter is almost four times the weight of a double butted 2mm-1.8mm-2mm spoke.
Locating this light, stiff spoke is done with Mavic’s well-known FORE system at the rim, which is threaded in the manner of a Ksyrium rim. A male threaded tip is bonded to the rim end of the spoke, and once inserted into the rim is resistant to both tension and compression. The hub end is a bit more complex. The spoke is inserted from the hub centre through a hole on the cylindrical flange; once tight, it pulls against the inside of the hub. To provide a seat to secure the spoke against pushing forces, a ring is placed inside the completed circle of spoke heads. A locking compound is used on the spoke head, presumably to lubricate the spoke as it is screwed into in the rim and to secure it at the hub once hardened.
Mavic says that the key to the whole idea is that the spokes are only lightly tensioned when the wheel is complete. Under normal rolling loads, each spoke remains in tension. Under sudden exceptional loading of the rim, as when hitting a bump or sprinting out of the saddle, the spoke may go from being in tension to being compressed. Since it is designed for this, it continues to contribute to the integrity of the wheel. Should a conventional tension spoke become unloaded, it no longer has any influence on the rim, which measurably loses stiffness when this happens. The ultimate outcome of such an unloading is seen when a wheel ‘crisps’, and all spokes lose their tension.
At the front, all the spokes are the same. However, since the rear wheel must be dished to accommodate the cassette, the tension in the drive side spokes has to be much greater than in the non-drive spokes. The R-SYS design gets around this by using Ksyrium-type aluminium spokes on the gear side. These are highly tensioned as per the Ksyrium design, and provide the tangent cross pattern that transmits drive torque. The TraComp spokes on the non-drive side need only be lightly tensioned to provide the balancing pull, and therefore resist loads in the same way as in the front wheel. Mavic says this way of locating the spokes also avoids problems with crossing, which is inevitable somewhere in the back wheel. Furthermore, prototypes with tangent TraComp spokes on the drive side proved incredibly stiff and uncomfortable to ride.
Mavic claims several advantages to the new technology (naturally). Since the spokes exert only a light pull on the rim, it can be made with less material and is therefore lighter than, say, the Ksyrium SSC rim. The wheel is much more resistant to crisping under severe side loading, since the compressed spokes still support the rim. The light tension in the spokes allows them to absorb road shock more effectively than steel spokes. Furthermore, should a spoke break, the wheel will stay fairly true because lower tension in the remaining spokes is less able to distort it. Due to the use of the aluminium drive side spokes on the back wheel, some of this must apply more to the front wheel than the rear.
On the minus side, the thick, round spokes are not good for aerodynamics, and each spoke is vulnerable to crash damage in the same way as the composite spokes of a Lightweight wheel. On the other hand, they can be replaced.
RCUK will be reporting on the real-life performance of these wheels as soon as a pair arrives on the editor’s desk. So far they have been ridden by Saunier Duval in the Giro, and have been spotted on Saunier team bikes at the Tour de France prologue. Apparently they are great for climbing and, thanks to great rim stability, provide very precise handling on mountain descents.
Projected price is €1100 per pair, and the wheels will be offered with Mavic’s usual freehub body options in clincher or tub format at wheel pair weights of 1355g and 1330g respectively.
www.mavic.com
www.chickencycles.co.uk
