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November 15, 2007
Plenty of hoopla at this year's trade shows. Some new products are recycled versions of previous ideas that are coming around on a fashion cycle. Nothing wrong with that, especially if the idea is good and prospective users examine the previous incarnation.
Some ideas are genuinely new technology. Usually crossing over from another industry, these include materials, processes, and alternative ways to do familiar things. In truth, we enjoy very few such arrivals each year. Like other consumer product scenes, cycling's buzz is mostly static, clinking glasses and loud talking at the party.
The party we enjoy, sharing the excitement of cycling, is receptive to innovation, value, and cleverness. "Very clever" rates high on the "I want you at my next party" list. Few companies deserve the label "very clever" as well as Mavic. They bring a sense of style, timing, and an appreciation for the spectacle that's positively, how shall we say...French.
This year's R-Sys wheels represent another clever expression of their passion. Will this system show the promise of Ksyrium? Mavic certainly hopes so. But, in any case, R-Sys is a smooth evolution of the clever, alternative thinking that generated Ksyrium.
In Ksyrium's case, a group of designers (figuratively speaking) sat around a table and puzzled why aluminum is so well suited to rims and so unsuited to spokes. If you answer that question completely, you learn two things: one, steel wire is dramatically more capable of economically and efficiently handling the tensions of bicycle wheels. Two, you learn what you'd have to do with aluminum to make it a rival to steel wire for spokes. Well, this designer group answered the questions and, rather than stop with number one, they acted upon number two, creating a niche market that cycling will never forget.
It's worth pointing to spokes, because that's where R-Sys' uniqueness lies. After almost ten years of unchallenged success with Ksyrium, Mavic asked "what material might perform for us in the future like aluminum has?" Someone must have plopped some composite rod on the table saying "Hey, look at this stuff." A spoke made from it has good strength to weight compared to aluminum or steel. It's not too expensive, and it has great cachet in a market swooning over carbon fiber.
Mavic settled on a 4mm, pultruded, unidirectional, carbon fiber tube with aluminum end fittings to enable it to mimick a Ksyrium spoke. Would the classic tensioned spoke wheel need modification for this new material? With Ksyrium, the key to success came from ensuring:
(1) Enough material to safely carry the load.
(2) No spoke elbow, to spare the material from high, local stress.
(3) Perfect alignment of the anchor points at both hub and rim.
(4) High, super uniform tension, so loads are shared and spokes don't come to zero in riding since aluminum is more vulnerable to fatigue than steel.
With composite tubes for spokes instead of aluminum #1, #2, and #3 are less critical, but still beneficial. Interestingly, #4 is critical but for an entirely different reason. Composites are much less likely to suffer fatigue failure, so keeping the wheel out of zero tension is not so crucial. However, unlike metals, the carbon fiber tube is ill suited to a compressive force. In your hand it seems stiff, as if it can carry compression with ease. But in a wheel, it's hard to limit the force.
A big jump or pothole can temporarily (or permanently) deform a rim, forcing potentially giant loads on the spoke. Metal spokes simply bend out of the way and spring back. A composite tube is not so flexible. Forced to change its length very much and it will fracture and splinter. So, the success of R-Sys depends on high, uniform tension. The spokes must be protected from compression loads. Fortunately, Mavic is masterful at high end building so you can relax.
Some PR around their introduction explained that R-Sys is built at lower tensions, unlike other wheels. The key, according to the explanation, is that the stiffer composite spoke can carry compressive loads so successfully that less tension is needed. Essentially, when the rim deforms so much that the spoke is loose, it can still support some load compressively, impossible for thin steel wire. Well, that's basically true but not the whole story.
Perhaps the most important feature of the R-Sys system is not the compressive stiffness of the spokes but their amazing tension stiffness. They are far less elastic than steel wire. For a given applied load, the length doesn't change. When a wheel receives side (lateral) loads, one side of spokes goes up in tension and the other goes down. If the spokes doing the pulling do not increase length, then the rim will stay centered. This is the case with R-Sys. It's the reluctance of spokes in tension to increase length that makes the wheel so stable, even at lower tension. The compressive stiffness of the spokes going slack is hardly a factor. I hope I've explained this subtlety.
The only limitation to the system is the round shape of the thick spokes. Spheres, first, and cylinders, next, are the worst possible shapes for aerodynamics. I wouldn't want to see the wind resistance of R-Sys at racing speeds. No thanks. But for speeds encountered in vigorous training, endurance riding, and touring, it should be no problem. A few mph missing on descents is surely acceptable.
As to the advertised extra stiffness, most have described Ksyrium as near the limit of stiffness in terms of comfort. A measurable increase seems an improvement in a questionable direction. And the difference begs the question of how often side loads as large as Mavic tests occur. Single track vehicles, by leaning into turns, avoid most side loads compared to other vehicles.
The R-Sys wheels are lighter, too. However, demands upon the rims are potentially just as rigorous as with wire spokes. When a spoke is stiff and anchored at both ends, like R-Sys, the rim must be prepared to endure a push, not just pull, if high loads are encountered. I'd guess the rims are simply made lighter.
Once again, Mavic combines its special insight into wheels to pioneer an unusual feature, mastering the details, engineering weak points, and presenting the package with panache, like a Paris fashion premiere. Bien fait!
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