Why use aluminium spokes?
Notes on an increasingly popular fitment
Posted 6 September 2012
Words by Richard Hallett
Why, several respondents have asked apropos the recent Remerx Alcyon wheel test, bother with aluminium spokes? Why, indeed? After all, until Mavic came up with the Ksyrium wheel concept in 1999, nobody had managed to make tensioned aluminium spokes that worked reliably as traditional bicycle wheel spokes. Maybe nobody had tried; after all, steel spokes work perfectly well in an application where aluminium would appear to offer no advantage whatsoever.
Aluminium – or, more accurately, high-strength aluminium alloy – is valued for its combination of light weight, which is about one third that of steel, and comparatively high yield strength – the point at which permanent deformation of a part takes place. Although even the strongest aluminium alloys are not as strong as high-strength alloy steels, their strength-to-weight ratio may be superior.
On that basis, using a suitable aluminium alloy to make bicycle wheel spokes sounds like a good idea. An aluminium spoke with the same strength but less weight than the equivalent in steel has to be better, right? On the face of it, yes; or why not have an aluminium spoke with the same weight as the steel spoke but with greater strength? This would allow the former reliably to be given a greater tension, which should enhance power transfer from hub to rim.
Such is the claim made by Remerx for the aluminium spokes used in the Alcyon wheels; “Is possible to achieve the best results with Aluminium spokes because their strength is about 4000N and have large seating area… Stainless steel spokes have higher weight, their strength is only 3000N and critical point is head bending.”
In other words, the Czech firm’s aluminium spokes are stronger than their steel ones – but not necessarily stronger than the best steel spokes. Sapim’s Race double-butted spokes, for example, have a quoted strength (whether breaking or yield is unstated) of around 4,200N; even the super-light Laser, with 1.5mm middle section, reaches 3375N.
Note, too, that the figure for the Remerx spokes is achieved thanks to a fat cross-section of around 10 sq mm, which is about three times that of the narrow part of Sapim’s Race. No spoke weights are given but, if three times fatter, the aluminium spokes will be about the same weight to within a few grammes, the difference coming from the greater weight of the steel spokes’ butted lengths.
Strength, however, is only part of the equation – and not the most important since spoke breakage through exceeding its breaking strength is almost unknown. More important is stiffness – the fundamental property of the elastic modulus, which in the case of spokes is simply how much they stretch under a given load.
This is where it comes into play. That of aluminium is roughly one third that of steel and for the two metals is in almost exact proportion to their weights, so if an aluminium spoke is to be as stiff as steel it must have a cross-section three times greater. Which is the case with aluminium spokes from both Mavic and Remerx – and quite probably from Fulcrum and any other manufacturer using them. So, they have the same stretch under the same tensile load; again, as with weight, there’s a small advantage to aluminium since steel spokes usually have a small, thicker butted length at each end and won’t stretch quite as much.
If there is little discernible difference in mechanical properties, what real advantage might aluminium offer? Remerx claims that the large diameter of the spoke requires a similarly oversized nipple that offers a “large seating area” against the rim. It must also, by definition, require a larger hole in the rim, which must then be weaker.
In any case, it is not obvious that, in this respect, bigger is better. Besides, aluminium spokes are always straight-pull, since forming the J-bend traditionally used with steel would put unwanted stresses into the spoke material. Since the metal on the outside of the bend in an aluminium spoke would have to stretch – deform – further than with steel, it would be “worked” to a greater extent.
This is one difficulty that has to be overcome with aluminium; the other is that of forming a thread that does not render aluminium susceptible to fatigue. Mavic’s Ksyrium spoke has a nipple that slides over the spoke and screws directly into the rim. The technology, called Fore, means the spoke does not need an external thread. It’s also a problem for steel, but one that rolled threads and accurate assembly mostly overcome.
Which leaves aluminium, where? Is there a damping effect on ride quality? A search of the published research suggests not; that there is no noteworthy difference in “loss factor” – how much energy is lost to internal friction – between steel and aluminium – and titanium, for that matter. On the other hand, wheels with aluminium spokes do feel less springy, perhaps due to some facet of the interface between spoke and either hub or nipple. Let’s agree there is a difference in ride feel; whether it’s liked or not is a question for personal preference.
So why is aluminium preferred over steel for components such as cranks and brake caliper arms? It scores on stiffness over steel and titanium when used in a component subjected to bending or twisting loads, since the aluminium member must be thicker for the same weight. This doesn’t apply with spokes, which are simply stretched, not bent.
Add in the fact that aluminium spokes are fatter than steel ones; fatter means more frontal area, which means slower through the air. Of course, it is possible to flatten the spokes to a more-or-less aero section; those of Mavic’s Ksyrium wheels are 5mmx2mm, those of the Remerx wheels 4mm x 2.5mm (including paint). Without a true airfoil shape, however, the result is no better than a round spoke of the same diameter and clearly inferior to a similarly shaped steel spoke of narrower cross-section.
In short, there’s a reason most high-performance wheels still come with steel spokes – usually flattened or bladed; steel does the mechanical job every bit as well and makes for a thinner, more aero spoke. And it’s less expensive. Aluminium spokes aren’t superior to top-end steel spokes. But they do look good and, on the evidence, they build into a fine-riding, seemingly well-damped wheel.